Call/cc for C programmers (2012)

For frontend, there are some nontrivial transformations needed to compile continuation based code for a JS target. You can't use the native stack, etc. (Same for WebAssembly I gather, judging from the recent Go/WebAssembly implementation discussion)

The point is, if you can serialize closures, I think you can serialize continuations.

    0. recognize that function return in C (and many langs)
       is just like implicitly calling a function of one
       argument (the return value)
    1. further recognize that these "return functions" are
       *closures*!  on the stack they are passed as both,
       an instruction pointer *and* the frame pointer of the
       frame into which to return!
    2. add closures (somewhat like GCC local functions)
    3. make a builtin macro-like thing that lets you call
       a C function with an alternative "return-function"
       argument that is like any other function (of one
       argument), and particularly allows local function
       references to be given as a return-function
    4. make a builtin macro-like thing to get at the current
       frame's return-function "argument" -- i.e., reify
       the previously implicit current continuation
    5. screw the function call stack, change the code
       generator and run-time to allocate function frames
       on a heap (and add garbage collection)

Shorter: allocate function frames on the stack, reify the return function argument closure and call it the continuation. To "reify" means to give reality to something that was previously hidden / implied.

call/cc looks really awesome until you realize that it has / causes problems. One problem is that if you hold on to a reference to a continuation then you're actually preventing GC of all the stack frames captured by that continuation. Then there are thread safety issues... Any language that has variables (as opposed to lexical, immutable bindings) will have thread safety problems. Imagine using closures as callbacks, and having said closures change variables they close over, and now imagine them racing against each other to modify the same variables. Well, in a Scheme continuations are strung-up closures, so now imagine multiple threads trying to call the same continuation. These thread-safety problems run deep, and will tend to push one towards Haskell or Rust.

Really, call/cc is a bit of a parlor trick :) -- a fascinating parlor trick, but still a parlor trick.

If you want much much more detail, I recommend two great books that deal with this subject:

     - On Lisp (by Paul Graham)
     - Lisp In Small Pieces

Lisp In Small Pieces is all about writing Lisp/Scheme interpreters and compilers, so it touches on all of this depth and great detail.

These questions usually have straightforward answers, but some are counterintuitive.

[1] https://web.archive.org/web/20180128095717/http://community....