if

Documentation

;;; (if guard consequent alternate) -> any?
;;;   guard : expression?
;;;   consequent : expression?
;;;   alternate : expression?
;;; Evaluate `guard`. If it holds (is not `#f`), evaluate `consequent`
;;; and return its value. If `guard` evaluates to `#f`, evavluate the
;;; `alternate` and return its value.

Formatting / Style

We traditionally format if expressions as follows.

(if guard
    consequent
    alternate)

That is,

• The guard is on the same line as the if.
• The consequent and alternate or on separate lines.

Tracing

In tracing if expressions, we first evaluate the guard. We then replace (if #f consequent alternate) with alternate and (if #t consequent alterante) with `consequent.

For example,

(define square-larger
  (lambda (x y)
    (if (> x y)
        (sqr x)
        (sqr y))))

    (square-larger (+ 1 2) (+ 4 -3))
--> (square-larger 3 (+ 4 -3))
--> (square-larger 3 1)
--> (if (> 3 1) (sqr 3) (sqr 1))
--> (if #t (sqr 3) (sqr 1))
--> (sqr 3)
--> 9

Notes

if expressions usually have exactly one consequent and exactly one alternate. If you want multiple consequents or alternates, you should use a cond expression. If you don’t want an alternate, you should use a when expression.

Examples

Forthcoming

See also

cond, when

Questions

Forthcoming