A recursive definition refers directly or indirectly to itself. In Haskell, it commonly combines a base case with a recursive case that processes a smaller part of the input. section 4.1.2
Numbering every line
A local function processes a list of lines. Its empty-list case returns [], while its recursive case emits the current line number and calls itself with the remaining lines and an incremented counter. Starting with 1 produces [(Just 1,"Hello"),(Just 2,""),(Just 3,"World")]. section 4.1.2
Listing 4.3 A function that numbers every line in order
numberAllLines :: [String] -> NumberedLines
numberAllLines lines =
let go :: Int -> [String] -> NumberedLines #1
go _ [] = [] #2
go counter (x : xs) = (Just counter, x) : go (counter + 1) xs #3
in go 1 lines #4
Inspect the complete recursive definition, including its base case, recursive pattern match, counter update, and initial call. section 4.1.2
Use recursive definitions to:
- Pattern match lists such as Strings, which are lists of characters. section 4.1.2
- Define related helpers with additional parameters inside a let binding. section 4.1.2
- Implement utilities such as lines, unlines, words, and unwords. section 4.1.2
