DData

[Daan Leijen]

Ok, here is a new "IntMap" module that might improve the
performance of "lookup". Please, let's first test it before
adding the changes to the real distribution.

I have:
1) removed calls to "natFromInt" and "intFromNat". Supposedly,
   they should expand to nothing but who knows...
2) I have commented out the guard that tests if the integer
is in the tree at all. When a key is present, lookup will be
faster -- if the key is not present, lookup might be slower.

Simon, I would be interested to see how it performs with and
without the comment (2) (line 269). If performance is better
without the comment, it means that ghc is spending time doing
intFromNat/natFromInt which means that we might improve matters
by storing Nat's instead of Int's in the IntMap. (I don't know
if such tweaking is worth the trouble though.)

All the best,
  Daan.
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