On Wed, Oct 7, 2015 at 4:43 PM, Mark Lentczner
If you don't intend to actually deprecate it. Why bother?
But seriously, why do you think that "you would still be able to use it"? That is true for only the simplest of code - and untrue for anyone who has a library that defines a Monad - or anyone who has a library that they want to keep "up to date". Do you really want to have a library where all your "how to use this" code has return in the examples? Shouldn't now be pure? Do I now need -XCPP just for Haddock? and my wiki page? And what gets shown in Hackage? This is just a nightmare for a huge number of libraries, and especially many commonly used ones.
Why bother!
This is explained in the original proposal. In particular, it eliminates opportunities for errors and simplifies ApplicativeDo. I don’t believe anyone has proposed removing return from base. The only proposed change is turning return into a stand-alone function instead of a method in Monad. There is no proposal for removing return.