Bool

A Bool can be either True or False.

Gleam defines a handful of operators that work with Bools.

False && False // => False
False && True  // => False
True && False  // => False
True && True   // => True

False || False // => False
False || True  // => True
True || False  // => True
True || True   // => True

&& and || are short circuiting, meaning they don't evaluate the right hand side if they don't have to.

&& evaluates the right hand side if the left hand side is True.

|| evaluates the right hand side if the left hand side is False.

Gleam supports negation of Bools using either the ! operator or the bool.negate function from the gleam/bool module.

!True  // => False
!False // => True

Erlang interop

While written in the code using a capital letter, they are represented at runtime with the atoms true and false, making them compatible with Elixir and Erlang's booleans.

This is important if you want to use Gleam and Elixir or Erlang together in one project.

// Gleam
True
False

% Erlang
true.
false.