Exercise 5: Write a function

Aim: give a calculation a name and separate returning from printing. Save ex05.fib. Predict what each call returns before predicting the whole output.

Type and predict

(defun total (price: i64 quantity: i64 delivery: i64) -> i64
  (+ (* price quantity) delivery))

(defun receipt (amount: i64) -> str
  (str "total: " amount))

(defun main () -> i64
  (do
    (println (receipt (total 7 3 4)))
    (println (receipt (total 7 0 4)))
    0))

Each parameter has a name and type. A function body returns its value; no return keyword is needed here. Neither total nor receipt prints anything. Main decides what to print. Keeping this boundary makes calculations testable.

Check your prediction

total: 25
total: 4
0

Change it

1. Call total for three orders using the same function. Check the arithmetic.
2. Add a discounted function taking an amount and a discount, returning their difference. Compose it with total; (discounted (total 7 3 4) 2) must be 23.
3. Call receipt in main without wrapping it in println. Predict what disappears.

Break and repair

Call total with two arguments in a copy. Read the argument-count diagnostic. Then supply three arguments, but use a string for quantity. Explain why adding an argument repairs the first error without repairing the second.

Checkpoint

Draw a table of each function's inputs, result type and side effects. Recreate total from its input/output description without looking at the implementation.

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