Bug in undecidable instances?

[Claus Reinke]

> You are misunderstanding what 'undecidable instances' does.

s/undecidable/incoherent/g

Claus

> GHC wants to solve the constraint (D Foo beta) where beta is 
> as-yet-unconstrained type variable.  It finds that one instance *matches* 
> (by instantiating only the instance declaration, not the constraint we are 
> solving):
> D a b
> but another instance *unifies*:
> D Foo Foo
> (That is, if 'beta' was later discovered to be 'Foo' then this latter 
> instance would match.)
>
> Without undecidable instances, GHC will refrain from choosing either. 
> With undecidable instances, GHC chooses the one that matches (ie D a b) 
> ignoring the possiblity that beta might later instantiate to Foo.
>
> You want to unify beta with Foo, but that wouldn't work if there was 
> another instances
> D Foo Bar
>
> That's the design at the moment.  If you are in undecidable-instance 
> territory the ice is pretty thin.
>
> Simon
>
> | -----Original Message-----
> | From: glasgow-haskell-users-bounces at haskell.org [mailto:glasgow-haskell-
> | users-bounces at haskell.org] On Behalf Of Roman Cheplyaka
> | Sent: 31 January 2011 22:01
> | To: glasgow-haskell-users at haskell.org
> | Subject: Bug in undecidable instances?
> |
> | The following looks like a bug in (undecidable) instances resolution.
> |
> |     {-# LANGUAGE
> | MultiParamTypeClasses,FlexibleInstances,UndecidableInstances,
> |                  OverlappingInstances,IncoherentInstances #-}
> |     class C a b
> |
> |     instance C a (a,b)
> |
> |     class D a b
> |
> |     instance (D a b, C b c) => D a c
> |
> |     data Foo = Foo deriving Show
> |     data Bar = Bar deriving Show
> |
> |     instance D Foo Foo
> |
> |     c :: C x y => x -> y -> ()
> |     c _ _ = ()
> |
> |     d :: D x y => x -> y -> ()
> |     d _ _ = ()
> |
> |
> | *Main> d Foo Foo
> | () -- as expected
> | *Main> c Foo (Foo,Bar)
> | () -- as expected
> | *Main> d Foo (Foo,Bar)
> |
> | <interactive>:1:1:
> |     Context reduction stack overflow; size = 21
> |     Use -fcontext-stack=N to increase stack size to N
> |       [skip]
> |       $dD :: D Foo b1
> |       $dD :: D Foo b
> |       $dD :: D Foo (Foo, Bar)
> |     In the expression: d Foo (Foo, Bar)
> |     In an equation for `it': it = d Foo (Foo, Bar)
> |
> | I.e. for some reason on the second step resolver fails to pick up the
> | most specific (and the most obvious) instance D Foo Foo and continues to
> | apply the instance (D a b, C b c) => D a c.
> |
> | Reproduced with GHC 6.12.1 and 7.0.1.
> |
> | --
> | Roman I. Cheplyaka :: http://ro-che.info/
> | Don't worry what people think, they don't do it very often.
> |
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> | Glasgow-haskell-users at haskell.org
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>
>
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