Hi,

If in a guard I use :erlang.map_get/2 on a map that does not have the key, and use a or to put another guard condition in second position, the second condition will not be evaluated.

defmodule T do
  def check_nil_1(map, key) when :erlang.map_get(key, map) == nil or not is_map_key(map, key),
    do: true

  def check_nil_1(_, _),
    do: false

  def check_nil_2(map, key) when not is_map_key(map, key) or :erlang.map_get(key, map) == nil,
    do: true

  def check_nil_2(_, _),
    do: false
end

map = %{a: 1}
key = :b

T.check_nil_1(map, key)
|> IO.inspect(label: "check 1")

T.check_nil_2(map, key)
|> IO.inspect(label: "check 2")

# Output:
# check 1: false
# check 2: true

I expected both checks to be true. The only difference is the order of the conditions in the guards. The documentation says:

The call fails with a {badmap,Map} exception if Map is not a map, or with a {badkey,Key} exception if no value is associated with Key. […] Allowed in guard tests.

Obviously as it is in a guard it does not fails with an error, but the docs do not tell that it also skips further evaluation of the guard conditions.

So, are there any more docs on that subject ? And any other special cases in guards that would be good to know ?

Thank you

(edit: if the key exists in the map with a nil value, both checks return true as expected.)