To be specific, the right argument of the in/not in operator must be statically known at compile-time when used in guard position, so you cannot do something like:
l = [1, 2, 3]
case 2 do
b when b in l -> b # Fails compile with an ArgumentError
_ -> nil
end
This is because in guard position the in operator works differently than in non-guard position (I really dislike inconsistencies like this), specifically it gets expanded, so the above example when written like:
case 2 do
b when b in [1,2,3] -> b
_ -> nil
end
Actually gets compiled to (you can check with a core erlang dump):
case 2 do
b when b == 1 -> b
b when b == 2 -> b
b when b == 3 -> b
_ -> nil
EDIT: It also works with ranges:
case 2 do
b when b in 1..3 -> b
_ -> nil
end
Gets turned into:
case 2 do
b when b >= 1 and b<=3 -> b
_ -> nil
end
