[plt-scheme] local special syntax confusion

From: Ryan Culpepper (ryanc at ccs.neu.edu)
Date: Thu Jan 3 01:20:49 EST 2008

Psy-Kosh wrote:
> First, thank you for taking the time to reply.
> Spent a bit of time reading and thinking about your explanation, and I 
> think I have a _partial_ grasp, but only a partial one.
>> The application isn't in the scope of the nonhygienically introduced 
>> binding, because it's in the right-hand side of 'let-syntax'.
> Sorry, that's one (the main) bit I don't follow. ie, in the let-syntax 
> expression, shouldn't the #%app that's explicitly there be replaced with 
> the syntax object from the with-syntax form? That is, I don't quite 
> follow why it's not. (or did you mean it is, but somehow the implicit 
> #%app later doesn't see it?

Yes, the explicit occurrence of '#%app' in the template is replaced. And 
it affects the '(3 5)' application, because that's in the *body* of the 
'let-syntax' form. The '(+ a b)' expression, however, is in the 
*right-hand side* of the 'let-syntax' expression, which is not in the 
scope of the new '#%app' binding. Instead, it's in the scope of the 
binding of '#%app' as a pattern variable (via 'with-syntax').

(define-syntax (blah stx)
   (syntax-case stx ()
     ((_ exp)
          ((#%app (datum->syntax-object #'exp '#%app)))
        #'(let-syntax ((#%app  ;; <-- this binding of '#%app' is ...
                        (syntax-rules ()
                          ((_ a b) (+ a b))))) ;; <-- not in scope here
            exp))))) ;; <-- in scope here

>> In this case, the reference to '#%app' is implicit. And if it were 
>> explicit, it would be substituted away as a pattern variable. So it's 
>> an unusual case. My advice would be not to use the implicit syntax 
>> names (like '#%app', '#%datum', '#%top') as pattern variable names.
> *blinks* is this a matter of timing? ie, when the implicit #%app is 
> "explicited"?


> If so, why does version #1 work at all? That is, why wouldn't the same 
> principle cause the implicit #%app in the first version to see the 
> standard #%app rather than the locally introduced one?

Yes, and it does.

(let-syntax ((#%app
               (syntax-rules ()
                 ((_ a b) (+ a b)))))  ;; <-- standard #%app
   (3 5)) ;; <-- new #%app

After all, you do want to really apply the '+' procedure to those 
arguments; you have to get to core syntax eventually.

> And again, thank you.

You're welcome.


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