diff --git a/CHANGELOG.md b/CHANGELOG.md
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,6 +2,45 @@
 
 See full history at: <https://github.com/faylang/fay/commits>
 
+### 0.24.2.0 (2021-01-10)
+
+* Drop GHC 8.0 support.
+* Cabal upgraded to 2.0 spec-version.
+* Travis CI refactored to allow cabal-install 3.2.0.0 builds.
+
+### 0.24.1.1 (2021-01-08)
+
+* Relax boundaries for aeson, tasty, base-compat. Allow builds for Gentoo (#469).
+
+### 0.24.1.0 (2020-03-26)
+
+* GHC 8.10.1 support.
+
+#### 0.24.0.5 (2020-03-10)
+
+* GHC 8.8 support.
+
+#### 0.24.0.4 (2019-12-03)
+
+* Fixes stack nix integration broken.
+
+#### 0.24.0.3 (2019-04-29)
+
+* Dependency updates including GHC-8.6 support.
+* Added stack compatibility.
+
+#### 0.24.0.2
+
+* Fix dependent compilation fail when building project with stack (#457).
+
+#### 0.24.0.1
+
+* Dependency updates incl GHC 8.4 support
+
+## 0.24.0.0
+
+* Add the option to generate typescript output, thanks to Junji Hashimoto.
+
 ### 0.23.2.0
 
 * GHC 8.2 support
diff --git a/Setup.hs b/Setup.hs
--- a/Setup.hs
+++ b/Setup.hs
@@ -7,24 +7,10 @@
 import Distribution.PackageDescription
 import Distribution.Simple
 
-main = do
+main =
   defaultMainWithHooks simpleUserHooks
-    { preConf = \_args _flags -> do putStrLn reminder
-                                    return emptyHookedBuildInfo
-    , postInst = (\_ _ _ _ -> putStrLn fayBaseReminder)
+    { postInst = \_ _ _ _ -> putStrLn fayBaseReminder
     }
-
-reminder =
-  "                                                                               \n\
-  \- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -\n\
-  \                                                                               \n\
-  \  REMEMBER: This compiler is in flux, supercalifragelistic style. You should   \n\
-  \            read the CHANGELOG for this release as the changes probably        \n\
-  \            affect you.                                                        \n\
-  \                                                                               \n\
-  \                                                                               \n\
-  \                                                                               \n\
-  \- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -\n"
 
 fayBaseReminder =
   "                                                                               \n\
diff --git a/examples/calc.hs b/examples/calc.hs
--- a/examples/calc.hs
+++ b/examples/calc.hs
@@ -32,7 +32,7 @@
                                       return ()
                               else do setVal (show n) display
                                       writeRef appendMore True)
-      getInput = getVal display >>= (return . parseDouble 10)
+      getInput = getVal display >>= (return . parseDouble)
       operator op = do
         calculate
         num <- getInput
@@ -104,8 +104,8 @@
 --------------------------------------------------------------------------------
 -- Utilities
 
-parseDouble :: Int -> String -> Double
-parseDouble = ffi "parseFloat(%2,%1) || 0"
+parseDouble :: String -> Double
+parseDouble = ffi "parseFloat(%1) || 0"
 
 --------------------------------------------------------------------------------
 -- Refs
diff --git a/fay.cabal b/fay.cabal
--- a/fay.cabal
+++ b/fay.cabal
@@ -1,5 +1,6 @@
+cabal-version:       2.0
 name:                fay
-version:             0.23.2.0
+version:             0.24.2.0
 synopsis:            A compiler for Fay, a Haskell subset that compiles to JavaScript.
 description:         Fay is a proper subset of Haskell which is type-checked
                      with GHC, and compiled to JavaScript. It is lazy, pure, has a Fay monad,
@@ -22,9 +23,8 @@
 copyright:           2012 Chris Done, Adam Bergmark
 category:            Development, Web, Fay
 build-type:          Custom
-cabal-version:       >=1.8
+Tested-with:         GHC ==8.4.4 || ==8.6.5 || ==8.8.3
 data-files:
-  js/runtime.js
   src/Fay/FFI.hs
 extra-source-files:
   CHANGELOG.md
@@ -72,11 +72,19 @@
   manual:            True
   default:           False
 
+custom-setup
+  setup-depends:
+      base
+    , Cabal
+
 library
+  default-language:  Haskell2010                           
   ghc-options:       -O2 -Wall
   hs-source-dirs:
     src
     src/haskell-names
+  autogen-modules:
+    Paths_fay
   exposed-modules:
     Fay
     Fay.Compiler
@@ -112,6 +120,7 @@
     Fay.Exts
     Fay.Exts.NoAnnotation
     Fay.Exts.Scoped
+    Fay.Runtime
     Fay.Types.FFI
     Fay.Types.Js
     Fay.Types.ModulePath
@@ -134,18 +143,18 @@
     Language.Haskell.Names.Types
     Paths_fay
   build-depends:
-      base >= 4.9 && < 4.11
-    , base-compat >= 0.8 && < 0.10
-    , aeson > 0.6 && < 1.3
+      base >= 4.9 && < 4.15
+    , base-compat >= 0.10 && < 0.12
+    , aeson > 0.6 && < 1.6
     , bytestring >= 0.9 && < 0.11
-    , containers >= 0.4 && < 0.6
+    , containers >= 0.4 && < 0.7
     , data-default >= 0.2 && < 0.8
     , data-lens-light == 0.1.*
     , directory >= 1.1 && < 1.4
     , filepath >= 1.3 && < 1.5
     , ghc-paths == 0.1.*
-    , haskell-src-exts >= 1.18.1 && < 1.20
-    , language-ecmascript >= 0.15 && < 0.18
+    , haskell-src-exts >= 1.20
+    , language-ecmascript >= 0.15 && < 0.20
     , mtl >= 2.1 && < 2.3
     , mtl-compat >= 0.1 && < 0.3
     , process >= 1.1 && < 1.7
@@ -155,19 +164,21 @@
     , spoon >= 0.1 && < 0.4
     , syb >= 0.3 && < 0.8
     , text >= 0.11 && < 1.3
-    , time >= 1.4 && < 1.9
-    , transformers >= 0.3 && < 0.4 || > 0.4.1 && < 0.7
-    , transformers-compat >= 0.3 && < 0.6
+    , time >= 1.4 && < 1.10
+    , transformers >= 0.3 && < 0.6
+    , transformers-compat >= 0.3 && < 0.7
     , traverse-with-class >= 1.0 && < 1.1
     , uniplate >= 1.6.11 && < 1.7
     , unordered-containers == 0.2.*
     , utf8-string >= 0.1 && < 1.1
     , vector < 0.13
+    , shakespeare < 2.1
 
