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List:       haskell
Subject:    Multi-parameter type classes in GHC 3.01
From:       Simon L Peyton Jones <simonpj () dcs ! gla ! ac ! uk>
Date:       1998-02-25 9:49:45
Message-ID: 9802250916.AA14403 () avon
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> PS. Could somebody inform me what is the current status of
> multi-parametric classes?


	GHC 3.01 supports multi-parameter type classes
	in more or less the form described in the last section of
	"Type classes: an exploration of the design space"
	(http://www.dcs.gla.ac.uk/~simonpj/multi.ps.gz)
	

We only mentioned this eliptically in the announcement, but
it should really have been a headline!  (The reason is that
we'd previously released 3.0 to friends, so we'd forgotten
that not everyone knew..)

It now seems very likely that Standard Haskell will support
multi-parameter type classes, and GHC 3.01 is our attempt
to gather feedback about whether we are moving towards
the right set of design choices.

The main limitation at the moment is

	no overlapping instance declarations

There are ways to lift this restriction, but it's a conservative
first choice.

Please try it out, and let us know if you trip over
	(a) bugs
	(b) obscure error messages
	(c) design restrictions that cramp your programming style

The URL is

		http://www.dcs.gla.ac.uk/fp/software/ghc/

Simon



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