[racket] Structs and syntax-local-value ... how is the struct name overloaded?

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 <scott at 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 <scott at 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.eastlund at 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 <scott at 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.eastlund at 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 <scott at 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 scott at 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 scott at 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 scott at 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 scott at pointyhat.ca
> 200-1575 W. Georgia
> Vancouver, BC V6G2V3
>
> _______________________________________
> To iterate is human; to recur, divine
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.racket-lang.org/users/archive/attachments/20140120/682ca12f/attachment.html>