書名： Mastering Elixir
作者名： André Albuquerque Daniel Caixinha
本章字數： 309字
更新時間： 2021-08-05 10:42:46

Pattern matching on tuples

The following snippet shows how pattern matching can be done on tuples:

iex> {number, representation} = {3.1415, "π"}
{3.1415, "π"}
iex> number
3.1415
iex> representation
"π"

The process here is the same as we have described in the preceding snippet. By setting the {number, description} pattern on the left-hand side, we're stating that we expect a tuple with two values—again, if that's not the case, a MatchError will be raised. In this case, the match succeeds, and we can see that the variables number and representation are bound to the expected values.

Unlike Erlang, Elixir allows you to rebind a variable, which is why the following works:

iex> a = 1
1
iex> a = 7
7

However, a variable can only bind once per match:

iex> {a, a} = {3, 3}
{3, 3}
iex> {a, a} = {2, 3}
** (MatchError) no match of right hand side value: {2, 3}

On the first line, the match succeeds because each a is binding to the same value, 3. On the second line, we get a MatchError because we're binding a to two different values on the same match. Later in this chapter, we'll see how we can make Elixir behave like Erlang in this regard, by using the pin operator.

We can set our expectations even further, using literals on the left-hand side:

iex> {3.1415, representation} = {3.1415, "π"}
{3.1415, "π"}
iex> representation
"π"

Now our expectation is a tuple with two elements, where the first one is the 3.1415 float literal. We can use this on other Elixir types as well, such as lists or maps. This technique becomes even more fruitful when we apply it to functions, as we will see in the next section.

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Mastering Elixir

Pattern matching on tuples