A list is a homogeneous, ordered sequence: every element has the same type. Write values in square brackets, separated by commas, such as [1, 2, 3] or [True]. Its type also uses brackets: [Int], [Bool], and [Char]. An empty list still has an element type, so [] can be a [Float]. section 2.2.2
intList :: [Int] intList = [1, 2, 3] boolList :: [Bool] boolList = [True] floatList :: [Float] floatList = [] doubleList :: [Double] doubleList = [1, 1 + 1, 1 + 2]
These declarations pair list values with their element types, including an empty Float list and a Double list containing expressions. section 2.2.2
Lists can grow dynamically and do not have a fixed size. They are especially useful for collections such as letters in a message. A String is a list of characters, so functions over lists can also process strings. section 2.2.2 section 2.5
In Haskell, a list has two constructors: [] for the empty list and (:) for prepending one element to an existing list. For example, 1 : [2, 3] produces [1, 2, 3]. The pattern x : xs splits a nonempty list into its first element x and remaining tail xs. section 2.4.1
Compute a list length recursively
Pattern matching handles both possible list shapes. The empty list is the stopping case; a nonempty list contributes one, then continues with its tail. listLength [] = 0 listLength (x : xs) = 1 + listLength xs Leaving out the x : xs case makes the function partial: it works for [] but fails on nonempty lists because the patterns are nonexhaustive. section 2.4.1
Use lists with recursion when immutable data and the absence of loops require a repeated operation over successive tails:
- Match [] to define the base case. section 2.4.1
- Match x : xs to handle one element and recursively process xs. section 2.4.1
- Construct a new list rather than modifying the old one. section 2.5
Transform every character
A Caesar cipher creates a new string by transforming the first character, prepending it to the recursively transformed tail, and preserving the empty-list base case. caesar :: Int -> String -> String caesar n [] = [] caesar n (x : xs) = rotChar n x : caesar n xs For example, caesar 13 "Caesars Cipher!" yields "Pnrfnef Pvcure!". section 2.5
The same head-and-tail structure supports searches. An index function compares the current element, returns 0 when it matches, and otherwise adds 1 while searching the tail. Its empty-list case must be defined deliberately; using undefined reports failure when no matching character exists. section 2.4.1
