But there's no guarantee that if someone imports the original name somewhere, renames it somehow, etc., that they'll copy over a <name>-META binding as well. Using a binding that carries everything with it, or using a syntax-time hash table that carries out of band data that can be looked up based on the original binding, is reliable. Using an extra binding that _should_ be there in most namespaces, can be broken by contexts you have no control over.
Carl Eastlund On Mon, Jan 20, 2014 at 6:31 PM, Scott Klarenbach <sc...@pointyhat.ca>wrote: > If/when it does matter, instead you could use a hashtable on the side, >> mapping from the procedure to the info. Of course that way, you need >> to use `(lookup thing)` to get the info. > > > Not to mention that it's vastly simpler to do it this way. You get a > straightforward transformer proc and a hashtable bound as syntax. > > This is partly a learning exercise for me - so it's valuable to go through > - but the more I look at it the more I think it's a lot of complexity just > to be able to overload the struct name, when it would suffice for match and > other macros that wanted compile-time meta info to follow a convention like > struct-name-META or something. Afterall, structs already come with a lot > of magical bindings that you have to learn by convention, what's one more? > > Just my two cents. > > sk > > > On Mon, Jan 20, 2014 at 1:41 PM, <users-requ...@racket-lang.org> wrote: > >> Send users mailing list submissions to >> users@racket-lang.org >> >> To subscribe or unsubscribe via the World Wide Web, visit >> http://lists.racket-lang.org/users/listinfo >> or, via email, send a message with subject or body 'help' to >> users-requ...@racket-lang.org >> >> You can reach the person managing the list at >> users-ow...@racket-lang.org >> >> When replying, please edit your Subject line so it is more specific >> than "Re: Contents of users digest..." >> >> >> [Racket Users list: >> http://lists.racket-lang.org/users ] >> >> >> Today's Topics: >> >> 1. Re: Structs and syntax-local-value ... how is the struct name >> overloaded? (Carl Eastlund) >> 2. Re: Structs and syntax-local-value ... how is the struct name >> overloaded? (Greg Hendershott) >> >> >> ---------------------------------------------------------------------- >> >> Message: 1 >> Date: Mon, 20 Jan 2014 13:00:35 -0500 >> From: Carl Eastlund <carl.eastl...@gmail.com> >> To: "Alexander D. Knauth" <alexan...@knauth.org> >> Cc: Scott Klarenbach <sc...@pointyhat.ca>, Racket mailing list >> <users@racket-lang.org> >> Subject: Re: [racket] Structs and syntax-local-value ... how is the >> struct name overloaded? >> Message-ID: >> <CAEOPtY09_5EUFpOn6uYBcJAB+TXTdDk=EK-DUHbLU2wu= >> xz...@mail.gmail.com> >> Content-Type: text/plain; charset="utf-8" >> >> Yes, that's exactly it. >> >> Carl Eastlund >> >> >> On Mon, Jan 20, 2014 at 10:13 AM, Alexander D. Knauth >> <alexan...@knauth.org>wrote: >> >> > I'm just curious, is this what you mean? >> > >> > #lang racket >> > >> > (require rackunit >> > (for-syntax >> > syntax/parse)) >> > >> > (begin-for-syntax >> > (struct proc-with-info (proc info) #:property prop:procedure >> > (struct-field-index proc))) >> > >> > (define-syntax thing >> > (proc-with-info (lambda (stx) >> > (syntax-parse stx #:literals (thing) >> > [(thing x ...) >> > #'(#%app thing x ...)] >> > [thing >> > #'(lambda (x) x)])) >> > 'info)) >> > >> > (define-syntax get-info >> > (lambda (stx) >> > (syntax-parse stx >> > [(get-info x) >> > (datum->syntax stx `(quote ,(proc-with-info-info >> > (syntax-local-value #'x))))]))) >> > >> > (check-equal? (thing 1) 1) >> > (let ([x (random)]) >> > (check-equal? (thing x) x)) >> > >> > (check-equal? (get-info thing) >> > 'info) >> > >> > >> > On Jan 20, 2014, at 12:37 AM, Carl Eastlund wrote: >> > >> > It sounds like you've got it. A syntax transformer must be a procedure, >> > and prop:procedure is how you make a procedure that can also be >> something >> > else. So if you want something to be both a syntax transformer and a >> > struct binding, for instance, you need to use prop:procedure and >> > prop:struct-info. You can't make something both a procedure and a >> symbol, >> > because symbols don't work via struct properties, but you could make it >> > both a procedure and a struct that contains a symbol. >> > >> > Carl Eastlund >> > >> > On Mon, Jan 20, 2014 at 12:21 AM, Scott Klarenbach <sc...