   if impl(ghc < 7.8)
     build-depends: tagged
 
 executable fay
+  default-language:  Haskell2010                           
   hs-source-dirs:    src/main
   ghc-options:       -O2 -Wall
   main-is:           Main.hs
@@ -175,11 +186,13 @@
       base
     , fay
     , mtl
-    , optparse-applicative >= 0.11 && < 0.15
+    , optparse-applicative >= 0.11 && < 0.16
     , split
   other-modules:
     Paths_fay
+
 executable fay-tests
+  default-language:  Haskell2010                           
   ghc-options:       -O2 -Wall -threaded -with-rtsopts=-N
   hs-source-dirs:    src/tests
   main-is:           Tests.hs
@@ -201,8 +214,8 @@
       , fay
       , filepath
       , haskell-src-exts
-      , random >= 1.0 && < 1.2
-      , tasty >= 0.9 && < 0.12
+      , random >= 1.0 && < 1.3
+      , tasty >= 0.9 && < 1.5
       , tasty-hunit >= 0.8 && < 0.11
       , tasty-th == 0.1.*
       , text
diff --git a/js/runtime.js b/js/runtime.js
deleted file mode 100644
--- a/js/runtime.js
+++ /dev/null
@@ -1,821 +0,0 @@
-/*******************************************************************************
- * Misc.
- */
-
-// Workaround for missing functionality in IE 8 and earlier.
-if( Object.create === undefined ) {
-  Object.create = function( o ) {
-    function F(){}
-    F.prototype = o;
-    return new F();
-  };
-}
-
-// Insert properties of b in place into a.
-function Fay$$objConcat(a,b){
-  for (var p in b) if (b.hasOwnProperty(p)){
-    a[p] = b[p];
-  }
-  return a;
-}
-
-/*******************************************************************************
- * Thunks.
- */
-
-// Force a thunk (if it is a thunk) until WHNF.
-function Fay$$_(thunkish,nocache){
-  while (thunkish instanceof Fay$$$) {
-    thunkish = thunkish.force(nocache);
-  }
-  return thunkish;
-}
-
-// Apply a function to arguments (see method2 in Fay.hs).
-function Fay$$__(){
-  var f = arguments[0];
-  for (var i = 1, len = arguments.length; i < len; i++) {
-    f = (f instanceof Fay$$$? Fay$$_(f) : f)(arguments[i]);
-  }
-  return f;
-}
-
-// Thunk object.
-function Fay$$$(value){
-  this.forced = false;
-  this.value = value;
-}
-
-// Force the thunk.
-Fay$$$.prototype.force = function(nocache) {
-  return nocache ?
-    this.value() :
-    (this.forced ?
-     this.value :
-     (this.value = this.value(), this.forced = true, this.value));
-};
-
-
-function Fay$$seq(x) {
-  return function(y) {
-    Fay$$_(x,false);
-    return y;
-  }
-}
-
-function Fay$$seq$36$uncurried(x,y) {
-  Fay$$_(x,false);
-  return y;
-}
-
-/*******************************************************************************
- * Monad.
- */
-
-function Fay$$Monad(value){
-  this.value = value;
-}
-
-// This is used directly from Fay, but can be rebound or shadowed. See primOps in Types.hs.
-// >>
-function Fay$$then(a){
-  return function(b){
-    return Fay$$bind(a)(function(_){
-      return b;
-    });
-  };
-}
-
-// This is used directly from Fay, but can be rebound or shadowed. See primOps in Types.hs.
-// >>
-function Fay$$then$36$uncurried(a,b){
-  return Fay$$bind$36$uncurried(a,function(_){ return b; });
-}
-
-// >>=
-// This is used directly from Fay, but can be rebound or shadowed. See primOps in Types.hs.
-function Fay$$bind(m){
-  return function(f){
-    return new Fay$$$(function(){
-      var monad = Fay$$_(m,true);
-      return Fay$$_(f)(monad.value);
-    });
-  };
-}
-
-// >>=
-// This is used directly from Fay, but can be rebound or shadowed. See primOps in Types.hs.
-function Fay$$bind$36$uncurried(m,f){
-  return new Fay$$$(function(){
-    var monad = Fay$$_(m,true);
-    return Fay$$_(f)(monad.value);
-  });
-}
-
-// This is used directly from Fay, but can be rebound or shadowed.
-function Fay$$$_return(a){
-  return new Fay$$Monad(a);
-}
-
-// Allow the programmer to access thunk forcing directly.
-function Fay$$force(thunk){
-  return function(type){
-    return new Fay$$$(function(){
-      Fay$$_(thunk,type);
-      return new Fay$$Monad(Fay$$unit);
-    })
-  }
-}
-
-// This is used directly from Fay, but can be rebound or shadowed.
-function Fay$$return$36$uncurried(a){
-  return new Fay$$Monad(a);
-}
-
-// Unit: ().
-var Fay$$unit = null;
-
-/*******************************************************************************
- * Serialization.
- * Fay <-> JS. Should be bijective.
- */
-
-// Serialize a Fay object to JS.
-function Fay$$fayToJs(type,fayObj){
-  var base = type[0];
-  var args = type[1];
-  var jsObj;
-  if(base == "action") {
-    // A nullary monadic action. Should become a nullary JS function.
-    // Fay () -> function(){ return ... }
-    return function(){
-      return Fay$$fayToJs(args[0],Fay$$_(fayObj,true).value);
-    };
-
-  }
-  else if(base == "function") {
-    // A proper function.
-    return function(){
-      var fayFunc = fayObj;
-      var return_type = args[args.length-1];
-      var len = args.length;
-      // If some arguments.
-      if (len > 1) {
-        // Apply to all the arguments.
-        fayFunc = Fay$$_(fayFunc,true);
-        // TODO: Perhaps we should throw an error when JS
-        // passes more arguments than Haskell accepts.
-
-        // Unserialize the JS values to Fay for the Fay callback.
-        if (args == "automatic_function")
-        {
-          for (var i = 0; i < arguments.length; i++) {
-            fayFunc = Fay$$_(fayFunc(Fay$$jsToFay(["automatic"],arguments[i])),true);
-          }
-          return Fay$$fayToJs(["automatic"], fayFunc);
-        }
-
-        for (var i = 0, len = len; i < len - 1 && fayFunc instanceof Function; i++) {
-          fayFunc = Fay$$_(fayFunc(Fay$$jsToFay(args[i],arguments[i])),true);
-        }
-        // Finally, serialize the Fay return value back to JS.
-        var return_base = return_type[0];
-        var return_args = return_type[1];
-        // If it's a monadic return value, get the value instead.
-        if(return_base == "action") {
-          return Fay$$fayToJs(return_args[0],fayFunc.value);
-        }
-        // Otherwise just serialize the value direct.
-        else {
-          return Fay$$fayToJs(return_type,fayFunc);
-        }
-      } else {
-        throw new Error("Nullary function?");
-      }
-    };
-
-  }
-  else if(base == "string") {
-    return Fay$$fayToJs_string(fayObj);
-  }
-  else if(base == "list") {
-    // Serialize Fay list to JavaScript array.
-    var arr = [];
-    fayObj = Fay$$_(fayObj);
-    while(fayObj instanceof Fay$$Cons) {
-      arr.push(Fay$$fayToJs(args[0],fayObj.car));
-      fayObj = Fay$$_(fayObj.cdr);
-    }
-    return arr;
-  }
-  else if(base == "tuple") {
-    // Serialize Fay tuple to JavaScript array.
-    var arr = [];
-    fayObj = Fay$$_(fayObj);
-    var i = 0;
-    while(fayObj instanceof Fay$$Cons) {
-      arr.push(Fay$$fayToJs(args[i++],fayObj.car));
-      fayObj = Fay$$_(fayObj.cdr);
-    }
-    return arr;
-  }
-  else if(base == "defined") {
-    fayObj = Fay$$_(fayObj);
-    return fayObj instanceof Fay.FFI._Undefined
-      ? undefined
-      : Fay$$fayToJs(args[0],fayObj.slot1);
-  }
-  else if(base == "nullable") {
-    fayObj = Fay$$_(fayObj);
-    return fayObj instanceof Fay.FFI._Null
-      ? null
-      : Fay$$fayToJs(args[0],fayObj.slot1);
-  }
-  else if(base == "double" || base == "int" || base == "bool") {
-    // Bools are unboxed.
-    return Fay$$_(fayObj);
-  }
-  else if(base == "ptr")
-    return fayObj;
-  else if(base == "unknown")
-    return Fay$$fayToJs(["automatic"], fayObj);
-  else if(base == "automatic" && fayObj instanceof Function) {
-    return Fay$$fayToJs(["function", "automatic_function"], fayObj);
-  }
-  else if(base == "automatic" || base == "user") {
-    fayObj = Fay$$_(fayObj);
-
-    if(fayObj instanceof Fay$$Cons || fayObj === null){
-      // Serialize Fay list to JavaScript array.
-      var arr = [];
-      while(fayObj instanceof Fay$$Cons) {
-        arr.push(Fay$$fayToJs(["automatic"],fayObj.car));
-        fayObj = Fay$$_(fayObj.cdr);
-      }
-      return arr;
-    } else {
-      var fayToJsFun = fayObj && fayObj.instance && Fay$$fayToJsHash[fayObj.instance];
-      return fayToJsFun ? fayToJsFun(type,type[2],fayObj) : fayObj;
-    }
-  }
-
-  throw new Error("Unhandled Fay->JS translation type: " + base);
-}
-
-// Stores the mappings from fay types to js objects.
-// This will be populated by compiled modules.
-var Fay$$fayToJsHash = {};
-
-// Specialized serializer for string.
-function Fay$$fayToJs_string(fayObj){
-  // Serialize Fay string to JavaScript string.
-  var str = "";
-  fayObj = Fay$$_(fayObj);
-  while(fayObj instanceof Fay$$Cons) {
-    str += Fay$$_(fayObj.car);
-    fayObj = Fay$$_(fayObj.cdr);
-  }
-  return str;
-};
-function Fay$$jsToFay_string(x){
-  return Fay$$list(x)
-};
-
-// Special num/bool serializers.
-function Fay$$jsToFay_int(x){return x;}
-function Fay$$jsToFay_double(x){return x;}
-function Fay$$jsToFay_bool(x){return x;}
-
-function Fay$$fayToJs_int(x){return Fay$$_(x);}
-function Fay$$fayToJs_double(x){return Fay$$_(x);}
-function Fay$$fayToJs_bool(x){return Fay$$_(x);}
-
-// Unserialize an object from JS to Fay.
-function Fay$$jsToFay(type,jsObj){
-  var base = type[0];
-  var args = type[1];
-  var fayObj;
-  if(base == "action") {
-    // Unserialize a "monadic" JavaScript return value into a monadic value.
-    return new Fay$$Monad(Fay$$jsToFay(args[0],jsObj));
-  }
-  else if(base == "function") {
-    // Unserialize a function from JavaScript to a function that Fay can call.
-    // So
-    //
-    //    var f = function(x,y,z){ … }
-    //
-    // becomes something like:
-    //
-    //    function(x){
-    //      return function(y){
-    //        return function(z){
-    //          return new Fay$$$(function(){
-    //            return Fay$$jsToFay(f(Fay$$fayTojs(x),
-    //                                  Fay$$fayTojs(y),
-    //                                  Fay$$fayTojs(z))
-    //    }}}}};
-    var returnType = args[args.length-1];
-    var funArgs = args.slice(0,-1);
-
-    if (jsObj.length > 0) {
-      var makePartial = function(args){
-        return function(arg){
-          var i = args.length;
-          var fayArg = Fay$$fayToJs(funArgs[i],arg);
-          var newArgs = args.concat([fayArg]);
-          if(newArgs.length == funArgs.length) {
-            return new Fay$$$(function(){
-              return Fay$$jsToFay(returnType,jsObj.apply(this,newArgs));
-            });
-          } else {
-            return makePartial(newArgs);
-          }
-        };
-      };
-      return makePartial([]);
-    }
-    else
-      return function (arg) {
-        return Fay$$jsToFay(["automatic"], jsObj(Fay$$fayToJs(["automatic"], arg)));
-      };
-  }
-  else if(base == "string") {
-    // Unserialize a JS string into Fay list (String).
-    // This is a special case, when String is explicit in the type signature,
-    // with `Automatic' a string would not be decoded.
-    return Fay$$list(jsObj);
-  }
-  else if(base == "list") {
-    // Unserialize a JS array into a Fay list ([a]).
-    var serializedList = [];
-    for (var i = 0, len = jsObj.length; i < len; i++) {
-      // Unserialize each JS value into a Fay value, too.
-      serializedList.push(Fay$$jsToFay(args[0],jsObj[i]));
-    }
-    // Pop it all in a Fay list.
-    return Fay$$list(serializedList);
-  }
-  else if(base == "tuple") {
-    // Unserialize a JS array into a Fay tuple ((a,b,c,...)).
-    var serializedTuple = [];
-    for (var i = 0, len = jsObj.length; i < len; i++) {
-      // Unserialize each JS value into a Fay value, too.
-      serializedTuple.push(Fay$$jsToFay(args[i],jsObj[i]));
-    }
-    // Pop it all in a Fay list.
-    return Fay$$list(serializedTuple);
-  }
-  else if(base == "defined") {
-    return jsObj === undefined
-      ? new Fay.FFI._Undefined()
-      : new Fay.FFI._Defined(Fay$$jsToFay(args[0],jsObj));
-  }
-  else if(base == "nullable") {
-    return jsObj === null
-      ? new Fay.FFI._Null()
-      : new Fay.FFI.Nullable(Fay$$jsToFay(args[0],jsObj));
-  }
-  else if(base == "int") {
-    // Int are unboxed, so there's no forcing to do.
-    // But we can do validation that the int has no decimal places.
-    // E.g. Math.round(x)!=x? throw "NOT AN INTEGER, GET OUT!"
-    fayObj = Math.round(jsObj);
-    if(fayObj!==jsObj) throw "Argument " + jsObj + " is not an integer!";
-    return fayObj;
-  }
-  else if (base == "double" ||
-           base == "bool" ||
-           base ==  "ptr") {
-    return jsObj;
-  }
-  else if(base == "unknown")
-    return Fay$$jsToFay(["automatic"], jsObj);
-  else if(base == "automatic" && jsObj instanceof Function) {
-    var type = [["automatic"]];
-    for (var i = 0; i < jsObj.length; i++)
-      type.push(["automatic"]);
-    return Fay$$jsToFay(["function", type], jsObj);
-  }
-  else if(base == "automatic" && jsObj instanceof Array) {
-    var list = null;
-    for (var i = jsObj.length - 1; i >= 0; i--) {
-      list = new Fay$$Cons(Fay$$jsToFay([base], jsObj[i]), list);
-    }
-    return list;
-  }
-  else if(base == "automatic" || base == "user") {
-    if (jsObj && jsObj['instance']) {
-      var jsToFayFun = Fay$$jsToFayHash[jsObj["instance"]];
-      return jsToFayFun ? jsToFayFun(type,type[2],jsObj) : jsObj;
-    }
-    else
-      return jsObj;
-  }
-
-  throw new Error("Unhandled JS->Fay translation type: " + base);
-}
-
-// Stores the mappings from js objects to fay types.
-// This will be populated by compiled modules.
-var Fay$$jsToFayHash = {};
-
-/*******************************************************************************
- * Lists.
- */
-
-// Cons object.
-function Fay$$Cons(car,cdr){
-  this.car = car;
-  this.cdr = cdr;
-}
-
-// Make a list.
-function Fay$$list(xs){
-  var out = null;
-  for(var i=xs.length-1; i>=0;i--)
-    out = new Fay$$Cons(xs[i],out);
-  return out;
-}
-
-// Built-in list cons.
-function Fay$$cons(x){
-  return function(y){
-    return new Fay$$Cons(x,y);
-  };
-}
-
-// List index.
-// `list' is already forced by the time it's passed to this function.
-// `list' cannot be null and `index' cannot be out of bounds.
-function Fay$$index(index,list){
-  for(var i = 0; i < index; i++) {
-    list = Fay$$_(list.cdr);
-  }
-  return list.car;
-}
-
-// List length.
-// `list' is already forced by the time it's passed to this function.
-function Fay$$listLen(list,max){
-  for(var i = 0; list !== null && i < max + 1; i++) {
-    list = Fay$$_(list.cdr);
-  }
-  return i == max;
-}
-
-/*******************************************************************************
- * Numbers.
- */
-
-// Built-in *.
-function Fay$$mult(x){
-  return function(y){
-    return new Fay$$$(function(){
-      return Fay$$_(x) * Fay$$_(y);
-    });
-  };
-}
-
-function Fay$$mult$36$uncurried(x,y){
-
-  return new Fay$$$(function(){
-    return Fay$$_(x) * Fay$$_(y);
-  });
-
-}
-
-// Built-in +.
-function Fay$$add(x){
-  return function(y){
-    return new Fay$$$(function(){
-      return Fay$$_(x) + Fay$$_(y);
-    });
-  };
-}
-
-// Built-in +.
-function Fay$$add$36$uncurried(x,y){
-
-  return new Fay$$$(function(){
-    return Fay$$_(x) + Fay$$_(y);
-  });
-
-}
-
-// Built-in -.
-function Fay$$sub(x){
-  return function(y){
-    return new Fay$$$(function(){
-      return Fay$$_(x) - Fay$$_(y);
-    });
-  };
-}
-// Built-in -.
-function Fay$$sub$36$uncurried(x,y){
-
-  return new Fay$$$(function(){
-    return Fay$$_(x) - Fay$$_(y);
-  });
-
-}
-
-// Built-in /.
-function Fay$$divi(x){
-  return function(y){
-    return new Fay$$$(function(){
-      return Fay$$_(x) / Fay$$_(y);
-    });
-  };
-}
-
-// Built-in /.
-function Fay$$divi$36$uncurried(x,y){
-
-  return new Fay$$$(function(){
-    return Fay$$_(x) / Fay$$_(y);
-  });
-
-}
-
-/*******************************************************************************
- * Booleans.
- */
-
-// Are two values equal?
-function Fay$$equal(lit1, lit2) {
-  // Simple case
-  lit1 = Fay$$_(lit1);
-  lit2 = Fay$$_(lit2);
-  if (lit1 === lit2) {
-    return true;
-  }
-  // General case
-  if (lit1 instanceof Array) {
-    if (lit1.length != lit2.length) return false;
-    for (var len = lit1.length, i = 0; i < len; i++) {
-      if (!Fay$$equal(lit1[i], lit2[i])) return false;
-    }
-    return true;
-  } else if (lit1 instanceof Fay$$Cons && lit2 instanceof Fay$$Cons) {
-    do {
-      if (!Fay$$equal(lit1.car,lit2.car))
-        return false;
-      lit1 = Fay$$_(lit1.cdr), lit2 = Fay$$_(lit2.cdr);
-      if (lit1 === null || lit2 === null)
-        return lit1 === lit2;
-    } while (true);
-  } else if (typeof lit1 == 'object' && typeof lit2 == 'object' && lit1 && lit2 &&
-             lit1.instance === lit2.instance) {
-    for(var x in lit1) {
-      if(!Fay$$equal(lit1[x],lit2[x]))
-        return false;
-    }
-    return true;
-  } else {
-    return false;
-  }
-}
-
-// Built-in ==.
-function Fay$$eq(x){
-  return function(y){
-    return new Fay$$$(function(){
-      return Fay$$equal(x,y);
-    });
-  };
-}
-
-function Fay$$eq$36$uncurried(x,y){
-
-  return new Fay$$$(function(){
-    return Fay$$equal(x,y);
-  });
-
-}
-
-// Built-in /=.
-function Fay$$neq(x){
-  return function(y){
-    return new Fay$$$(function(){
-      return !(Fay$$equal(x,y));
-    });
-  };
-}
-
-// Built-in /=.
-function Fay$$neq$36$uncurried(x,y){
-
-  return new Fay$$$(function(){
-    return !(Fay$$equal(x,y));
-  });
-
-}
-
-// Built-in >.
-function Fay$$gt(x){
-  return function(y){
-    return new Fay$$$(function(){
-      return Fay$$_(x) > Fay$$_(y);
-    });
-  };
-}
-
-// Built-in >.
-function Fay$$gt$36$uncurried(x,y){
-
-  return new Fay$$$(function(){
-    return Fay$$_(x) > Fay$$_(y);
-  });
-
-}
-
-// Built-in <.
-function Fay$$lt(x){
-  return function(y){
-    return new Fay$$$(function(){
-      return Fay$$_(x) < Fay$$_(y);
-    });
-  };
-}
-
-
-// Built-in <.
-function Fay$$lt$36$uncurried(x,y){
-
-  return new Fay$$$(function(){
-    return Fay$$_(x) < Fay$$_(y);
-  });
-
-}
-
-
-// Built-in >=.
-function Fay$$gte(x){
-  return function(y){
-    return new Fay$$$(function(){
-      return Fay$$_(x) >= Fay$$_(y);
-    });
-  };
-}
-
-// Built-in >=.
-function Fay$$gte$36$uncurried(x,y){
-
-  return new Fay$$$(function(){
-    return Fay$$_(x) >= Fay$$_(y);
-  });
-
-}
-
-// Built-in <=.
-function Fay$$lte(x){
-  return function(y){
-    return new Fay$$$(function(){
-      return Fay$$_(x) <= Fay$$_(y);
-    });
-  };
-}
-
-// Built-in <=.
-function Fay$$lte$36$uncurried(x,y){
-
-  return new Fay$$$(function(){
-    return Fay$$_(x) <= Fay$$_(y);
-  });
-
-}
-
-// Built-in &&.
-function Fay$$and(x){
-  return function(y){
-    return new Fay$$$(function(){
-      return Fay$$_(x) && Fay$$_(y);
-    });
-  };
-}
-
-// Built-in &&.
-function Fay$$and$36$uncurried(x,y){
-
-  return new Fay$$$(function(){
-    return Fay$$_(x) && Fay$$_(y);
-  });
-  ;
-}
-
-// Built-in ||.
-function Fay$$or(x){
-  return function(y){
-    return new Fay$$$(function(){
-      return Fay$$_(x) || Fay$$_(y);
-    });
-  };
-}
-
-// Built-in ||.
-function Fay$$or$36$uncurried(x,y){
-
-  return new Fay$$$(function(){
-    return Fay$$_(x) || Fay$$_(y);
-  });
-
-}
-
-/*******************************************************************************
- * Mutable references.
- */
-
-// Make a new mutable reference.
-function Fay$$Ref(x){
-  this.value = x;
-}
-
-// Write to the ref.
-function Fay$$writeRef(ref,x){
-  ref.value = x;
-}
-
-// Get the value from the ref.
-function Fay$$readRef(ref,x){
-  return ref.value;
-}
-
-/*******************************************************************************
- * Dates.
- */
-function Fay$$date(str){
-  return window.Date.parse(str);
-}
-
-/*******************************************************************************
- * Data.Var
- */
-
-function Fay$$Ref2(val){
-  this.val = val;
-}
-
-function Fay$$Sig(){
-  this.handlers = [];
-}
-
-function Fay$$Var(val){
-  this.val = val;
-  this.handlers = [];
-}
-
-// Helper used by Fay$$setValue and for merging
-function Fay$$broadcastInternal(self, val, force){
-  var handlers = self.handlers;
-  var exceptions = [];
-  for(var len = handlers.length, i = 0; i < len; i++) {
-    try {
-      force(handlers[i][1](val), true);
-    } catch (e) {
-      exceptions.push(e);
-    }
-  }
-  // Rethrow the encountered exceptions.
-  if (exceptions.length > 0) {
-    console.error("Encountered " + exceptions.length + " exception(s) while broadcasing a change to ", self);
-    for(var len = exceptions.length, i = 0; i < len; i++) {
-      (function(exception) {
-        window.setTimeout(function() { throw exception; }, 0);
-      })(exceptions[i]);
-    }
-  }
-}
-
-function Fay$$setValue(self, val, force){
-  if (self instanceof Fay$$Ref2) {
-    self.val = val;
-  } else if (self instanceof Fay$$Var) {
-    self.val = val;
-    Fay$$broadcastInternal(self, val, force);
-  } else if (self instanceof Fay$$Sig) {
-    Fay$$broadcastInternal(self, val, force);
-  } else {
-    throw "Fay$$setValue given something that's not a Ref2, Var, or Sig"
-  }
-}
-
-function Fay$$subscribe(self, f){
-  var key = {};
-  self.handlers.push([key,f]);
-  var searchStart = self.handlers.length - 1;
-  return function(_){
-    for(var i = Math.min(searchStart, self.handlers.length - 1); i >= 0; i--) {
-      if(self.handlers[i][0] == key) {
-        self.handlers = self.handlers.slice(0,i).concat(self.handlers.slice(i+1));
-        return;
-      }
-    }
-    return _; // This variable has to be used, otherwise Closure
-              // strips it out and Fay serialization breaks.
-  };
-}
-
-/*******************************************************************************
- * Application code.
- */
diff --git a/src/Fay.hs b/src/Fay.hs
--- a/src/Fay.hs
+++ b/src/Fay.hs
@@ -2,7 +2,6 @@
 {-# LANGUAGE MultiParamTypeClasses #-}
 {-# LANGUAGE NoImplicitPrelude     #-}
 {-# LANGUAGE OverloadedStrings     #-}
-{-# LANGUAGE RecordWildCards       #-}
 {-# LANGUAGE TypeSynonymInstances  #-}
 {-# LANGUAGE ViewPatterns          #-}
 
