Random element from list Haskell

user_15547432 0
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I'm trying to get a random Int from a list and I'm having some issues because the only libraries that we're allowed to use are QuickCheck, GHC.IO and Data.List.

So far I managed to get a random IO Int by using:

ran :: Gen Int
ran = choose (0,3)

(The list has length=4)

But when I call

elem = list !! generate ran

I get: Couldn't match expected type ‘Int’ with actual type ‘IO Int’ And i'm pretty sure you can't "extract" only the Int from an IO Int so i'm kinda lost.

¿Any ideas?

haskell
1 Answers

You can work with an fmap such that elem is an IO Int:

elem :: IO Int
elem = fmap (list !!) (generate ran)

or we can work with the operator alias of fmap: (<$>):

elem :: IO Int
elem = (list !!) <$> generate ran

It is probably better, as @AndrewRay says, to use another name, since the Prelude already contains an elem function. For example randElem:

randElem :: IO Int
randElem = (list !!) <$> generate ran
user_675790
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