lack of improvement/reduction with TFs
{-# 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'
Trac metadata
Trac field | Value |
---|---|
Version | 6.9 |
Type | Bug |
TypeOfFailure | OtherFailure |
Priority | normal |
Resolution | Unresolved |
Component | Compiler (Type checker) |
Test case | |
Differential revisions | |
BlockedBy | |
Related | |
Blocking | |
CC | |
Operating system | Unknown |
Architecture | Unknown |