Data, Conditions, and Decisions

Programs do useful work when they can distinguish code from data and select one result. Lispex keeps both ideas visible: quote preserves data, while conditionals decide which expression is evaluated.

Quote keeps a form as data

Without a quote, Lispex treats a non-empty list as something to evaluate:

(+ 1 2)

With a quote, the same shape is returned as data:

'(+ 1 2)

(+ 1 2)

You can also construct a fresh list from evaluated values:

(list 'decision 'allow)

(decision allow)

Use quote for fixed literal data. Use list when one or more elements must be computed.

if makes one two-way choice

if has a test, a result for true, and a result for false:

(if (>= 62 50)
    '(decision free-shipping)
    '(decision standard-shipping))

Only the selected branch runs. Only #f is false; 0, "", and () are all true values. Use an explicit predicate when you mean “non-empty,” “positive,” or another domain condition.

and and or stop early

and evaluates from left to right and stops on #f. or stops on its first true value. That makes a compound eligibility test readable:

(and (<= 9 14)
     (not #f))

#t

Short-circuiting is part of the evaluation rule. An expression to the right is not evaluated after the result is already known.

cond names several cases

(define (shipping-decision total member?)
  (cond
    ((>= total 50) '(decision free-shipping))
    (member? '(decision discounted-shipping))
    (else '(decision standard-shipping))))

(shipping-decision 42 #t)

(decision discounted-shipping)

The clauses are tested from top to bottom. A high total wins before membership is considered. else is the fallback. The labelled list tells a reader what the answer means without requiring knowledge of the procedure that produced it.

Inspect a list when you need to

For a proper list, car returns the first value and cdr returns the remaining list:

(define answer '(decision allow))
(list (car answer) (car (cdr answer)))

(decision allow)

Prefer direct construction and predicates in ordinary rules. Manual car/cdr chains are useful for learning and low-level access, but named helpers often communicate intent better.

Exercise

Write age-decision so ages 18 and above return (decision allow) and lower ages return (decision deny under-age).

(define (age-decision age)
  (if (>= age 18)
      '(decision allow)
      '(decision deny under-age)))

(age-decision 17)

Ready for Step 4?

Continue when you can explain the difference between '(decision allow) and (decision allow), name the three positions in if, and say why 0 is not false. Next: First Decision Rule.