4.1.2 · Control is part of meaning

A special form chooses what gets evaluated.

An evaluator cannot treat if like an ordinary procedure because it must select one branch before evaluating that branch.

Guiding question

What would go wrong if an evaluator evaluated both branches of if?

Recognize syntax with its own evaluation rule
Evaluate the predicate before selecting a branch
Observe that an unselected expression performs no work

The if case first evaluates only the predicate. It then calls evaluate on either the consequent or the alternative, never both.

In the first example the alternative divides by zero. The program still returns 60 because the true predicate selects the addition branch and the invalid alternative remains expression data.

SICP code975 of 1,048,576 UTF-8 bytes

(begin
  (define (lookup name environment)
    (cdr (assoc name environment)))
  (define (evaluate expression environment)
    (cond ((number? expression) expression)
          ((symbol? expression) (lookup expression environment))
          ((eq? (car expression) 'if)
           (if (evaluate (cadr expression) environment)
               (evaluate (caddr expression) environment)
               (evaluate (cadddr expression) environment)))
          ((eq? (car expression) '<)
           (< (evaluate (cadr expression) environment)
              (evaluate (caddr expression) environment)))
          ((eq? (car expression) '+)
           (+ (evaluate (cadr expression) environment)
              (evaluate (caddr expression) environment)))
          ((eq? (car expression) '/)
           (/ (evaluate (cadr expression) environment)
              (evaluate (caddr expression) environment)))))
  (evaluate '(if (< score 60) (+ score 5) (/ 1 0))
            '((score . 55))))

Examples

Result—

Output: —
Value: —
Diagnostic: —

Execution trace0 / 0 events

Expected result

The first program returns 60 without evaluating the division by zero.

Trace focus

After the less-than comparison returns true, follow evaluation into the addition branch. No division application appears in this run.

Try it yourself

Change the program and compare the result.

Change score to 70 and replace the alternative with score. Predict which branch evaluate will visit.

Show hint

The host if inside evaluate enforces the evaluation rule of the interpreted if.

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