Proposal: add ifM and whenM to Control.Monad

[Mario Pastorelli]

On 04/21/2014 04:22 PM, Casey McCann wrote:
> On Mon, Apr 21, 2014 at 8:55 AM, Brandon Allbery <allbery.b at gmail.com> wrote:
>> On Mon, Apr 21, 2014 at 5:35 AM, Mario Pastorelli
>> <pastorelli.mario at gmail.com> wrote:
>>> On 04/21/2014 10:41 AM, Simon Hengel wrote:
>>>>> A quick heuristic grep over all Hackage packages results in quite a bit
>>>>> of packages containing the ifM/whenM/unlessM:
>>>> But that kind of shows that the "expected" names for those functions are
>>>> ifM/whenM/unlessM.  I would ask the question:
>>>>
>>>>       Are there any other useful combinators that would be named
>>>>       ifM/whenM/unlessM under the current naming convention?
>>>>
>>>> If no, then I'm not entirely convinced that we should decide against
>>>> what seems to be common intuition here.
>>>
>>> Breaking API consistency because a lot of people are already doing it
>>> doesn't feel right. If they
>>
>> The API is there to serve its users, not really to dictate to them. If the
>> common convention is counter to the API structure, perhaps it is the API
>> that should change.
> 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.

replicate    ::                       Int ->     a ->     [a]
replicateM :: Monad m => Int -> m a -> m [a]

if    ::                           Bool ->     a ->      a ->     a
ifM :: Monad m => m Bool -> m a -> m a -> m a

Only the name 'mif' respects the rules with this type. The more generic 
version of 'ifM' is

ifM :: Monad m => m Bool -> a -> a -> a

and I don't think there is a name conversion for this kind of function 
in Control.Monad.

>
> 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?