@@ -19,9 +18,9 @@
   ,compileFromTo
   ,compileFromToAndGenerateHtml
   ,toJsName
+  ,toTsName
   ,showCompileError
-  ,getConfigRuntime
-  ,getRuntime
+  ,readConfigRuntime
   ) where
 
 import           Fay.Compiler.Prelude
@@ -31,6 +30,7 @@
 import           Fay.Compiler.Packages
 import           Fay.Compiler.Typecheck
 import           Fay.Config
+import           Fay.Runtime
 import qualified Fay.Exts                               as F
 import           Fay.Types
 
@@ -39,7 +39,6 @@
 import           Language.Haskell.Exts        (prettyPrint)
 import           Language.Haskell.Exts.Syntax
 import           Language.Haskell.Exts.SrcLoc
-import           Paths_fay
 import           SourceMap                              (generate)
 import           SourceMap.Types
 import           System.FilePath
@@ -118,9 +117,8 @@
 -- Don't use this directly, it's only exposed for the test suite.
 compileFileWithState :: Config -> FilePath -> IO (Either CompileError (String,Maybe [Mapping],CompileState))
 compileFileWithState config filein = do
-  runtime <- getConfigRuntime config
+  raw <- readConfigRuntime config
   hscode <- readFile filein
-  raw <- readFile runtime
   config' <- resolvePackages config
   compileToModule filein config' raw (compileToplevelModule filein) hscode
 
@@ -139,14 +137,14 @@
             )
       where
         pw = execPrinter (runtime <> aliases <> printer <> main) pr
-        runtime = if (configExportRuntime config) then write raw else mempty
-        aliases = if (configPrettyThunks config)
+        runtime = if configExportRuntime config then write raw else mempty
+        aliases = if configPrettyThunks config
                   then write . unlines $ [ "var $ = Fay$$$;"
                                          , "var _ = Fay$$_;"
                                          , "var __ = Fay$$__;"
                                          ]
                   else mempty
-        main = if (not $ configLibrary config)
+        main = if not (configLibrary config)
                then write $ "Fay$$_(" ++ modulename ++ ".main, true);\n"
                else mempty
         pr = defaultPrintReader
@@ -161,6 +159,12 @@
   ('s':'h':'.': (reverse -> file)) -> file ++ ".js"
   _ -> x
 
+-- | Convert a Haskell filename to a TypeScript filename.
+toTsName :: String -> String
+toTsName x = case reverse x of
+  ('s':'h':'.': (reverse -> file)) -> file ++ ".ts"
+  _ -> x
+
 -- | Print a compile error for human consumption.
 showCompileError :: CompileError -> String
 showCompileError e = case e of
@@ -203,9 +207,8 @@
 
 -- | Get the JS runtime source.
 -- This will return the user supplied runtime if it exists.
-getConfigRuntime :: Config -> IO String
-getConfigRuntime cfg = maybe getRuntime return $ configRuntimePath cfg
-
--- | Get the default JS runtime source.
-getRuntime :: IO String
-getRuntime = getDataFileName "js/runtime.js"
+readConfigRuntime :: Config -> IO String
+readConfigRuntime cfg =
+  case configRuntimePath cfg of
+    Just path -> readFile path
+    Nothing -> return $ getRuntimeSource cfg
diff --git a/src/Fay/Compiler.hs b/src/Fay/Compiler.hs
--- a/src/Fay/Compiler.hs
+++ b/src/Fay/Compiler.hs
@@ -121,7 +121,7 @@
 -- | Compile a parse HSE module.
 compileModuleFromAST :: ([JsStmt], [JsStmt]) -> F.Module -> Compile ([JsStmt], [JsStmt])
 compileModuleFromAST (hstmts0, fstmts0) mod'@Module{} = do
-  mod@(Module _ _ pragmas _ decls) <- annotateModule Haskell2010 defaultExtensions mod'
+  ~mod@(Module _ _ pragmas _ decls) <- annotateModule Haskell2010 defaultExtensions mod'
   let modName = unAnn $ F.moduleName mod
   modify $ \s -> s { stateUseFromString = hasLanguagePragmas ["OverloadedStrings", "RebindableSyntax"] pragmas
                    }
@@ -153,16 +153,19 @@
 createModulePath :: ModuleName a -> Compile [JsStmt]
 createModulePath (unAnn -> m) = do
   cfg <- config id
-  reg <- liftM concat . mapM modPath . mkModulePaths $ m
+  let isTs = configTypeScript cfg
+  reg <- fmap concat . mapM (modPath isTs) . mkModulePaths $ m
   strict <-
     if shouldExportStrictWrapper m cfg
-      then liftM concat . mapM modPath . mkModulePaths $ (\(ModuleName i n) -> ModuleName i ("Strict." ++ n)) m
+      then fmap concat . mapM (modPath isTs) . mkModulePaths $ (\(ModuleName i n) -> ModuleName i ("Strict." ++ n)) m
        else return []
   return $ reg ++ strict
   where
-    modPath :: ModulePath -> Compile [JsStmt]
-    modPath mp = whenImportNotGenerated mp $ \(unModulePath -> l) -> case l of
-     [n] -> [JsVar (JsNameVar . UnQual () $ Ident () n) (JsObj [])]
+    modPath :: Bool -> ModulePath -> Compile [JsStmt]
+    modPath isTs mp = whenImportNotGenerated mp $ \(unModulePath -> l) -> case l of
+     [n] -> if isTs
+              then [JsMapVar (JsNameVar . UnQual () $ Ident () n) (JsObj [])]
+              else [JsVar (JsNameVar . UnQual () $ Ident () n) (JsObj [])]
      _   -> [JsSetModule mp (JsObj [])]
 
     whenImportNotGenerated :: ModulePath -> (ModulePath -> [JsStmt]) -> Compile [JsStmt]
diff --git a/src/Fay/Compiler/Decl.hs b/src/Fay/Compiler/Decl.hs
--- a/src/Fay/Compiler/Decl.hs
+++ b/src/Fay/Compiler/Decl.hs
@@ -25,6 +25,8 @@
 import           Control.Monad.RWS               (gets, modify)
 import           Language.Haskell.Exts hiding (binds, loc, name)
 
+{-# ANN module ("HLint: ignore Reduce duplication"::String) #-}
+
 -- | Compile Haskell declaration.
 compileDecls :: Bool -> [S.Decl] -> Compile [JsStmt]
 compileDecls toplevel = fmap concat . mapM (compileDecl toplevel)
@@ -206,10 +208,15 @@
                        (Just (srcInfoSpan (S.srcSpanInfo srcloc)))
                        name
                        (foldr (\arg inner -> JsFun Nothing [arg] [] (Just inner))
-                              (stmtsThunk (concat pats ++ basecase))
+                              (stmtsThunk $ deleteAfterReturn (concat pats ++ basecase))
                               args)
   return [bind]
   where
+    deleteAfterReturn :: [JsStmt] -> [JsStmt]
+    deleteAfterReturn [] = []
+    deleteAfterReturn (x@(JsEarlyReturn _):_) = [x]
+    deleteAfterReturn (x:xs) = x:deleteAfterReturn xs
+
     args = zipWith const uniqueNames argslen
 
     isWildCardMatch (Match _ _ pats          _ _) = all isWildCardPat pats
diff --git a/src/Fay/Compiler/Desugar.hs b/src/Fay/Compiler/Desugar.hs
--- a/src/Fay/Compiler/Desugar.hs
+++ b/src/Fay/Compiler/Desugar.hs
@@ -49,6 +49,7 @@
   >>= return . desugarInfixOp
   >>= return . desugarInfixPat
   >>= return . desugarExpParen
+{-# ANN desugar' "HLint: ignore Use <$>" #-}
 
 -- | (a `f`) => \b -> a `f` b
 --   (`f` b) => \a -> a `f` b
diff --git a/src/Fay/Compiler/Exp.hs b/src/Fay/Compiler/Exp.hs
--- a/src/Fay/Compiler/Exp.hs
+++ b/src/Fay/Compiler/Exp.hs
@@ -187,14 +187,21 @@
   exp <- compileExp e
   withScopedTmpJsName $ \tmpName -> do
     pats <- optimizePatConditions <$> mapM (compilePatAlt (JsName tmpName)) alts
+    let (xx,flag) = deleteAfterReturn (concat pats)
     return $
       JsApp (JsFun Nothing
                    [tmpName]
-                   (concat pats)
-                   (if any isWildCardAlt alts
+                   xx
+                   (if (flag || any isWildCardAlt alts)
                        then Nothing
                        else Just (throwExp "unhandled case" (JsName tmpName))))
             [exp]
+  where
+    deleteAfterReturn :: [JsStmt] -> ([JsStmt],Bool)
+    deleteAfterReturn [] = ([],False)
+    deleteAfterReturn (x@(JsEarlyReturn _):_) = ([x],True)
+    deleteAfterReturn (x:xs) = ((x:xx),flag)
+      where (xx,flag) = deleteAfterReturn xs
 
 -- | Compile the given pattern against the given expression.
 compilePatAlt :: JsExp -> S.Alt -> Compile [JsStmt]
@@ -220,7 +227,7 @@
          <*> if null rest then return [] else do
            gs' <- compileGuards rest
            return [gs']
-    where makeIf gs e gss = JsIf gs [JsEarlyReturn e] gss
+    where makeIf gs e = JsIf gs [JsEarlyReturn e]
 
 compileGuards rhss = throwError . UnsupportedRhs . GuardedRhss noI $ rhss
 
@@ -237,9 +244,13 @@
         generateStatements exp =
           foldM (\inner (param,pat) -> do
                   stmts <- compilePat (JsName param) pat inner
-                  return [JsEarlyReturn (JsFun Nothing [param] (stmts ++ [unhandledcase param | not allfree]) Nothing)])
+                  return [JsEarlyReturn (JsFun Nothing [param] (deleteAfterReturn $ stmts ++ [unhandledcase param | not allfree]) Nothing)])
                 [JsEarlyReturn exp]
                 (reverse (zip uniqueNames pats))
+        deleteAfterReturn :: [JsStmt] -> [JsStmt]
+        deleteAfterReturn [] = []
+        deleteAfterReturn (x@(JsEarlyReturn _):_) = [x]
+        deleteAfterReturn (x:xs) = x:deleteAfterReturn xs
 
 -- | Compile [e1..] arithmetic sequences.
 compileEnumFrom :: S.Exp -> Compile JsExp
@@ -283,7 +294,7 @@
   let unQualName = withIdent lowerFirst . unQualify $ unAnn name
   qname <- unsafeResolveName name
   let record = JsVar (JsNameVar unQualName) (JsNew (JsConstructor qname) [])
-  setFields <- liftM concat (forM fieldUpdates (updateStmt name))
+  setFields <- concat <$> forM fieldUpdates (updateStmt name)
   return $ JsApp (JsFun Nothing [] (record:setFields) (Just . JsName . JsNameVar . withIdent lowerFirst . unQualify $ unAnn name)) []
   where
     -- updateStmt :: QName a -> S.FieldUpdate -> Compile [JsStmt]
diff --git a/src/Fay/Compiler/FFI.hs b/src/Fay/Compiler/FFI.hs
--- a/src/Fay/Compiler/FFI.hs
+++ b/src/Fay/Compiler/FFI.hs
@@ -57,8 +57,9 @@
       TyEquals _ t1 t2  -> TyEquals () <$> rmNewtys t1 <*> rmNewtys t2
       TySplice {}       -> return $ unAnn typ
       TyBang _ bt unp t -> TyBang () (unAnn bt) (unAnn unp) <$> rmNewtys t
-      TyWildCard _ _    -> error "TyWildCard not supported"
-      TyQuasiQuote _ _ _ -> error "TyQuasiQuote not supported"
+      TyWildCard {}     -> error "TyWildCard not supported"
+      TyQuasiQuote {}   -> error "TyQuasiQuote not supported"
+      TyUnboxedSum {}   -> error "TyUnboxedSum not supported"
 
     compileFFI' :: N.Type -> Compile JsExp
     compileFFI' sig = do
diff --git a/src/Fay/Compiler/GADT.hs b/src/Fay/Compiler/GADT.hs
--- a/src/Fay/Compiler/GADT.hs
+++ b/src/Fay/Compiler/GADT.hs
@@ -9,13 +9,11 @@
 -- | Convert a GADT to a normal data type.
 convertGADT :: GadtDecl a -> QualConDecl a
 convertGADT d = case d of
-  GadtDecl s name Nothing typ ->
+  GadtDecl s name tyvars context Nothing typ ->
     QualConDecl s tyvars context (ConDecl s name (convertFunc typ))
-  GadtDecl s name (Just fs) _typ ->
+  GadtDecl s name tyvars context (Just fs) _typ ->
     QualConDecl s tyvars context (RecDecl s name fs)
   where
-    tyvars = Nothing
-    context = Nothing
     convertFunc :: Type a -> [Type a]
     convertFunc (TyCon _ _) = []
     convertFunc (TyFun _ x xs) = x : convertFunc xs
diff --git a/src/Fay/Compiler/Import.hs b/src/Fay/Compiler/Import.hs
--- a/src/Fay/Compiler/Import.hs
+++ b/src/Fay/Compiler/Import.hs
@@ -22,7 +22,7 @@
 
 import           Control.Monad.Except            (throwError)
 import           Control.Monad.RWS               (ask, get, gets, lift, listen, modify)
-import           Language.Haskell.Exts hiding (name, var)
+import           Language.Haskell.Exts hiding (name)
 import           System.Directory
 import           System.FilePath
 
