A better type signature for `forM_`

[davean]

On Fri, Apr 1, 2016 at 1:13 AM, Erik de Castro Lopo <mle+hs at mega-nerd.com>
wrote:

> Which is the exact problem. I suspect that most people use `forM_` as
> "assume
> the action returns unit" rather than "assume the return value of the
> action is
> ignored".
>

A review of my code shows I mostly use it as "assume the return value of
the action is ignored".

Many things return data that is sometimes useful, but not always. Rarely do
they have an '_' variant or the like to allow you to be more specific.
I wouldn't find it to be overly onerous to throw "void" in in those cases
though.

I will point out that "void" isn't exported from the Prelude, while "mapM_"
is.

-davean
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