On Mon, Apr 21, 2014 at 12:55 PM, Tom Ellis
I have been reading the discussion about 'whenM' and 'ifM' wondering where it is going to end. Presumably we will also have 'unlessM'. Then 'guardM' becomes a reasonable candidate, and indeed 'libraryFunctionM' for any 'libraryFunction' that happens to have a monadic return type and non-monadic argument. It is as if everyone is discussing how many 'liftAn' we should provide when (<$>) and (<*>) already do the job generically.
What other functions, aside from guard, would this make sense for? The only other "libraryFunctionM" candidates I can think of would require more complex control flow and would not work with your suggestion, I think. For example, how would you implement an unfoldM function? For ifM &c. specifically it would work, but I find the result much less readable. These are simple functions being reinvented to improve readability, not to create one-liners for their own sake. I suspect anyone using ifM now would be unlikely to replace it with what you suggest. As for guard, it's not necessary because it only takes one argument and can be applied using (>>=) with no additional hassle.
The generic alternative that I believe warrants further investigation is to add an operator called, for example, (<*>|) to be read as "apply and done". All these specific questions about whether a function should take a monadic argument or not are solved generically by (<$>), (<*>) and (<*>|) as demonstrated below.
It is no more complex than just using the Applicative combinators (<$>) and (<*>) we know and love.
It seems the discussion about 'mif' vs 'ifM' can be completely finessed. Those functions are no more convenient than (<*>|) syntax. There may be an even better syntax than the one I propose here but I think it's actually pretty good as it is.
Unrelated to the ifM &c. matter, however, I actually find this function useful and would be happy to see it included. For what it's worth, my preferred name for it is (=<*), intended as a portmanteau of (=<<) and (<*>). (<*>|) seems a bit awkward. - C.