Functional Programming: A PragPub Anthology

Pattern Matching Structured Data

Matching goes further—Elixir will look at the structure of the two sides when making a match:

[ a, b, c ] = [1, 2, 3]     # a = 1, b = 2, c = 3
[ head | tail ] = [1, 2, 3]   # head = 1, tail = [ 2, 3 ]

Once bound, a variable keeps the same value for the duration of the pattern match. So the following match will only succeed if the variable list is a three-element list where the first and last elements have the same value:

[ a, b, a ] = list

You can mix constants and variables on the left-hand side. To illustrate this, I’m going to introduce a new Elixir data type, the tuple. A tuple is a fixed-length collection of values. We write tuple constants between braces:

{ 1, 2, "cat" }
{ :ok, result }

(The ...