Re: manipulating continuations

[Andy Wingo <wingo@pobox.com>]

Hi Thomas,

On Sat 12 Feb 2011 15:10, Thomas Girod <girodt@gmail.com> writes:

> (define (foo)
>   (call/cc
>    (lambda (return)
>      (display "first part")
>      (newline)
>      (call/cc
>       (lambda (cont)
> 	(return cont)))
>      (display "second part")
>      (newline))))
>
> guile> (define c (foo))
> first part
> guile> (c)
> second part
>
> yay, it works. But now, what if I want to run the continuation directly,
> rather than storing it and calling it later ? This is what I get:
>
> guile> ((foo))
> first part
> second part
>
> Backtrace:
> In current input:
>   15: 0* [#<unspecified>]

In this expression, `foo' has returned twice.  The first time it
returned a continuation, which was applied.  The second time it returned
whatever `newline' returned: the unspecified value.  Applying the
unspecified value failed.

If you use Guile 1.9/2.0, partial continuations behave more in the way
you were thinking of:

    (use-modules (ice-9 control))

    (define (foo)
      (% (begin
           (display "first part\n")
           (abort)
           (display "second part\n"))
         (lambda (cont)
           ;; abort jumps back here, to the handler. return the partial
           ;; continuation. 
           cont)))

    scheme@(guile-user)> (foo)
    first part
    $1 = #<partial-continuation 29dede0>
    scheme@(guile-user)> ($1)
    second part
    scheme@(guile-user)> ((foo))
    first part
    second part
    scheme@(guile-user)> 

Have fun with Guile,

Andy
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