Discussion: forM for Data.Set

[hasufell at posteo.de]

This was posted a year ago already, but without many comments:
https://mail.haskell.org/pipermail/libraries/2019-January/029335.html

So after cleaning up the PR a bit, adding tests and benchmarks, let's
give it another go.

PR is at: https://github.com/haskell/containers/pull/592

New functions are: forM, mapM, for, traverse

This follows the other Data.Set exported functions, overwriting Prelude
ones like foldr etc, so I think that's exactly what we want.


Cheers,
Julian Ospald