Re: Proposal: add ifM and whenM to Control.Monad

21 Apr 2014

      On 04/21/2014 04:22 PM, Casey McCann wrote:
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On Mon, Apr 21, 2014 at 8:55 AM, Brandon Allbery  wrote:
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On Mon, Apr 21, 2014 at 5:35 AM, Mario Pastorelli
 wrote:
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On 04/21/2014 10:41 AM, Simon Hengel wrote:
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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.
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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?