Programming Elixir ≥ 1.6

Variables Bind Once (per Match)

Once a variable has been bound to a value in the matching process, it keeps that value for the remainder of the match.

iex> [a, a] = [1, 1]
[1, 1]
iex> a
1
iex> [b, b] = [1, 2]
** (MatchError) no match of right hand side value: [1, 2]

The first expression in this example succeeds because a is initially matched with the first 1 on the right side. The value in a is then used in the second term to match the second 1 on the right side.

In the next expression, the first b matches the 1. But the second b corresponds to the value 2 on the right. b cannot have two different values, and so the match fails.

However, a variable can be bound to a new value in a subsequent match, and its current value does not ...