Conditional define-syntax

Questions & Answers
1

I'm trying to not load an FFI and define all functions when it's not available, defining the functions to be something like 'error' on call.

Is it possible to do a define-syntax based on the value of a variable? e.g., something like this:

(define-ffi-definer def-rktwebview webview-lib)

(define-syntax def-rktwebiew-dummy
  (syntax-rules ()
    ((_ f def)
     (λ args (error "Not implemented")))))

(define define-rktwebview #f)

(if (eq? do-ffi #t)
    (set! define-rktwebview def-rktwebview)
    (set! define-rktwebview def-rktwebview-dummy))
2

You can use #:default-make-fail make-not-available in define-ffi-definer and it will do what I think you want.

1 Like
3

Thanks, I just created this (read the manual of ffi-definer).

(define (make-ffi-repl id err . ret)
  (let ((warned #f)
        (msg (if (eq? do-ffi #t)
                 (string-append
                  "'~a' could not be loaded from "
                  (format "~a" webview-lib-file))
                 (string-append
                  "'~a' could not be loaded.\n"
                  reason)))
        )
    (λ () (λ args
            (if err
                (error (format msg id))
                (begin
                  (unless warned
                    (displayln (format msg id))
                    (set! warned #t))
                  (car ret)))))))

(define-ffi-definer define-rktwebview
                    webview-lib
                    #:default-make-fail (λ (id)
                                          (if (eq? do-ffi #f)
                                              (cond
                                                ((eq? id 'rkt_webview_env)
                                                 (make-ffi-repl id #f #t))
                                                ((eq? id 'rkt_webview_events_waiting)
                                                 (make-ffi-repl id #f 0))
                                                ((eq? id 'rkt_webview_init)
                                                 (make-ffi-repl id #f #t))
                                                ((eq? id 'rkt_webview_cleanup)
                                                 (make-ffi-repl id #f #t))
                                                (else
                                                 (make-ffi-repl id #t))
                                                )
                                              (make-ffi-repl id #t)))
  )