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 > <g9ks1...@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 > > _______________________________________________ > Glasgow-haskell-users mailing list > Glasgow-haskell-users@haskell.org > http://mail.haskell.org/cgi-bin/mailman/listinfo/glasgow-haskell-users > > _______________________________________________ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/glasgow-haskell-users
