Hello, You should be able to use fundeps to do exactly what you describe below. Can you make a relatively small self-contained example which exemplifies the ugliness you see? -Jeff haskell-cafe-bounces@haskell.org wrote on 12/07/2007 11:24:35 AM:
I have some type-level sets using fundeps working whereby equality and membership etc are predicate functions. This seems to leads to an
explosion
of ugly code, with `If' class constraints etc getting out of hand -- I want to treat these as relations instead so providing the definition describes everything that is 'in' and nothing that is 'out'. I've been using Oleg's paper on lightweight static resources [1] as a template for this. I want to do something like this (supposing I have an EQ relation, (:::) for consing):
class Member x y instance EQ x y => Member x (y:::ys) instance Member x ys => Member x (y:::ys)
But I can certainly see why this isn't possible (It's the equivalent of pattern-matching on the constraints I suppose). Do type families provide a way to do this kind of thing or do I need a different strategy altogether, involving GADTs or whatever?
Thanks,
[1] http://okmij.org/ftp/Computation/resource-aware-prog/tfp.pdf -- View this message in context: http://www.nabble.com/fundeps-and- overlapping-undecidable-instances-tf4962996.html#a14215583 Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.
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