Re: [racket-users] Access pattern variable in macro

['Shakin Billy' via Racket Users]

Thanks a lot.

Am 03.10.2017 9:05 nachm. schrieb "Matthew Butterick" :

>
> > On Oct 3, 2017, at 10:31 AM, 'Shakin Billy' via Racket Users <
> racket-users@googlegroups.com> wrote:
> >
> > how do i access the pattern variable var at compile time (phase 1?)
> > additional to an answer, could you point me to the right part of the
> documentation? i really couldn't find it.
>
>
> `var` isn't a `procedure?` during phase 1. It's just a piece of syntax. So
> your `cond`, which is evaluated in phase 1, will always fail.
>
> But if you shift the `cond` to phase 0, by putting it inside the syntax
> template, it will work:
>
>
> #lang racket
> (require rackunit)
> (define inp (open-input-string "12+34"))
>
> (define-syntax (terminal stx)
>   (syntax-case stx ()
> ((_ name rx)
>  (if (pregexp? (syntax->datum #'rx))
>  #'(define (name)
>  (regexp-try-match rx inp))
>  #'(error "Pregexp expected.")
>
> (define-syntax (prod stx)
>   (syntax-case stx ()
> ((_ prod-name (name var))
>  #'(cond
>  ((procedure? var) 0)
>  (else (error 'PROD-ARGUMENTS "Pregexp oder Procedure
> expected."))
>
> (terminal digit #px"\\d")
> (check-equal? (prod term (op1 digit)) 0)
>
> (define not-a-procedure 42)
> (check-exn exn:fail? (λ () (prod term (op1 not-a-procedure

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Re: [racket-users] Access pattern variable in macro

[Matthew Butterick]

> On Oct 3, 2017, at 10:31 AM, 'Shakin Billy' via Racket Users 
>  wrote:
> 
> how do i access the pattern variable var at compile time (phase 1?)
> additional to an answer, could you point me to the right part of the 
> documentation? i really couldn't find it.


`var` isn't a `procedure?` during phase 1. It's just a piece of syntax. So your 
`cond`, which is evaluated in phase 1, will always fail.

But if you shift the `cond` to phase 0, by putting it inside the syntax 
template, it will work:


#lang racket
(require rackunit)
(define inp (open-input-string "12+34"))

(define-syntax (terminal stx)
  (syntax-case stx ()
((_ name rx)
 (if (pregexp? (syntax->datum #'rx))
 #'(define (name)
 (regexp-try-match rx inp))
 #'(error "Pregexp expected.")

(define-syntax (prod stx)
  (syntax-case stx ()
((_ prod-name (name var))
 #'(cond
 ((procedure? var) 0)
 (else (error 'PROD-ARGUMENTS "Pregexp oder Procedure expected."))

(terminal digit #px"\\d")
(check-equal? (prod term (op1 digit)) 0)

(define not-a-procedure 42)
(check-exn exn:fail? (λ () (prod term (op1 not-a-procedure

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[racket-users] Access pattern variable in macro

['Shakin Billy' via Racket Users]

hello,

how do i access the pattern variable var at compile time (phase 1?)
additional to an answer, could you point me to the right part of the 
documentation? i really couldn't find it.

#lang racket
(define inp (open-input-string "12+34"))

(define-syntax (terminal stx)
  (syntax-case stx ()
((_ name rx)
 (if (pregexp? (syntax->datum #'rx))
 #'(define (name)
 (regexp-try-match rx inp))
 #'(error "Pregexp expected.")

(define-syntax (prod stx)
  (syntax-case stx ()
((_ prod-name (name var))
 (cond ((procedure? (syntax->datum #'var)) #'1)
   ((procedure? #'var) #'2)
   ((procedure? 'var) #'2)
   ((procedure? #''var) #'3)
   ((procedure? #''var) #'4)
   ((procedure? (syntax 'var)) #'5)
   ((pregexp? #'var) #'0)
   (else #'(raise-syntax-error 'PROD-ARGUMENTS "Pregexp oder 
Procedure expected."))

(terminal digit #px"\\d")
(prod term (op1 digit))

thanks for your help! :-)

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