@pointyhat.ca >> >wrote: >> > >> >> That doesn't look like a complete program; what does #'done refer to? >> >>> And where did the "val is: " printout go? >> >> >> >> >> >> That's just a quick hack for illustration purposes. #''done is just >> >> something to return. (note the two quotes) The output is: >> >> val is: #<procedure:self-ctor-checked-struct-info>#<procedure:posn> >> >> 'done >> >> >> >> But your supposition is correct: posn is always bound as syntax to a >> >>> self-ctor-checked-struct-info-object. That object works as a syntax >> >>> transformer; run time references to posn are transformed into >> references to >> >>> the actual procedure value you're seeing as #<procedure:posn>. >> >> >> >> >> >> Thanks Carl, it's starting to make sense. So the prop:procedure of the >> >> struct is actually the transformer? And so in expression context it >> acts >> >> as a macro, but in syntax-local-value context it acts as a struct? I >> was >> >> trying to produce something similar, but ran into the following issues: >> >> >> >> Say I want (define-syntax (posn) ...) to transform syntax, but I also >> >> want (syntax-local-value #'posn) to return 'something. >> >> Without the struct trick I can only have one but not the other. I >> could >> >> either have (define-syntax posn 'something), and lose the ability to >> call >> >> it as a macro (illegal syntax), or have (define-syntax (posn) >> #'something), >> >> and then (syntax-local-value #'posn) returns the transformer, rather >> than >> >> 'something. >> >> >> >> >> >> >> >> >> >> >> >> On Sun, Jan 19, 2014 at 8:57 PM, Scott Klarenbach <sc...@pointyhat.ca >> >wrote: >> >> >> >>> It's not changing it, I'm just trying to figure out the implementation >> >>> and understand what I'm seeing. >> >>> For example, given this: >> >>> >> >>> (struct posn (x y)) >> >>> >> >>> (define-syntax (test stx) >> >>> (syntax-case stx () >> >>> [(_ x) >> >>> (printf "val is: ~s" (syntax-local-value #'posn)) >> >>> #''done])) >> >>> >> >>> > posn >> >>> #<procedure:posn> >> >>> >> >>> > (test x) >> >>> #<procedure:self-ctor-checked-struct-info> >> >>> >> >>> I'm surprised that the values are different. Is posn actually always >> a >> >>> self-ctor-checked-struct-info object, but it's prop:procedure is >> defined to >> >>> allow for being used in an expression in the first case? >> >>> >> >>> >> >>> >> >>> >> >>> On Sun, Jan 19, 2014 at 8:40 PM, Carl Eastlund < >> carl.eastl...@gmail.com>wrote: >> >>> >> >>>> If syntax-local-value is returning something other than the value you >> >>>> put in, that's a bug. It shouldn't be wrapping it or changing it in >> any >> >>>> way. Do you have a program where you bind something via >> define-syntax that >> >>>> satisfies struct-info?, and get something out via syntax-local-value >> that >> >>>> doesn't? >> >>>> >> >>>> Carl Eastlund >> >>>> >> >>>> On Sun, Jan 19, 2014 at 11:27 PM, Scott Klarenbach < >> sc...@pointyhat.ca>wrote: >> >>>> >> >>>>> But I don't see how the same binding can be a transformer and also >> >>>>> return something else (like a list, or a checked-struct-info-thing) >> via >> >>>>> syntax-local-value. >> >>>>> >> >>>>> If I bind my-fn as a transformer, then any other macros that use it >> >>>>> with syntax-local-value will receive the transformer procedure >> back, not >> >>>>> any special meta data. And if I bind it as meta data directly, ie >> >>>>> (define-syntax my-fn 'something) then it works with >> syntax-local-value but >> >>>>> any attempts to use it as a transformer result in illegal syntax. >> >>>>> >> >>>>> Even if I create a transformer that returns a struct which >> implements >> >>>>> both prop:procedure and prop:struct-info, using that binding with >> >>>>> syntax-local-value will return the transformer procedure itself, >> rather >> >>>>> than the final struct. >> >>>>> >> >>>>> >> >>>>> >> >>>>> On Sun, Jan 19, 2014 at 8:04 PM, Carl Eastlund < >> >>>>> carl.eastl...@gmail.com> wrote: >> >>>>> >> >>>>>> Yes, I believe that the name of a structure defined by "struct" is >> >>>>>> bound at syntax-time to a value that implements both >> prop:procedure, so >> >>>>>> that it can expand to a use of the constructor when used in an >> expression, >> >>>>>> and prop:struct-info so that it can be use to look up static >> information >> >>>>>> when passed to relevant macros. >> >>>>>> >> >>>>>> Carl Eastlund >> >>>>>> >> >>>>>> >> >>>>>> On Sun, Jan 19, 2014 at 11:00 PM, Scott Klarenbach < >> >>>>>> sc...