Op 29 sep. 2016, om 22:28  heeft Jake <jake.waksbaum@gmail.com> het volgende geschreven:

take 3 $ many $ Just 1

doesn't type check. Did you mean this?

take 3 <$> (many $ Just 1)

I think this may have something to do with the default definition of many in the definition of Alternative:

many :: f a -> f [a]
many v = many_v
  where
    many_v = some_v <|> pure []
    some_v = (fmap (:) v) <*> many_v

many_v and some_v are mutually recursive functions, and it may be that this prevents the thunks from being made available to take in some way. I'm really not sure though, this is just an idea about why this is not quite the same as (take $ repeat 1)

On Thu, Sep 29, 2016 at 3:51 PM Corentin Dupont <corentin.dupont@gmail.com> wrote:

Hi guys,

I'm playing with the mysterious "some" and "many" from Control.Applicative.

If I try:

many $ Just 1

It just loops, I understand why:
http://stackoverflow.com/questions/18108608/what-are-alternatives-some-and-many-useful-for

It seems that some and many are usually used in a context where something is consumed, and can be depleted, so the loop ends.

But why doesn't this terminates?

take 3 $ many $ Just 1

It's a recursive call, but the construction of the result should be lazy...

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