diff --git a/System/Console/ParseArgs.hs b/System/Console/ParseArgs.hs
--- a/System/Console/ParseArgs.hs
+++ b/System/Console/ParseArgs.hs
@@ -1,4 +1,4 @@
-{-# LANGUAGE FlexibleInstances #-}
+{-# LANGUAGE FlexibleInstances, DeriveDataTypeable #-}
 -- Full-featured argument parsing library for Haskell programs
 -- Bart Massey <bart@cs.pdx.edu>
 
@@ -186,12 +186,7 @@
 -- when argument parsing fails.  The first argument is the usage
 -- message, the second the actual error message from the parser.
 data ParseArgsException = ParseArgsException String String
-     deriving Eq
-
-instance Typeable ParseArgsException where
-    typeOf _ = mkTyConApp e [s, s] where
-        e = mkTyCon "ParseArgsException"
-        s = typeOf ""
+     deriving (Eq, Typeable)
 
 instance Exception ParseArgsException
 
@@ -552,12 +547,13 @@
                           ++ show index ++ "not supplied")
 
 getArgPrimitive decons (Args { args = ArgRecord am }) k =
-    case Map.lookup k am of
-      Just v -> Just (decons v)
-      Nothing -> Nothing
+  Map.lookup k am >>= decons
 
+instance ArgType () where
+  getArg = getArgPrimitive (\ArgvalFlag -> return ())
+
 instance ArgType ([] Char) where
-  getArg = getArgPrimitive (\(ArgvalString s) -> s)
+  getArg = getArgPrimitive (\(ArgvalString s) -> return s)
 
 -- |[Deprecated]  Return the `String` value, if any, of the given argument.
 getArgString :: (Show a, Ord a) =>
@@ -567,7 +563,7 @@
 getArgString = getArg
 
 instance ArgType Integer where
-  getArg = getArgPrimitive (\(ArgvalInteger i) -> i)
+  getArg = getArgPrimitive (\(ArgvalInteger i) -> return i)
 
 -- |[Deprecated] Return the `Integer` value, if any, of the given argument.
 getArgInteger :: (Show a, Ord a) =>
@@ -577,7 +573,7 @@
 getArgInteger = getArg
 
 instance ArgType Int where
-  getArg = getArgPrimitive (\(ArgvalInt i) -> i)
+  getArg = getArgPrimitive (\(ArgvalInt i) -> return i)
 
 -- |[Deprecated] Return the `Int` value, if any, of the given argument.
 getArgInt :: (Show a, Ord a) =>
@@ -587,7 +583,7 @@
 getArgInt = getArg
 
 instance ArgType Double where
-  getArg = getArgPrimitive (\(ArgvalDouble i) -> i)
+  getArg = getArgPrimitive (\(ArgvalDouble i) -> return i)
 
 -- |[Deprecated] Return the `Double` value, if any, of the given argument.
 getArgDouble :: (Show a, Ord a) =>
@@ -597,7 +593,7 @@
 getArgDouble = getArg
 
 instance ArgType Float where
-  getArg = getArgPrimitive (\(ArgvalFloat i) -> i)
+  getArg = getArgPrimitive (\(ArgvalFloat i) -> return i)
 
 -- |[Deprecated] Return the `Float` value, if any, of the given argument.
 getArgFloat :: (Show a, Ord a) =>
@@ -613,9 +609,8 @@
 
 instance ArgType ArgFileOpener where
     getArg args index =
-        case getArg args index of
-          Nothing -> Nothing
-          Just s -> Just (ArgFileOpener { argFileOpener = openFile s })
+        getArg args index >>= 
+          (\s -> return $ ArgFileOpener { argFileOpener = openFile s })
 
 -- |[Deprecated] Treat the `String` value, if any, of the given argument as
 -- a file handle and try to open it as requested.
diff --git a/parseargs.cabal b/parseargs.cabal
--- a/parseargs.cabal
+++ b/parseargs.cabal
@@ -1,8 +1,8 @@
 Name: parseargs
 Build-Type: Simple
 Description: Parse command-line arguments
-Version: 0.1.3.2
-Cabal-Version: >= 1.2
+Version: 0.1.3.4
+Cabal-Version: >= 1.6
 License: BSD3
 License-File: COPYING
 Author: Bart Massey <bart@cs.pdx.edu>
@@ -21,3 +21,12 @@
   Build-Depends:   base < 5
   Main-Is:         parseargs-example.hs
   Other-Modules:   System.Console.ParseArgs
+
+Source-repository head
+  Type:     git
+  Location: git://svcs.cs.pdx.edu/git/parseargs.git
+
+Source-repository this
+  Type:     git
+  Location: git://svcs.cs.pdx.edu/git/parseargs.git
+  Tag:      v0.1.3.4