On Fri, Oct 1, 2021 at 11:58 AM Hendrik Boom <hend...@topoi.pooq.com> wrote:
> On Fri, Oct 01, 2021 at 02:22:14PM +0000, Jesse Alama wrote:
> > Hello,
> >
> > Have you ever wished you could do a C-style return in the middle
> > of a block of Racket code? When you're in the heat of things with
> > a complicated problem where input values need a fair amount of
> > multi-stage extraction and validation, using cond quickly pulls
> > code to the right. My procedure is:
> >
> > * cond/case. In each branch:
> > * Define some new values safe in the knowledge of where you are
> > (extract) and perhaps check them, if necessasry (validate)
> > * Make sure you have an else branch.
> > * return to 1 and repeat as many times as necessary.
> >
> > The result:
> >
> > (cond [(foo? x)
> > (define y (bar x))
> > (define z (jazz x y))
> > (cond [(loopy? z)
> > (define a (yowza z))
> > (cond [(string? a)
> > (define b (bonkers a))
> > (cond [(number? (hoop x b))
> > (define ...)]
> > [else
> > (error 'um)])]
> > [else
> > (error 'ugh)])]
> > [else #f])]
> > [else #f])
>
>
I'm presuming that this code should either return #f, return a calculated
value, or raise an exception. If so, here's a version that runs in plain
racket that I find pretty easy to read. It has the advantage that the
'return #f' parts aren't way far away from what causes them.
(with-handlers ([false? identity] ; return #f
[any/c raise]) ; re-raise everything else
(let* ([x (if (foo? x)
x
(raise #f))]
[z (jazz x (bar x))]
[a (if (loopy? z)
(yowza z)
(raise #f))]
[b (if (string? a)
(bonkers a)
(error 'ugh))])
(if (number? (hoop x b))
'all-good
(error 'um))))
If instead you want to return the exn that comes from error instead of
re-raising it then you can do that by removing the false? clause from the
with-handlers. NOTE: You should re-raise exn:break since otherwise the
user cannot ^C the program.
(with-handlers ([exn:break? raise]
[any/c identity])
...put the let* code here...)
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