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Re: while break and continue
From: |
Kevin Ryde |
Subject: |
Re: while break and continue |
Date: |
Thu, 05 Jun 2003 11:42:46 +1000 |
User-agent: |
Gnus/5.090019 (Oort Gnus v0.19) Emacs/21.2 (gnu/linux) |
Marius Vollmer <address@hidden> writes:
>
> you shouldn't use procedure->memoizing-macro (use
> define-macro instead) and that you inject literal procedures into the
> expanded code.
Ah right, yep. New code and prospective documentation below.
The code is still pretty ugly, I suppose that's what you get for
non-local exits :-). Maybe should have a caveat in the docs about
style, to discourage their use.
I wonder if the value parameter for break should be optional, and have
while return unspecified when not given. That might be a reasonably
common usage. Unless the whole thing is emulating some established
convention.
Incidentally, where would be a good place to put some tests?
syntax.test maybe, or a new file?
(define-macro (while cond . body)
(let ((break-key (gensym " while break-key"))
(continue-key (gensym " while continue-key")))
`(catch ',break-key
(lambda ()
(do ((break (lambda (value) (throw ',break-key value)))
(continue (lambda () (throw ',continue-key))))
((not (catch ',continue-key
(lambda ()
,(if (null? body)
cond ;; avoid some code if body empty
`(and ,cond
(begin
,@body
#t))))
(lambda args #t))))))
(lambda (key value)
value))))
- syntax: while cond body ...
Run a loop executing BODY while COND is true. COND is tested at
the start of each iteration, so if it's `#f' the first time then
BODY is not executed at all. The return value is unspecified.
Within `while' two additional bindings are provided, and can be
used from both COND and BODY.
- Scheme Procedure: break value
Break out of the `while' form, and have it return the given
VALUE.
- Scheme Procedure: continue
Abandon the current iteration, and continue with the next.
Ie. go to the top of the loop, test COND again, etc.
Each `while' provides separate `break' and `continue' procedures,
and they operate on that `while'. So for instance when `while'
loops are nested the outer `break' can be used from within the
inner loop to get all the way out.
(while (test1)
(let ((outer-break break))
(while (test2)
(if (something)
(outer-break #f)))))
Note that each `break' and `continue' procedure can only be used
within the dynamic extent of their corresponding `while'. Once
the `while' has been exited their behaviour is unspecified.