Fix a problem with fundeps on GHC 7.2

Introduced IsFunction type class, which is quoted from http://okmij.org/ftp/Haskell/typecast.html#is-function-type.

Data/AttoLisp.hs

@@ -1,7 +1,7 @@
 {-# LANGUAGE OverloadedStrings, Rank2Types, DeriveDataTypeable, BangPatterns #-}
 -- The following is for the ParseList stuff
 {-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies, FlexibleInstances,
-             UndecidableInstances #-}
+             UndecidableInstances, ScopedTypeVariables, OverlappingInstances, EmptyDataDecls #-}
 -- | Efficient parsing and serialisation of S-Expressions (as used by Lisp).
 --
 -- This module is intended to be imported qualified, e.g.:
@@ -305,15 +305,48 @@ typeMismatch expected actual =
 class ParseList a b | a -> b where
   parseList :: String -> a -> [Lisp] -> Parser b
 
-instance (FromLisp a, ParseList b c) => ParseList (a -> b) c where
-  parseList msg _ []     = fail $ "Too few arguments for object: " ++ msg
-  parseList msg f (x:xs) = do
+instance (IsFunction a f, ParseList' f a b) => ParseList a b where
+  parseList = parseList' (undefined :: f)
+
+class ParseList' f a b | f a -> b where
+  parseList' :: f -> String -> a -> [Lisp] -> Parser b
+
+instance (FromLisp a, IsFunction b f, ParseList' f b c, ParseList b c)
+  => ParseList' HTrue (a -> b) c where
+  parseList' _ msg _ []     = fail $ "Too few arguments for object: " ++ msg
+  parseList' _ msg f (x:xs) = do
     y <- parseLisp x
     parseList msg (f y) xs
 
-instance ParseList a a where
-  parseList _msg r [] = return r
-  parseList msg  _ (_:_) = fail $ "Too many arguments for object: " ++ msg
+instance ParseList' HFalse a a where
+  parseList' _ _msg r [] = return r
+  parseList' _ msg  _ (_:_) = fail $ "Too many arguments for object: " ++ msg
+
+data HTrue
+data HFalse
+
+class IsFunction a b | a -> b
+
+instance TypeCast f HTrue => IsFunction (x -> y) f
+instance TypeCast f HFalse => IsFunction a f
+
+class TypeCast a b | a -> b, b -> a where
+  typeCast :: a -> b
+
+class TypeCast' t a b | t a -> b, t b -> a where
+  typeCast' :: t -> a -> b
+
+class TypeCast'' t a b | t a -> b, t b -> a where
+  typeCast'' :: t -> a -> b
+
+instance TypeCast' () a b => TypeCast a b where
+  typeCast x = typeCast' () x
+
+instance TypeCast'' t a b => TypeCast' t a b where
+  typeCast' = typeCast''
+
+instance TypeCast'' () a a where
+  typeCast'' _ x  = x
 
 -- | Decode structure serialised with 'mkStruct'.
 --