On Fri, Oct 01, 2021 at 02:32:52PM -0400, David Storrs wrote: > 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))))
Yes. But different semantics if bar, yowza, and bonkers have side effects. If they don't, they're quite equivalent. -- hendrik > > 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...) > > -- > You received this message because you are subscribed to the Google Groups > "Racket Users" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to racket-users+unsubscr...@googlegroups.com. > To view this discussion on the web visit > https://groups.google.com/d/msgid/racket-users/CAE8gKodxas7jtze%2BttcFA%2BG0ATKUFZD3rhK%2B%3Dn2U1md1zQPJSg%40mail.gmail.com. -- You received this message because you are subscribed to the Google Groups "Racket Users" group. To unsubscribe from this group and stop receiving emails from it, send an email to racket-users+unsubscr...@googlegroups.com. To view this discussion on the web visit https://groups.google.com/d/msgid/racket-users/20211001195320.4wtga4tjwgo75jhz%40topoi.pooq.com.
