Truthiness: only false and nil

Only two false values

In Erlang exactly two values count as false: the atom false and the empty list nil. Everything else is true, including zero and the empty binary.

This is stricter than most languages, so conditions never trip over a zero by accident.

0.

0

2> is_atom([]).

true

Short-circuit and

andalso and orelse are the short-circuit forms. The right side runs only when the left side cannot decide the answer.

Use them to guard access, just like the and and or of other languages.

X = nil, X =/= nil andalso X > 0.

false

2> true orelse error(boom).

true

In guards

Guard expressions in case and if see the same truthiness. A guard sequence succeeds when its last test holds.

Lesson 17 covers guards in full, including which functions may appear inside one.

case [] of

[] -> empty;

_ -> nonempty

end.

Type checks

The is functions answer type questions: is_atom, is_integer, is_binary, and the rest of the family. They are the honest way to test a value.

Pair them with the comparison operators of lesson 7 and every branch of your code states exactly what it expects.

is_integer(42).

true

2> is_binary("text").

true

Next: Conditions: case and if

Article author: Arthur Isaev