Lists

Lists are sequences of elements of the same type and are a key component of Haskell. This means that a list can only hold elements of the same type, e.g. a list of Ints as we used in our example function - sum. To create lists in Haskell, we put their elements in square brackets and separate them with commas:

[False, False, True] :: [Bool] -- a list of booleans
[1, 3, 5] :: [Int] -- a list of integers
['a', 'b', 'c'] :: [Char] -- a list of characters

Lists can also contain other lists:

[[1, 2, 3], [4, 5, 6]] :: [[Int]] -- a list of lists of integers

But remember - lists are sequences of elements of the same type, so a list of lists must not contain lists of different types. For example, we cannot combine a list of Ints and a list of Chars into a single list of lists:

ghci> x = [[1, 2, 3], ['a', 'b', 'c']]

<interactive>:2:7: error:
    • No instance for (Num Char) arising from the literal ‘1’
    • In the expression: 1
      In the expression: [1, 2, 3]
      In the expression: [[1, 2, 3], ['a', 'b', 'c']]

Lists can also be empty ([]) and a special case called a singleton list is ([[]]), which is a list with its single element being an empty list. Lists in Haskell can also be infinite.