An engine is a

continuation In computer science, a continuation is an abstract representation of the control state of a computer program. A continuation implements ( reifies) the program control state, i.e. the continuation is a data structure that represents the computati ...

-based construct that provides timed preemption. Engines which can contain other engines are sometimes called Nesters and engines which do not have this ability are then called flat engines or "solo engines". To implement timed preemption there needs to be a clock. This clock can measure real time or simulated time. Simulated time can be implemented in a language like

Scheme A scheme is a systematic plan for the implementation of a certain idea. Scheme or schemer may refer to: Arts and entertainment * ''The Scheme'' (TV series), a BBC Scotland documentary series * The Scheme (band), an English pop band * ''The Schem ...

, by making each function start with decrementing the clock. (define-syntax timed-lambda ((_ formals exp1 exp2 ...) (lambda formals (decrement-timer) exp1 exp2 ...))))

References

Control flow Continuations {{compsci-stub