How is Ruby's throw-catch implemented?

Question

In ruby you can throw :label so long as you've wrapped everything in a catch(:label) do block.

I want to add this to a custom lispy language but I'm not sure how it's implemented under the hood. Any pointers?

Answer 1

This is an example of a non-local exit. If you are using your host language's (in this case C) call stack for function calls in your target language (so e.g. a function call in your lisp equates to a function call in C), then the easiest way is to use your host language's form of non-local exit. In C that means setjmp/longjmp.

If, however, you are maintaining your target language's call stack separately then you have many options for how to do this. One really simple way would be to have each lexical-scope exit yield two values; the actual value returned, and an exception state, if any. Then you can check for the exception at runtime and propagate this value up. This has the downside of incurring extra cost to function calls when no condition is signaled, but may be sufficient for a toy language.

The book "Lisp In Small Pieces" covers about a half-dozen ways of handling this, if you're interested.