[Haskell-cafe] fundeps and overlapping/undecidable instances

[Jeff Polakow]

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 at 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
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> 
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