<div dir="ltr">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. </div><div class="gmail_extra"> <div>Carl Eastlund</div> <div class="gmail_quote">On Mon, Jan 20, 2014 at 6:31 PM, Scott Klarenbach <<a href="mailto:scott@pointyhat.ca" target="_blank">scott@pointyhat.ca</a>> wrote: <blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"> <div dir="ltr"><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-color:rgb(204,204,204);border-left-style:solid;padding-left:1ex">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.</blockquote> <div> </div><div>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.</div><div> </div><div>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?</div> <div> </div><div>Just my two cents.</div><div> </div><div>sk</div><div class="gmail_extra"> <div class="gmail_quote">On Mon, Jan 20, 2014 at 1:41 PM, <<a href="mailto:users-request@racket-lang.org" target="_blank">users-request@racket-lang.org</a>> wrote: <blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-color:rgb(204,204,204);border-left-style:solid;padding-left:1ex">Send users mailing list submissions to <a href="mailto:users@racket-lang.org" target="_blank">users@racket-lang.org</a> To subscribe or unsubscribe via the World Wide Web, visit <a href="http://lists.racket-lang.org/users/listinfo" target="_blank">http://lists.racket-lang.org/users/listinfo</a> or, via email, send a message with subject or body 'help' to <a href="mailto:users-request@racket-lang.org" target="_blank">users-request@racket-lang.org</a> You can reach the person managing the list at <a href="mailto:users-owner@racket-lang.org" target="_blank">users-owner@racket-lang.org</a> When replying, please edit your Subject line so it is more specific than "Re: Contents of users digest..." [Racket Users list: <a href="http://lists.racket-lang.org/users" target="_blank">http://lists.racket-lang.org/users</a> ] 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 <<a href="mailto:carl.eastlund@gmail.com" target="_blank">carl.eastlund@gmail.com</a>> To: "Alexander D. Knauth" <<a href="mailto:alexander@knauth.org" target="_blank">alexander@knauth.org</a>> Cc: Scott Klarenbach <<a href="mailto:scott@pointyhat.ca" target="_blank">scott@pointyhat.ca</a>>, Racket mailing list <<a href="mailto:users@racket-lang.org" target="_blank">users@racket-lang.org</a>> Subject: Re: [racket] Structs and syntax-local-value ... how is the struct name overloaded? Message-ID: <CAEOPtY09_5EUFpOn6uYBcJAB+TXTdDk=EK-DUHbLU2wu=<a href="mailto:XzDZw@mail.gmail.com" target="_blank">XzDZw@mail.gmail.com</a>> 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 <<a href="mailto:alexander@knauth.org" target="_blank">alexander@knauth.org</a>>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 <<a href="mailto:scott@pointyhat.ca" target="_blank">scott@pointyhat.ca</a>>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 <<a href="mailto:scott@pointyhat.ca" target="_blank">scott@pointyhat.ca</a>>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 <<a href="mailto:carl.eastlund@gmail.com" target="_blank">carl.eastlund@gmail.com</a>>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 <<a href="mailto:scott@pointyhat.ca" target="_blank">scott@pointyhat.ca</a>>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 < >>>>> <a href="mailto:carl.eastlund@gmail.com" target="_blank">carl.eastlund@gmail.com</a>> 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 < >>>>>> <a href="mailto:scott@pointyhat.ca" target="_blank">scott@pointyhat.ca</a>> 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. >>>>>>> <a href="http://www.pointyhat.ca" target="_blank">www.pointyhat.ca</a> >>>>>>> p <a