Booleans and nil

Elixir supports true and false as booleans.

true

false

Are they equal?

true == false

To operate on booleans, Elixir provides and/2, or/2 and not/1.

Logical and of two true values:

true and true

true or false

Note that and and or are short-circuit: if the left-hand side determines the result, the right-hand side won’t even be evaluated. It’s especially important when used expressions have side-effects.

# 💡 Try changing `true` to `IO.inspect(true)` to see if it gets evaluated.
false and true

Logical or:

false or is_boolean(true)

Logical not:

not true

The nil value

Elixir also provides the concept of nil, to indicate the absence of a value, and a set of logical operators that also manipulate nil: ||, &&, and !. For these operators, false and nil are considered “falsy”, all other values are considered “truthy”.

Using && with booleans:

true && false

nil is falsy, so && short-circuits:

# 💡 Try changing `&&` to `and`. What happens?
nil && 13

true is truthy, so && returns the right-hand side:

true && 17

|| returns the first truthy value:

# 💡 Try changing `1` to `IO.inspect("foo")`. Is "foo" printed out?
1 || true

false is falsy, so || returns the right-hand side:

false || 11

The ! operator:

# 💡 Try `!1` and `!nil`.
!true

As a rule of thumb, use and, or, and not when you are expecting booleans. If any of the arguments are non-boolean, use &&, ||, and !.