[racket] Adjusting behavior based on version of Racket?

For version specific behavior i use this macro. No dynamic require and no  
runtime overhead.

(define-syntax (version>=? stx)
   (syntax-case stx ()
     [(_ min-version body body-else)
      (if (string>=? (version) (syntax-e #'min-version))
          #'body
          #'body-else)]))

(version>=? "5.3.6"
   (begin impl)
   (begin fallback))

Tobias


On Mon, 07 Oct 2013 19:58:03 +0200, Greg Hendershott  
<greghendershott at gmail.com> wrote:

> This is a general question with a specific motivating example.
>
> For 5.3.6 and earlier I wrote this:
>
> https://github.com/greghendershott/frog/blob/master/frog/watch-dir.rkt
>
> ;; This is a poor person's version of OS-specific mechanisms like
> ;; FindFirstFileChangeNotification on Windows and fsevents on OS X.
> ;; Instead this does a checksum of a path's contents, running a thread
> ;; to check periodically.
>
> A thread periodically wakes up, looks for changes, then goes back to  
> sleep.
>
> Next, in the upcoming version of Racket there will be a  
> `filesystem-change-evt`:
>
> http://www.cs.utah.edu/plt/snapshots/current/doc/reference/Filesystem.html?q=change-evt#%28def._%28%28quote._%7E23%7E25kernel%29._filesystem-change-evt%29%29
>
> To detect that _something_ changed, I could use this instead of a
> sleeping thread. Should be more efficient. I would still need to
> determine _what_ changed, so I'd keep using that part of the code. And
> in theory `filesystem-change-evt` might be unsupported on a given OS,
> in which case I should fall back to my status quo code, completely.
>
> And also: If I want to remain compatible with 5.3.6 and earlier, I
> can't statically require `filesystem-change-evt`. If 5.3.5 and
> earlier, obviously another case to use the status quo code,
> completely.
>
> I haven't done this (a runtime check for a Racket-version-dependent
> capability) before. What's the recommended way -- use
> `dynamic-require`?
>
> As in the following pseudo-code (which also is supposed to handle the
> OS-doesn't-support case)?
>
> (define (our-filesystem-change-evt path)
>   ;; An equivalent using thread that sleeps. Works in 5.3.6 and earlier.
>   ....)
>
> (define filesystem-change-evt
>   ;; 1. filesystem-change-evt doesn't exist i.e. 5.3.6 or earlier
>   ;; 2. filesystem-change-evt exists but OS doesn't support it
>   ;; 3. filesystem-change-evt exists and OS supports it
>   (with-handlers ([exn:fail:contract? (const  
> our-filesystem-change-evt)]) ;1
>     (define f (dynamic-require 'racket/base 'filesystem-change-evt))
>     (match (f (build-path 'same) (const our-filesystem-change-evt))
>       [(? procedure? f) f] ;2
>       [(? evt?) f]))) ;3
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>   http://lists.racket-lang.org/users


-- 
---------------------------------------------------------
Tobias Hammer
DLR / Robotics and Mechatronics Center (RMC)
Muenchner Str. 20, D-82234 Wessling
Tel.: 08153/28-1487
Mail: tobias.hammer at dlr.de