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 >
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