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From: net@opal.cs.tu-berlin.de (Oliver Laumann)
Newsgroups: comp.lang.scheme
Subject: `return' in Scheme (was: C->Scheme mappings)
Message-ID: <3535@kraftbus.cs.tu-berlin.de>
Date: 31 May 91 09:26:56 GMT
References: <1991May28.191834.15025@apd.mentorg.com> <9105302308.AA18221@cymbal.reasoning.com.>
Organization: Technical University of Berlin, Germany
Lines: 39

In article <9105302308.AA18221@cymbal.reasoning.com.> Gyro@reasoning.com writes:
>    I'm trying to map the various C control statements into
>    Scheme. The translation of break, continue, and return
>    are giving me trouble. [..]
> 
>    I suspect I need to do something with call-with-current-continuation
> 
> You don't need call-with-current-continuation.
> 
> `return' is implicit in Scheme: a lambda body always returns the value of its
> last form.  So you don't have to do anything special with it; e.g.

I suppose what the original poster wanted is to be able to write

   (return expression)

in the middle of a function body to terminate the function and return
the value of an expression.

If Scheme had macros, this could be implemented easily by defining a
new version of `define' that wraps a call/cc around the body of the
function:

  (define-macro (improved-define head . body)
    `(define ,head
      (call-with-current-continuation
        (lambda (return)
	  ,@body))))

`improved-define' could then be used like this:

   (improved-define (redraw window)
     (if (invisible? window)
         (return #f))
     ;; do something complex...
   )

--
Oliver Laumann    net@tub.cs.tu-berlin.de  net@tub.UUCP  ol@gnu.ai.mit.edu