Procedures As Arguments

     (arithmetic + 3 4)	⇒ 7
     (arithmetic * 3 4)	⇒ 12
     (arithmetic remainder 30 4) ⇒ 2

     (define arithmetic
       (lambda (operate op1 op2)
         (cond ((equal? operate +) (+ op1 op2))
               ((equal? operate *) (* op1 op2))
               ((equal? operate remainder) (remainder op1 op2)))))

• arithmetic takes an arithmetic operator as an argument, so it is a higher-order procedure.

• equal? is a more general equality predicate than =, which works only on numbers.

• This use of equal? has a procedure as its second argument, so equal? is also a higher-order procedure.

Q: Why is arithmetic inconvenient?