Dynamic Wind

(dynamic-wind pre-thunk value-thunk post-thunk) applies its three thunk arguments in order. The value of a dynamic-wind expression is the value returned by value-thunk. The pre-thunk procedure is invoked before calling value-thunk and post-thunk is invoked after value-thunk returns. The special properties of dynamic-wind are manifest when control jumps into or out of the value-thunk application (either due to an exception or a continuation invocation): every time control jumps into the value-thunk application, pre-thunk is invoked, and every time control jumps out of value-thunk, post-thunk is invoked. (No special handling is performed for jumps into or out of the pre-thunk and post-thunk applications.)

When dynamic-wind calls pre-thunk for normal evaluation of value-thunk, the continuation of the pre-thunk application calls value-thunk (with dynamic-wind's special jump handling) and then post-thunk. Similarly, the continuation of the post-thunk application returns the value of the preceding value-thunk application to the continuation of the entire dynamic-wind application.

When pre-thunk is called due to a continuation jump, the continuation of pre-thunk

1. calls more deeply nested pre-thunks, then
2. jumps to the destination continuation, then
3. continues with the context of the dynamic-wind call.

Example:

  (let ([v (let/ec out 
             (dynamic-wind 
              (lambda () (display "in ")) 
              (lambda () 
                (display "pre ") 
                (display (call/cc out)) 
                #f) 
              (lambda () (display "out "))))]) 
    (when v (v "post "))) 
  ; => displays in pre out in post out 

  (let/ec k0
    (let/ec k1
      (dynamic-wind
       void
       (lambda () (k0 'cancel))
       (lambda () (k1 'cancel-canceled)))))
  ; => 'cancel-canceled