@@ -34,7 +34,7 @@
 
 -- | Compile a module
 compileWith
-  :: Monoid a
+  :: (Monoid a, Semigroup a)
   => FilePath
   -> (a -> F.Module -> Compile a)
   -> (FilePath -> String -> Compile a)
@@ -50,7 +50,7 @@
       st
       (parseResult (throwError . uncurry ParseError)
                    (\mod' -> do
-                     mod@(Module _ _ _ imports _) <-
+                     ~mod@(Module _ _ _ imports _) <-
                        either throwError return =<< io (before F.noI mod')
                      res <- foldr (<>) mempty <$> mapM (compileImport compileModule) imports
                      modify $ \s -> s { stateModuleName = unAnn $ F.moduleName mod }
diff --git a/src/Fay/Compiler/InitialPass.hs b/src/Fay/Compiler/InitialPass.hs
--- a/src/Fay/Compiler/InitialPass.hs
+++ b/src/Fay/Compiler/InitialPass.hs
@@ -22,7 +22,7 @@
 import           Control.Monad.Except            (throwError)
 import           Control.Monad.RWS               (modify)
 import qualified Data.Map                        as M
-import           Language.Haskell.Exts hiding (name, var)
+import           Language.Haskell.Exts hiding (name)
 import qualified Language.Haskell.Names          as HN (getInterfaces)
 
 -- | Preprocess and collect all information needed during code generation.
@@ -51,7 +51,7 @@
 preprocessAST :: () -> F.Module -> Compile ()
 preprocessAST () mod@(Module _ _ _ _ decls) = do
   -- This can only return one element since we only compile one module.
-  ([exports],_) <- HN.getInterfaces Haskell2010 defaultExtensions [mod]
+  ~([exports],_) <- HN.getInterfaces Haskell2010 defaultExtensions [mod]
   modify $ \s -> s { stateInterfaces = M.insert (stateModuleName s) exports $ stateInterfaces s }
   forM_ decls scanTypeSigs
   forM_ decls scanRecordDecls
@@ -85,6 +85,7 @@
                cs{stateNewtypes=(qcname,qdname,unAnn ty):nts})
 compileNewtypeDecl q = error $ "compileNewtypeDecl: Should be impossible (this is a bug). Got: " ++ show q
 
+{-# ANN scanRecordDecls ("HLint: ignore Redundant flip" :: String) #-}
 -- | Add record declarations to the state
 scanRecordDecls :: F.Decl -> Compile ()
 scanRecordDecls decl = do
diff --git a/src/Fay/Compiler/Misc.hs b/src/Fay/Compiler/Misc.hs
--- a/src/Fay/Compiler/Misc.hs
+++ b/src/Fay/Compiler/Misc.hs
@@ -282,7 +282,7 @@
 hasLanguagePragmas pragmas modulePragmas = (== length pragmas) . length . filter (`elem` pragmas) $ flattenPragmas modulePragmas
   where
     flattenPragmas :: [ModulePragma l] -> [String]
-    flattenPragmas ps = concatMap pragmaName ps
+    flattenPragmas = concatMap pragmaName
     pragmaName (LanguagePragma _ q) = map unname q
     pragmaName _ = []
 
diff --git a/src/Fay/Compiler/ModuleT.hs b/src/Fay/Compiler/ModuleT.hs
--- a/src/Fay/Compiler/ModuleT.hs
+++ b/src/Fay/Compiler/ModuleT.hs
@@ -47,7 +47,7 @@
     components' = split string
 
     split cs = case break (=='.') cs of
-      (chunk,[])     -> chunk : []
+      (chunk,[])     -> [chunk]
       (chunk,_:rest) -> chunk : split rest
 
     validModuleComponent :: String -> Bool
@@ -105,9 +105,9 @@
 --
 -- @i@ is the type of module info, @m@ is the underlying monad.
 newtype ModuleT i m a =
-  ModuleT (
+  ModuleT
     (StateT (Map.Map ModuleName i)
-            (ReaderT ([FilePath] -> ModuleName -> m i) m) a))
+            (ReaderT ([FilePath] -> ModuleName -> m i) m) a)
   deriving (Functor, Applicative, Monad)
 
 instance MonadTrans (ModuleT i) where
diff --git a/src/Fay/Compiler/Packages.hs b/src/Fay/Compiler/Packages.hs
--- a/src/Fay/Compiler/Packages.hs
+++ b/src/Fay/Compiler/Packages.hs
@@ -12,6 +12,7 @@
 import           GHC.Paths
 import           System.Directory
 import           System.FilePath
+import           System.Environment
 
 -- | Given a configuration, resolve any packages specified to their
 -- data file directories for importing the *.hs sources.
@@ -55,12 +56,23 @@
 describePackage :: Maybe FilePath -> String -> IO String
 describePackage db name = do
   exists <- doesFileExist ghc_pkg
-  result <- readAllFromProcess (if exists then ghc_pkg else "ghc-pkg") args ""
+  stackInNixShell <- fmap isJust (lookupEnv "STACK_IN_NIX_SHELL")
+  let command = if exists
+        then if (isInfixOf ".stack" ghc_pkg || stackInNixShell)
+             then "stack"
+             else ghc_pkg
+        else "ghc-pkg"
+      extraArgs = case command of
+        "stack" -> ["exec","--","ghc-pkg"]
+        _       -> []
+      args = extraArgs ++ ["describe",name] ++ ["--expand-env-vars", "-v2"]
+             ++ ["--package-db=" ++ db' | Just db' <- [db]]
+  when stackInNixShell (unsetEnv "STACK_IN_NIX_SHELL")
+  result <- readAllFromProcess command args ""
+  when stackInNixShell (setEnv "STACK_IN_NIX_SHELL" "1")
   case result of
     Left  (err,out) -> error $ "ghc-pkg describe error:\n" ++ err ++ "\n" ++ out
     Right (_err,out) -> return out
-
-  where args = ["describe",name] ++ ["-f" ++ db' | Just db' <- [db]]
 
 -- | Get the package version from the package description.
 packageVersion :: String -> Maybe String
diff --git a/src/Fay/Compiler/Parse.hs b/src/Fay/Compiler/Parse.hs
--- a/src/Fay/Compiler/Parse.hs
+++ b/src/Fay/Compiler/Parse.hs
@@ -3,7 +3,7 @@
   , defaultExtensions
   ) where
 
-import           Language.Haskell.Exts hiding (name)
+import           Language.Haskell.Exts
 
 -- | Parse some Fay code.
 parseFay :: Parseable ast => FilePath -> String -> ParseResult ast
diff --git a/src/Fay/Compiler/Pattern.hs b/src/Fay/Compiler/Pattern.hs
--- a/src/Fay/Compiler/Pattern.hs
+++ b/src/Fay/Compiler/Pattern.hs
@@ -47,7 +47,7 @@
 -- | Compile a record field pattern.
 compilePatFields :: JsExp -> S.QName -> [S.PatField] -> [JsStmt] -> Compile [JsStmt]
 compilePatFields exp name pats body = do
-  c <- liftM (++ body) (compilePats' [] pats)
+  c <- (++ body) <$> compilePats' [] pats
   qname <- unsafeResolveName name
   return [JsIf (force exp `JsInstanceOf` JsConstructor qname) c []]
   where
diff --git a/src/Fay/Compiler/Prelude.hs b/src/Fay/Compiler/Prelude.hs
--- a/src/Fay/Compiler/Prelude.hs
+++ b/src/Fay/Compiler/Prelude.hs
@@ -39,7 +39,7 @@
 import           Data.Function        (on)
 import           Data.List.Compat
 import           Data.Maybe
-import           Data.Monoid          (Monoid (..), (<>))
+import           Data.Monoid          (Monoid (..))
 import           Data.Ord
 import           Data.Traversable
 import           Prelude.Compat       hiding (exp, mod)
@@ -55,7 +55,7 @@
 
 -- | Do any of the (monadic) predicates match?
 anyM :: (Functor m, Applicative m, Monad m) => (a -> m Bool) -> [a] -> m Bool
-anyM p l = return . not . null =<< filterM p l
+anyM p l = not . null <$> filterM p l
 
 -- | Read from a process returning both std err and out.
 readAllFromProcess :: FilePath -> [String] -> String -> IO (Either (String,String) (String,String))
diff --git a/src/Fay/Compiler/PrimOp.hs b/src/Fay/Compiler/PrimOp.hs
--- a/src/Fay/Compiler/PrimOp.hs
+++ b/src/Fay/Compiler/PrimOp.hs
@@ -29,7 +29,7 @@
 
 import           Data.Map                        (Map)
 import qualified Data.Map                        as M
-import           Language.Haskell.Exts hiding (binds, name)
+import           Language.Haskell.Exts
 
 -- | Make an identifier from the built-in HJ module.
 fayBuiltin :: a -> String -> QName a
diff --git a/src/Fay/Compiler/Print.hs b/src/Fay/Compiler/Print.hs
--- a/src/Fay/Compiler/Print.hs
+++ b/src/Fay/Compiler/Print.hs
@@ -89,7 +89,7 @@
         UnitCon _ -> "unit"
         Cons    _ -> "cons"
         _         -> error $ "Special constructor not supported: " ++
-                   show (fmap (const ()) specialCon)
+                   show (void specialCon)
 
 
 -- | Print a list of statements.
@@ -101,7 +101,9 @@
   printJS (JsExpStmt e) =
     printJS e <> ";" <> newline
   printJS (JsBlock stmts) =
-    "{ " <> (printStmts stmts) <> "}"
+    "{ " <> printStmts stmts <> "}"
+  printJS (JsMapVar name expr) =
+    "var " <> printJS name <> " : {[key: string]: any;} = " <> printJS expr <> ";" <> newline
   printJS (JsVar name expr) =
     "var " <> printJS name <> " = " <> printJS expr <> ";" <> newline
   printJS (JsUpdate name expr) =
@@ -122,7 +124,7 @@
     "if (" <> printJS expr <> ") {" <> newline <>
     indented (printStmts thens) <>
     "}" <>
-    (if (null elses)
+    (if null elses
       then mempty
       else " else {" <> newline <>
            indented (printStmts elses) <>
@@ -152,7 +154,7 @@
   printJS (JsLit lit) = printJS lit
   printJS (JsParen expr) = "(" <> printJS expr <> ")"
   printJS (JsList exprs) = "[" <> mintercalate "," (map printJS exprs) <> "]"
-  printJS (JsNew name args) = "new " <> (printJS $ JsApp (JsName name) args)
+  printJS (JsNew name args) = "new " <> printJS (JsApp (JsName name) args)
   printJS (JsIndex i expr) = "(" <> printJS expr <> ")[" <> write (show i) <> "]"
   printJS (JsEq expr1 expr2) = printJS expr1 <> " === " <> printJS expr2
   printJS (JsNeq expr1 expr2) = printJS expr1 <> " !== " <> printJS expr2
@@ -186,7 +188,7 @@
                    Nothing   -> mempty)
     <> "}"
   printJS (JsApp op args) =
-    printJS (case op of JsFun _ _ _ _ -> JsParen op; _ -> op)
+    printJS (case op of JsFun {} -> JsParen op; _ -> op)
     <> "("
     <> mintercalate "," (map printJS args)
     <> ")"
@@ -209,8 +211,8 @@
       JsThunk             -> askIf prPrettyThunks "$" "Fay$$$"
       JsForce             -> askIf prPrettyThunks "_" "Fay$$_"
       JsApply             -> askIf prPrettyThunks "__" "Fay$$__"
-      JsParam i           -> "$p" <> (write $ show i)
-      JsTmp i             -> "$tmp" <> (write $ show i)
+      JsParam i           -> "$p" <> write (show i)
+      JsTmp i             -> "$tmp" <> write (show i)
       JsConstructor qname -> printCons qname
       JsBuiltIn qname     -> "Fay$$" <> printJS qname
       JsParametrizedType  -> "type"
@@ -220,13 +222,13 @@
 printCons :: QName l -> Printer
 printCons (UnQual _ n) = printConsName n
 printCons (Qual _ (ModuleName _ m) n) = write m <> "." <> printConsName n
-printCons (Special {}) = error "qname2String Special"
+printCons Special {} = error "qname2String Special"
 
 -- | Print an unqualified name.
 printConsUnQual :: QName l -> Printer
 printConsUnQual (UnQual _ x) = printJS x
 printConsUnQual (Qual _ _ n) = printJS n
-printConsUnQual (Special {}) = error "printConsUnqual Special"
+printConsUnQual Special {} = error "printConsUnqual Special"
 
 -- | Print a constructor name given a Name. Helper for printCons.
 printConsName :: Name l -> Printer
@@ -252,7 +254,7 @@
   -- The problem only occurs if there is a module A.B and a constructor B in module A.
    ++ ["__defineGetter__", "__defineSetter__", "__lookupGetter__", "__lookupSetter__", "constructor", "force", "forced", "hasOwnProperty", "isPrototypeOf", "propertyIsEnumerable", "toLocaleString", "toString", "value", "valueOf"]
 
-allowedNameChars :: [Char]
+allowedNameChars :: String
 allowedNameChars = ['a'..'z'] ++ ['A'..'Z'] ++ ['0'..'9'] ++ "_"
 
 -- | Encode a Haskell name to JavaScript.
diff --git a/src/Fay/Compiler/Typecheck.hs b/src/Fay/Compiler/Typecheck.hs
--- a/src/Fay/Compiler/Typecheck.hs
+++ b/src/Fay/Compiler/Typecheck.hs
@@ -12,6 +12,7 @@
 import qualified GHC.Paths             as GHCPaths
 
 import           System.Directory
+import           System.Environment
 
 -- | Call out to GHC to type-check the file.
 typecheck :: Config -> FilePath -> IO (Either CompileError String)
@@ -40,10 +41,20 @@
           , "-i" ++ intercalate ":" includeDirs
           , fp ] ++ ghcPackageDbArgs ++ wallF ++ map ("-package " ++) packages
   exists <- doesFileExist GHCPaths.ghc
-  let ghcPath = if exists then GHCPaths.ghc else "ghc"
+  stackInNixShell <- fmap isJust (lookupEnv "STACK_IN_NIX_SHELL")
+  let ghcPath = if exists
+        then if (isInfixOf ".stack" GHCPaths.ghc || stackInNixShell)
+             then "stack"
+             else GHCPaths.ghc
+        else "ghc"
+      extraFlags = case ghcPath of
+        "stack" -> ["exec","--","ghc"]
+        _       -> []
   when (configShowGhcCalls cfg) $
-    putStrLn . intercalate " " $ ghcPath : flags
-  res <- readAllFromProcess ghcPath flags ""
+    putStrLn . unwords $ ghcPath : (extraFlags ++ flags)
+  when stackInNixShell (unsetEnv "STACK_IN_NIX_SHELL")
+  res <- readAllFromProcess ghcPath (extraFlags ++ flags) ""
+  when stackInNixShell (setEnv "STACK_IN_NIX_SHELL" "1")
   either (return . Left . GHCError . fst) (return . Right . fst) res
    where
     wallF | configWall cfg = ["-Wall"]
diff --git a/src/Fay/Config.hs b/src/Fay/Config.hs
--- a/src/Fay/Config.hs
+++ b/src/Fay/Config.hs
@@ -26,6 +26,7 @@
       , configPrettyThunks
       , configPrettyOperators
       , configShowGhcCalls
+      , configTypeScript
       )
   , defaultConfig
   , defaultConfigWithSandbox
@@ -78,6 +79,7 @@
   , configPrettyThunks       :: Bool                        -- ^ Use pretty thunk names?
   , configPrettyOperators    :: Bool                        -- ^ Use pretty operators?
   , configShowGhcCalls       :: Bool                        -- ^ Print commands sent to GHC?
+  , configTypeScript         :: Bool                        -- ^ Output a TypeScript file.
   } deriving (Show)
 
 defaultConfig :: Config
@@ -109,6 +111,7 @@
     , configPrettyThunks       = False
     , configPrettyOperators    = False
     , configShowGhcCalls       = False
+    , configTypeScript         = False
     }
 
 defaultConfigWithSandbox :: IO Config
diff --git a/src/Fay/Convert.hs b/src/Fay/Convert.hs
--- a/src/Fay/Convert.hs
+++ b/src/Fay/Convert.hs
@@ -3,9 +3,7 @@
 {-# LANGUAGE PatternGuards       #-}
 {-# LANGUAGE RankNTypes          #-}
 {-# LANGUAGE ScopedTypeVariables #-}
-{-# LANGUAGE TupleSections       #-}
 {-# LANGUAGE ViewPatterns        #-}
-{-# OPTIONS -fno-warn-type-defaults #-}
 
 -- | Convert a Haskell value to a (JSON representation of a) Fay value.
 
