Conal Elliott <conal@conal.net> wrote in article <ea8ae9fb0707311208j9f3fcf7m92bef8f54aea3dd8@mail.gmail.com > in gmane.comp.lang.haskell.general:
> I keep running into situations in which I want more powerful search in
> selecting type class instances.
I agree that it's quite useful for instance search to backtrack, if not
desirable in all cases. Proof search is program search, after all.
Of course, allowing undecidable instances, we can build backtracking
into instance search ourselves by representing the state of a
backtracking machine as a type. http://okmij.org/ftp/Haskell/poly2.txt
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