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

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
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