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
