A simple program in Scheme to solve expressions step by step

sexps.ss

#lang racket


(define expr-operator
  (lambda (expr)
    (car (cdr expr))))


(define expr-operand1
  (lambda (expr)
    (car expr)))


(define expr-operand2
  (lambda (expr)
    (car (cdr (cdr expr)))))


(define build-expr
  (lambda (operator operand1 operand2)
    (list operand1 operator operand2)))


(define get-operator
  (lambda (operator)
    (cond
      ((eq? operator '+) +)
      ((eq? operator '-) -)
      ((eq? operator '*) *)
      ((eq? operator '/) /)
      ((eq? operator '^) expt)))) 


(define solve-step
  (lambda (expr)
    (cond
      ((and (number? (expr-operand1 expr))
            (number? (expr-operand2 expr)))
       ((get-operator (expr-operator expr))
        (expr-operand1 expr)
        (expr-operand2 expr)))
      ((number? (expr-operand1 expr))
       (build-expr (expr-operator expr)
                   (expr-operand1 expr)
                   (solve-step (expr-operand2 expr))))
      ((number? (expr-operand2 expr))
       (build-expr (expr-operator expr)
                   (solve-step (expr-operand1 expr))
                   (expr-operand2 expr)))
      (else 
        (build-expr (expr-operator expr)
                    (solve-step (expr-operand1 expr))
                    (solve-step (expr-operand2 expr)))))))


(define solve
  (lambda (expr)
    (if (number? expr)
      (list expr)
      (cons expr (solve (solve-step expr))))))


(define main
  (lambda ()
    (display "[IN] ")
    (for-each 
     (lambda (x) (display x) (newline))
     (solve (read)))
    (main)))

(main)