@pointyhat.ca> wrote: >> >>>>>> >> >>>>>>> How is it that the definition of (struct my-name (x y)) can bind >> >>>>>>> *my-name* both as a #<procedure:my-name> at runtime and a >> >>>>>>> transformer-binding *my-name* that at compile time (via >> >>>>>>> syntax-local-value) produces >> #<procedure:self-ctor-checked-struct-info>.? >> >>>>>>> >> >>>>>>> Or, put another way, how can I define a transformer *my-fn* that >> >>>>>>> produces syntax, but that also exposes hidden meta-data under the >> same >> >>>>>>> binding to other macros that might wish to know about the binding >> at >> >>>>>>> compile time? >> >>>>>>> >> >>>>>>> I'm specifically wondering how the overloading works. Is it some >> >>>>>>> clever use of prop:procedure? >> >>>>>>> >> >>>>>>> Thanks. >> >>>>>>> >> >>>>>>> -- >> >>>>>>> Talk to you soon, >> >>>>>>> >> >>>>>>> Scott Klarenbach >> >>>>>>> >> >>>>>>> PointyHat Software Corp. >> >>>>>>> www.pointyhat.ca >> >>>>>>> p 604-568-4280 >> >>>>>>> e sc...@pointyhat.ca >> >>>>>>> 200-1575 W. Georgia >> >>>>>>> Vancouver, BC V6G2V3 >> >>>>>>> >> >>>>>>> _______________________________________ >> >>>>>>> To iterate is human; to recur, divine >> >>>>>>> >> >>>>>>> ____________________ >> >>>>>>> Racket Users list: >> >>>>>>> http://lists.racket-lang.org/users >> >>>>>>> >> >>>>>>> >> >>>>>> >> >>>>> >> >>>>> >> >>>>> -- >> >>>>> Talk to you soon, >> >>>>> >> >>>>> Scott Klarenbach >> >>>>> >> >>>>> PointyHat Software Corp. >> >>>>> www.pointyhat.ca >> >>>>> p 604-568-4280 >> >>>>> e sc...@pointyhat.ca >> >>>>> 200-1575 W. Georgia >> >>>>> Vancouver, BC V6G2V3 >> >>>>> >> >>>>> _______________________________________ >> >>>>> To iterate is human; to recur, divine >> >>>>> >> >>>> >> >>>> >> >>> >> >>> >> >>> -- >> >>> Talk to you soon, >> >>> >> >>> Scott Klarenbach >> >>> >> >>> PointyHat Software Corp. >> >>> www.pointyhat.ca >> >>> p 604-568-4280 >> >>> e sc...@pointyhat.ca >> >>> 200-1575 W. Georgia >> >>> Vancouver, BC V6G2V3 >> >>> >> >>> _______________________________________ >> >>> To iterate is human; to recur, divine >> >>> >> >> >> >> >> >> >> >> -- >> >> Talk to you soon, >> >> >> >> Scott Klarenbach >> >> >> >> PointyHat Software Corp. >> >> www.pointyhat.ca >> >> p 604-568-4280 >> >> e sc...@pointyhat.ca >> >> 200-1575 W. Georgia >> >> Vancouver, BC V6G2V3 >> >> >> >> _______________________________________ >> >> To iterate is human; to recur, divine >> >> >> > >> > ____________________ >> > Racket Users list: >> > http://lists.racket-lang.org/users >> > >> > >> > >> -------------- next part -------------- >> An HTML attachment was scrubbed... >> URL: < >> http://lists.racket-lang.org/users/archive/attachments/20140120/1cbff376/attachment-0001.html >> > >> >> ------------------------------ >> >> Message: 2 >> Date: Mon, 20 Jan 2014 16:40:56 -0500 >> From: Greg Hendershott <greghendersh...@gmail.com> >> To: users@racket-lang.org >> Subject: Re: [racket] Structs and syntax-local-value ... how is the >> struct name overloaded? >> Message-ID: >> <CAGspUn3DeoWR7Qfw=uPamDuV+6ig9asRt=- >> dcajh+uvibbt...@mail.gmail.com> >> Content-Type: text/plain; charset=ISO-8859-1 >> >> >> and prop:procedure is how you make a procedure that can also be >> something >> >> else. >> >> One thing to keep in mind is that there is _some_ overhead. In my >> recent experience, applying via prop:procedure took roughly 1.6X the >> time as applying a plain procedure. (This with the variant of >> prop:procedure where its value is an index to the field in the struct >> holding the procedure.) >> >> I don't know how much of the extra time is due to memory reference, >> missing the optimizer, and/or other? >> >> For many applications that doesn't matter, and it's worth it to have >> `thing` self-evaluate to the info. >> >> If/when it does matter, instead you could use a hashtable on the side, >> mapping from the procedure to the info. Of course that way, you need >> to use `(lookup thing)` to get the info. >> >> >> End of users Digest, Vol 101, Issue 60 >> ************************************** >> > > > > -- > Talk to you soon, > > Scott Klarenbach > > PointyHat Software Corp. > www.pointyhat.ca > p 604-568-4280 > e sc...@pointyhat.ca > 200-1575 W. Georgia > Vancouver, BC V6G2V3 > > _______________________________________ > To iterate is human; to recur, divine > > ____________________ > Racket Users list: > http://lists.racket-lang.org/users > >
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