Solution to cassidoo newsletter baby lisp problem

baby-lisp.rkt

#lang racket/base

; unit tests
(require rackunit)

; Give (eval expr namespace) access to  
; the current namespace via the label ns
(define-namespace-anchor a)
(define ns (namespace-anchor->namespace a))
 
(define add +)
(define subtract -)
(define multiply *)
(define divide /)

(define (baby-lisp expr)
  (eval expr ns))

(check-equal?
 30 (baby-lisp '(multiply 5 6)))

(check-equal?
 -1 (baby-lisp '(subtract 5 6)))

(check-equal?
 5/6 (baby-lisp '(divide 5 6)))

(check-equal?
 11 (baby-lisp '(add 5 6)))

;5 * (6 + 5 - (5 / 6)) = 50.833...
(define complex-baby-lisp-expr
  '(multiply
    5 (add
     6 (subtract
      5 (divide 5 6)))))

(check-equal?
 50 5/6 (baby-lisp complex-baby-lisp-expr))