A better type signature for `forM_`

[Michael Snoyman]

For what it's worth, we made that change in mono-traversable already,
although the primary motivation was performance, not safety.

https://github.com/snoyberg/mono-traversable/issues/28
http://www.yesodweb.com/blog/2014/05/foldable-mapm-maybe-recursive

On Fri, Apr 1, 2016, 2:37 AM Artyom <yom at artyom.me> wrote:

> For me the convenience of for_ (without having to use void) is more
> important than increased safety, but I accept that others’ needs are
> different – sometimes avoiding such nasty surprises is incredibly
> important, sometimes not so, and it depends both on the developer and the
> project.
>
> In an ideal world it’d probably be an optional warning (which I’d be able
> to disable, just like I do with fwarn-unused-do-bind), but I have no idea
> how hard it would be to implement and I can't imagine how it might look
> anyway.
>
> Bottom line: if it will result in an error and not a warning (i.e.
> changing the type of mapM_/forM_/for_, as originally proposed), I’m
> mildly against this idea.
> ​
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