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An engine represents computation that is subject to timed preemption. In other words, an engine's underlying computation is an ordinary thunk that runs as a timer-preemptable process.

An engine is called with three arguments: (1) a number of time units or ticks, (2) a success procedure, and (3) a failure procedure. If the engine computation finishes within the allotted ticks, the success procedure is applied to the computation result and the remaining ticks. If the engine computation could not finish within the allotted ticks, the failure procedure is applied to a new engine representing the unfinished portion of the engine computation.

For example, consider an engine whose underlying computation is a loop that printed the nonnegative integers in sequence. It is created as follows, with the soon-to-be-defined make-engine procedure. make-engine is called on an argument thunk representing the underlying computation, and it returns the corresponding engine:

(define printn-engine
    (lambda ()
      (let loop ((i 0))
        (display i)
        (display " ")
        (loop (+ i 1))))))

Here is a call to printn-engine:

(define *more* #f)
(printn-engine 50 list (lambda (ne) (set! *more* ne)))
=> 0 1 2 3 4 5 6 7 8 9

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