Why cannot inferred type signatures restrict (potentially) ambiguous type variables?
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
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
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 _______________________________________________ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
AllowAmbiguousTypes at this point only extends to signatures that are
explicitly written.
This would need a new "AllowInferredAmbiguousTypes" or something.
On Sat, Oct 12, 2013 at 5:34 PM, adam vogt
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 _______________________________________________ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
participants (3)
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adam vogt -
Edward Kmett -
Nicolas Frisby