Exercise from SICP:

Exercise 1.4.

Observe that our model of evaluation allows for combinations whose operators are compound expressions. Use this observation to describe the behavior of the following procedure:

(define (a-plus-abs-b a b)
  ((if (> b 0) + -) a b))

Solution

There are two possible branches, if b is positive then we get:

((if (> b 0) + -) a b)
((if #t + -) a b)
(+ a b)

Wheras if b is non-positive we get

((if (> b 0) + -) a b)
((if #f + -) a b)
(- a b)

So we always get a added to the absolute value of b. This is a nice use of a first order function.