What to do about divMod'.
Christopher Lane Hinson
lane at downstairspeople.org
Fri Nov 14 22:58:13 EST 2008
Data.Fixed contains divMod' :: (Real a, Integral b) => a -> a -> (b, a)
as well as div' and mod'.
There are two problems:
1) Implementation of this type signature requires conversion to Rational,
which is extremely slow. It really isn't usable in any kind of
significant loop at all.
2) This has nothing specific to do with fixed precision arithmetic
despite being in the Data.Fixed module.
I played with writing a patch but I think this requires discussion. I
believe the original author was aware of the problems and didn't consider
Should there be a RULES to address the performance issues? Should this be
based on RealFrac or specific to Float, Double, etc? Alternately OR
additionally should there be a separate set of functions created based on
Another possibility is to change the signature of the divMod' family
itself to use RealFrac instead of Real. This might break at compile time,
but Rational is still an instance of RealFrac, so the fix would easy, and
the performance penalty of calling to/fromRational would be visible in the
calling code instead of hidden away.
If we use RULES, this may make programs that depend on it effectively
unusable without the optimizer. Whatever we do will probably change the
semantics a bit in terms of infinite/NaN, but I don't think that is likely
I personally prefer changing the signature of the divMod' family, but I
I also think that we should make sure that the divMod' family gets inlined
as my understanding is this will avoid dictionary lookups and aid
the strictness analyzer, but I'm not 100% certain on this point.
Does a module (Data.Num?) need to be created to keep these?
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