Is it a good idea that we can synthesize “original looking code”?
> On Sep 20, 2017, at 4:48 PM, Alexis King <lexi.lam...@gmail.com> wrote:
> To understand this, note that (syntax-original? stx) is only #t when
> *both* of the following things are true:
> 1. stx has the special, opaque syntax property that syntax-original?
> knows how to look for.
> 2. stx has no macro-introduction scopes.
> The first point is satisfied by read-syntax, which attaches the syntax
> property to syntax it reads. The second is a secondary check, which
> ensures macro-introduced syntax will not be treated as original, even
> though the syntax objects produced by quote-syntax *will* have the
> “originalness” property.
> Remember, however, that the way the Racket macro system tracks which
> syntax objects are macro-introduced is by applying the
> macro-introduction scope to the syntax object that is the input to a
> syntax transformer, then flipping the scope on the result. This means
> that one must apply syntax-local-introduce to any syntax objects that
> come from a macro transformer’s input to remove the macro-introduction
> scope. In your example, stx contains the macro-introduction scope for
> the use of exp, so syntax-original? does not treat the syntax as
> original. This slightly modified program exhibits the behavior you
> expect by adding a use of syntax-local-introduce:
> #lang racket
> (define-syntax (exp stx)
> (define a (local-expand (cadr (syntax->list stx)) 'expression '()))
> (define one (syntax-local-introduce (cadr (syntax->list a))))
> (displayln (list one (syntax-original? one)))
> (exp 1)
> Since the macro-introduction scope is flipped by the expander on the
> result of each macro, the syntax object returned by your macro has its
> syntax macro-introduction scope removed, and the macro stepper sees that
> expanded piece of syntax. Therefore, it reports that it is original even
> though your macro does not.
>> On Sep 20, 2017, at 11:48 AM, Spencer Florence
>> <spencerflore...@gmail.com> wrote:
>> The program:
>> #lang racket
>> (define-syntax (exp stx)
>> (define a (local-expand (cadr (syntax->list stx)) 'expression '()))
>> (define one (cadr (syntax->list a)))
>> (displayln (list one (syntax-original? one)))
>> (exp 1)
>> prints `(#<tmp.rkt:10:5 1> #f)`.
>> However if I open the macro stepper in and step to the end, the macro
>> stepper shows the corresponding `1` as `original?: #t`.
>> Why are these different?
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