because left || right actually compiles to something like

case left do
  v when v == false or v == nil -> right
  _ -> left
end

Which is not allowed in guards

Only || and && are not allowed in guards. All other boolean operators are allowed

Yeah && and || have what is called a “short circuit” behavior where they will only evaluate the right hand side of the operand if they need to. This allows you to do things like:

value = params[:some_opt_key] || raise "this parameter is required!

If the left hand side is “truthy” then || short circuits and doesn’t evaluate the raise at all. This sort of branching behavior doesn’t even make sense in guards.

Worth noting that in guards, or is precisely the same as calling :erlang.orelse/2.

This might work for literals, but I’m not sure it works for arbitrary expressions—in general, nor successful at not evaluating the second branch.

In general there’s not much reason to want || in these cases, as we are not working with arbitrary expressions—we know that functions allowed in guards are always idempotent, and don’t have side effects, so there’s little gain in suppressing them!

This is because orelse will just err with badarg, whereas in Elixir we want more accurate errors. But besides that this is the same behaviour as orelse : only booleans on the left.