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Fluid bindings

These procedures implement dynamic binding and are in structure fluids. A fluid is a cell whose value can be bound dynamically. Each fluid has a top-level value that is used when the fluid is unbound in the current dynamic environment.

Make-fluid returns a new fluid with value as its initial top-level value. Fluid returns fluid's current value. Let-fluid calls thunk, with fluid bound to value until thunk returns. Using a continuation to throw out of the call to thunk causes fluid to revert to its original value, while throwing back in causes fluid to be rebound to value. Let-fluid returns the value(s) returned by thunk. Let-fluids is identical to let-fluid except that it binds an arbitrary number of fluids to new values.

(let* ((f (make-fluid 'a))
       (v0 (fluid f))
       (v1 (let-fluid f 'b
             (lambda ()
               (fluid f))))
       (v2 (fluid f)))
  (list v0 v1 v2))
  -> '(a b a)

(let ((f (make-fluid 'a))
      (path '())
      (c #f))
  (let ((add (lambda ()
               (set! path (cons (fluid f) path)))))
    (let-fluid f 'b
      (lambda ()
          (lambda (c0)
            (set! c c0)))
    (if (< (length path) 5)
        (reverse path))))
  -> '(a b a b a)

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