Yes, being forced to use 'stx' as context is suspicious, too. It injects the identifier into the scope and a program may accidentally see it then.
On May 15, 2014, at 7:20 PM, Robby Findler wrote: > We have a work around. (In the larger program there is one is that comes from > the input and one that doesn't. We can just bind it to a pair instead.) > > This program seems quite suspicious. I can't quite bring myself to call it a > bug because it is sucks widely used part of things tho! > > Robby > > On Thursday, May 15, 2014, Matthias Felleisen <[email protected]> wrote: > > > On May 15, 2014, at 5:56 PM, Robby Findler wrote: > >> How does this help us understand the original, strange program? > > > I am providing a work-around because I recognize the idiom. > I can't explain the original. > > > > On May 15, 2014, at 6:23 PM, Spencer Florence wrote: > >> I'm not sure how gensym fixes this: > > > Not #f, stx: > > (define-syntax (make-funny-set! stx) > (syntax-parse stx > [(_ v) #`(define #,(begin (set! funny (datum->syntax stx (gensym))) > funny) v)])) > > > > > > > > > > > > >> >> #lang racket >> >> (require (for-syntax syntax/parse racket/syntax racket/format)) >> >> (define-for-syntax funny #f) >> >> (define-syntax (make-funny-set! stx) >> (syntax-parse stx >> [(_ v) #`(define #,(begin (set! funny (datum->syntax #f (gensym))) >> funny) v)])) >> >> (define-syntax (funny-ref stx) >> (syntax-parse stx >> [(_) funny])) >> >> (define-syntax (funny-set! stx) >> (syntax-parse stx >> [(_ v) #`(set! #,funny v)])) >> >> (make-funny-set! 2) >> (void (void (void (funny-set! 3)))) >> (funny-ref) >> >> errors with: >> >> g27591: unbound identifier; >> also, no #%top syntax transformer is bound in: g27591 >> >> regardless of how many calls to void there are (inclusive of 0). This makes >> sense, as the syntax marks don't add up. The calls to >> "syntax-local-introduce" in the original program aught to solve this, but as >> we saw... `/me points to robby's email` >> >> >> >> And removing the datum->syntax errors with: >> >> funny-ref: received value from syntax expander was not syntax >> received: 'g27876 >> >> Which also makes sense: a symbol is not Syntax. >> >> >> >> >> On Thu, May 15, 2014 at 4:56 PM, Robby Findler <[email protected]> >> wrote: >> How does this help us understand the original, strange program? >> >> Robby >> >> On Thu, May 15, 2014 at 4:51 PM, Matthias Felleisen >> <[email protected]> wrote: >> > >> > Then use gensym instead of g and "~a" instead. >> > >> > >> > >> > On May 15, 2014, at 5:38 PM, Matthias Felleisen <[email protected]> >> > wrote: >> > >> >> >> >> Did you want this: >> >> >> >> #lang racket >> >> >> >> (require (for-syntax syntax/parse racket/syntax)) >> >> >> >> (define-for-syntax funny #f) >> >> >> >> (define-syntax (make-funny-set! stx) >> >> (syntax-parse stx >> >> [(_ v) #`(define #,(begin (set! funny (format-id stx "g")) funny) v)])) >> >> >> >> (define-syntax (funny-ref stx) >> >> (syntax-parse stx >> >> [(_) funny])) >> >> >> >> (define-syntax (funny-set! stx) >> >> (syntax-parse stx >> >> [(_ v) #`(set! #,funny v)])) >> >> >> >> (make-funny-set! 2) >> >> (void (void (void (funny-set! 3)))) >> >> (funny-ref) >> >> >> >> [I had to write such a macro a while back, and the above is roughly what >> >> I remember doing. Note the lexical context] >> >> >> >> >> >> On May 15, 2014, at 5:25 PM, Spencer Florence <[email protected]> wrote: >> >> >> >>> I'm attempting to write a macro which introduces a new id, then another >> >>> macro that set!s that id. >> >>> Example: >> >>> >> >>> #lang racket >> >>> (require (for-syntax syntax/parse racket/syntax)) >> >>> (define-for-syntax funny #f) >> >>> (define-syntax (make-funny-set! stx) >> >>> (syntax-parse stx >> >>> [(_ v) >> >>> (define unmarked (generate-temporary)) >> >>> (set! funny (syntax-local-introduce unmarked)) >> >>> #`(define #,unmarked v)])) >> >>> (define-syntax (funny-ref stx) >> >>> (syntax-parse stx >> >>> [(_) >> >>> funny])) >> >>> (define-syntax (funny-set! stx) >> >>> (syntax-parse stx >> >>> [(_ v) >> >>> #`(set! #,(syntax-local-introduce funny) v)])) >> >>> >> >>> (make-funny-set! 2) >> >>> (funny-set! 3) >> >>> (funny-ref) >> >>> >> >>> This program works as I expect, evaluating to 3. However if I change >> >>> (funny-set! 3) to (void (funny-set! 3)) I get the error: "set!: unbound >> >>> identifier in module in: g1" >> >>> >> >>> I do not get this error if I change (funny-ref) to (void (funny-ref)). >> >>> >> >>> If I look at the expansion of the (void (funny-set! 3)) program in >> >>> drracket's macro stepper the the g1 in (define g1 2) and the g1 in (void >> >>> (set! g1 3)) have the >
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