diff --git a/ats-format.cabal b/ats-format.cabal
--- a/ats-format.cabal
+++ b/ats-format.cabal
@@ -1,5 +1,5 @@
 name:                ats-format
-version:             0.1.0.12
+version:             0.1.0.14
 synopsis:            A source-code formatter for ATS
 description:         An opinionated source-code formatter for ATS (http://www.ats-lang.org/).
 homepage:            https://hub.darcs.net/vmchale/ats-format#readme
diff --git a/src/Language/ATS/Parser.y b/src/Language/ATS/Parser.y
--- a/src/Language/ATS/Parser.y
+++ b/src/Language/ATS/Parser.y
@@ -33,6 +33,7 @@
 %token
     fun { Keyword $$ KwFun }
     fn { Keyword $$ KwFn }
+    castfn { Keyword $$ KwCastfn }
     prfun { Keyword $$ KwPrfun }
     fnx { Keyword $$ KwFnx }
     and { Keyword $$ KwAnd }
@@ -489,12 +490,16 @@
 
 -- | Function declaration
 FunDecl : fun PreFunction { [ Func $1 (Fun $2) ] }
-        | fn PreFunction { [ Func $1 (Fn $2) ] }
         | prfun PreFunction { [ Func $1 (PrFun $2) ] }
         | fnx PreFunction { [ Func $1 (Fnx $2) ] }
-        | extern FunDecl { over _head (Extern $1) $2 }
+        | castfn PreFunction { [ Func $1 (CastFn $2) ] }
+        | fn PreFunction { [ Func $1 (Fn $2) ] }
         | FunDecl and PreFunction { Func $2 (And $3) : $1 }
+        | extern FunDecl { over _head (Extern $1) $2 }
         | extern fun PreFunction eq {% Left $ Expected $1 "Declaration" "Function body" }
+        | extern fnx PreFunction eq {% Left $ Expected $1 "Declaration" "Function body" }
+        | extern praxi PreFunction eq {% Left $ Expected $1 "Declaration" "Function body" }
+        | extern prfun PreFunction eq {% Left $ Expected $1 "Declaration" "Function body" }
         | lambda {% Left $ Expected $1 "Function declaration" "lam" }
         | llambda {% Left $ Expected $1 "Function declaration" "llam" }
 
diff --git a/src/Language/ATS/PrettyPrint.hs b/src/Language/ATS/PrettyPrint.hs
--- a/src/Language/ATS/PrettyPrint.hs
+++ b/src/Language/ATS/PrettyPrint.hs
@@ -181,8 +181,8 @@
         a (BeginF _ e)
             | not (startsParens e) = linebreak <> indent 2 ("begin" <$> indent 2 e <$> "end")
             | otherwise = e
-        a (FixAtF (PreF n s [] [] as t Nothing (Just e))) = "fix@" <+> pretty n <+> prettyArgs as <+> pretty s <> ":" <+> pretty t <+> "=>" </> pretty e
-        a (LambdaAtF (PreF Unnamed{} s [] [] as t Nothing (Just e))) = "lam@" <+> prettyArgs as <+> pretty s <> ":" <+> pretty t <+> "=>" </> pretty e
+        a (FixAtF (PreF n s [] [] as t Nothing (Just e))) = "fix@" <+> pretty n <+> prettyArgs as <+> ":" <> pretty s <+> pretty t <+> "=>" </> pretty e
+        a (LambdaAtF (PreF Unnamed{} s [] [] as t Nothing (Just e))) = "lam@" <+> prettyArgs as <+> ":" <> pretty s <+> pretty t <+> "=>" </> pretty e
         a (AddrAtF _ e)                = "addr@" <> e
         a (ViewAtF _ e)                = "view@" <> e
         a _ = "FIXME"
@@ -438,6 +438,7 @@
     pretty (OverloadOp _ o n)    = "overload" <+> pretty o <+> "with" <+> pretty n
     pretty (Func _ (Fn pref))    = "fn" </> pretty pref
     pretty (Func _ (Fun pref))   = "fun" </> pretty pref
+    pretty (Func _ (CastFn pref)) = "castfn" </> pretty pref
     pretty (Func _ (Fnx pref))   = "fnx" </> pretty pref
     pretty (Func _ (And pref))   = "and" </> pretty pref
     pretty (Func _ (Praxi pref)) = "praxi" </> pretty pref
diff --git a/src/Language/ATS/Types.hs b/src/Language/ATS/Types.hs
--- a/src/Language/ATS/Types.hs
+++ b/src/Language/ATS/Types.hs
@@ -269,6 +269,7 @@
               | PrFun PreFunction
               | PrFn PreFunction
               | Praxi PreFunction
+              | CastFn PreFunction
               deriving (Show, Eq, Generic, NFData)
 
 data PreFunction = PreF { fname         :: Name -- ^ Function name