Nice filesystem testing pattern

I've come across a useful pattern for unit testing with the filesystem: frequently I want to create some "sandbox" directory where I'll stick a bunch of temporary files and subdirectories for testing, and when the test is over, I want to delete the whole thing. But any intermediate files or directories I create during testing should not be deleted, because I may still need them later in the test. So only the first directory I create should be automatically deleted.

This requires two things: 1) a macro for creating a new directory and evaluating subexpressions in the context of that directory, and 2) a facility for enabling and disabling the automatic deletion of temporary directories.

(define keep-new-directories? (make-parameter #f))

(define-syntax in-new-directory
 (syntax-rules ()
   [(_ dir-e e1 e2 ...)
    (let ([dir dir-e])
      (rm-rf dir)
      (make-directory dir)
      (let ([result (parameterize ([current-directory dir]
                                   [keep-new-directories? #t])
                      e1 e2 ...)])
        (unless (keep-new-directories?)
          (rm-rf dir))
        result))]))

Another nice property of this pattern is that if I want to test my test, i.e., to inspect the files it's creating, I can temporarily disable the automatic deletion of the sandbox directory by wrapping the whole thing in a (parameterize ([keep-new-directories? #t]) ...).

Meta-conclusion: dynamic scope is useful.