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Pattern matching

Pattern matching in function definition Pattern matching consists of

  • specifying patterns to which some data should conform
  • and then checking the shape of input data
  • and deconstructing the data according to those patterns.
lucky :: Int -> String
lucky 7 = "LUCKY NUMBER SEVEN!"
lucky x = "Sorry, you're out of luck, pal!"

During execution, the patterns will be checked from top to bottom and when it conforms to a pattern, the corresponding function body will be used.

Which is similar to piecewise function in mathematics below:

math

catch-all pattern

binding​

deconstructing​

ignore with _​

pattern matching on Array​

takeFive :: Array Int -> Int
takeFive [0, 1, a, b, _] = a * b
takeFive _ = 0

The first pattern only matches arrays with five elements, whose first and second elements are 0 and 1, respectively. In that case, the function returns the product of the third and fourth elements. In every other case, the function returns zero.

info
  • List supports proper pattern matching (Cons/Nil).
  • Array does not support pattern matching directly; use uncons instead

pattern matching on Records​

pattern matching on List​

tip

A pattern like x:xs will bind the head of the list to x and the rest of it to xs, even if there's only one element so xs ends up being an empty list. Note: The x:xs pattern is used a lot, especially with recursive functions.

Pattern matching used outside of function, is not just limited in function

  • case
  • ADT