[plt-scheme] finally

Eli Barzilay writes:
 > On Dec 19, Doug Orleans wrote:
 > > What is the PLT equivalent of Java's "finally"?  I think I could write
 > > something using dynamic-wind, but is there a more efficient way to do
 > > it if you just want to guard exits and not entrances?
 > 
 > You can give dynamic-wind `void' as a first argument.

Well, yes, I knew that, I was just wondering if there was something
more efficient/idiomatic.  Anyway, here's my macro:

  (define-syntax try
    (syntax-rules (finally)
      ((_ expr catch-clause ... (finally finally-expr ...))
       (dynamic-wind void
           (lambda () (try expr catch-clause ...))
           (lambda () finally-expr ...)))
      ((_ expr ((exn? exn) catch-expr ...) ...)
       (with-handlers ((exn? (lambda (exn) catch-expr ...))
                       ...)
         expr))))

> (try (/ 1 0)
    ((exn:fail? exn)
     (printf "caught: ~a~%" (exn-message exn))
     0)
    (finally (printf "hi~%")))
caught: /: division by zero
hi
0
> (try (/ 1 10)
    ((exn:fail? exn)
     (printf "caught: ~a~%" (exn-message exn))
     0)
    (finally (printf "hi~%")))
hi
1/10


--dougo at place.org