Ambiguity check and type families

[Wolfgang Jeltsch]

Hi Adam,

yes, this seems to be the same bug.

I just annotated ticket #10009. I hope the fix will make it into
GHC 7.10.2.

Can anyone say when GHC 7.10.2 will be released approximately?

All the best,
Wolfgang

Am Dienstag, den 02.06.2015, 13:00 -0400 schrieb adam vogt:
> Hi Wolfgang,
> 
> 
> https://ghc.haskell.org/trac/ghc/ticket/10009 might be the same
> regression (fixed in HEAD)
> 
> 
> Regards,
> 
> Adam
> 
> 
> On Tue, Jun 2, 2015 at 12:28 PM, Wolfgang Jeltsch
> <g9ks157k at acme.softbase.org> wrote:
>         Hi,
>         
>         the following (contrived) code is accepted by GHC 7.8.3, but
>         not 7.10.1:
>         
>         > {-# LANGUAGE TypeFamilies #-}
>         >
>         > type family F a :: *
>         >
>         > type family G b :: *
>         >
>         > x :: G (F a) ~ a => F a
>         > x = undefined
>         
>         GHC 7.10.1 reports:
>         
>         > Could not deduce (F a0 ~ F a)
>         > from the context (G (F a) ~ a)
>         >   bound by the type signature for x :: (G (F a) ~ a) => F a
>         >   at Test.hs:7:6-23
>         > NB: ‘F’ is a type function, and may not be injective
>         > The type variable ‘a0’ is ambiguous
>         > In the ambiguity check for the type signature for ‘x’:
>         >   x :: forall a. (G (F a) ~ a) => F a
>         > To defer the ambiguity check to use sites, enable
>         AllowAmbiguousTypes
>         > In the type signature for ‘x’: x :: G (F a) ~ a => F a
>         
>         At a first look, this complaint seems reasonable, and I have
>         already
>         wondered why GHC 7.8.3 actually accepts the above code.
>         
>         From an intuitive standpoint, however, the code seems actually
>         acceptable to me. While it is true that type families are
>         generally not
>         injective, it is possible to derive the type a from F a by
>         applying G.
>         
>         It would great if this code would be accepted by GHC again and
>         if there
>         was a workaround to make it work with GHC 7.10.1. At the
>         moment, this
>         change in the type checker from 7.8.3 to 7.10.1 breaks the
>         incremental-computing package in a rather fundamental way.
>         
>         All the best,
>         Wolfgang
>         
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