I also think the explicit type parameters on Map.empty<int, int> are not needed.

You could replace x::xs when x = '[' with '['::xs I believe.

I realize they're not going to change, but it avoids writing the yield 6 times, like you said.

I updated my array solution.

I don't know about the naming thing. One thing to keep in mind is that a let binding, parameter, etc. will shadow any previous one (at least in the current scope), which might actually be desirable in some cases if you want to avoid accidentally referencing the original.

Putting the type extensions in a library might be appropriate if you plan on having many of these these plugins and a common codebase to support them all. Or if not, that is fine as well - type extensions are really nice for this type of ad-hoc, throwaway functionality.