Suppose I have a struct which implements a generic interface. I'll copy and
paste the interface from the guide for the example. What I'd like to do is
have automatically-introduced syntax which allows you to skip a lot of the
struct-accessor forms. So for instance, for a struct like
(struct test (a b c))
I'd like, in a method, to be able to say
(define (some-method genericable)
(define-accessors (a b)))
Which is essentially a (define-values ...) expression, taking care of the
messy work of typing out all the accessor methods like (test-a genericable).
I have this example, which doesn't raise any syntax errors, and looks
exactly like I'd expect from the macro stepper's output, yet fails when run:
#lang racket/base
(require (for-syntax racket/base)
racket/generic)
(define-generics printable
(gen-print printable [port])
(gen-port-print port printable)
(gen-print* printable [port] #:width width #:height [height])
#:defaults ([string?
(define/generic super-print gen-print)
(define (gen-print s [port (current-output-port)])
(fprintf port "String: ~a" s))
(define (gen-port-print port s)
(super-print s port))
(define (gen-print* s [port (current-output-port)]
#:width w #:height [h 0])
(fprintf port "String (~ax~a): ~a" w h s))]))
(define-syntax (pstruct stx)
(syntax-case stx ()
((_ struct-name (accessors ...) [body ...])
(with-syntax ((sv (datum->syntax stx 'struct-values)))
#'(struct struct-name (accessors ...)
#:methods gen:printable
[(define-syntax (sv stx)
(syntax-case stx ()
((_ (acc (... ...)))
(let ((accs (for/list ((id (syntax->datum #'(acc (...
...)))))
(string->symbol (format "~a-~a" 'struct-name
id)))))
(with-syntax ((s (datum->syntax stx 'ps))
(as (datum->syntax stx accs)))
#'(define-values (acc (... ...)) (apply values (map
(λ(proc) (proc s)) as))))))))
body ...])))))
(pstruct test (a b c)
[(define (gen-print ps)
(struct-values (a))
a)])
Welcome to DrRacket, version 6.2 [3m].
Language: racket/base [custom]; memory limit: 1024 MB.
> (gen-print (test 1 2 3))
. . gen-print: not implemented for #<test>
How am I going wrong here?
Thanks,
Deren
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