# Unit 5: Higher-order functions

## The functions `map` and `filter`

The higher-order function `map` takes a function `f` and a list `xs` as its arguments and it applies `f` to each element of `xs`:

``````map f [x1, x2, ..., xn] = [f x1, f x2, ..., f xn]
``````

It can be defined as follows:

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

Let `square x = x * x`. Then we have the following:

``````map square [1 .. 7] = [1, 4, 9, 16, 25, 36, 49]
``````

The higher-order function `filter` takes a Boolean-valued expression `pred` and a list `xs` as its arguments and it produces a sublist of `xs` as its value such that each element in the value satisfies the Boolean-valued expression `pred`. For example:

``````filter even [1 .. 10] = [2, 4, 6, 8, 10]
``````

The higher-order function `filter` can be defined like this:

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

## Function composition

Composition is a binary operator represented by an infix full stop: `(f.g) x` is equivalent to `f (g x)`. The type of the section `(.)` is `(a -> b) -> (c -> a) -> c -> b`. Function composition is useful for many reasons. One of them is that `f (g ( h (i (j (k x)))))`, say, can be written as `(f . g . h . i . j . k ) x`; noting that function composition is associative. This usefulness can be illustrated by means of the following problem: Find the sum of the cubes of all the numbers divisible by 7 in a list `xs` of integers. The solution is as follows:

``````answer :: [Int] -> Int
answer xs = sum (map cube (filter by7 xs))

cube :: Int -> Int
cube x = x * x * x

by7 :: Int -> Bool
by7 x = x `mod` 7 == 0
``````

But using function composition this can be written more clearly as follows:

``````answer :: [Int] -> Int
answer = sum . map cube . filter by7
``````

This version of `answer` illustrates function-level definition. When a parameter appears on the extreme left of both sides of the definition symbol `=`, both of its occurrences can be deleted. The version in which the parameter occurs is know as an object-level definition. For example, the following two definitions are equivalent; the first is the object-level definition and the second the function-level definition:

``````fun xs = (filter odd . map square) xs
fun    =  filter odd . map square
``````

Function-level definitions are also know as point-free ones and even sometimes as pointless ones; object-level definitions are also known as point-level or point-wise ones.

## The fusion law for `map`

Fusion laws in Haskell combine two or more computations into a single computation. The fusion law for `map` states that:

``map f . map g = map (f.g)``

Applying the left-hand side of this law to a list results in two traversals of its argument, whereas applying the right-hand version only traverses the list once.

Combining two uses of `filter` is a bit more complicated. It requires us to define a function `andPred`:

``````andPred :: (a -> Bool) -> (a -> Bool) -> a -> Bool
andPred p q x = (p x) && (q x)
``````

Using `andPred` we have the following law in both point-level and point-free versions:

``````filter p (filter q xs) = filter (andPred p q ) xs
filter p . filter q    = filter (andPred p q )
``````

## Function application \$

Function application in Haskell is simply represented by writing a function `f` before its argument `x`, thus `f x`. However, there is also an explicit function application operator represented by the dollar sign:

``(\$) :: (a -> b) -> a -> b``

Thus, `f \$ x` is the same as `f x`. Such an operator may seem unnecessary, but its use can reduce the number of parentheses used in writing complicated expressions. This is because it has the lowest precedence of any operator and is right associative whereas normal function application has the highest precedence of any Haskell operator and is left associative. The function `answer` defined above can be written as follows:

``answer xs = sum \$ map cube \$ filter by7 xs``

Note that you cannot remove the occurrences of `xs` in the above definition since `filter by7 xs` is the argument of the function `map cube`.

## List comprehensions and `map` and `filter`

Every list that can be defined using a list comprehension can also be defined using `map` and `filter` and visa versa. First, I define `map` and `filter` using list comprehensions:

``````map f xs = [ f x | x <- xs ]
filter pred xs = [ x | x <- xs, pred x ]
``````

Next, I show the idea behind defining arbitrary list comprehensions using `map` and `filter`: `[ f x | x <- xs, pred x ]` is equivalent to `map f (filter pred xs)`, which can also be written as `(map f . filter pred) xs`, using function composition.

## The higher-order function `iterate`

### Newton's method for finding positive square roots

Let `x` be the positive number whose square root you are trying to find. Then if `y > 0` is a guess, then `(y + x/y)/2` is a better guess. For example, say we want to find the positive square root of 27.3. Let us guess 1. Applying Newton's method, a better guess is 14.15. Applying Newton's method again, a still better guess is 8.03966. Applying Newton's method again, a still better guess is 5.71766. Applying Newton's method again, a still better guess is 5.24617. And so on. Newton's method can be programmed straightforwardly in Haskell as follows:

``````root :: Float -> Float
root x = rootiter x 1

rootiter :: Float -> Float -> Float
rootiter x y
| satisfactory x y = y
| otherwise        = rootiter x (improve x y)

satisfactory :: Float -> Float -> Bool
satisfactory x y = abs (y*y - x) < 0.01

improve :: Float -> Float -> Float
improve x y = (y + x/y)/2
``````

This, however, is quite an "imperative" solution. A more "functional" solution uses the predefined Haskell function `iterate`:

``````iterate :: (a -> a) -> a -> [a]
iterate f x = x : iterate f (f x)
``````

The function `iterate` generates an infinite list in the following way:

``````iterate f x = [x, f x, f (f x), f (f (f x)), f (f (f (f x))), ...]
``````

Using `iterate` it is possible to produce a more "functional" implementation of Newton's method of finding positive square roots:

``````root :: Float -> Float
root x = head (filter (satisfactory x) (iterate (improve x) 1))

satisfactory :: Float -> Float -> Bool
satisfactory x y = abs (y*y - x) < 0.01

improve :: Float -> Float -> Float
improve x y = (y + x/y)/2
``````

### The sieve of Eratosthenes for generating primes

1. Make a list of all the positive integers starting at 2.
2. The first number on the list is prime; call it `p`.
3. Construct a new list in which all multiples of `p` have been removed.
4. Repeat the above from step (2).

The above procedure can be thought of as generating an infinite list of infinite lists. The prime numbers are the first elements of lists of integers:

``````[ [ 2,  3,  4,  5,  6,  7,  8,  9, 10, 11, 12, ... ],
[ 3,  5,  7,  9, 11, 13, 15, 17, 19, 21, 23, ... ],
[ 5,  7, 11, 13, 17, 19, 23, 25, 29, 31, 35, ... ],
[ 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, ... ],
[11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, ... ], ... ]
``````

In Haskell the sieve can be programmed like this:

``````primes       = map head (iterate sieve [2..])
sieve (p:xs) = [ x | x <- xs, x `mod` p /= 0 ]
``````

Step (1) is represented by `[2..]`. Step (2) is represented by `map head`. Step (3) is represented by `sieve` and step (4) is represented by `iterate`.

© Antoni Diller (20 September 2021)