bug#14347: reset, shift, continuation values truncated inconsistently

[Stefan Israelsson Tampe]

Hmm, we have

(let ((k (cal-with-prompt tag (-- expr that ends with (values a b c) --)  (-- expr that is of unknown values length --))))

  (k))

So it looks like peval.scm in language/tree-il does not handle this well.

Looking in that file we have

(($ <prompt> src tag body handler)

       (define (make-prompt-tag? x)

         (match x

           (($ <application> _ ($ <primitive-ref> _ 'make-prompt-tag)

               (or () ((? constant-_expression_?))))

            #t)

           (_ #f)))

       (let ((tag  (for-value tag))

             (body (for-tail body)))

   

       ... cases where we can optimize sue to knowledge of prompt-tag ...

       (else

           (make-prompt src tag body (for-value handler))))))

So to me it looks like peval does a for-tail on body, and uses the value constraint on the body. I'm not sure how to fix this though.

But that this is the problem can be shown by changing the code to

(($ <prompt> src tag body-in handler)

       (define (make-prompt-tag? x)

         (match x

           (($ <application> _ ($ <primitive-ref> _ 'make-prompt-tag)

               (or () ((? constant-_expression_?))))

            #t)

           (_ #f)))

       (let ((tag  (for-value tag))

             (body (for-tail body-in)))

   

       ... cases where we can optimize sue to knowledge of prompt-tag ...

       (else

   (make-prompt src tag (for-values body-in) (for-value handler))))))

/Stefan

On Mon, May 6, 2013 at 9:06 PM, Ian Price <address@hidden> wrote:

Jussi Piitulainen <address@hidden> writes:

> The documentation for reset and shift in the
> manual does not quite say, but I believe the
> captured continuation in these examples should be
> the continuation of the shift _expression_ inside
> the reset _expression_, that is, it should simply
> return the three values in all cases.

Totally agree, this is the behaviour I expect

> scheme@(guile-user)> (let ((k (reset (shift k k) (values 3.1 2 3)))) (k))
> $7 = 3.1

scheme@(guile−user)> (import (only (rnrs) let-values))
scheme@(guile−user)> (let ((k (reset (shift k k) (values 3.1 2 3)))) (k))
$40 = 3.1
scheme@(guile−user)> (let-values (((k) (reset (shift k k) (values 3.1 2 3)))) (k))
$41 = 3.1
$42 = 2
$43 = 3

So, my first suspicion was that there is some part of the code that
receives the multiple values in a let or something, but neither the
code, nor the ,expand command revealed that. However, when we check with
,optimize

(let ((k (call-with-prompt
           ((@@ (ice-9 control) default-prompt-tag))
           (lambda ()
             (apply abort
                    ((@@ (ice-9 control) default-prompt-tag))
                    (lambda (cont)
                      (call-with-prompt
                        ((@@ (ice-9 control) default-prompt-tag))
                        (lambda ()
                          (lambda vals
                            (call-with-prompt
                              ((@@ (ice-9 control) default-prompt-tag))
                              (lambda () (@apply cont vals))
                              (lambda (cont f) (f cont)))))
                        (lambda (cont f) (f cont))))
                    '())
             3.1)
           (lambda (cont f) (f cont)))))
  (k))

Gotcha. The optimizer is getting rid of the multiple values.
On #guile, mark_weaver reminded me of
http://debbugs.gnu.org/cgi/bugreport.cgi?bug=13966 which I think is the
same issue. But I haven't tested that yet.

--
Ian Price -- shift-reset.com

"Programming is like pinball. The reward for doing it well is
the opportunity to do it again" - from "The Wizardy Compiled"