Ambiguity check and type families

[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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