[plt-scheme] Catch an unknown method error

From: Robby Findler (robby at cs.uchicago.edu)
Date: Wed Feb 20 16:31:49 EST 2008

On Wed, Feb 20, 2008 at 3:25 PM, Filipe Cabecinhas <filcab at gmail.com> wrote:
>  On 20 Feb, 2008, at 19:19, Robby Findler wrote:
>  > On Feb 20, 2008 1:15 PM, Filipe Cabecinhas <filcab at gmail.com> wrote:
>  >> is it possible, in scheme/class to catch the error for an unknown
>  >> method, from inside the object?
>  >> Kind of Objective-C's forwardInvocation:, Ruby's method_missing (or
>  >> something), and many others ;-)
>  >
>  > There's not any such thing, but maybe if we knew some larger context
>  > for what you're doing, we might able to suggest an alternative.
>  >
>  I'm trying to make some proxies for MysterX (COM classes) in a quick
>  way, for rapid prototyping. If I had that functionality I could just
>  implement the forwardInvocation: method and just change that method
>  call to a COM method call. And then incrementally making the objects
>  more Scheme-like and rearranging the interface.
>  I hope that paragraph isn't very confusing ;-)

It's clear! I guess the easiest thing would be to write a macro that
consumes the names of the methods and generates classes that respond
to those methods. Something like this:

#lang scheme/base

(require scheme/class)

(define-syntax (stub stx)
  (syntax-case stx ()
    [(_ name . names)
     (identifier? #'name)
         (define/public (name . args)
           (do-com-call-here 'name args))
         (stub . names))]

(define (do-com-call-here name args)
  (printf "calling com method ~s with args ~s\n" name args))

(define my-class%
  (class object%
    (stub a b c)
    (stub d e f)
    (define/public (m x) (printf "not a com method\n"))

(define o (new my-class%))
(send o a 1 2 3)
(send o b 'p 'q)
(send o m 12)

>  >> P.S: Where can I find the documentation for the missing sections in
>  >> the PLT Guide (v3.99)
>  >
>  > What you see is what's written, I'm sorry to say. You might try v372
>  > docs for some things, but not for missing parts of the Guide.
>  Hmm. Any docs on reader extensibility or must I read the code? Also,
>  where is that stuff defined, I couldn't find it :s

Oh! everything is documented! Of course. You asked about the Guide
specifically, which isn't complete. But the basic information is all
there (in the reference or in v372's docs (our latest non pre-release)
if the docs aren't ported yet).

Try search in Help Desk for "reader" or "reader macro".


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