gen:custom-write macro

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Dimaugh Silvestris

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Sep 1, 2021, 1:26:16 PMSep 1
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Tired of writing:

#:methods gen:custom-write
  [(define write-proc
     (make-constructor-style-printer
      (lambda (x) blablabla)
      (lambda (x) blablabla)))]

I made this macro:

(define-syntax (print-as stx)
  (syntax-case stx []
    {[_ lam1 lam2]
     #'[(define write-proc
          (make-constructor-style-printer
           lam1 lam2))]}))

So I could just:

(struct point (x y)
  #:methods gen:custom-write
  [print-as
   (lambda (obj) 'p)
   (lambda (obj)
     (list [point-x obj] [point-y obj]))])

But I get a bad syntax error. I've tried many subtle changes to no avail. What am I doing wrong?

Sorawee Porncharoenwase

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Sep 1, 2021, 1:42:44 PMSep 1
to Dimaugh Silvestris, Racket list

There are positions that macros can’t operate. https://lexi-lambda.github.io/blog/2018/10/06/macroexpand-anywhere-with-local-apply-transformer/ explains this issue really well, so I recommend you to read it.

There’s also another issue, which is that you want write-proc to be named write-proc, but Racket, which has a hygienic macro system, will “rename” it automatically for you. So you need to explicitly indicate that you want the name write-proc. Like this:

#lang racket

(require racket/struct)

(define-syntax (print-as stx)
  (syntax-case stx ()
    [(_ lam1 lam2)
     #`(define #,(datum->syntax stx 'write-proc)
         (make-constructor-style-printer
          lam1 lam2))]))

(struct point (x y)
  #:methods gen:custom-write
  [(print-as
   (lambda (obj) 'p)
   (lambda (obj)
     (list [point-x obj] [point-y obj])))])

(point 1 2)
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