Exercise 4: Make a decision

Aim: test a boundary and both branches of if. Save ex04.fib.

Type and predict

(defun main () -> i64
  (let ((temperature 18)
        (limit 20))
    (do
      (println (< temperature limit))
      (println (= temperature limit))
      (println (if (< temperature limit) "bring a coat" "leave the coat"))
      0)))

Comparisons produce booleans, true or false. if chooses one of its two branches and produces that branch's value. Both branches here produce str. It does not execute both branches and discard one answer.

Check your prediction

true
false
bring a coat
0

Change it

1. Run temperatures 19, 20 and 21. Write the full expected output first.
2. Replace < with <=. Which of those three cases changes?
3. Use (and (>= temperature 10) (< temperature 20)) as the condition. Test 9, 10, 19 and 20.

Break and repair

Replace the string in one branch with a number. The two result types no longer agree. Restore a string in that branch; do not “fix” it by deleting the test.

Checkpoint

Describe the exact interval in drill 3 in words, including both ends. Explain why testing only temperature 18 would miss a boundary mistake.

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