href="tel:604-568-4280" value="+16045684280" target="_blank">604-568-4280</a> >>>>>>> e <a href="mailto:scott@pointyhat.ca" target="_blank">scott@pointyhat.ca</a> >>>>>>> 200-1575 W. Georgia >>>>>>> Vancouver, BC V6G2V3 >>>>>>> >>>>>>> _______________________________________ >>>>>>> To iterate is human; to recur, divine >>>>>>> >>>>>>> ____________________ >>>>>>> Racket Users list: >>>>>>> <a href="http://lists.racket-lang.org/users" target="_blank">http://lists.racket-lang.org/users</a> >>>>>>> >>>>>>> >>>>>> >>>>> >>>>> >>>>> -- >>>>> Talk to you soon, >>>>> >>>>> Scott Klarenbach >>>>> >>>>> PointyHat Software Corp. >>>>> <a href="http://www.pointyhat.ca" target="_blank">www.pointyhat.ca</a> >>>>> p <a href="tel:604-568-4280" value="+16045684280" target="_blank">604-568-4280</a> >>>>> e <a href="mailto:scott@pointyhat.ca" target="_blank">scott@pointyhat.ca</a> >>>>> 200-1575 W. Georgia >>>>> Vancouver, BC V6G2V3 >>>>> >>>>> _______________________________________ >>>>> To iterate is human; to recur, divine >>>>> >>>> >>>> >>> >>> >>> -- >>> Talk to you soon, >>> >>> Scott Klarenbach >>> >>> PointyHat Software Corp. >>> <a href="http://www.pointyhat.ca" target="_blank">www.pointyhat.ca</a> >>> p <a href="tel:604-568-4280" value="+16045684280" target="_blank">604-568-4280</a> >>> e <a href="mailto:scott@pointyhat.ca" target="_blank">scott@pointyhat.ca</a> >>> 200-1575 W. Georgia >>> Vancouver, BC V6G2V3 >>> >>> _______________________________________ >>> To iterate is human; to recur, divine >>> >> >> >> >> -- >> Talk to you soon, >> >> Scott Klarenbach >> >> PointyHat Software Corp. >> <a href="http://www.pointyhat.ca" target="_blank">www.pointyhat.ca</a> >> p <a href="tel:604-568-4280" value="+16045684280" target="_blank">604-568-4280</a> >> e <a href="mailto:scott@pointyhat.ca" target="_blank">scott@pointyhat.ca</a> >> 200-1575 W. Georgia >> Vancouver, BC V6G2V3 >> >> _______________________________________ >> To iterate is human; to recur, divine >> > > ____________________ > Racket Users list: > <a href="http://lists.racket-lang.org/users" target="_blank">http://lists.racket-lang.org/users</a> > > > -------------- next part -------------- An HTML attachment was scrubbed... URL: <<a href="http://lists.racket-lang.org/users/archive/attachments/20140120/1cbff376/attachment-0001.html" target="_blank">http://lists.racket-lang.org/users/archive/attachments/20140120/1cbff376/attachment-0001.html</a>> ------------------------------ Message: 2 Date: Mon, 20 Jan 2014 16:40:56 -0500 From: Greg Hendershott <<a href="mailto:greghendershott@gmail.com" target="_blank">greghendershott@gmail.com</a>> To: <a href="mailto:users@racket-lang.org" target="_blank">users@racket-lang.org</a> Subject: Re: [racket] Structs and syntax-local-value ... how is the struct name overloaded? Message-ID: <CAGspUn3DeoWR7Qfw=uPamDuV+6ig9asRt=-<a href="mailto:DcAjh%2BuViBbTYnA@mail.gmail.com" target="_blank">DcAjh+uViBbTYnA@mail.gmail.com</a>> 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 ************************************** </blockquote></div> <div> </div>-- Talk to you soon, Scott Klarenbach PointyHat Software Corp. <a href="http://www.pointyhat.ca" target="_blank">www.pointyhat.ca</a> p <a href="tel:604-568-4280" value="+16045684280" target="_blank">604-568-4280</a> e <a href="mailto:scott@pointyhat.ca" target="_blank">scott@pointyhat.ca</a> 200-1575 W. Georgia Vancouver, BC V6G2V3 _______________________________________ To iterate is human; to recur, divine </div></div> ____________________ Racket Users list: <a href="http://lists.racket-lang.org/users" target="_blank">http://lists.racket-lang.org/users</a> </blockquote></div> </div>