Dan Liebgold wrote on 02/16/2017 02:33 PM:
How can I rewrite this so that it either eats the exception or is atomic?
I think the difficulty here is that you don't know for certain why a
`make-directory` failed, so maybe conditionally re-raising the exception
is the way:
(define (make-directory-if-does-not-exist dir)
(or (directory-exists? dir)
(with-handlers ((exn:fail:filesystem?
(lambda (exn)
(if (directory-exists? dir)
;; make-directory exception, but directory
exists
;; now, so assume all is OK.
(void)
;; make-directory exception, but directory
does
;; NOT exist, so re-raise the exception.
(raise exn)))))
(make-directory dir))))
(make-directory-if-does-not-exist "/tmp/my-dir")
BTW, the first `directory-exists?` call is not functionally necessary,
but (since the exact low-level behavior is unspecified) a general rule
of thumb is that it's gentler to do read first, when that might preclude
having to do a write. And you only do the second `directory-exists?` if
you already had to do a likely more expensive `make-directory`.
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