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])
the Racket package parendown (see https://docs.racket-lang.org/parendown/index.html ) can reduce this a lot: (if (not #/ foo? x) #f #/ begin (define y (bar z)) (define z (jazz x y)) #/if (not #/ loopy? z) #f #/ begin (define a (yowza z)) #/if (not (string a)) (error 'ugh) #/begin (define b (bonkers a)) #/if (not (number? (hoop x b))) (error 'um) (define ...) ) Or, using let instead of define (if (not #/ foo? x) #f #/ let ((y (bar z))) #/ let ((z (jazz x y))) #/ if (not #/ loopy? z) #f #/ let ((a (yowza z))) #/ if (not (string a)) (error 'ugh) #/ let ((b (bonkers a))) #/ if (not (number? (hoop x b))) (error 'um) #/ let ((...)) -- and presumably you wanted to do someting inside all there define's ) Is this clearar? There remain * the ugly extra parentheses aroung each let-pair, which is an unnecessary tradition once you use parendown, and * the 'not's after the 'if's. which could be resolved with a three-argument check operator: (check requirement errormessage stufftodoifok) -- essentially an argument-reversed if. but this is enough to eliminate the push off the right side of the page. A heuristic when programming this way: When a function takes several arguments, define it so that the argument that is likely to be textually longer is at the end. -- hendrik > > That's an awful lot of whitespace. We're getting dragged to the > right. Pretty soon we're dealing with lines that have three dozen > spaces at the front or more. > > At times, it can even get a bit silly just how deeply nested code > can get. It can even degrade program comprehension for you & > others. It can even impede your own coding by requiring you to > scroll up to mentally re-construct the state you're in. > > This isn't necessarily a problem with Racket. I think it reflects > of the inherent fussiness of some problems. And certainly, you > can reformat the code differently to make the problem somewhat > less severe. > > Nonetheless, I certainly have found myself envying those working > in other languages where they can just bail out of a complicated > computation by just returning a value. I mean, sure, I could > raise an exception and bail out that way, right? It turns out > there's a way: escape continuations. They permit a kind of > C-style return-y programming in Racket. I've got an article ( > https://click.convertkit-mail.com/92u52qdvngtnh535n9s9/3ohphkhq39xwevtr/aHR0cHM6Ly9saXNwLnNoL2VzY2FwZS1jb250aW51YXRpb25zLWZvci1mdXNzeS1jb2RlLw== > ) up introducing escape continuations and give a simple > real-world example from web programming. > > Happy hacking, > > Jesse > > Unsubscribe ( > https://unsubscribe.convertkit-mail.com/92u52qdvngtnh535n9s9 ) | > Update your profile ( > https://preferences.convertkit-mail.com/92u52qdvngtnh535n9s9 ) | > Moltkestrasse 3d, Mainz, RP 55118 -- 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/20211001155821.hdbqgzfkn6gfjw67%40topoi.pooq.com.
