[GHC] #2239: lack of improvement/reduction with TFs

[GHC]

#2239: lack of improvement/reduction with TFs
-------------------------+--------------------------------------------------
    Reporter:  claus     |       Owner:                         
        Type:  bug       |      Status:  new                    
    Priority:  normal    |   Component:  Compiler (Type checker)
     Version:  6.9       |    Severity:  normal                 
    Keywords:  TF vs FD  |    Testcase:                         
Architecture:  Unknown   |          Os:  Unknown                
-------------------------+--------------------------------------------------
 {{{
 {-# LANGUAGE MultiParamTypeClasses #-}
 {-# LANGUAGE FunctionalDependencies #-}
 {-# LANGUAGE ScopedTypeVariables #-}
 {-# LANGUAGE FlexibleContexts #-}
 {-# LANGUAGE TypeFamilies #-}
 data A = A
 data B = B

 class C a where c :: a -> String
 instance C Bool where c _ = "Bool"
 instance C Char where c _ = "Char"

 -- via TFs
 type family TF a
 type instance TF A = Char
 type instance TF B = Bool

 tf :: forall a b. (b ~ TF a,C b) => a -> String
 tf a = c (undefined:: b)

 -- via FDs
 class FD a b | a -> b
 instance FD A Char
 instance FD B Bool

 fd :: forall a b. (FD a b,C b) => a -> String
 fd a = c (undefined:: b)
 }}}
 for some reason, the TF version doesn't work as well as the FD version:
 {{{
 *Main> fd A
 "Char"
 *Main> fd B
 "Bool"
 *Main> tf A

 <interactive>:1:0:
     No instance for (C (TF A))
       arising from a use of `tf' at <interactive>:1:0-3
     Possible fix: add an instance declaration for (C (TF A))
     In the expression: tf A
     In the definition of `it': it = tf A
 *Main> :t undefined :: (b~TF A)=>b
 undefined :: (b~TF A)=>b :: TF A
 *Main> :t undefined :: (FD A b)=>b
 undefined :: (FD A b)=>b :: Char
 }}}
 this is with `GHCi, version 6.9.20080217`.

 the result of the TF is "known", even if not used:
 {{{
 *Main> :t undefined :: (b~TF A,b~Char)=>b
 undefined :: (b~TF A,b~Char)=>b :: TF A
 *Main> :t undefined :: (b~TF A,b~Bool)=>b

 <interactive>:1:0:
     Couldn't match expected type `Bool' against inferred type `Char'
 }}}

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/2239>
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