Let's start with simple quiz.
Given module defined as:
defmodule Foo do
def a(x) when is_integer(x) or is_map_key(x, :foo), do: true
def a(x), do: false
def b(x) when is_map_key(x, :foo) or is_integer(x), do: true
def b(x), do: false
endTry to answer these questions.
Q: What will be result of Foo.a(%{foo: 21})?
true
false
WRONG
RIGHT
This one is straightforward.
We check guard, it has one condition is_integer(x) or is_map_key(x, :foo). First one returns false, second returns true, Boolean's alternative results in true and first case is matched.
Q: What will be result of Foo.a(37)?
true
false
WRONG
RIGHT
This one is straightforward as well.
We check guard, it has one condition is_integer(x) or is_map_key(x, :foo). First one returns true, second one isn't fired at all, because or operator is short circuiting.
Q: What will be result of Foo.b(%{foo: 21})?
true
false
WRONG
RIGHT
Again, similar to the previous questions.
We check guard, it has one condition is_map_key(x, :foo) or is_integer(x). First one returns true and the rest is short circuited.
Q: What will be result of Foo.b(37)?
true
false
WRONG
RIGHT
Ouch, something changed…
Again, we check guard, one condition is_map_key(x, :foo) or is_integer(x). We hit first clause is_map_key(x, :foo) and this doesn't return false, instead it fail. Failure in one of guard functions isn't converted to false but instead makes whole guard expression fail. This mean that is_integer(x) part will never be called.
This behaviour is often surprising for a lot of Elixir developers, as it seemingly breaks commutative property of boolean operators. However, to be honest, these never were commutative because of short circuiting.
It seems that Elixir at the time of writing (Elixir 1.20.1, OTP 29) do not warn about this issue.
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