The filter Function

The filter function is another higher-order function for working with lists. As the name suggests, it is used for filtering lists by selecting only elements that satisfy a predicate defined by the function passed in. Like map, we could also define filter using a list comprehension:

filter :: (a -> Bool) -> [a] -> [a]
filter f xs = [x | x <- xs, f x]

Unlike map, the filter function must receive a function that returns a boolean as its argument, and the resulting list from filter will always be of the same type as the list we passed in because we are only selecting elements from that list, rather than generating new ones. We can filter a list using our squareGt100 function from before in order to get the elements for which squareGt100 returns True:

ghci> filter squareGt100 [7..12]

Here are some other examples of using filter with other pre-defined functions:

ghci> filter odd [1..5] -- get all odd numbers from a list

ghci> filter (\x -> length x > 2) ["a", "abc"] -- elements with length greater than 2

ghci> filter (\(x:xs) -> x == 'a') ["cardano", "ada"] -- elements staring with 'a'

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