Re: [racket-users] Beginners question: implementing existing generics for existing structs

[Ronie Uliana]

Thank you, Jon and andmkent!

Jon, I understand the reason. However, I guess it's possible to fix that kind 
of problem using precedence the same way some languages do with traits and 
mixins.

:)

Anyway, thank you again!

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[racket-users] Beginners question: implementing existing generics for existing structs

[Ronie Uliana]

Let's assume I have a struct that is not mine (like ddict 
[https://pkgn.racket-lang.org/package/ddict], or some of pfds 
[https://pkgn.racket-lang.org/package/pfds]). I'd like to implement generics 
for them that also are not mine :p (like data/collection from Alexis King).

1 - Is it possible?
2 - How do I do that?

I searched through generics documentation, but I think I completely missed that 
part =/

[]s an thx!
Ronie

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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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> "Racket Users" group.
> 
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> 
> For more options, visit https://groups.google.com/d/optout.

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] Digest for racket-users@googlegroups.com - 10 updates in 1 topic

[Ronie Uliana]

I made this macro for exactly for the same reason :)

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)

Em ter, 22 de nov de 2016 às 05:24, 
escreveu:

> racket-users@googlegroups.com
> 
>  Google
> Groups
> 
> 
> Topic digest
> View all topics
> 
>
>- What is the Racket equivalent to 'return' for early exit?
><#m_-7182901911078251811_group_thread_0> - 10 Updates
>
> What is the Racket equivalent to 'return' for early exit?
> 
> David Storrs : Nov 21 09:37AM -0800
>
> 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?
> David Storrs : Nov 21 09:41AM -0800
>
> 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:
>
> George Neuner : Nov 21 01:04PM -0500
>
> On 11/21/2016 12:41 PM, David Storrs wrote:
>
> > 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
> Leif Andersen : Nov 21 01:07PM -0500
>
> 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:
>
> "John Clements" : Nov 21 01:30PM -0500
>
> > ... 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?
>
> Playing the role of irritating ideologue, it’s … me!
>
> Yes, there’s let/ec, but my (limited) experience in industry suggests that
> reading functions that use ‘return’ liberally can be massively more
> difficult than one that uses ‘if’ or a related form. I would argue that the
> call/ec doesn’t actually make the code less clunky, it just hides the
> clunkiness, and makes the code harder to read, to boot. In the absence of
> return, you can generally easily deduce when control flow reaches a
> particular point (“we only get here if x is > 0 and the string is empty”).
> In the presence of return, this becomes “we only get here if x is > 0 and
> the string is empty and one of these 19 incomprehensible clauses didn’t
> trigger a return.”
>
> I do think that call/ec makes sense in some circumstances; for instance,
> if you want to bail out of a deeply nested call, and the int

[racket-users] A Racket talk in Rubyconf Brazil

[Ronie Uliana]

Hi, there!

I gave a talk about Racket in Rubyconf Brazil yesterday (saturday 24).

Here  are the slides
translated to English and here
 the original in Portuguese
:)

Not sure I gave the right importance to the right points, but people liked
it a lot.

Thank you all for the amazing job in the Racket!

[]s
Ronie

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[racket-users] Re: Order of argument conventions, or ... conventions of argument ordering

[Ronie Uliana]

I like a lot this advice from Haskell:

"... in Haskell it is not possible to alter objects, but there are many 
functions which return a somehow altered input object. This object should be 
the last parameter because then you can compose a sequence of operations on 
this object" (https://wiki.haskell.org/Parameter_order)

That makes threading macros (~>>) easier to apply, and also to remember the 
argument order. :)

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Re: [racket-users] Is it possible to implement generics to an already defined struct?

[Ronie Uliana]

Thank you, Andrew! :)

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[racket-users] Is it possible to implement generics to an already defined struct?

[Ronie Uliana]

Sorry for the beginner question :(

I'm learning Racket and it is an amazing language. Documentation is great, but 
I couldn't find this.

Let's say I have a struct defined somewhere I have no control:

(struct blah (x y))

Now, in my module, I have this generics:

(define-generics my-thing
  (level my-thing))

Is it possible to make "blah" to implement "my-thing"? So I can use it like:

(level (blah 1 2))

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