Programming Elixir 1.2

More Complex List Patterns

Not every list problem can be easily solved by processing one element at a time. Fortunately, the join operator, |, supports multiple values to its left. Thus, you could write

iex> [ 1, 2, 3 | [ 4, 5, 6 ]]
[1, 2, 3, 4, 5, 6]

The same thing works in patterns, so you can match multiple individual elements as the head. For example, the following program swaps pairs of values in a list.

lists/swap.exs

defmodule Swapper do
  def swap([]), do: []
  def swap([ a, b | tail ]), do: [ b, a | swap(tail) ]
  def swap([_]), do: raise "Can't swap a list with an odd number of elements"
end

We can play with it in iex:

iex> c "swap.exs"
[Swapper]
iex> Swapper.swap [1,2,3,4,5,6]
[2, 1, 4, 3, ...