Fix a problem with fundeps on GHC 7.2

[maoe]

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

The problem is that we cannot compile Data.AttoLisp.Test with GHC 7.2.1:

maoe@maoe.local
% ghc Data/AttoLisp/Test.hs                         /Users/maoe/coding/haskell/atto-lisp
[1 of 2] Compiling Data.AttoLisp    ( Data/AttoLisp.hs, Data/AttoLisp.o )
[2 of 2] Compiling Data.AttoLisp.Test ( Data/AttoLisp/Test.hs, Data/AttoLisp/Test.o )

Data/AttoLisp/Test.hs:24:17:
    Couldn't match type `T.Text -> Integer -> Msg' with `Msg'
    When using functional dependencies to combine
      Data.AttoLisp.ParseList a a,
        arising from the dependency `a -> b'
        in the instance declaration at Data/AttoLisp.hs:314:10
      Data.AttoLisp.ParseList (T.Text -> Integer -> Msg) Msg,
        arising from a use of `struct' at Data/AttoLisp/Test.hs:24:17-22
    In the expression: struct "msg" Msg e
    In an equation for `parseLisp': parseLisp e = struct "msg" Msg e

[nominolo]

Thanks. I noticed that the IsFunction stuff is perhaps a bit too magical.

Did you test these changes with GHC 6.12 and 7.0? I played around with similar tricks, but I didn't get it to work reliably.

[maoe]

Hi,

I agree with you. This is a bit magical but works well.

I've compiled Data.AttoLisp.Test on GHC 6.12.3, 7.0.3 and 7.2.1. All of these have passed the tests. As Oleg mentioned in isFunction.hs, this trick should work on GHCi 6.2.1 and GHCi 6.4.