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
<greghendersh...@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
____________________
Racket Users list:
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.ham...@dlr.de
____________________
Racket Users list:
http://lists.racket-lang.org/users
