Question

Suppose we want those elements of list x for which the corresponding element of list y is strictly positive. Any of the three solutions below work:

let x = [1..4]
let y = [1, -1, 2, -2]

[ snd both | both <- zip (map (> 0) y) x, fst both ]

or

map snd $ filter fst $ zip (map (>0) y) x

or

sel :: [Bool] -> [a] -> [a]
sel [] _ = []
sel (True : xs) (y : ys) = y : sel xs ys
sel (False : xs) (y : ys) = sel xs ys

sel (map (> 0) y) x

however, what prompted this was that in the R language this can be written compactly like this:

x[y > 0]

and given how much shorter that is I was wondering if there is a shorter/better way to do this in Haskell?

Answer 1

I’m not a haskell specialist, but why not use list comprehension?

 [i | (i,j)  <-  zip x y, j > 0 ]