[racket] prompts and aborts

[Danny Yoo]

I'm a little confused by the tail behavior I'm seeing in
call-with-continuation-prompt and abort-current-continuation. Here's
my test code:

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
#lang racket

(provide (all-defined-out))

(define (f)
(call-with-continuation-prompt
(lambda ()
(abort-current-continuation (default-continuation-prompt-tag)
(lambda ()
(f))))
(default-continuation-prompt-tag)
(lambda (thunk)
(thunk))))

(define (f2)
(call-with-continuation-prompt
(lambda ()
(abort-current-continuation (default-continuation-prompt-tag)
(lambda ()
(f2))))))
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

(I know this is fairly useless code, but I'm using it to test my
implementations of continuation prompts in WeScheme.)

I expect both f and f2 to define infinite loops that consume bounded
stack space. I expect this because, for f, the definition of
call-with-continuation-prompt says that the handler for any aborts
gets called in the same tail-calling context as
call-with-continuation-prompt. For f2, I expected the same behavior.

Under DrRacket 5.0.1 (as well as under regular console racket), I'm
seeing that f has bounded memory, but f2 seems to use unbounded
memory. Why does f2 consume memory?
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[Matthew Flatt]

The default prompt handler is

(lambda (abort-thunk)
(call-with-continuation-prompt abort-thunk prompt-tag #f))

When the `abort-thunk' is `(lambda () (f2))', as in `f2', then the
prompts stack up, because `f2' installs a prompt as well.

Here's an infinite loop that uses the default prompt handler:

(call-with-continuation-prompt
(letrec ([loop
(lambda ()
(abort-current-continuation (default-continuation-prompt-tag)
loop))])
loop))