Method contexts

[Marcin 'Qrczak' Kowalczyk]

Thu, 25 Jan 2001 09:55:08 -0800, Simon Peyton-Jones <simonpj at microsoft.com> pisze:

> 	The HEAD, which will become GHCi 5.0, does not have functional
> 	dependencies, and I think they work properly

You meant that it does have them :-)

Indeed it seems to finally work with my collection experiments which
heavily use fundeps. It used to work with Hugs only. Thanks!


Could ghc relax the requirement we talked about some time ago, that
a method context must constrain a type variable which is quantified
there? For example:

------------------------------------------------------------------------
class Member c k where
    member :: Eq k => c -> k -> Bool

class Member c a => Seq c a

instance Member [a] a where
    member s x = x `elem` s

instance Seq [a] a
------------------------------------------------------------------------

I see no reason to disallow this, it is needed to express requirements
about sequences specified above, and hugs -98 accepts this.


BTW, ghc prints the type in an unusual way:

    The constraint `Eq k' does not mention any of the universally quantified type variables
        in the type `{Eq k} -> c -> k -> Bool'

-- 
 __("<  Marcin Kowalczyk * qrczak at knm.org.pl http://qrczak.ids.net.pl/
 \__/
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QRCZAK