On Fri, Oct 01, 2021 at 02:32:52PM -0400, David Storrs wrote: > On Fri, Oct 1, 2021 at 11:58 AM Hendrik Boom <[email protected]> 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 [email protected]. > 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 [email protected]. To view this discussion on the web visit https://groups.google.com/d/msgid/racket-users/20211001195320.4wtga4tjwgo75jhz%40topoi.pooq.com.
