[xjm]

A modular and safe way to achieve this is probably effect handlers. It's like python's yield but can return a value and is scoped like an exception, it's not local to a function call. If you're unfamiliar with it, this article is a good motivation.

Each function, written in direct style, can perform an "effect" when the function wants control to go somewhere else (for c=getchar() and emit(c) here).

Control then goes to the effect handler, in this case probably the caller of the two functions, which decides what to do next: decompressor emits a char? Let's resume the parser's code with the char until it asks for more, then resume decompressor again, etc.

Effects can be efficiently implemented, especially if the continuation is only allowed to be called once (which is the case in OCaml), and allow writing code in direct style, together with type/memory safety. They are also very helpful in a concurrent setting.

An example here : https://effekt-lang.org/docs/casestudies/lexer