[Haskell] [Fwd: undecidable & overlapping instances: a bug?]

[Iavor Diatchki]

Hi,
Mark is quite right, and there is a bug report that documents the problem:
http://hackage.haskell.org/trac/ghc/ticket/1241
The trac ticket is targeting GHC 6.8 but the ticket is still open.  I
have not had a chance to try out any of the 6.8 release candidates
yet, so I am not sure if there have been changes in this area.
-Iavor

On 10/17/07, Mark P Jones <mpj at cs.pdx.edu> wrote:
> Simon Peyton-Jones wrote:
> > | I am quite intrigued at the behaviour examplified in the attached module.
> > | It's true I am a newbie and probably don't quite get the whole consequence
> > | spectrum of -fallow-undecidable-instances, but why providing that dummy
> > | instance (commented out) get the thing to compile?
> >
> > Sorry I must have missed this.  It's a nice example of the trickiness of
> > functional dependencies.  Here's what is happening.  First a very cut-down
> > version of your example:
> >
> > class Concrete a b | a -> b where
> >         bar :: a -> String
> >
> > instance (Show a) => Concrete a b
> >
> > wib :: Concrete a b => a -> String
> > wib x = bar x
> >
> > Now consider type inference for 'wib'. ...
>
> Hold on a second!  There's a more serious problem here, before we
> get to 'wib'.  The definition of class Concrete asserts that there
> is a dependency from a to b.  In other words, it promises that, for
> any a, there must be at most one b such that Concrete a b holds.
> But then the following instance declaration says that Concrete a b
> can be instantiated for *any* a and b, the only proviso being that a
> is an instance of Show.  In particular, there is no functional
> relationship between the parameters.  As such, these two
> declarations are in direct conflict with one another!  To quote the
> error message that Hugs produces, the "Instance is more general than
> a dependency allows".
>
> I thought this must be a typo in your email, but then I discovered
> that the ghci (6.6.1) installed on my machine accepts this code, at
> least once the Concrete Bool Bool instance was added.  If the
> instance declarations are not consistent with the functional
> dependency, then improvement is unsound, and all bets are off!
>
> Further experiments suggest that this behavior occurs only when
> the-fallow-undecidable-instances flag is specified.  But the reason you
> need to check for consistency between instance declarations and
> dependencies is to ensure soundness, not decidability.
>
> I don't know if this was the problem in the original example, but
> perhaps we should debug this cut down version first :-)
>
> All the best,
> Mark
>
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