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

>
> 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
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