Exercise 1.04

This the 4th Exercise in SICP

The Question

Exercise 1.04: Observe that our model of evaluation allows for more 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))

The Answer

Now in this case, all that is being done is:

1. Check if b is positive or negative
2. And run (+ a b) if b is positive, or run (- a b) if negative.

In the if statement, notice that it is surrounded by two brackets instead of one. This is how the returned function is run.

So in short:

If b is larger than 0 (positive) (+ a b) is returned, Else (- a b) is returned.