Re: Proposal: Add a strict version of <$> for monads

On 2013-11-30 at 12:05:06 +0100, Roman Cheplyaka wrote:

* Nicolas Trangez [2013-11-30 02:11:02+0100]

...

I'm without a doubt overlooking something, but couldn't this be (otoh)

fmap' :: Functor f => (a -> b) -> f a -> f b fmap' f = fmap (strictify f) where strictify s = (($!) id) . s

No. Evaluation of your 'strictify' function will be delayed for the very same reason that evaluation of 'f' is delayed in the first place.

It doesn't matter what 'strictify' *does* if it's not even evaluated.

jfyi, if compiled with ghc -O2 -dsuppress-all -dsuppress-uniques -ddump-simpl it can be seen that fmap' is really almost the same as a plain fmap, except for an left-over eta-expansion which ghc doesn't optimize away: fmap' = \ @ a @ b @ f $dFunctor f1 -> fmap $dFunctor (\ x -> f1 x)