Proposal: Add log1p and expm1 to GHC.Float.Floating

[Henning Thielemann]

Am 17.04.2014 19:15, schrieb Edward Kmett:

> log1p and expm1 are C standard library functions that are important for
> work with exponentials and logarithms.
>
> I propose adding them to the Floating class where it is defined in
> GHC.Float.

No question, these functions are useful. But I think there should be two 
proposals:

1) Add log1pFloat, log1pDouble, expm1Float, expm1Double to GHC.Float
2) Extend Floating class with log1p and expm1 methods.

I think the first item is unproblematic since it is a simple addition. 
Since FPUs sometimes directly implement log1p and expm1 functions, I 
wonder whether GHC also should support the according machine 
instructions. E.g. x86 has F2XM1 and FYL2XP1 and good old MC68882 had 
FETOXM1 and FLOGNP1.

The second item means to alter the Floating class which affects all 
custom Floating instances. I think one should add default 
implementations. They don't have an numerical advantage but they save 
programmers from code breakage.


> We do not have to export these from Prelude. My knee-jerk reaction is
> that we probably shouldn't.

Not exporting them from Prelude still means to export them from 
GHC.Float, right? I mean, users must be able to implement these methods 
for custom types like extended precision floating point numbers as 
provided by libqd. But there should also be a non-GHC module that 
exports the full Floating class.