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 dynmaic-wind call.

Normally, the third part of this continuation is never reached, due to the jump in the second part. However, the third part is important because it enables jumps to escape continuations that are contained in the context of the dynamic-wind call. Similarly, when post-thunk is called due to a continuation jump, the continuation of post-thunk calls less deeply nested post-thunks, them jumps to the destination continuation, then continues from the dynamic-wind application.

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