@@ -40,7 +38,7 @@
 --   values aren't handled by explicit cases.  'encodeFay' can be used to
 --   resolve this issue.
 showToFay :: Data a => a -> Maybe Value
-showToFay = spoon . encodeFay (\x -> x)
+showToFay = spoon . encodeFay id
 
 -- | Convert a Haskell value to a Fay json value.  This can fail when primitive
 --   values aren't handled by explicit cases.  When this happens, you can add
@@ -92,12 +90,12 @@
 
 -- | Convert a Fay json value to a Haskell value.
 readFromFay :: Data a => Value -> Maybe a
-readFromFay = either (\_ -> Nothing) Just . decodeFay (\_ -> id)
+readFromFay = either (const Nothing) Just . decodeFay (const id)
 
 -- | Convert a Fay json value to a Haskell value.  This is like readFromFay,
 --   except it yields helpful error messages on failure.
 readFromFay' :: Data a => Value -> Either String a
-readFromFay' = decodeFay (\_ -> id)
+readFromFay' = decodeFay (const id)
 
 -- | Convert a Fay json value to a Haskell value.
 --
@@ -141,7 +139,7 @@
 parseTuple :: Data a => GenericParser -> DataType -> Vector Value -> Either String a
 parseTuple rec typ arr =
   case dataTypeConstrs typ of
-    [cons] -> evalStateT (fromConstrM (do i:next <- get
+    [cons] -> evalStateT (fromConstrM (do ~(i:next) <- get
                                           put next
                                           value <- lift (Vector.indexM arr i)
                                           lift (rec value))
@@ -166,22 +164,23 @@
 -- | Make a simple ADT constructor from an object: { "slot1": 1, "slot2": 2} -> Foo 1 2
 makeSimple :: Data a => GenericParser -> HashMap Text Value -> Constr -> Either String a
 makeSimple rec obj cons =
-  evalStateT (fromConstrM (do i:next <- get
+  evalStateT (fromConstrM (do ~(i:next) <- get
                               put next
                               value <- lift (lookupField obj (Text.pack ("slot" ++ show i)))
                               lift (rec value))
                           cons)
-             [1..]
+             [(1::Integer)..]
 
 -- | Make a record from a key-value: { "x": 1 } -> Foo { x = 1 }
 makeRecord :: Data a => GenericParser -> HashMap Text Value -> Constr -> [String] -> Either String a
-makeRecord rec obj cons fields =
-  evalStateT (fromConstrM (do key:next <- get
-                              put next
-                              value <- lift (lookupField obj (Text.pack key))
-                              lift (rec value))
-                          cons)
-             fields
+makeRecord rec obj cons =
+  evalStateT $
+    fromConstrM
+      (do ~(key:next) <- get
+          put next
+          value <- lift (lookupField obj (Text.pack key))
+          lift $ rec value)
+      cons
 
