Lists and List Processing

Lists are the fundamental data structure in functional programming. In Haskell, almost all data processing involves manipulating lists using recursion, pattern matching, and higher-order functions like map, filter, and fold.

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Lists in Haskell

A list in Haskell is a sequence of elements of the same type, enclosed in square brackets:

numbers = [1, 2, 3, 4, 5]          -- List of integers
names = ["Alice", "Bob", "Carol"]   -- List of strings
empty = []                          -- Empty list
single = [42]                       -- List with one element

List Types

Every list has a type written as [a], where a is the type of the elements:

[1, 2, 3]           :: [Int]
["hello", "world"]  :: [String]
[True, False, True]  :: [Bool]

All elements must be the same type — you cannot mix integers and strings.

Strings are Lists of Characters

In Haskell, a String is simply a list of Char:

"hello" == ['h', 'e', 'l', 'l', 'o']    -- True

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Head, Tail, and Cons

Head and Tail

• head returns the first element of a list.
• tail returns everything except the first element.