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]
[11,12]``````

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
[1,3,5]

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

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