[Haskell-cafe] Possible bug in GHC 7.0.3

[Alberto G. Corona]

http://hackage.haskell.org/trac/ghc/ticket/5287
El 29/06/2011 10:22, "Yitzchak Gale" <gale at sefer.org> escribió:
> Ryan Ingram wrote:
>> So this is definitely a GHC bug, but I think the problem is probably
>> triggered by this line:
>>
>> instance  Serializable a b => IResource a
>>
>> I don't think this is a valid instance declaration without a functional
>> dependency on Serializable, as it's impossible to know which type 'b' to
use
>> in the methods of IResource.
>
> That's not exactly the reason why that line is a problem.
> The methods of IResource don't use b. Presumably, some of
> the methods of Serializable don't mention b either, and those
> could be used in the instance.
>
> But what exactly does this instance mean? Where does the
> implicit "forall b" go? For this to make any sense, it would
> need to be:
>
> -- Illegal syntax for instance
> instance (forall b. Serializable a b => IResource a)
>
> I don't think it has ever been specified whether it is legal
> to have free type variables as parameters of a multi-parameter
> class in the superclass context of an instance declaration,
> and if so, how to interpret that.
>
> Haskell 98/2010 does seem to allow it for single-parameter
> classes, but there the obvious meaning would be just to
> ignore those parts of the context.
>
> In any case, GHC does seem to be getting very confused.
> Here is a smaller test case. While it does not trigger the
> crash, it does cause GHC to give a seemingly nonsensical
> error message:
>
> module ClassContextBug where
> class A a
> class B b
> instance A a => B b
>
> ClassContextBug.hs:5:17:
> Illegal instance declaration for `B b'
> (All instance types must be of the form (T a1 ... an)
> where a1 ... an are *distinct type variables*,
> and each type variable appears at most once in the instance head.
> Use -XFlexibleInstances if you want to disable this.)
> In the instance declaration for `B b'
>
> I believe this is valid Haskell 98, so GHC should accept it.
> But even if not, what GHC is claiming in the error message
> is simply not true.
>
> I would like to add this to the bug report.
> Alberto, have you filed the bug report yet? Can you please post a link?
>
> Thanks,
> Yitz
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