[davidw]

> custom guards

I don't recall 100%, but I think this is a BEAM feature that exists because they don't want to run arbitrary code as part of guards that could have side effects, delays and so on. I don't remember the specifics.

[el_oni]

You can write custom guards out of any built in function that can already be used in a guard.

Like defguard is_list_or_even_int(val) do is_list(val) or (is_integer(val) and val % 2 ==0) end