# Higher order functions

## List-based recursion

Consider the length function that finds the length of a list. Below is a recursive definition.

length :: [a] -> Int
length [] = 0
length (x:xs) = 1 + length xs

The type signature tells us that length is a function that accepts a list of an arbitrary type a, and returns an Int. The (x:xs) term binds x to the head of the list and xs to the tail.

ghci> head [1,2,3]
1
ghci> tail [1,2,3]
[2,3]

This works by pattern matching on the type constructor (:). If we had recursively defined our list as

data List a = Nil
| Cons a (List a)

then the same length function would be written as follows.

length :: List a -> Int
length Nil = 0
length (Cons x xs) = 1 + length xs

## Abstracting to map

Suppose we’re writing a function that doubles every element in a list of Ints. We might write the following.

doubleList :: [Int] -> [Int]
doubleList [] = []
doubleList (x:xs) = (2 * x) : doubleList xs

We can easily generalize this, for example to a function that multiplies every element in a list of Ints by $$m$$.

multiplyList :: Int -> [Int] -> [Int]
multiplyList _ [] = []
multiplyList m (x:xs) = (m * x) : multiplyList m xs

Here, the naive reading of the type signature is as a function from an Int and a [Int] to a [Int]. However, since the -> operator is right associative, we can read the type signature as

multiplyList :: Int -> ([Int] -> [Int])

meaning multiplyList takes an Int and returns a function from [Int] to [Int]. In particular, note that we can recover doubleList by passing one argument to multiplyList. The following code gives us a function equivalent to the original.

doubleList' = multiplyList 2

Functions in Haskell are “first-class citizens,” and behave exactly like any other value. As we’ve seen, functions can return other functions. Similarly, functions can accept other functions as arguments. To generalize multiplyList further, we can write

applyToInts :: (Int -> Int) -> [Int] -> [Int]
applyToInts _ [] = []
applyToInts f (x:xs) = (f x) : applyToInts f xs

applyToInts accepts a function from integers to integers, with type signature Int -> Int, and returns a function that applies that function to each element in a list.

ghci> applyToInts (* 2) [1,2,3]
[2,4,6]

We can further generalize applyToIntegers. While its type signature is (Int -> Int) -> [Int] -> [Int], the definition is not integer specific. We can make a polymorphic version with the type signature (a -> b) -> [a] -> [b]. This function is called map, which you might be familiar with.

map :: (a -> b) -> [a] -> [b]
map _ [] = []
map f (x:xs) = (f x) : map f xs

## The filter function

Besides map, Haskell contains numerous other higher-order functions for list operations. As in other languages, filter is a function that takes a predicate and a list, and returns the list of elements that satisfy the predicate.

filter :: (a -> Bool) -> [a] -> [a],
filter _ [] = []
filter pred (x:xs)
| pred x         = x : filter pred xs
| otherwise      = filter pred xs

Looking at the type signature, we can see that the predicate, which is the first argument, has type (a -> Bool) where a is the type of the list elements.

Here are some examples.

ghci> filter even [1..10]
[2,4,6,8,10]
ghci> filter (>2) [1,6,2,3,8,3]
[6,3,8,3]

As with map, we can define more specific functions using filter. For example, the following function justEvens takes a list of type [Int] and returns a list of its even elements.

justEvens :: [Int] -> [Int]
justEvens = filter even