Em segunda-feira, 21 de novembro de 2016 16:08:40 UTC-2, Leif Andersen
escreveu:
> Honestly, I personally like to use let/ec for this. I know it's still using
> continuations, but it is much more lightweight, both syntactically and in
> terms of run-time costs.
>
> (define (do-something)
>
> (let/ec return
> (unless (some-condition)
> (return NO))
>
> (do-the-thing)))
>
> Although honestly, with this pattern, I find that errors work better here, as
> they return early, and you can decide how they get handled with a
> with-handlers.
>
> (define (do-something)
> (unless (some-condition)
> (error "NO"))
> (do-the-thing))
>
> (with-handlers ([exn:fail? (lambda (e) (displayln "I returned early"))])
> (do-something))
>
> But that is specifically because I prefer the workflow of catching errors,
> rather than always having to manually check the output of the function....
> (I'm looking at you C...)
>
> Hope that helps.
>
>
>
>
>
>
>
> ~Leif Andersen
>
>
> On Mon, Nov 21, 2016 at 12:41 PM, David Storrs <david....@gmail.com> wrote:
>
>
> Edit: I know I could also use call/cc and invoke the continuation to escape,
> but that still adds another layer of indentation for something that in the
> normal case won't be called.
>
> It's not a big deal, but I was wondering about it.
>
>
>
>
>
> On Mon, Nov 21, 2016 at 9:37 AM, David Storrs <david....@gmail.com> wrote:
>
>
>
>
> In Perl I would often write:
>
> sub do_something {
> return unless ( some necessary condition is met );
>
> ... do the thing ...
>
> }
>
> In Racket I could wrap the rest of the procedure in an (if), but that adds an
> unnecessary level of indentation and feels clunky. Is there a clean solution?
>
>
>
>
>
>
>
>
>
>
>
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Just notice I replied the digest email instead of post a response. :p
So, let me try again and sorry for the mess...
I made this macro for exactly for the same reason (it's basically Leif's
answer) :)
https://gist.github.com/ruliana/80d7bb46225a22a3682711ca8bd11a1d
@JohnClements, I guess you have a point :) but I think David is mostly talking
about what we call "Guard Clauses"
(http://refactoring.com/catalog/replaceNestedConditionalWithGuardClauses.html),
good or bad, they are a tool for simplifying code. But I agree, don't overdo it
^_^
Macro copied below for convenience (btw, sorry for the code, Racket is not my
main programming language... yet)
#lang racket
(require racket/stxparam)
(provide return
define/guard)
(define-syntax-parameter return
(λ (stx)
(raise-syntax-error (syntax-e stx) "can only used inside
\"define/guard\"")))
(define-syntax-rule (define/guard (name args ...) body0 body ...)
(define (name args ...)
(let/cc ret
(syntax-parameterize ([return (make-rename-transformer #'ret)])
body0
body ...))))
(define/guard (test2 a b)
(when (> a b) (return 456))
(displayln "hehehe")
123)
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