 lookupField :: HashMap Text Value -> Text -> Either String Value
 lookupField obj key =
diff --git a/src/Fay/Runtime.hs b/src/Fay/Runtime.hs
new file mode 100644
--- /dev/null
+++ b/src/Fay/Runtime.hs
@@ -0,0 +1,842 @@
+{-# LANGUAGE OverloadedStrings #-}
+{-# LANGUAGE TemplateHaskell #-}
+{-# LANGUAGE QuasiQuotes #-}
+
+module Fay.Runtime where
+
+import Text.Shakespeare.Text
+import Data.Text.Lazy as T
+import Fay.Config
+
+-- | Get the default runtime source.
+getRuntimeSource :: Config -> String
+getRuntimeSource cfg =
+  let ifTs :: T.Text -> T.Text
+      ifTs a = if configTypeScript cfg then a else ""
+      ifTsJs :: T.Text -> T.Text -> T.Text
+      ifTsJs a b = if configTypeScript cfg then a else b
+  in T.unpack [lt|
+/*******************************************************************************
+ * Misc.
+ */
+#{ifTs "var Fay:{[key:string]: any;} = {};"}
+
+// Workaround for missing functionality in IE 8 and earlier.
+if( Object.create === undefined ) {
+  Object.create = function( o ) {
+    function F(){}
+    F.prototype = o;
+    return new F();
+  };
+}
+
+// Insert properties of b in place into a.
+function Fay$$objConcat(a,b){
+  for (var p in b) if (b.hasOwnProperty(p)){
+    a[p] = b[p];
+  }
+  return a;
+}
+
+/*******************************************************************************
+ * Thunks.
+ */
+
+// Force a thunk (if it is a thunk) until WHNF.
+function Fay$$_(thunkish,nocache#{ifTs "?: boolean"}){
+  while (thunkish instanceof Fay$$$) {
+    thunkish = thunkish.force(nocache);
+  }
+  return thunkish;
+}
+
+// Apply a function to arguments (see method2 in Fay.hs).
+function Fay$$__(){
+  var f = arguments[0];
+  for (var i = 1, len = arguments.length; i < len; i++) {
+    f = (f instanceof Fay$$$? Fay$$_(f) : f)(arguments[i]);
+  }
+  return f;
+}
+
+// Thunk object.
+function Fay$$$(value){
+  this.forced = false;
+  this.value = value;
+}
+
+// Force the thunk.
+Fay$$$.prototype.force = function(nocache) {
+  return nocache ?
+    this.value() :
+    (this.forced ?
+     this.value :
+     (this.value = this.value(), this.forced = true, this.value));
+};
+
+
+function Fay$$seq(x) {
+  return function(y) {
+    Fay$$_(x,false);
+    return y;
+  }
+}
+
+function Fay$$seq$36$uncurried(x,y) {
+  Fay$$_(x,false);
+  return y;
+}
+
+/*******************************************************************************
+ * Monad.
+ */
+
+function Fay$$Monad(value){
+  this.value = value;
+}
+
+// This is used directly from Fay, but can be rebound or shadowed. See primOps in Types.hs.
+// >>
+function Fay$$then(a){
+  return function(b){
+    return Fay$$bind(a)(function(_){
+      return b;
+    });
+  };
+}
+
+// This is used directly from Fay, but can be rebound or shadowed. See primOps in Types.hs.
+// >>
+function Fay$$then$36$uncurried(a,b){
+  return Fay$$bind$36$uncurried(a,function(_){ return b; });
+}
+
+// >>=
+// This is used directly from Fay, but can be rebound or shadowed. See primOps in Types.hs.
+function Fay$$bind(m){
+  return function(f){
+    return new Fay$$$(function(){
+      var monad = Fay$$_(m,true);
+      return Fay$$_(f)(monad.value);
+    });
+  };
+}
+
+// >>=
+// This is used directly from Fay, but can be rebound or shadowed. See primOps in Types.hs.
+function Fay$$bind$36$uncurried(m,f){
+  return new Fay$$$(function(){
+    var monad = Fay$$_(m,true);
+    return Fay$$_(f)(monad.value);
+  });
+}
+
+// This is used directly from Fay, but can be rebound or shadowed.
+function Fay$$$_return(a){
+  return new Fay$$Monad(a);
+}
+
+// Allow the programmer to access thunk forcing directly.
+function Fay$$force(thunk){
+  return function(type){
+    return new Fay$$$(function(){
+      Fay$$_(thunk,type);
+      return new Fay$$Monad(Fay$$unit);
+    })
+  }
+}
+
+// This is used directly from Fay, but can be rebound or shadowed.
+function Fay$$return$36$uncurried(a){
+  return new Fay$$Monad(a);
+}
+
+// Unit: ().
+var Fay$$unit = null;
+
+/*******************************************************************************
+ * Serialization.
+ * Fay <-> JS. Should be bijective.
+ */
+
+// Serialize a Fay object to JS.
+function Fay$$fayToJs(type,fayObj){
+  var base = type[0];
+  var args = type[1];
+  var jsObj;
+  if(base == "action") {
+    // A nullary monadic action. Should become a nullary JS function.
+    // Fay () -> function(){ return ... }
+    return function(){
+      return Fay$$fayToJs(args[0],Fay$$_(fayObj,true).value);
+    };
+
+  }
+  else if(base == "function") {
+    // A proper function.
+    return function(){
+      var fayFunc = fayObj;
+      var return_type = args[args.length-1];
+      var len = args.length;
+      // If some arguments.
+      if (len > 1) {
+        // Apply to all the arguments.
+        fayFunc = Fay$$_(fayFunc,true);
+        // TODO: Perhaps we should throw an error when JS
+        // passes more arguments than Haskell accepts.
+
+        // Unserialize the JS values to Fay for the Fay callback.
+        if (args == "automatic_function")
+        {
+          for (var i = 0; i < arguments.length; i++) {
+            fayFunc = Fay$$_(fayFunc(Fay$$jsToFay(["automatic"],arguments[i])),true);
+          }
+          return Fay$$fayToJs(["automatic"], fayFunc);
+        }
+
+        for (var i = 0, len = len; i < len - 1 && fayFunc instanceof Function; i++) {
+          fayFunc = Fay$$_(fayFunc(Fay$$jsToFay(args[i],arguments[i])),true);
+        }
+        // Finally, serialize the Fay return value back to JS.
+        var return_base = return_type[0];
+        var return_args = return_type[1];
+        // If it's a monadic return value, get the value instead.
+        if(return_base == "action") {
+          return Fay$$fayToJs(return_args[0],fayFunc.value);
+        }
+        // Otherwise just serialize the value direct.
+        else {
+          return Fay$$fayToJs(return_type,fayFunc);
+        }
+      } else {
+        throw new Error("Nullary function?");
+      }
+    };
+
+  }
+  else if(base == "string") {
+    return Fay$$fayToJs_string(fayObj);
+  }
+  else if(base == "list") {
+    // Serialize Fay list to JavaScript array.
+    var arr = [];
+    fayObj = Fay$$_(fayObj);
+    while(fayObj instanceof Fay$$Cons) {
+      arr.push(Fay$$fayToJs(args[0],fayObj.car));
+      fayObj = Fay$$_(fayObj.cdr);
+    }
+    return arr;
+  }
+  else if(base == "tuple") {
+    // Serialize Fay tuple to JavaScript array.
+    var arr = [];
+    fayObj = Fay$$_(fayObj);
+    var i = 0;
+    while(fayObj instanceof Fay$$Cons) {
+      arr.push(Fay$$fayToJs(args[i++],fayObj.car));
+      fayObj = Fay$$_(fayObj.cdr);
+    }
+    return arr;
+  }
+  else if(base == "defined") {
+    fayObj = Fay$$_(fayObj);
+    return fayObj instanceof Fay.FFI._Undefined
+      ? undefined
+      : Fay$$fayToJs(args[0],fayObj.slot1);
+  }
+  else if(base == "nullable") {
+    fayObj = Fay$$_(fayObj);
+    return fayObj instanceof Fay.FFI._Null
+      ? null
+      : Fay$$fayToJs(args[0],fayObj.slot1);
+  }
+  else if(base == "double" || base == "int" || base == "bool") {
+    // Bools are unboxed.
+    return Fay$$_(fayObj);
+  }
+  else if(base == "ptr")
+    return fayObj;
+  else if(base == "unknown")
+    return Fay$$fayToJs(["automatic"], fayObj);
+  else if(base == "automatic" && fayObj instanceof Function) {
+    return Fay$$fayToJs(["function", "automatic_function"], fayObj);
+  }
+  else if(base == "automatic" || base == "user") {
+    fayObj = Fay$$_(fayObj);
+
+    if(fayObj instanceof Fay$$Cons || fayObj === null){
+      // Serialize Fay list to JavaScript array.
+      var arr = [];
+      while(fayObj instanceof Fay$$Cons) {
+        arr.push(Fay$$fayToJs(["automatic"],fayObj.car));
+        fayObj = Fay$$_(fayObj.cdr);
+      }
+      return arr;
+    } else {
+      var fayToJsFun = fayObj && fayObj.instance && Fay$$fayToJsHash[fayObj.instance];
+      return fayToJsFun ? fayToJsFun(type,type[2],fayObj) : fayObj;
+    }
+  }
+
+  throw new Error("Unhandled Fay->JS translation type: " + base);
+}
+
+// Stores the mappings from fay types to js objects.
+// This will be populated by compiled modules.
+var Fay$$fayToJsHash = {};
+
+// Specialized serializer for string.
+function Fay$$fayToJs_string(fayObj){
+  // Serialize Fay string to JavaScript string.
+  var str = "";
+  fayObj = Fay$$_(fayObj);
+  while(fayObj instanceof Fay$$Cons) {
+    str += Fay$$_(fayObj.car);
+    fayObj = Fay$$_(fayObj.cdr);
+  }
+  return str;
+};
+function Fay$$jsToFay_string(x){
+  return Fay$$list(x)
+};
+
+// Special num/bool serializers.
+function Fay$$jsToFay_int(x){return x;}
+function Fay$$jsToFay_double(x){return x;}
+function Fay$$jsToFay_bool(x){return x;}
+
+function Fay$$fayToJs_int(x){return Fay$$_(x);}
+function Fay$$fayToJs_double(x){return Fay$$_(x);}
+function Fay$$fayToJs_bool(x){return Fay$$_(x);}
+
+// Unserialize an object from JS to Fay.
+function Fay$$jsToFay(type,jsObj){
+  var base = type[0];
+  var args = type[1];
+  var fayObj;
+  if(base == "action") {
+    // Unserialize a "monadic" JavaScript return value into a monadic value.
+    return new Fay$$Monad(Fay$$jsToFay(args[0],jsObj));
+  }
+  else if(base == "function") {
+    // Unserialize a function from JavaScript to a function that Fay can call.
+    // So
+    //
+    //    var f = function(x,y,z){ … }
+    //
+    // becomes something like:
+    //
+    //    function(x){
+    //      return function(y){
+    //        return function(z){
+    //          return new Fay$$$(function(){
+    //            return Fay$$jsToFay(f(Fay$$fayTojs(x),
+    //                                  Fay$$fayTojs(y),
+    //                                  Fay$$fayTojs(z))
+    //    }}}}};
+    var returnType = args[args.length-1];
+    var funArgs = args.slice(0,-1);
+
+    if (jsObj.length > 0) {
+      var makePartial = function(args){
+        return function(arg){
+          var i = args.length;
+          var fayArg = Fay$$fayToJs(funArgs[i],arg);
+          var newArgs = args.concat([fayArg]);
+          if(newArgs.length == funArgs.length) {
+            return new Fay$$$(function(){
+              return Fay$$jsToFay(returnType,jsObj.apply(this,newArgs));
+            });
+          } else {
+            return makePartial(newArgs);
+          }
+        };
+      };
+      return makePartial([]);
+    }
+    else
+      return function (arg) {
+        return Fay$$jsToFay(["automatic"], jsObj(Fay$$fayToJs(["automatic"], arg)));
+      };
+  }
+  else if(base == "string") {
+    // Unserialize a JS string into Fay list (String).
+    // This is a special case, when String is explicit in the type signature,
+    // with `Automatic' a string would not be decoded.
+    return Fay$$list(jsObj);
+  }
+  else if(base == "list") {
+    // Unserialize a JS array into a Fay list ([a]).
+    var serializedList = [];
+    for (var i = 0, len = jsObj.length; i < len; i++) {
+      // Unserialize each JS value into a Fay value, too.
+      serializedList.push(Fay$$jsToFay(args[0],jsObj[i]));
+    }
+    // Pop it all in a Fay list.
+    return Fay$$list(serializedList);
+  }
+  else if(base == "tuple") {
+    // Unserialize a JS array into a Fay tuple ((a,b,c,...)).
+    var serializedTuple = [];
+    for (var i = 0, len = jsObj.length; i < len; i++) {
+      // Unserialize each JS value into a Fay value, too.
+      serializedTuple.push(Fay$$jsToFay(args[i],jsObj[i]));
+    }
+    // Pop it all in a Fay list.
+    return Fay$$list(serializedTuple);
+  }
+  else if(base == "defined") {
+    return jsObj === undefined
+      ? new Fay.FFI._Undefined()
+      : new Fay.FFI._Defined(Fay$$jsToFay(args[0],jsObj));
+  }
+  else if(base == "nullable") {
+    return jsObj === null
+      ? new Fay.FFI._Null()
+      : new Fay.FFI.Nullable(Fay$$jsToFay(args[0],jsObj));
+  }
+  else if(base == "int") {
+    // Int are unboxed, so there's no forcing to do.
+    // But we can do validation that the int has no decimal places.
+    // E.g. Math.round(x)!=x? throw "NOT AN INTEGER, GET OUT!"
+    fayObj = Math.round(jsObj);
+    if(fayObj!==jsObj) throw "Argument " + jsObj + " is not an integer!";
+    return fayObj;
+  }
+  else if (base == "double" ||
+           base == "bool" ||
+           base ==  "ptr") {
+    return jsObj;
+  }
+  else if(base == "unknown")
+    return Fay$$jsToFay(["automatic"], jsObj);
+  else if(base == "automatic" && jsObj instanceof Function) {
+    #{ifTsJs "let type: string[][]" "var type"} = [["automatic"]];
+    for (var i = 0; i < jsObj.length; i++)
+      type.push(["automatic"]);
+    return Fay$$jsToFay(["function", type], jsObj);
+  }
+  else if(base == "automatic" && jsObj instanceof Array) {
+    var list = null;
+    for (var i = jsObj.length - 1; i >= 0; i--) {
+      list = new Fay$$Cons(Fay$$jsToFay([base], jsObj[i]), list);
+    }
+    return list;
+  }
+  else if(base == "automatic" || base == "user") {
+    if (jsObj && jsObj['instance']) {
+      var jsToFayFun = Fay$$jsToFayHash[jsObj["instance"]];
+      return jsToFayFun ? jsToFayFun(type,type[2],jsObj) : jsObj;
+    }
+    else
+      return jsObj;
+  }
+
+  throw new Error("Unhandled JS->Fay translation type: " + base);
+}
+
+// Stores the mappings from js objects to fay types.
+// This will be populated by compiled modules.
+var Fay$$jsToFayHash = {};
+
+/*******************************************************************************
+ * Lists.
+ */
+
+// Cons object.
+function Fay$$Cons(car,cdr){
+  this.car = car;
+  this.cdr = cdr;
+}
+
+// Make a list.
+function Fay$$list(xs){
+  var out = null;
+  for(var i=xs.length-1; i>=0;i--)
+    out = new Fay$$Cons(xs[i],out);
+  return out;
+}
+
+// Built-in list cons.
+function Fay$$cons(x){
+  return function(y){
+    return new Fay$$Cons(x,y);
+  };
+}
+
+// List index.
+// `list' is already forced by the time it's passed to this function.
+// `list' cannot be null and `index' cannot be out of bounds.
+function Fay$$index(index,list){
+  for(var i = 0; i < index; i++) {
+    list = Fay$$_(list.cdr);
+  }
+  return list.car;
+}
+
+// List length.
+// `list' is already forced by the time it's passed to this function.
+function Fay$$listLen(list,max){
+  for(var i = 0; list !== null && i < max + 1; i++) {
+    list = Fay$$_(list.cdr);
+  }
+  return i == max;
+}
+
+/*******************************************************************************
+ * Numbers.
+ */
+
+// Built-in *.
+function Fay$$mult(x){
+  return function(y){
+    return new Fay$$$(function(){
+      return Fay$$_(x) * Fay$$_(y);
+    });
+  };
+}
+
+function Fay$$mult$36$uncurried(x,y){
+
+  return new Fay$$$(function(){
+    return Fay$$_(x) * Fay$$_(y);
+  });
+
+}
+
+// Built-in +.
+function Fay$$add(x){
+  return function(y){
+    return new Fay$$$(function(){
+      return Fay$$_(x) + Fay$$_(y);
+    });
+  };
+}
+
+// Built-in +.
+function Fay$$add$36$uncurried(x,y){
+
+  return new Fay$$$(function(){
+    return Fay$$_(x) + Fay$$_(y);
+  });
+
+}
+
+// Built-in -.
+function Fay$$sub(x){
+  return function(y){
+    return new Fay$$$(function(){
+      return Fay$$_(x) - Fay$$_(y);
+    });
+  };
+}
+// Built-in -.
+function Fay$$sub$36$uncurried(x,y){
+
+  return new Fay$$$(function(){
+    return Fay$$_(x) - Fay$$_(y);
+  });
+
+}
+
+// Built-in /.
+function Fay$$divi(x){
+  return function(y){
+    return new Fay$$$(function(){
+      return Fay$$_(x) / Fay$$_(y);
+    });
+  };
+}
+
+// Built-in /.
+function Fay$$divi$36$uncurried(x,y){
+
+  return new Fay$$$(function(){
+    return Fay$$_(x) / Fay$$_(y);
+  });
+
+}
+
+/*******************************************************************************
+ * Booleans.
+ */
+
+// Are two values equal?
+function Fay$$equal(lit1, lit2) {
+  // Simple case
+  lit1 = Fay$$_(lit1);
+  lit2 = Fay$$_(lit2);
+  if (lit1 === lit2) {
+    return true;
+  }
+  // General case
+  if (lit1 instanceof Array) {
+    if (lit1.length != lit2.length) return false;
+    for (var len = lit1.length, i = 0; i < len; i++) {
+      if (!Fay$$equal(lit1[i], lit2[i])) return false;
+    }
+    return true;
+  } else if (lit1 instanceof Fay$$Cons && lit2 instanceof Fay$$Cons) {
+    do {
+      if (!Fay$$equal(lit1.car,lit2.car))
+        return false;
+      lit1 = Fay$$_(lit1.cdr), lit2 = Fay$$_(lit2.cdr);
+      if (lit1 === null || lit2 === null)
+        return lit1 === lit2;
+    } while (true);
+  } else if (typeof lit1 == 'object' && typeof lit2 == 'object' && lit1 && lit2 &&
+             lit1.instance === lit2.instance) {
+    for(var x in lit1) {
+      if(!Fay$$equal(lit1[x],lit2[x]))
+        return false;
+    }
+    return true;
+  } else {
+    return false;
+  }
+}
+
+// Built-in ==.
+function Fay$$eq(x){
+  return function(y){
+    return new Fay$$$(function(){
+      return Fay$$equal(x,y);
+    });
+  };
+}
+
+function Fay$$eq$36$uncurried(x,y){
+
+  return new Fay$$$(function(){
+    return Fay$$equal(x,y);
+  });
+
+}
+
+// Built-in /=.
+function Fay$$neq(x){
+  return function(y){
+    return new Fay$$$(function(){
+      return !(Fay$$equal(x,y));
+    });
+  };
+}
+
+// Built-in /=.
+function Fay$$neq$36$uncurried(x,y){
+
+  return new Fay$$$(function(){
+    return !(Fay$$equal(x,y));
+  });
+
+}
+
+// Built-in >.
+function Fay$$gt(x){
+  return function(y){
+    return new Fay$$$(function(){
+      return Fay$$_(x) > Fay$$_(y);
+    });
+  };
+}
+
+// Built-in >.
+function Fay$$gt$36$uncurried(x,y){
+
+  return new Fay$$$(function(){
+    return Fay$$_(x) > Fay$$_(y);
+  });
+
+}
+
+// Built-in <.
+function Fay$$lt(x){
+  return function(y){
+    return new Fay$$$(function(){
+      return Fay$$_(x) < Fay$$_(y);
+    });
+  };
+}
+
+
+// Built-in <.
+function Fay$$lt$36$uncurried(x,y){
+
+  return new Fay$$$(function(){
+    return Fay$$_(x) < Fay$$_(y);
+  });
+
+}
+
+
+// Built-in >=.
+function Fay$$gte(x){
+  return function(y){
+    return new Fay$$$(function(){
+      return Fay$$_(x) >= Fay$$_(y);
+    });
+  };
+}
+
+// Built-in >=.
+function Fay$$gte$36$uncurried(x,y){
+
+  return new Fay$$$(function(){
+    return Fay$$_(x) >= Fay$$_(y);
+  });
+
+}
+
+// Built-in <=.
+function Fay$$lte(x){
+  return function(y){
+    return new Fay$$$(function(){
+      return Fay$$_(x) <= Fay$$_(y);
+    });
+  };
+}
+
+// Built-in <=.
+function Fay$$lte$36$uncurried(x,y){
+
+  return new Fay$$$(function(){
+    return Fay$$_(x) <= Fay$$_(y);
+  });
+
+}
+
+// Built-in &&.
+function Fay$$and(x){
+  return function(y){
+    return new Fay$$$(function(){
+      return Fay$$_(x) && Fay$$_(y);
+    });
+  };
+}
+
+// Built-in &&.
+function Fay$$and$36$uncurried(x,y){
+
+  return new Fay$$$(function(){
+    return Fay$$_(x) && Fay$$_(y);
+  });
+  ;
+}
+
+// Built-in ||.
+function Fay$$or(x){
+  return function(y){
+    return new Fay$$$(function(){
+      return Fay$$_(x) || Fay$$_(y);
+    });
+  };
+}
+
+// Built-in ||.
+function Fay$$or$36$uncurried(x,y){
+
+  return new Fay$$$(function(){
+    return Fay$$_(x) || Fay$$_(y);
+  });
+
+}
+
+/*******************************************************************************
+ * Mutable references.
+ */
+
+// Make a new mutable reference.
+function Fay$$Ref(x){
+  this.value = x;
+}
+
+// Write to the ref.
+function Fay$$writeRef(ref,x){
+  ref.value = x;
+}
+
+// Get the value from the ref.
+function Fay$$readRef(ref){
+  return ref.value;
+}
+
+/*******************************************************************************
+ * Dates.
+ */
+function Fay$$date(str){
+  return Date.parse(str);
+}
+
+/*******************************************************************************
+ * Data.Var
+ */
+
+function Fay$$Ref2(val){
+  this.val = val;
+}
+
+function Fay$$Sig(){
+  this.handlers = [];
+}
+
+function Fay$$Var(val){
+  this.val = val;
+  this.handlers = [];
+}
+
+// Helper used by Fay$$setValue and for merging
+function Fay$$broadcastInternal(self, val, force){
+  var handlers = self.handlers;
+  var exceptions = [];
+  for(#{ifTsJs "let" "var"} len = handlers.length, i = 0; i < len; i++) {
+    try {
+      force(handlers[i][1](val), true);
+    } catch (e) {
+      exceptions.push(e);
+    }
+  }
+  // Rethrow the encountered exceptions.
+  if (exceptions.length > 0) {
+    console.error("Encountered " + exceptions.length + " exception(s) while broadcasing a change to ", self);
+    for(#{ifTsJs "let len: number" "var len"} = exceptions.length, i = 0; i < len; i++) {
+      (function(exception) {
+        setTimeout(function() { throw exception; }, 0);
+      })(exceptions[i]);
+    }
+  }
+}
+
+function Fay$$setValue(self, val, force){
+  if (self instanceof Fay$$Ref2) {
+    self.val = val;
+  } else if (self instanceof Fay$$Var) {
+    self.val = val;
+    Fay$$broadcastInternal(self, val, force);
+  } else if (self instanceof Fay$$Sig) {
+    Fay$$broadcastInternal(self, val, force);
+  } else {
+    throw "Fay$$setValue given something that's not a Ref2, Var, or Sig"
+  }
+}
+
+function Fay$$subscribe(self, f){
+  var key = {};
+  self.handlers.push([key,f]);
+  var searchStart = self.handlers.length - 1;
+  return function(_){
+    for(var i = Math.min(searchStart, self.handlers.length - 1); i >= 0; i--) {
+      if(self.handlers[i][0] == key) {
+        self.handlers = self.handlers.slice(0,i).concat(self.handlers.slice(i+1));
+        return;
+      }
+    }
+    return _; // This variable has to be used, otherwise Closure
+              // strips it out and Fay serialization breaks.
+  };
+}
+
+/*******************************************************************************
+ * Application code.
+ */
+
+|]
diff --git a/src/Fay/Types.hs b/src/Fay/Types.hs
--- a/src/Fay/Types.hs
+++ b/src/Fay/Types.hs
@@ -1,5 +1,6 @@
 {-# LANGUAGE CPP                        #-}
 {-# LANGUAGE GeneralizedNewtypeDeriving #-}
+{-# LANGUAGE NoImplicitPrelude          #-}
 {-# LANGUAGE TypeFamilies               #-}
 
 -- | All Fay types and instances.
@@ -40,6 +41,8 @@
   , mkModulePathFromQName
   ) where
 
