Martijn van Steenbergen wrote:
Ashley Yakeley wrote:
Have a look at these:
http://hackage.haskell.org/cgi-bin/hackage-scripts/package/witness
http://hackage.haskell.org/cgi-bin/hackage-scripts/package/open-witness
Ah, nice! It seems most we came up with is already in there. Even Any
which I use in my project but didn't think of putting in the package is
there. No use anymore for a new package now, I guess. On the other hand,
I can't find the comm, trans, coerce, subst and resp. Would it be an
idea to add those to your package?
Well, trans is the same as (.), if you import Control.Category. But by
and large you don't need those functions. You just match MkEqualType
where you need it. Since matchWitness returns (Maybe (EqualType a b)),
you can do this easily with "do" notation. For instance:
do
MkEqualType <- matchWitness a1 b1
MkEqualType <- matchWitness a2 b2
return MkEqualType
Current code is in darcs here:
http://code.haskell.org/witness/
http://code.haskell.org/open-witness/
They now target base 4.0.
Also, my paper that explains it:
http://semantic.org/stuff/Open-Witnesses.pdf
--
Ashley Yakeley
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