Proposal: add ifM and whenM to Control.Monad

[Casey McCann]

On Mon, Apr 21, 2014 at 11:33 AM, Mario Pastorelli
<pastorelli.mario at gmail.com> wrote:
> On 04/21/2014 04:22 PM, Casey McCann wrote:
>> Has anyone noticed yet that replicateM is already breaking the
>> ostensibly established naming scheme, and should properly be named
>> mreplicate?
>
>
> Sadly, replicateM breaks the naming scheme in a different way than ifM. The
> first doesn't justify the existence of the latter.

But it does somewhat deflate the notion that the fooM convention is
consistently used, especially since replicateM and replicateM_ are
probably more frequently used than many of the properly-named
functions.

>> But perhaps we should in fact rename replicateM. Certainly
>> "mreplicate" would fit in better with other monadic variants of list
>> functions, like "mconcat"! Consistency is important, after all.
>>
>> Also, mfix should probably be fixM, for what that's worth.
>
>
> Don't you think that for compatibility reason this can be very problematic?
> How many people already used replicateM (and replicateM_) in their code? How
> many times?

Ah, so you're arguing that API consistency is sometimes outweighed by
other concerns? I suppose I can't disagree with that. ;]

Naming things is a matter of human language, and as a rule of thumb if
everyone consistently breaks a linguistic "rule" in exactly the same
way, it was the wrong rule. Figuring out what the actual rule being
followed is and making it explicit is far more useful than trying to
force a rule that runs counter to intuition.

These functions have been reinvented many, many times under the names
ifM &c., and between that and the strong association between the m
prefix and MonadPlus/Monoid (as demonstrated by the fact you
apparently didn't notice I compared "mreplicate" to "mconcat" above) I
would argue that mif &c. are actually more misleading in this case,
for the clear and simple reason that they're not the names most people
would expect.

Unless we plan to be absolutely and completely consistent, including
renaming replicateM and mfix, we should use the widely established
names in this case.

- C.