+import           Fay.Compiler.Prelude
+
 import           Fay.Compiler.ModuleT
 import           Fay.Config
 import qualified Fay.Exts.NoAnnotation   as N
@@ -56,10 +59,11 @@
 #endif
 import           Control.Monad.Except    (ExceptT, MonadError)
 import           Control.Monad.Identity  (Identity)
-import           Control.Monad.RWS
+import           Control.Monad.RWS       (MonadIO, MonadReader, MonadState, MonadWriter, RWST, lift)
 import           Data.Map                (Map)
 import           Data.Set                (Set)
 import           Language.Haskell.Names  (Symbols)
+import           Data.Semigroup          (Semigroup)
 
 --------------------------------------------------------------------------------
 -- Compiler types
@@ -87,10 +91,14 @@
   } deriving (Show)
 
 -- | Simple concatenating instance.
+instance Semigroup CompileWriter where
+  (CompileWriter a b c) <> (CompileWriter x y z) =
+    CompileWriter (a++x) (b++y) (c++z)
+
+-- | Simple concatenating instance.
 instance Monoid CompileWriter where
   mempty = CompileWriter [] [] []
-  mappend (CompileWriter a b c) (CompileWriter x y z) =
-    CompileWriter (a++x) (b++y) (c++z)
+  mappend = (<>)
 
 -- | Configuration and globals for the compiler.
 data CompileReader = CompileReader
diff --git a/src/Fay/Types/CompileError.hs b/src/Fay/Types/CompileError.hs
--- a/src/Fay/Types/CompileError.hs
+++ b/src/Fay/Types/CompileError.hs
@@ -37,3 +37,4 @@
   | UnsupportedWhereInAlt S.Alt
   | UnsupportedWhereInMatch S.Match
   deriving (Show)
+{-# ANN module "HLint: ignore Use camelCase" #-}
diff --git a/src/Fay/Types/Js.hs b/src/Fay/Types/Js.hs
--- a/src/Fay/Types/Js.hs
+++ b/src/Fay/Types/Js.hs
@@ -16,6 +16,7 @@
 -- | Statement type.
 data JsStmt
   = JsVar JsName JsExp
+  | JsMapVar JsName JsExp
   | JsIf JsExp [JsStmt] [JsStmt]
   | JsEarlyReturn JsExp
   | JsThrow JsExp
diff --git a/src/Fay/Types/Printer.hs b/src/Fay/Types/Printer.hs
--- a/src/Fay/Types/Printer.hs
+++ b/src/Fay/Types/Printer.hs
@@ -16,11 +16,15 @@
   , mapping
   ) where
 
-import Control.Monad.RWS
+import Fay.Compiler.Prelude
+
+import Control.Monad.RWS               (RWS, asks, execRWS, get, modify, put, tell)
 import Data.List                       (elemIndex)
+import Data.Maybe                      (fromMaybe)
 import Data.String
 import Language.Haskell.Exts
 import SourceMap.Types
+import qualified Data.Semigroup as SG
 
 -- | Global options of the printer
 data PrintReader = PrintReader
@@ -42,10 +46,13 @@
 pwOutputString :: PrintWriter -> String
 pwOutputString (PrintWriter _ out) = out ""
 
+instance SG.Semigroup PrintWriter where
+  (PrintWriter a b) <> (PrintWriter x y) = PrintWriter (a ++ x) (b . y)
+
 -- | Output concatenation
 instance Monoid PrintWriter where
   mempty =  PrintWriter [] id
-  mappend (PrintWriter a b) (PrintWriter x y) = PrintWriter (a ++ x) (b . y)
+  mappend = (<>)
 
 -- | The state of the pretty printer.
 data PrintState = PrintState
@@ -66,10 +73,12 @@
 execPrinter :: Printer -> PrintReader -> PrintWriter
 execPrinter (Printer p) r = snd $ execRWS p r defaultPrintState
 
+instance SG.Semigroup Printer where
+  (Printer p) <> (Printer q) = Printer (p >> q)
 
 instance Monoid Printer where
   mempty = Printer $ return ()
-  mappend (Printer p) (Printer q) = Printer (p >> q)
+  mappend = (<>)
 
 -- | Print some value.
 class Printable a where
@@ -85,7 +94,7 @@
 --   Does nothing when prPretty is False
 newline :: Printer
 newline = Printer $ asks prPretty >>= flip when writeNewline
-  where writeNewline = (writeRWS "\n" >> modify (\s -> s {psNewline = True}))
+  where writeNewline = writeRWS "\n" >> modify (\s -> s { psNewline = True })
 
 -- | Write out a raw string, respecting the indentation
 --   Note: if you pass a string with newline characters, it will print them
@@ -106,9 +115,7 @@
 
   let newLines = length (filter (== '\n') x)
   put ps { psLine    = psLine ps + newLines
-         , psColumn  = case elemIndex '\n' (reverse x) of
-                        Just i  -> i
-                        Nothing -> psColumn ps + length x
+         , psColumn  = fromMaybe (psColumn ps + length x) . elemIndex '\n' $ reverse x
          , psNewline = False
          }
 
diff --git a/src/haskell-names/Language/Haskell/Names/GetBound.hs b/src/haskell-names/Language/Haskell/Names/GetBound.hs
--- a/src/haskell-names/Language/Haskell/Names/GetBound.hs
+++ b/src/haskell-names/Language/Haskell/Names/GetBound.hs
@@ -79,7 +79,7 @@
     getBound ctx (QualConDecl _ _ _ d) = getBound ctx d
 
 instance (Data l) => GetBound (GadtDecl l) l where
-    getBound _ctx (GadtDecl _l conName mbFieldDecls _ty) =
+    getBound _ctx (GadtDecl _l conName _tyvarBinds _context mbFieldDecls _ty) =
       -- GADT constructor name
       [conName] ++
       -- GADT selector names
diff --git a/src/haskell-names/Language/Haskell/Names/GlobalSymbolTable.hs b/src/haskell-names/Language/Haskell/Names/GlobalSymbolTable.hs
--- a/src/haskell-names/Language/Haskell/Names/GlobalSymbolTable.hs
+++ b/src/haskell-names/Language/Haskell/Names/GlobalSymbolTable.hs
@@ -1,5 +1,6 @@
 {-# OPTIONS -fno-warn-name-shadowing #-}
 {-# LANGUAGE DeriveDataTypeable #-}
+{-# LANGUAGE NoImplicitPrelude  #-}
 -- | This module is designed to be imported qualified.
 module Language.Haskell.Names.GlobalSymbolTable
   ( Table
@@ -21,6 +22,7 @@
 import qualified Data.Map                           as Map
 import qualified Data.Set                           as Set
 import           Language.Haskell.Exts    as HSE
+import           Data.Semigroup (Semigroup)
 
 -- | Global symbol table — contains global names
 data Table =
@@ -35,9 +37,8 @@
 tyLens :: Lens Table (Map.Map GName (Set.Set (SymTypeInfo OrigName)))
 tyLens = lens (\(Table _ ts) -> ts) (\ts (Table vs _) -> Table vs ts)
 
-instance Monoid Table where
-  mempty = empty
-  mappend (Table vs1 ts1) (Table vs2 ts2) =
+instance Semigroup Table where
+  (Table vs1 ts1) <> (Table vs2 ts2) =
     Table (j vs1 vs2) (j ts1 ts2)
     where
       j :: (Ord i, Ord k)
@@ -45,6 +46,9 @@
         -> Map.Map k (Set.Set i)
         -> Map.Map k (Set.Set i)
       j = Map.unionWith Set.union
+instance Monoid Table where
+  mempty = empty
+  mappend = (<>)
 
 toGName :: QName l -> GName
 toGName (UnQual _ n) = GName "" (nameToString n)
diff --git a/src/haskell-names/Language/Haskell/Names/Imports.hs b/src/haskell-names/Language/Haskell/Names/Imports.hs
--- a/src/haskell-names/Language/Haskell/Names/Imports.hs
+++ b/src/haskell-names/Language/Haskell/Names/Imports.hs
@@ -189,8 +189,6 @@
               (ENotExported Nothing n mod)
               matches
               spec
-    IAbs _ (TypeNamespace {}) _ -> error "'type' namespace is not supported yet" -- FIXME
-
     -- FIXME
     -- What about things like:
     -- head(..)
diff --git a/src/haskell-names/Language/Haskell/Names/LocalSymbolTable.hs b/src/haskell-names/Language/Haskell/Names/LocalSymbolTable.hs
--- a/src/haskell-names/Language/Haskell/Names/LocalSymbolTable.hs
+++ b/src/haskell-names/Language/Haskell/Names/LocalSymbolTable.hs
@@ -14,10 +14,11 @@
 
 import qualified Data.Map                           as Map
 import           Language.Haskell.Exts
+import           Data.Semigroup ()
 
 -- | Local symbol table — contains locally bound names
 newtype Table = Table (Map.Map NameS SrcLoc)
-  deriving Monoid
+  deriving Semigroup
 
 addValue :: SrcInfo l => Name l -> Table -> Table
 addValue n (Table vs) =
diff --git a/src/haskell-names/Language/Haskell/Names/ModuleSymbols.hs b/src/haskell-names/Language/Haskell/Names/ModuleSymbols.hs
--- a/src/haskell-names/Language/Haskell/Names/ModuleSymbols.hs
+++ b/src/haskell-names/Language/Haskell/Names/ModuleSymbols.hs
@@ -1,4 +1,5 @@
 {-# OPTIONS -fno-warn-name-shadowing #-}
+{-# LANGUAGE NoImplicitPrelude   #-}
 {-# LANGUAGE ScopedTypeVariables #-}
 {-# LANGUAGE ViewPatterns        #-}
 module Language.Haskell.Names.ModuleSymbols
@@ -7,6 +8,8 @@
   )
   where
 
+import           Fay.Compiler.Prelude
+
 import           Language.Haskell.Exts
 import           Language.Haskell.Names.GetBound
 import qualified Language.Haskell.Names.GlobalSymbolTable as Global
@@ -14,13 +17,8 @@
 import           Language.Haskell.Names.SyntaxUtils
 import           Language.Haskell.Names.Types
 
-import           Control.Monad
-import           Data.Data
-import           Data.Either
 import           Data.Lens.Light
 import qualified Data.Map                                 as Map
-import           Data.Maybe
-import           Data.Monoid
 import qualified Data.Set                                 as Set
 
 -- | Compute module's global table. It contains both the imported entities
@@ -103,7 +101,7 @@
 
         cons :: Constructors
         cons = do -- list monad
-          GadtDecl _ cn (fromMaybe [] -> fields) _ty <- gadtDecls
+          GadtDecl _ cn _tyvarBinds _context (fromMaybe [] -> fields) _ty <- gadtDecls
           return (void cn , [void f | FieldDecl _ fNames _ <- fields, f <- fNames])
 
         infos = constructorsToInfos dq cons
diff --git a/src/haskell-names/Language/Haskell/Names/Open/Derived.hs b/src/haskell-names/Language/Haskell/Names/Open/Derived.hs
--- a/src/haskell-names/Language/Haskell/Names/Open/Derived.hs
+++ b/src/haskell-names/Language/Haskell/Names/Open/Derived.hs
@@ -47,7 +47,6 @@
 deriveGTraversable ''GuardedRhs
 deriveGTraversable ''Type
 deriveGTraversable ''TyVarBind
-deriveGTraversable ''Kind
 deriveGTraversable ''FunDep
 deriveGTraversable ''Context
 deriveGTraversable ''Asst
@@ -85,3 +84,5 @@
 deriveGTraversable ''Unpackedness
 deriveGTraversable ''ResultSig
 deriveGTraversable ''InjectivityInfo
+deriveGTraversable ''DerivStrategy
+deriveGTraversable ''MaybePromotedName
diff --git a/src/haskell-names/Language/Haskell/Names/Recursive.hs b/src/haskell-names/Language/Haskell/Names/Recursive.hs
--- a/src/haskell-names/Language/Haskell/Names/Recursive.hs
+++ b/src/haskell-names/Language/Haskell/Names/Recursive.hs
@@ -1,4 +1,5 @@
 {-# OPTIONS -fno-warn-name-shadowing #-}
+{-# LANGUAGE NoImplicitPrelude   #-}
 {-# LANGUAGE ScopedTypeVariables #-}
 {-# LANGUAGE TypeFamilies        #-}
 module Language.Haskell.Names.Recursive
@@ -7,6 +8,8 @@
   , annotateModule
   ) where
 
+import           Fay.Compiler.Prelude
+
 import           Fay.Compiler.ModuleT
 import           Language.Haskell.Names.Annotated
 import           Language.Haskell.Names.Exports
@@ -17,12 +20,9 @@
 import           Language.Haskell.Names.SyntaxUtils
 import           Language.Haskell.Names.Types
 
-import           Control.Monad                        hiding (forM_)
 import           Data.Data                            (Data)
 import           Data.Foldable
 import           Data.Graph                           (flattenSCC, stronglyConnComp)
-import           Data.Maybe
-import           Data.Monoid
 import qualified Data.Set                             as Set
 import           Language.Haskell.Exts
 
diff --git a/src/haskell-names/Language/Haskell/Names/SyntaxUtils.hs b/src/haskell-names/Language/Haskell/Names/SyntaxUtils.hs
--- a/src/haskell-names/Language/Haskell/Names/SyntaxUtils.hs
+++ b/src/haskell-names/Language/Haskell/Names/SyntaxUtils.hs
@@ -82,6 +82,7 @@
 specialConToString (TupleCon _ Unboxed n) = '#':replicate (n-1) ','
 specialConToString (Cons _)               = ":"
 specialConToString (UnboxedSingleCon _)   = "#"
+specialConToString (ExprHole _)           = "_"
 
 unCName :: CName l -> Name l
 unCName (VarName _ n) = n
diff --git a/src/haskell-names/Language/Haskell/Names/Types.hs b/src/haskell-names/Language/Haskell/Names/Types.hs
--- a/src/haskell-names/Language/Haskell/Names/Types.hs
+++ b/src/haskell-names/Language/Haskell/Names/Types.hs
@@ -4,6 +4,7 @@
 {-# LANGUAGE DeriveFoldable     #-}
 {-# LANGUAGE DeriveFunctor      #-}
 {-# LANGUAGE DeriveTraversable  #-}
+{-# LANGUAGE NoImplicitPrelude  #-}
 {-# LANGUAGE StandaloneDeriving #-}
 module Language.Haskell.Names.Types
   ( Error (..)
@@ -36,6 +37,7 @@
 import qualified Data.Set                                 as Set
 import           Language.Haskell.Exts
 import           Text.Printf
+import qualified Data.Semigroup                           as SG
 
 type ExtensionSet = Set.Set KnownExtension
 
@@ -118,10 +120,13 @@
 data Symbols = Symbols (Set.Set (SymValueInfo OrigName)) (Set.Set (SymTypeInfo OrigName))
   deriving (Eq, Ord, Show, Data, Typeable)
 
