Proposal: add ifM and whenM to Control.Monad

[Mario Pastorelli]

Thank you.

So bool takes the predicate as last argument, good for combining it with 
a monadic operation:

doesDirectoryExists path >>=
bool (putStrLn ("Creating dir " ++ path) >> createDirectory path)
         (putStrLn ("Directory already exists"))

I still prefer the version using mif/ifM, in particular when working 
with IO:

ifM (doesDirectoryExists path)
       (putStrLn ("Directory already exists"))
       (putStrLn ("Creating dir " ++ path) >> createDirectory path)


On 04/21/2014 02:16 PM, Edward Kmett wrote:
> http://hackage.haskell.org/package/base-4.7.0.0/docs/src/Data-Bool.html#bool
>
>
> On Mon, Apr 21, 2014 at 5:35 AM, Mario Pastorelli 
> <pastorelli.mario at gmail.com <mailto: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 are like me, they probably were
>     ignoring the naming convention and used the most intuitive name.
>     Once you know forM then it's obvious that you append 'M' to
>     functions that "are more monadic". Probably mif, mwhen and munless
>     are more compatible with the API rules.
>
>
>
>         In general, I'm not sure about ifM (as it does not line up
>         with `bool`).
>
>
>     This is the second time that I read about `bool` but I can't find
>     it. Can somebody provide a link to it? mbool can be a solution,
>     but not as intuitive as mif.
>
>     Mario
>
>

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