Elixir Guards

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Guard clauses allow Elixir to determine which function to invoke based not only on which arguments are passed, but also based on type or some tests involving their values. Guard clauses are defined using the when keyword.

Examples:

defmodule Guard do
  def kind_of(x) when is_binary(x) do
    "#{x} is a binary"
  end

  def kind_of(x) when is_atom(x) do
    "#{x} is an atom"
  end

  def is_ten(x) when x > 10, do: "Greater than ten"

  def is_ten(x) when x < 10, do: "Less than ten"

  def is_ten(x), do: "Yes"
end
max = fn
  x, y when x > y -> x
  x, y -> y
end
case 42 do
  x when is_binary(x) -> "Nope"
  x                   -> "Yep"
end

Additionally, users may define their own guards. Example: the Bitwise module defines bnot, ~~~, band, &&&, bor, |||, bxor, ^^^, bsl <<<, bsr >>>.

Operators

Comparison

==, !=, ===, !==, <, >, <=, >=

Arithmetic

+, -, *, /

Unary operators +, and - are also allowed.

Boolean and Negation

or, and, not

Short-circuiting operators &&, ||, and ! are not allowed.

Join

<>, ++ as long as left side is a literal.

Membership

Test membership in a list or range with in.

Example:

def is_animal(animal) when animal in [:dog, :cat, :fish] do
  true
end

def in_range(x) when x in 1..6 do
  true
end

Functions

Type-check

See main Elixir documentation for more information.

is_atom/1, is_binary/1, is_bitstring/1, is_boolean/1, is_float/1, is_function/1, is_function/2, is_integer/1, is_list/1, is_map/1, is_number/1, is_pid/1, is_port/1, is_record/1, is_record/2, is_reference/1, is_tuple/1

Other

See main Elixir documentation for more information.

abs(number), binary_part(binary, start, length), bit_size(bitstring), byte_size(bitstring), div(number, number), elem(tuple, n), hd(list), length(list), map_size(map), node(), node(pid | ref | port), rem(integer, integer), round(number), self(), tl(list), trunc(number), tuple_size(tuple)

Notes