[Haskell-cafe] Alternative: you can fool many people some time, and some people many time, but...
Corentin Dupont
corentin.dupont at gmail.com
Fri Sep 30 08:02:42 UTC 2016
Thanks for the replies...
What I'm trying to do is a simple input system:
query :: IO (Maybe String)
query = do
putStrLn "Enter text or press q:"
r <- getLine
return $ if r == "q" then Nothing else Just r
This will ask an input to the user which is returned, unless "q" is pressed.
I want to repeat this query "some" or "many" times:
main = do
qs <- some $ query
-- qs <- many $ query
putStrLn qs
This should ask the query multiple times until "q" is pressed.
The type of qs is Maybe [String].
The expected result is that with "some", returning zero results will not be
permitted, while with "many" it is.
Probably I should defined a newtype for IO Maybe:
data IOMaybe a = IOMaybe {getIOMaybe :: IO (Maybe a)}
And define all the instances.
Or use Data.Fucntor.Compose:
type IOMaybe = Compose IO Maybe
On Thu, Sep 29, 2016 at 11:15 PM, Doaitse Swierstra <
doaitse.swierstra at gmail.com> wrote:
> The type of the last part of the expression is:
>
> many $ Just 1 :: Num a => Maybe [a]
>
> So in order to be able to return the “Just” constructor which inspected by
> the application of (take 3 <$>) we have somehow to know for sure that all
> the <*> executions will indeed see a “Just” in both of their arguments.
> This forces more and more evaluations.
>
> Doaitse
>
>
> Op 29 sep. 2016, om 22:28 heeft Jake <jake.waksbaum at 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 <http://hackage.haskell.org/package/base-4.9.0.0/docs/src/GHC.Base.html#Alternative>:
>
> many :: f <http://hackage.haskell.org/package/base-4.9.0.0/docs/src/GHC.Base.html#local-1627395956> a <http://hackage.haskell.org/package/base-4.9.0.0/docs/src/GHC.Base.html#local-1627395960> -> f <http://hackage.haskell.org/package/base-4.9.0.0/docs/src/GHC.Base.html#local-1627395956> [a <http://hackage.haskell.org/package/base-4.9.0.0/docs/src/GHC.Base.html#local-1627395960>]many <http://hackage.haskell.org/package/base-4.9.0.0/docs/src/GHC.Base.html#many> v <http://hackage.haskell.org/package/base-4.9.0.0/docs/src/GHC.Base.html#local-1627395964> = many_v <http://hackage.haskell.org/package/base-4.9.0.0/docs/src/GHC.Base.html#local-1627395965> where many_v <http://hackage.haskell.org/package/base-4.9.0.0/docs/src/GHC.Base.html#local-1627395965> = some_v <http://hackage.haskell.org/package/base-4.9.0.0/docs/src/GHC.Base.html#local-1627395966> <|> <http://hackage.haskell.org/package/base-4.9.0.0/docs/src/GHC.Base.html#%3C%7C%3E> pure <http://hackage.haskell.org/package/base-4.9.0.0/docs/src/GHC.Base.html#pure> [] some_v <http://hackage.haskell.org/package/base-4.9.0.0/docs/src/GHC.Base.html#local-1627395966> = (fmap <http://hackage.haskell.org/package/base-4.9.0.0/docs/src/GHC.Base.html#fmap> (:) v <http://hackage.haskell.org/package/base-4.9.0.0/docs/src/GHC.Base.html#local-1627395964>) <*> <http://hackage.haskell.org/package/base-4.9.0.0/docs/src/GHC.Base.html#%3C%2A%3E> many_v <http://hackage.haskell.org/package/base-4.9.0.0/docs/src/GHC.Base.html#local-1627395965>
>
> 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>
> 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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