Re: [Haskell-cafe] problem with compiler error message

Hi Dmitry, This is indeed a GHC bug. Here's an existing ticket about it, it seems to be fixed for the next release of GHC. https://ghc.haskell.org/trac/ghc/ticket/14000 Cheers, Li-yao On 11/21/2017 04:51 AM, Dmitry Olshansky wrote:

GHCi 8.2.1.

2017-11-21 10:48 GMT+03:00 Dmitry Olshansky :

...

Hello cafe!

It seems that this behavior is wrong:

...

class Foo a where { type Bar a; baz :: a -> Bar a } baz zzz -- zzz is not in scope <interactive>:17:1: error: • Couldn't match expected type ‘Bar a’ with actual type ‘Bar a0’ NB: ‘Bar’ is a type function, and may not be injective The type variable ‘a0’ is ambiguous • In the ambiguity check for the inferred type for ‘it’ To defer the ambiguity check to use sites, enable AllowAmbiguousTypes When checking the inferred type it :: forall a. Bar a

As opposite - correct behavior:

...

:t negate negate :: Num a => a -> a negate zzz <interactive>:19:8: error: Variable not in scope: zzz

Best regards, Dmitry

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