Github: ArrayFilter.hs
We’ll continue list manipulation by considering how to filter a list.
Specifically, given a number n and an array, we’ll return all elements of the array less than n.

First we declare the type of our function, which takes a number and an array, and returns an array.

type ArrFilter = Int -> [Int] -> [Int]

Using Recursion

f :: ArrFilter
f _ [] = []
f n (x:xs) =
    if x < n
        then x : f n xs
    else
        f n xs

f 10 [2,4,6,8,10,12,14]

[2,4,6,8]

Using ListComp

List comprehensions might look a bit familiar if you’ve used them in Python before.

f2 :: ArrFilter
f2 n arr = [num | num <- arr, num < n]

f2 10 [2,4,6,8,10,12,14]

[2,4,6,8]

Using Filter

f3 :: ArrFilter
f3 n arr = filter (< n) arr

f3 10 [2,4,6,8,10,12,14]

[2,4,6,8]

Using Filter - Short

f4 :: ArrFilter
f4 n = filter (< n)

f4 10 [2,4,6,8,10,12,14]

[2,4,6,8]

Using Filter - Shorter

f5 :: ArrFilter
f5 = filter . (>)

f5 10 [2,4,6,8,10,12,14]

[2,4,6,8]