Re: [racket-users] Re: What is the Racket equivalent to 'return' for early exit?

[Ronie Uliana]

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  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  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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Re: [racket-users] Re: What is the Racket equivalent to 'return' for early exit?

[David Storrs]

Okay, that makes sense.  Thanks, all.

On Mon, Nov 21, 2016 at 10:07 AM, Leif Andersen 
wrote:

> 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 
> 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 
>> 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?
>>>
>>>
>> --
>> 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.
>> For more options, visit https://groups.google.com/d/optout.
>>
>
>

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Re: [racket-users] Re: What is the Racket equivalent to 'return' for early exit?

[Leif Andersen]

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 
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 
> 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?
>>
>>
> --
> 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.
> For more options, visit https://groups.google.com/d/optout.
>

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Re: [racket-users] Re: What is the Racket equivalent to 'return' for early exit?

[George Neuner]

On 11/21/2016 12:41 PM, David Storrs 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 > 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?




In Racket, IF requires both alternatives be specified.   What's wrong 
with WHEN or UNLESS?


(define (do-something)
(unless (some necessary condition is met)
  ... do the thing(s) ...)
)

George

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