Tuples

Pairs and more

Curly braces build a tuple, a fixed-size group of values. Tuples hold related items of different types, like a status with its payload.

The size is fixed at creation, so a tuple never grows or shrinks in place.

{ok, 42}.

{ok,42}

2> {point, 1, 2}.

{point,1,2}

3> {}.

{}

Access by index

The element function reads a position starting from one, and tuple_size counts the items. The size function answers the same for tuples and lists.

Index access is fine for ad-hoc work, but pattern matching, the next section, reads better.

T = {ok, 42}.

2> element(2, T).

42

3> tuple_size(T).

2

Match inside

Tuples match deeply: a pattern with a tuple inside matches only tuples of the same shape. The values inside bind to names in the same move.

This is how Erlang unpacks results: match the tuple, and the parts arrive as variables.

{ok, Value} = {ok, 42}.

{ok,42}

2> Value.

42

3> {error, _} = {ok, 42}.

** exception error: no match of right hand side value {ok,42}

The ok and error shape

Erlang functions answer expected failures with tuples instead of exceptions. Callers match {ok, Value} for success and {error, Reason} for failure, and the compiler checks nothing: the pattern does.

The convention is so common that it has a name: the ok tuple. Lesson 34 shows how a GenServer answers calls with the same shape.

divide(_, 0) -> {error, div_by_zero};

divide(A, B) -> {ok, A / B}.

Next: Maps

Article author: Arthur Isaev