;; top-level function -- simplify an expression (define (simplify expr) (cond ((number? expr) expr) ; number simplifies to itself ((symbol? expr) expr) ; can't further simplify a symbol either ;; if the expression is an addition or a multiplication, ;; first simplify each argument, using a recursive call, ;; then simplify the sum or product ((plus-expr? expr) (plus-simplify (simplify (second expr)) (simplify (third expr)))) ((times-expr? expr) (times-simplify (simplify (second expr)) (simplify (third expr)))) (else (error "unknown type of expression")))) ;; test whether an expression is an addition (define (plus-expr? expr) (and (list? expr) (eq? (car expr) '+))) ;; test whether an expression is a multiplication (define (times-expr? expr) (and (list? expr) (eq? (car expr) '*)))

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Last modified: Wed Mar 29 20:29:13 PST 2000