Re: A better type signature for `forM_`

1 Apr 2016

      John Wiegley wrote:
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Erik de Castro Lopo  writes:
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As pointed out to me on IRC (thanks pjdeport), chaning the type signature of
`forM_` to
    forM_' :: (Monad m, Foldable t) => t a -> (a -> m ()) -> m ()
would have resulted in an error.
What about mapM_, replicateM_, etc.? In each case, '_' means "if you return a
value from your action, it is ignored", and not "you must return unit".
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".

FWIW, ignoring a list of unit is perfectly fine, but ignoring a list of `IO ()`
is almost certainly not what people meant.

Erik
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Erik de Castro Lopo
http://www.mega-nerd.com/