03 Lists

Michael Saelee

Feb 1, 2019


On data structures and immutability

Most languages have built-in aggregate types – i.e., data structures – used to store, access, and manipulate collections of values. The most common data structure used in imperative languages is the array.

An array is a fixed-size structure made up of “slots”, each of which can hold an element. Depending on the implementation, the elements in an array may be homogeneous (all of the same type) or heterogeneous (of different types). Arrays also typically support indexing for both retrieval and updates.


int[] arr = new int[100];
for (int i=0; i<arr.length; i++)
    arr[i] = 0;

But remember, in a functional language, in-place mutations are not possible!

So how might an operation like arr[N] = X be carried out?

If arrays existed in Haskell, “updating” an index would require making a new copy of the entire array except for the value at that index (which would be different from the original). Also, empty “slots” would be pointless, as we couldn’t actually “place” elements there — they would only be useful if they served a semantic purpose.

Arrays are not part of the standard Haskell repertoire. Instead, we have lists.

Lists and list operations

: operator is used for list construction. (It is an example of a “data/value constructor”.)

(:) :: a -> [a] -> [a]

In the type of : (which is polymorphic), [a] represents a list of some homogeneous type a. I.e., : takes a value of type a and a list of as and returns a list of as.

The empty list, [], is also polymorphic:

[] :: [t]

So we can construct a list of Booleans like this:

True : []
True : False : True : []

(check operator associativity of : with :i (:))

There is also syntactic sugar for defining lists:

['a'..'z'] -- works because of `Enum` type class

List comprehensions

-- to try: 
[2*x | x <- [1..10]]
[(i,j) | i <- [1..5], j <- ['a'..'e']]
[p | p <- [1..100], p `mod` 9 == 0]
integerRightTriangles p = [(a,b,p-(a+b)) | a <- [1..(p-2)], b <- [a..(p-1)],
                           let c = p-(a+b) in a^2 + b^2 == c^2]

Basic list operations _____________________

length, head, tail, last, init, !!, null, elem, take, drop, reverse, ++

Defining functions on lists: pattern matching & recursion

When defining functions, we can provide multiple patterns for the function call to be matched against, so long as the results are all of the same type. The first pattern to match the actual argument will have its result chosen.

fib :: Integer -> Integer
fib 0 = 1
fib 1 = 1
fib x = fib (x-1) + fib (x-2)

Any data/value constructor can be used in pattern matches, so:

empty [] = "empty"
empty _  = "not empty" -- the variable `_` means "we don't care about this value"

Our own head and tail implementations:

head' (x:_) = x
tail' (_:xs) = xs

Implement last and init?

last' :: [a] -> a
last' [] = error "empty list"
last' [x] = x
last' (_:xs) = last' xs

init' :: [a] -> [a]
init' [] = error "empty list"
init' [x] = []
init' (x:xs) = x : init' xs

Recursion is a common list programming pattern!