Prompts¶

The prompts library (import (prompts)) provides dynamic delimited continuations.

Delimited continuations in Scheme are created by enclosing some procedure in what is called a prompt, so-called because it resembles the REPL prompt. Code running in a prompt has the ability to abort to a prompt handler, passing along its current continuation at the time. Unlike the continuation obtained from call/cc, the continuation of a prompt does not extent beyond the prompt handler.

Dynamic delimited continuations further extend this by allowing prompts to be attached with a prompt tag. The prompt tag allows for code to decide at runtime which prompt they should extend to by invoking abort-to-prompt with the desired prompt tag.

Continuations can be quite mind-bending by themselves, but dynamic delimited continuations take them to a whole new level. Experiment with them and most importantly have fun!

call-with-prompt procedure¶

(call-with-prompt tag thunk handler)

Installs a prompt with the given tag and handler and invokes thunk. tag must be a Symbol.

abort-to-prompt procedure¶

(abort-to-prompt tag)

Calls the handler of the prompt installed with tag with the current continuation such that the current continuation does not extend past the prompt.

Examples¶

 1
 2
 3
 4
 5
 6
 7
 8
 9
10

(import (prompts))

(define print-goodbye
  (call-with-prompt
   'prompt
   (lambda ()
     (display "hello\n")
     (abort-to-prompt 'prompt)
     (display "goodbye\n"))
   (lambda (k) k)))

This code will print "hello", and every subsequent call to print-goodbye will print "goodbye". This code can be thought of as functionally equivalent to the following:

1
2
3
4
5

(define print-goodbye
  (begin
    (display "hello\n")
    (lambda ()
      (display "goodbye\n"))))