lane:
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 them urgent.
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 RealFrac?
Almost certainly. Things like fromIntegral and truncate already have extensive type-based rules.
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 to matter.
Well, we make them the same as they currently are.
I personally prefer changing the signature of the divMod' family, but I anticipate objections.
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.
Check the core! -- Donn