Of course you can: you just have to define it in your type. The output set becomes a union type of the normal output and whatever you want as an exception.

If you write this as a monad, your get very similar syntax to procedural code.

Then allow partial functions too. Maybe even require them to be tagged as such. (Is that within the capabilities of Zig's programmable type system?)

I don't mind escape hatches - as long as they're visible/greppable in the source code. You can always write undefined/error/panic/trace directives while you're coding, then come back and remove them later.