Design Pattern for Auxiliary Function

A design pattern for recursive auxiliary functions in Lisp:

(define (myfun arg)
      (myfunb arg initanswer initstate ))

(define (myfunb arg answer state )
      (if (done arg state)
           answer
           (myfunb (newarg arg)
                (combine arg answer)
                (smaller state))))

In this pattern,

• initanswer and initstate are initial values for the answer and state variables.

• (done arg state) is a test to determine whether the problem is finished. If so, answer is returned.

• (newarg arg) may produce a new argument.

• (combine arg answer) computes an updated answer.

• (smaller state) computes a new value of state, e.g. by decrementing a counter.

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