isolate in SMLofNJ.Cont

Question

I was reading about continuations in Standard ML (SMLofNJ.Cont). I understood what callcc and throw does, but could not understand isolate. The documentation says

Discard all live data from the calling context (except what is reachable from f or x), then call f(x), then exit. This may use much less memory then something like f(x) before exit().

However this does not make any sense to me. I just wanted to know what this function does, with some examples.

Answer 1

MLton does a better job of explaining an implementation of isolate using callcc and throw:

val isolate: ('a -> unit) -> 'a t =
  fn (f: 'a -> unit) =>
  callcc
  (fn k1 =>
   let
      val x = callcc (fn k2 => throw (k1, k2))
      val _ = (f x ; Exit.topLevelSuffix ())
              handle exn => MLtonExn.topLevelHandler exn
   in
      raise Fail "MLton.Cont.isolate: return from (wrapped) func"
   end)

We use the standard nested callcc trick to return a continuation that is ready to receive an argument, execute the isolated function, and exit the program. [...]

The page continues to explain how to achieve the same effect with less space leaking.

MLton's CONT signature has a different documentation line than SML/NJ's CONT signature:

• isolate f creates a continuation that evaluates f in an empty context.

  This is a constant time operation, and yields a constant size stack.