A better type signature for `forM_`

[David Feuer]

Suppose you have something like

data Free f a = Pure a | Free (f (Free f a))
instance Functor f => Functor (Free f) where
  fmap = mapFree

-- Separated out for clarity
mapFree :: Functor f
  => (a -> b)
  -> Free f a -> Free f b
mapFree f (Pure a) = Pure (f a)
mapFree f (Free m) = Free (fmap (mapFree f) m)

Now void x = () <$ x, and `Free f` doesn't offer any potential for a cheap
<$. So voiding out Free f a is going to cost you some. Laziness will help,
but you'll face extra allocation.
On Apr 1, 2016 1:02 AM, "Erik de Castro Lopo" <mle+hs at mega-nerd.com> wrote:

> David Feuer wrote:
>
> > I think it's an interesting idea from a safety standpoint. Unfortunately,
> > as Ed pointed out to me in a similar context, `void` isn't always free.
>
> I would love to see an elaboration of that.
>
> Erik
> --
> ----------------------------------------------------------------------
> Erik de Castro Lopo
> http://www.mega-nerd.com/
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