[Haskell-cafe] Alternative: you can fool many people some time, and some people many time, but...

Jake jake.waksbaum at gmail.com
Thu Sep 29 20:28:09 UTC 2016

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 <http://hackage.haskell.org/package/base->
a <http://hackage.haskell.org/package/base->
-> f <http://hackage.haskell.org/package/base->
[a <http://hackage.haskell.org/package/base->]many
v <http://hackage.haskell.org/package/base->
= many_v <http://hackage.haskell.org/package/base->
 where    many_v
= some_v <http://hackage.haskell.org/package/base->
<|> <http://hackage.haskell.org/package/base->
pure <http://hackage.haskell.org/package/base->
[]    some_v <http://hackage.haskell.org/package/base->
= (fmap <http://hackage.haskell.org/package/base->
(:) v <http://hackage.haskell.org/package/base->)
<*> <http://hackage.haskell.org/package/base->
many_v <http://hackage.haskell.org/package/base->

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 at gmail.com>

> 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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