It is unfortunately much trickier because keyword lists are also optional. So what happens if opt2 is not given? We could raise but... since they are optional, we wouldn't want it to raise but rather have a default value.

Of course, we could then say "let's allow a default value to be given when pattern matching keyword lists". The problem is that no other pattern works like this and it would be inconsistent.

I think optional arguments are fundamentally incompatible with pattern matching (in Elixir). I understand why we would want that but, if we did have it, we would be adding tons of complexity and it would stand out as a sore thumb. I am also wary of going towards the same direction as Ruby and Python which have quite complex argument handling (albeit we have slightly different feature sets).