+instance SG.Semigroup Symbols where
+  (Symbols s1 t1) <> (Symbols s2 t2) =
+    Symbols (s1 <> s2) (t1 <> t2)
+
 instance Monoid Symbols where
   mempty = Symbols mempty mempty
-  mappend (Symbols s1 t1) (Symbols s2 t2) =
-    Symbols (s1 `mappend` s2) (t1 `mappend` t2)
+  mappend = (<>)
 
 valSyms :: Lens Symbols (Set.Set (SymValueInfo OrigName))
 valSyms = lens (\(Symbols vs _) -> vs) (\vs (Symbols _ ts) -> Symbols vs ts)
diff --git a/src/main/Main.hs b/src/main/Main.hs
--- a/src/main/Main.hs
+++ b/src/main/Main.hs
@@ -49,6 +49,7 @@
   , optVersion            :: Bool
   , optWall               :: Bool
   , optShowGhcCalls       :: Bool
+  , optTypeScript         :: Bool
   , optFiles              :: [String]
   }
 
@@ -81,11 +82,12 @@
           , configPrettyThunks     = optPrettyThunks opts || optPrettyAll opts
           , configPrettyOperators  = optPrettyOperators opts || optPrettyAll opts
           , configShowGhcCalls     = optShowGhcCalls opts
+          , configTypeScript       = optTypeScript opts
           }
   if optVersion opts
     then runCommandVersion
     else if optPrintRuntime opts
-      then getConfigRuntime config >>= readFile >>= putStr
+      then readConfigRuntime config >>= putStr
       else do
         void $ incompatible htmlAndStdout opts "Html wrapping and stdout are incompatible"
         case optFiles opts of
@@ -97,7 +99,7 @@
     parser = info (helper <*> options) (fullDesc <> header helpTxt)
 
     outPutFile :: FayCompilerOptions -> String -> FilePath
-    outPutFile opts file = fromMaybe (toJsName file) $ optOutput opts
+    outPutFile opts file = fromMaybe (if optTypeScript opts then toTsName file else toJsName file) $ optOutput opts
 
 -- | All Fay's command-line options.
 options :: Parser FayCompilerOptions
@@ -135,6 +137,7 @@
   <*> switch (long "version" <> help "Output version number")
   <*> switch (long "Wall" <> help "Typecheck with -Wall")
   <*> switch (long "show-ghc-calls" <> help "Print commands sent to ghc")
+  <*> switch (long "ts" <> help "Output TypeScript instead of JavaScript")
   <*> many (argument (ReadM ask) (metavar "<hs-file>..."))
   where
     strsOption :: Mod OptionFields [String] -> Parser [String]
@@ -150,10 +153,9 @@
 
 -- | The basic help text.
 helpTxt :: String
-helpTxt = concat
-  ["fay -- The fay compiler from (a proper subset of) Haskell to Javascript\n\n"
-  ,"  fay <hs-file>... processes each .hs file"
-  ]
+helpTxt
+  =  "fay -- The fay compiler from (a proper subset of) Haskell to Javascript\n\n"
+  ++ "  fay <hs-file>... processes each .hs file"
 
 -- | Print the command version.
 runCommandVersion :: IO ()
diff --git a/src/tests/Test/Compile.hs b/src/tests/Test/Compile.hs
--- a/src/tests/Test/Compile.hs
+++ b/src/tests/Test/Compile.hs
@@ -3,13 +3,10 @@
 {-# LANGUAGE RecordWildCards   #-}
 {-# LANGUAGE TemplateHaskell   #-}
 
-module Test.Compile (tests) where
+module Test.Compile (tests,runScriptFile) where
 
 import           Fay
 import           Fay.Compiler.Prelude
-#if !MIN_VERSION_base(4,7,0)
-import           Test.Util                       (isRight)
-#endif
 
 import           Language.Haskell.Exts
 import           Test.Tasty
@@ -87,24 +84,29 @@
 case_strictWrapper = do
   cfg <- defConf
   res <- compileFile cfg { configTypecheck = True, configFilePath = Just "tests/Compile/StrictWrapper.hs", configStrict = ["StrictWrapper"] } "tests/Compile/StrictWrapper.hs"
+  let isTs = configTypeScript cfg
+      suffix = if isTs then ".ts" else ".js"
   (\a b -> either a b res) (assertFailure . show) $ \js -> do
-    writeFile "tests/Compile/StrictWrapper.js" js
-    (err, out) <- either id id <$> readAllFromProcess "node" ["tests/Compile/StrictWrapper.js"] ""
+    writeFile ("tests/Compile/StrictWrapper" ++ suffix) js
+    (err, out) <- either id id <$> runScriptFile isTs ("tests/Compile/StrictWrapper" ++ suffix)
     when (err /= "") $ assertFailure err
     expected <- readFile "tests/Compile/StrictWrapper.res"
     assertEqual "strictWrapper node stdout" expected out
 
 assertPretty :: Config -> String -> Assertion
 assertPretty cfg flagName = do
+  let isTs = configTypeScript cfg
+      suffix = if isTs then ".ts" else ".js"
   res <- compileFile cfg $ "tests/Compile/" ++ flagName ++ ".hs"
   case res of
     Left l  -> assertFailure $ "Should compile, but failed with: " ++ show l
     Right js -> do
-    writeFile ("tests/Compile/" ++ flagName ++ ".js") js
-    (err, out) <- either id id <$> readAllFromProcess "node" ["tests/Compile/" ++ flagName ++ ".js"] ""
-    when (err /= "") $ assertFailure err
-    expected <- readFile $ "tests/Compile/" ++ flagName ++ ".res"
-    assertEqual (flagName ++ " node stdout") expected out
+      writeFile ("tests/Compile/" ++ flagName ++ suffix) js
+      (err, out) <- either id id <$> runScriptFile isTs
+        ("tests/Compile/" ++ flagName ++ suffix)
+      when (err /= "") $ assertFailure err
+      expected <- readFile $ "tests/Compile/" ++ flagName ++ ".res"
+      assertEqual (flagName ++ " node stdout") expected out
 
 case_pretty :: Assertion
 case_pretty = do
@@ -134,3 +136,15 @@
 defConf = do
   cfg <- defaultConfigWithSandbox
   return $ addConfigDirectoryIncludePaths ["tests/"] cfg { configTypecheck = False }
+
+-- | Run a JS or TS file.
+runScriptFile :: Bool -- ^ If a file-format is TypeScript, this is True.
+              -> String -- ^ A name of script file
+              -> IO (Either (String,String) (String,String))
+runScriptFile True file = do
+  tsc_ret <- readAllFromProcess "tsc" [file] ""
+  case tsc_ret of
+    Left _ -> return tsc_ret
+    Right _ -> readAllFromProcess "node" [(reverse (drop 3 (reverse file))) ++ ".js" ] ""
+
+runScriptFile False file = readAllFromProcess "node" [file] ""
diff --git a/src/tests/Test/Desugar.hs b/src/tests/Test/Desugar.hs
--- a/src/tests/Test/Desugar.hs
+++ b/src/tests/Test/Desugar.hs
@@ -13,7 +13,7 @@
 import           Fay.Compiler.Parse     (parseFay)
 import           Fay.Types.CompileError (CompileError (..))
 
-import           Language.Haskell.Exts  hiding (alt, binds, loc, name)
+import           Language.Haskell.Exts  hiding (name)
 import           Test.Tasty
 import           Test.Tasty.HUnit
 -- import           Text.Groom
@@ -95,7 +95,7 @@
   case parseFay "test" s :: ParseResult (Module SrcSpanInfo) of
     ParseFailed a b -> error $ show (name, a, b)
     ParseOk (fmap srcSpanInfoToSrcLoc -> m) -> do
-      d <- desugar' "gen" noLoc $ m
+      d <- desugar' "gen" noLoc m
       return (m, d)
   where
     srcSpanInfoToSrcLoc :: SrcSpanInfo -> SrcLoc
@@ -147,7 +147,7 @@
   when (unAnn desugaredExpected /= unAnn originalExpected ) $ putStrLn "desugaredExpected /= undesugared"
 
 g :: Show a => a -> IO ()
-g = putStrLn . show -- groom
+g = print -- putStrLn . groom
 
 unAnn :: Functor f => f a -> f ()
 unAnn = void
diff --git a/src/tests/Tests.hs b/src/tests/Tests.hs
--- a/src/tests/Tests.hs
+++ b/src/tests/Tests.hs
@@ -33,8 +33,11 @@
   (packageConf,args)     <- prefixed (== "-package-conf") <$> getArgs
   let (basePath,args')    = prefixed (== "-base-path"   ) args
   let (testCount, args'') = first (readMay =<<) $ prefixed (== "-random") args'
-  (runtime,codegen) <- makeCompilerTests (packageConf <|> sandbox) basePath testCount
-  withArgs args'' $ defaultMain $ testGroup "Fay"
+  let (isTs, args''') = case args'' of
+                          ("-ts":xs) -> (True,xs)
+                          _ -> (False,args'')
+  (runtime,codegen) <- makeCompilerTests (packageConf <|> sandbox) basePath testCount isTs
+  withArgs args''' $ defaultMain $ testGroup "Fay"
     [ Desugar.tests
     , Convert.tests
     , codegen
@@ -48,19 +51,19 @@
 prefixed f (break f -> (x,y)) = (listToMaybe (drop 1 y),x ++ drop 2 y)
 
 -- | Make the case-by-case unit tests.
-makeCompilerTests :: Maybe FilePath -> Maybe FilePath -> Maybe Int -> IO (TestTree,TestTree)
-makeCompilerTests packageConf basePath rand = do
+makeCompilerTests :: Maybe FilePath -> Maybe FilePath -> Maybe Int -> Bool -> IO (TestTree,TestTree)
+makeCompilerTests packageConf basePath rand isTs = do
   runtimeFiles' <- runtimeTestFiles
   runtimeFiles  <- maybe (return runtimeFiles') (randomize runtimeFiles') rand
   codegenFiles  <- codegenTestFiles
   return
-    (makeTestGroup "Runtime tests"
-                   runtimeFiles
-                   (\file -> do testFile packageConf basePath False file
-                                testFile packageConf basePath True file)
-    ,makeTestGroup "Codegen tests"
-                   codegenFiles
-                   (\file -> testCodegen packageConf basePath file))
+    ( makeTestGroup "Runtime tests"
+                    runtimeFiles
+                    (\file -> do testFile packageConf basePath False isTs file
+                                 testFile packageConf basePath True isTs file)
+    , makeTestGroup "Codegen tests"
+                    codegenFiles
+                    (testCodegen packageConf basePath isTs))
   where
     makeTestGroup title files inner =
       testGroup title $ flip map files $ \file ->
@@ -86,11 +89,18 @@
         then return s'
         else randomizeAux (S.insert i s) count b
 
-testFile :: Maybe FilePath -> Maybe FilePath -> Bool -> String -> IO ()
-testFile packageConf basePath opt file = do
-  let root = (reverse . drop 1 . dropWhile (/='.') . reverse) file
-      out = toJsName file
-      resf = root <.> "res"
+fns :: Bool -> String -> (String, String, FilePath)
+fns isTs file =
+  ( root
+  , if isTs then toTsName file else toJsName file
+  , root <.> "res"
+  )
+  where
+    root = reverse . drop 1 . dropWhile (/='.') . reverse $ file
+
+testFile :: Maybe FilePath -> Maybe FilePath -> Bool -> Bool -> String -> IO ()
+testFile packageConf basePath opt isTs file = do
+  let (root, out, resf) = fns isTs file
       config =
         addConfigDirectoryIncludePaths ["tests/"]
           defaultConfig
@@ -98,12 +108,13 @@
             , configTypecheck   = False
             , configPackageConf = packageConf
             , configBasePath    = basePath
+            , configTypeScript  = isTs
             }
   resExists <- doesFileExist resf
   let partialName = root ++ "_partial.res"
   partialExists <- doesFileExist partialName
   compileFromTo config file (Just out)
-  result <- runJavaScriptFile out
+  result <- Compile.runScriptFile isTs out
   if resExists
      then do output <- readFile resf
              assertEqual file output (either show snd result)
@@ -119,11 +130,9 @@
 -- | Test the generated code output for the given file with
 -- optimizations turned on. This disables runtime generation and
 -- things like that; it's only concerned with the core of the program.
-testCodegen :: Maybe FilePath -> Maybe FilePath -> String -> IO ()
-testCodegen packageConf basePath file = do
-  let root = (reverse . drop 1 . dropWhile (/='.') . reverse) file
-      out = toJsName file
-      resf = root <.> "res"
+testCodegen :: Maybe FilePath -> Maybe FilePath -> Bool -> String -> IO ()
+testCodegen packageConf basePath isTs file = do
+  let (_, out, resf) = fns isTs file
       config =
         addConfigDirectoryIncludePaths ["tests/codegen/"]
           defaultConfig
@@ -135,14 +144,12 @@
             , configPrettyPrint   = True
             , configLibrary       = True
             , configExportRuntime = False
+            , configTypeScript    = isTs
             }
   compileFromTo config file (Just out)
   actual <- readStripped out
-  expected <- readStripped resf
+  expected <- readStripped $ resf ++ (if isTs then "_ts" else "")
   assertEqual file expected actual
   where readStripped =
           fmap (unlines . filter (not . null) . lines) . readFile
 
--- | Run a JS file.
-runJavaScriptFile :: String -> IO (Either (String,String) (String,String))
-runJavaScriptFile file = readAllFromProcess "node" [file] ""
diff --git a/tests/Nullable.hs b/tests/Nullable.hs
--- a/tests/Nullable.hs
+++ b/tests/Nullable.hs
@@ -40,4 +40,4 @@
 r2 = ffi "{ instance : 'R', slot1 : null }"
 
 parseInt :: String -> Nullable Int
-parseInt = ffi "(function () { var n = global.parseInt(%1, 10); if (isNaN(n)) return null; return n; })()"
+parseInt = ffi "(function () { var n = parseInt(%1, 10); if (isNaN(n)) return null; return n; })()"
diff --git a/tests/String.hs b/tests/String.hs
deleted file mode 100644
--- a/tests/String.hs
+++ /dev/null
@@ -1,3 +0,0 @@
-module String where
-
-main = putStrLn "Hello, World!"
diff --git a/tests/String.res b/tests/String.res
deleted file mode 100644
--- a/tests/String.res
+++ /dev/null
@@ -1,1 +0,0 @@
-Hello, World!
diff --git a/tests/Strings.hs b/tests/Strings.hs
new file mode 100644
--- /dev/null
+++ b/tests/Strings.hs
@@ -0,0 +1,3 @@
+module Strings where
+
+main = putStrLn "Hello, World!"
diff --git a/tests/Strings.res b/tests/Strings.res
new file mode 100644
--- /dev/null
+++ b/tests/Strings.res
@@ -0,0 +1,1 @@
+Hello, World!
diff --git a/tests/WhenUnlessRecursion.hs b/tests/WhenUnlessRecursion.hs
--- a/tests/WhenUnlessRecursion.hs
+++ b/tests/WhenUnlessRecursion.hs
@@ -3,7 +3,7 @@
 import FFI
 
 getStackSize :: Fay Int
-getStackSize = ffi "(Error.stackTraceLimit = Infinity, new Error().stack.split('\\n').length)"
+getStackSize = ffi "(Error[\"stackTraceLimit\"] = Infinity, new Error().stack.split('\\n').length)"
 
 checkGrowth :: Maybe Int -> Int -> Fay ()
 checkGrowth Nothing _      = return ()