Why cannot inferred type signatures restrict (potentially) ambiguous type variables?
adam vogt
vogt.adam at gmail.com
Sat Oct 12 21:34:13 UTC 2013
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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