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

[John Lato]

On Sat, Apr 19, 2014 at 5:49 AM, Edward Kmett <ekmett at gmail.com> wrote:

> With the defaults the code is never worse than it is forced to be right
> now and users do not need to create CPP blocked code to work around this
> addition.
>

I usually like defaults, and avoiding CPP is good, however with the
defaults users will expect better code than they get.  We aren't doing
anyone any favors by introducing the possibility of silent floating-point
precision loss from 'exp1m'.  An "error" default would be better.

Besides, the code would be worse than it's forced to be now.  At least now
users who care about this run headlong into the issue.  If we provide exp1m
and log1p, users who use those functions should get the advertised
behavior, not loss of precision (I realize not all types would lose
precision, but some will).


>
> Without the defaults this becomes a much bigger request, as I'd be asking
> _every_ author of Floating to add CPP to their packages for a feature they
> never heard of and probably will never use, and in that situation we'd have
> to export it from Prelude.
>

It's perfectly fine to leave some methods blank; IMHO the resulting
run-time error is better than an incorrect default.  Plus, it's useful for
library authors to know that the class has changed; if a default is
provided everything will build properly and there is no compile-time
indication that library authors should adjust their code.

Originally I was +1 for everything except the defaults, but I'm
reconsidering.  If this is something that most Floating instance authors
don't know about and probably won't ever use, do these functions really
belong in that class?  Why not make a separate class for fused algorithms?

John L.



>
> -Edward
>
>
> On Sat, Apr 19, 2014 at 5:42 AM, Scott Turner <2haskell at pkturner.org>wrote:
>
>>  On 2014-04-17 15:08, Edward Kmett wrote:
>>
>>  On Thu, Apr 17, 2014 at 2:48 PM, Henning Thielemann <
>> schlepptop at henning-thielemann.de> wrote:
>>
>>> I think one should add default implementations. They don't have an
>>> numerical advantage but they save programmers from code breakage.
>>
>>
>>  I included the default definitions in code snippet in the proposal, so
>> user code that remains unaware of them would be unaffected, while packages
>> like compensated, or a wrapper around libqd could implement them as needed.
>>
>>  expm1 :: Floating a => a -> a
>> expm1 x = exp x - 1
>>
>>  log1p :: Floating a => a -> a
>> log1p x = log (1 + x)
>>
>> On the contrary, code that explicitly uses these functions is likely to
>> need the precision. Defaults would cause subtle breakage.
>>
>> -- Scott
>>
>
>
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