Why cannot inferred type signatures restrict (potentially) ambiguous type variables?

[Nicolas Frisby]

An observation: on GHC 7.6.3, if I remove c2 entirely, then ghci cooperates.

*Main> :t \x -> c (c x)
\x -> c (c x) :: (C a b, C a1 a) => a1 -> b

At first blush, I also expected the definition

> -- no signature, no ascriptions
> c2 x = c (c x)

to type-check. Perhaps GHC adopted a trade-off giving helpful error
messages at the cost of conveniently supporting the "local type
refinements" like the one Adam used in his instance of C?


On Sat, Oct 12, 2013 at 4:34 PM, adam vogt <vogt.adam at gmail.com> wrote:

> Hello,
>
> I have code:
>
> {-# LANGUAGE FlexibleInstances, MultiParamTypeClasses,
> ScopedTypeVariables, TypeFamilies #-}
>
> class C a b where c :: a -> b
> instance (int ~ Integer) => C Integer int where c = (+1)
>
> c2 :: forall a b c. (C a b, C b c) => a -> c
> c2 x = c (c x :: b)
> c2 x = c ((c :: a -> b) x)
>
>
> Why are the type signatures needed? If I leave all of them off, I get:
>
>     Could not deduce (C a1 a0)
>       arising from the ambiguity check for 'c2'
>     from the context (C a b, C a1 a)
>       bound by the inferred type for 'c2': (C a b, C a1 a) => a1 -> b
>
> from the line: c2 x = c (c x)
>
>
> From my perspective, it seems that the type signature ghc infers
> should be able to restrict the ambiguous types as the hand-written
> signature does.
>
> These concerns apply to HEAD (using -XAllowAmbiguousTypes) and ghc-7.6 too.
>
> Regards,
> Adam
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