chickadee » continuations

continuations

This library contains two interfaces to continuations, Marc Feeley's and Matt Might's, both with some syntactic sugar.

In Mark Feeley's interface, continuations are a datatype separate from procedures. Hence it provides a continuation? predicate. I've stripped the prefix from the procedures continuation-capture and continuation-graft and renamed continuation-return throw. This latter procedure is accompanied in this module by a macro named catch, so that the usual catch-throw-semantics of other languages is available. But note, that in this pattern, the continuation is a Scheme object, and like every Scheme object it has indefinite extent, hence can be exported, saved in other data-structures etc. So this pattern is much more powerful than the corresponding pattern in other languages.

Some other procedures are provided as well, in particular continuation->procedure, which does what the name says, and continuations, which provides offline documentation to the module. Like in my other modules, the call (continuations) lists the exported symbols, and (continuations sym) provides documentation of the exported symbol sym.

And then there is the infamous goto, which albeit dangerous is sometimes useful, e.g. in backtracking ....

Matt Might's interface is proveded in two flavors, with continuations, checked by continuation?, and escape procedures, checked by escape-procedure?. This makes possible code written in an idiom similar to the setjmp-longjmp-pair in C.

The escape-procedure interface

escape-procedure

escape-procedureprocedure

captures and returns the current continuation as an escape procedure. Typically used as follows

(let ((cc (escape-procedure)))
  (cond
    ((escape-procedure? cc)
     ;; normal body
     ;; possibly calling (cc val) ...
    ((ok? cc)
     ;; exceptional case
     ;; do something with cc ...))

Note, that the let is invoked twice, first after the call to escape-procedure, then with the object val, which was bound to cc by calling (cc val).

escape-procedure?

escape-procedure? xprprocedure

type predicate, defined simultaneously with escape-procedure

call/ep

call/ep receiverprocedure

like call/cc, but reifies the captured continuation as a typed escape-procedure

let/ep

(let/ep k xpr . xprs)syntax

like let/cc from miscmacros, but reifies the captured continuation k as a typed escape-procedure

The continuation interface

continuation

continuationprocedure

captures and returns the current continuation. Typically used as follows

(let ((cc (continuation)))
  (if (continuation? cc)
    ... (throw cc val) ...
    ... do something with val ...))

Note, that the let is invoked twice, first after the call to continuation, then with the object val, which was thrown to cc.

continuation?

continuation? xprprocedure

type predicate

continuation->procedure

continuation->procedure contprocedure

transforms a continuation into a procedure

capture

capture procprocedure

The same as call/cc but with a different datatype: Captures the current continuation as a continuation datatype (contrary to a procedure datatype in call/cc) and calls proc with that continuation as its only argument.

graft

graft cont thunkprocedure

tail-calls thunk with the implicit continuation cont.

throw

throw cont val ....procedure

throws the values val .... to the continuation cont.

catch

(catch cont xpr ....)syntax

The same as let/cc of miscmacros but with a different datatype: Binds the cont variable to the current continuation as a continuation and executes the body xpr .... in this context. Typically used as follows

(catch k
  ...
  (if ...
    (throw k val)
    ...))

goto

goto ccprocedure

The infamous goto, but with a continuation as argument instead of a label.

call

call receiverprocedure

The same as call/cc, but implemented via capture.

continuations

continuations sym ..procedure

documentation procedure

escape procedures

</enscript>

Requirements

none

Examples

(import continuations)

(define 1+ #f)

;; ... with catch
(define (init)
  (+ 1 (catch cont
              (set! 1+ (continuation->procedure cont))
              (throw cont 0))))

;; ... with continuation
(define (init-again)
  (+ 1 (let ((cc (continuation)))
         (cond
           ((continuation? cc)
            (set! 1+ (continuation->procedure cc))
            (throw cc 0))
           (else cc)))))

(define (search ok? lst)
  (catch return
         (for-each (lambda (item)
                     (if (ok? item)
                       (throw return item)))
                   lst)
         #f))

(define (search-with-goto ok? lst)
  (let ((start (continuation)))
    (cond
      ((null? lst) #f)
      ((ok? (car lst)) (car lst))
      (else
        (set! lst (cdr lst))
        (goto start)))))

;; nonlocal return: throw and catch in different procedures
(define (treat ok?)
  (lambda (item cont)
    (if (ok? item)
      (throw cont item))))

(define (handled-search handle lst)
  (catch return
         (for-each (lambda (item)
                     (handle item return))
                   lst)
         #f))

(define (product . nums)
  (let ((cc (escape-procedure)))
    (cond
      ((escape-procedure? cc) ; continuation cc just created
       (print "NORMAL BODY")
       (cond 
         ((null? nums) 1)
         ((zero? (car nums))
          (cc 0))
         (else
           (* (car nums) (apply product (cdr nums))))))
      ((number? cc) ; cc has been thrown a number
       (print "EXCEPTIONAL CASE")
       cc)
      )))

Last update

Mar 31, 2020

Author

Juergen Lorenz

Repository

This egg is hosted on the CHICKEN Subversion repository:

https://anonymous@code.call-cc.org/svn/chicken-eggs/release/5/continuations

If you want to check out the source code repository of this egg and you are not familiar with Subversion, see this page.

License

Copyright (c) 2013-2020, Juergen Lorenz
All rights reserved.

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Version History

1.2

dependency on (chicken memory representation) removed

1.1

call/ep and let/ep added

1.0.1

typo in documentation procedure corrected

1.0

ported and simplyfied from chicken-4 version 1.4.1