Guards add conditions to pattern matches with
when. They work in function heads, case, fn
and comprehensions.
defmodule Shape do
def area({:circle, r}) when is_number(r) and r >= 0, do: 3.14159 * r * r
def area({:rect, w, h}) when is_number(w) and is_number(h), do: w * h
def area(other), do: {:error, {:unknown_shape, other}}
end
IO.inspect(Shape.area({:circle, 2}))
IO.inspect(Shape.area({:rect, 3, 4}))
IO.inspect(Shape.area({:circle, -1}))12.56636
12
{:error, {:unknown_shape, {:circle, -1}}}Only a fixed set of expressions is allowed in guards:
comparisons, boolean operators, arithmetic, type
checks (is_*) and a few functions like
length/1, map_size/1, elem/2, is_map_key/2.
check = fn
list when is_list(list) and length(list) > 2 -> :long_list
map when is_map(map) and is_map_key(map, :id) -> :has_id
bin when is_binary(bin) and byte_size(bin) == 0 -> :empty_string
_ -> :other
end
IO.inspect(Enum.map([[1, 2, 3], %{id: 1}, "", 42], check))[:long_list, :has_id, :empty_string, :other]in with a literal list or range works in guards.
weekday? = fn
day when day in [:sat, :sun] -> false
_ -> true
end
IO.inspect({weekday?.(:mon), weekday?.(:sun)}){true, false}An error inside a guard doesn't raise; the guard just fails and the next clause is tried.
safe = fn
x when hd(x) == :a -> :starts_with_a
_ -> :no
end
IO.inspect(safe.(:not_a_list)):noName reusable guards with defguard.
defmodule Checks do
defguard is_even(n) when is_integer(n) and rem(n, 2) == 0
end
defmodule Parity do
import Checks
def of(n) when is_even(n), do: :even
def of(n) when is_integer(n), do: :odd
end
IO.inspect(Enum.map(1..4, &Parity.of/1))[:odd, :even, :odd, :even]Pitfall: you can't call your own functions in a guard (
when valid?(x)is a compile error). Usedefguard, or match first and check in the body.
Pitfall: an error anywhere in a guard fails the whole guard.
when map_size(x) == 0 or is_list(x)does not match a list, becausemap_size/1raises first. Chainwhenclauses instead (when map_size(x) == 0 when is_list(x)): each one is tried on its own.