Duplicate instance declarations of a methodless class

[mm]

On Tue, Mar 06, 2007 at 08:49:25PM +0100, Alfonso Acosta wrote:
> Did you try to compile with -fallow-overlapping-instances

Same effect:

ghc -fallow-overlapping-instances --make test5.lhs
[1 of 1] Compiling Main             ( test5.lhs, test5.o )

test5.lhs:22:1:
    Duplicate instance declarations:
      instance [overlap ok] (Succ a b) => GT a b
        -- Defined at test5.lhs:22:1
      instance [overlap ok] (Succ a b, GT b c) => GT a c
        -- Defined at test5.lhs:23:1


Does it work with another compiler version?


> 
> On 3/6/07, mm <schneegloeckchen at gmx.li> wrote:
> >Hello,
> >
> >I was thinking that it could maybe be useful to allow duplicate instance
> >declarations, if the class has no methods. From my naive point of view, i
> >can not see problems, scince there are no conflicting methods to choose
> >from. But the following program fails to be compiled by ghc-6.6:
> >
> >> {-# OPTIONS -fglasgow-exts -fallow-undecidable-instances #-}
> >
> >> class GT a b
> >> class Succ a b
> >
> >> data Zero
> >> data One
> >> data Two
> >> data Three
> >
> >> instance Succ One Zero
> >> instance Succ Two One
> >> instance Succ Three Two
> >
> >> instance Succ a b => GT a b
> >> instance (Succ a b, GT b c) => GT a c
> >
> >The former two lines lead to:
> >
> >test5.lhs:22:1:
> >    Duplicate instance declarations:
> >      instance (Succ a b) => GT a b -- Defined at test5.lhs:22:1
> >      instance (Succ a b, GT b c) => GT a c -- Defined at test5.lhs:23:1
> >_______________________________________________
> >Glasgow-haskell-users mailing list
> >Glasgow-haskell-users at haskell.org
> >http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
> >