dynamic-wind -- r7rs Definition

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Kind

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procedure;

Implemented by

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• dynamic-wind (from vonuvoli);

Procedure signature

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Procedure variants:

• (((before procedure-0) (thunk procedure-0) (after procedure-0)) -> (any))
  • inputs:
    • before of type procedure-0;
    • thunk of type procedure-0;
    • after of type procedure-0;
  • output: a value of type any;

Exports

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• scheme:base -- (scheme base);

Exports recursive

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• scheme -- (scheme);

Description

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(dynamic-wind before thunk after)

Calls thunk without arguments, returning the result(s) of this call. Before and after are called, also without arguments, as required by the following rules. Note that, in the absence of calls to continuations captured using call-with-current-continuation, the three arguments are called once each, in order. Before is called whenever execution enters the dynamic extent of the call to thunk and after is called whenever it exits that dynamic extent. The dynamic extent of a procedure call is the period between when the call is initiated and when it returns. The before and after thunks are called in the same dynamic environment as the call to dynamic-wind. In Scheme, because of call-with-current-continuation, the dynamic extent of a call is not always a single, connected time period. It is defined as follows:

• The dynamic extent is entered when execution of the body of the called procedure begins.

• The dynamic extent is also entered when execution is not within the dynamic extent and a continuation is invoked that was captured (using call-with-current-continuation) during the dynamic extent.

• It is exited when the called procedure returns.

• It is also exited when execution is within the dynamic extent and a continuation is invoked that was captured while not within the dynamic extent.

If a second call to dynamic-wind occurs within the dynamic extent of the call to thunk and then a continuation is invoked in such a way that the afters from these two invocations of dynamic-wind are both to be called, then the after associated with the second (inner) call to dynamic-wind is called first.

If a second call to dynamic-wind occurs within the dynamic extent of the call to thunk and then a continuation is invoked in such a way that the befores from these two invocations of dynamic-wind are both to be called, then the before associated with the first (outer) call to dynamic-wind is called first.

If invoking a continuation requires calling the before from one call to dynamic-wind and the after from another, then the after is called first.

The effect of using a captured continuation to enter or exit the dynamic extent of a call to before or after is unspecified.

(let ((path '())
      (c #f))
  (let ((add (lambda (s)
               (set! path (cons s path)))))
    (dynamic-wind
      (lambda () (add 'connect))
      (lambda ()
        (add (call-with-current-continuation
               (lambda (c0)
                 (set! c c0)
                 'talk1))))
      (lambda () (add 'disconnect)))
    (if (< (length path) 4)
        (c 'talk2)
        (reverse path))))
    ===> (connect talk1 disconnect
               connect talk2 disconnect)

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The text herein was sourced and adapted as described in the "R7RS attribution of various text snippets" appendix.

Referenced-types

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• procedure-0;
• any;