[Haskell-cafe] Strange type error with associated type synonyms

[Matt Morrow]

On Mon, Apr 6, 2009 at 7:39 PM, Manuel M T Chakravarty <chak at cse.unsw.edu.au
> wrote:

> Peter Berry:
>
>> 3) we apply appl to x, so Memo d1 a = Memo d a. unify d = d1
>>
>> But for some reason, step 3 fails.
>>
>
> Step 3 is invalid - cf, <
> http://www.haskell.org/pipermail/haskell-cafe/2009-April/059196.html>.
>
> More generally, the signature of memo_fmap is ambiguous, and hence,
> correctly rejected.  We need to improve the error message, though.  Here is
> a previous discussion of the subject:
>
>  http://www.mail-archive.com/haskell-cafe@haskell.org/msg39673.html
>
> Manuel


The thing that confuses me about this case is how, if the type sig on
memo_fmap is omitted, ghci has no problem with it, and even gives it the
type that it rejected:

------------------------------------------------

{-# LANGUAGE TypeFamilies #-}

class Fun d where
  type Memo d :: * -> *
  abst :: (d -> a) -> Memo d a
  appl :: Memo d a -> (d -> a)

memo_fmap f x = abst (f . appl x)

-- [m at monire a]$ ghci -ignore-dot-ghci
-- GHCi, version 6.10.1: http://www.haskell.org/ghc/  :? for help
--
-- Prelude> :l ./Memo.hs
-- [1 of 1] Compiling Main             ( Memo.hs, interpreted )
-- Ok, modules loaded: Main.
--
-- *Main> :t memo_fmap
-- memo_fmap :: (Fun d) => (a -> c) -> Memo d a -> Memo d c

-- copy/paste the :t sig

memo_fmap_sig :: (Fun d) => (a -> c) -> Memo d a -> Memo d c
memo_fmap_sig f x = abst (f . appl x)

-- and,

-- *Main> :r
-- [1 of 1] Compiling Main             ( Memo.hs, interpreted )
--
-- Memo.hs:26:35:
--     Couldn't match expected type `Memo d'
--            against inferred type `Memo d1'
--     In the first argument of `appl', namely `x'
--     In the second argument of `(.)', namely `appl x'
--     In the first argument of `abst', namely `(f . appl x)'
-- Failed, modules loaded: none.

------------------------------------------------

Matt
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