Exercise 10: Handle alternatives

Aim: describe data with named fields and handle optional values explicitly. Save ex10.fib. Predict the successful parse and the failed parse separately.

Type and predict

(ns main (:use fib.core))
(defstruct Order (name: str quantity: i64))

(defun quantity-or-zero (input: str) -> i64
  (match (parse-long input)
    ((some n) n)
    (nil 0)))

(defun main () -> i64
  (let ((order (Order "tea" (quantity-or-zero "12"))))
    (do
      (println (str (. order name) ": " (. order quantity)))
      (println (quantity-or-zero "twelve"))
      0)))

defstruct defines named fields; (. order name) reads one. parse-long returns an Option i64: (some n) has a value, while nil has none. match selects a pattern and binds its payload. Here the function deliberately turns absence into zero; that is a policy, not proof that the input was valid.

Check your prediction

tea: 12
0
0

Change it

1. Try "0", "-3" and "". Explain the difference between a parsed zero and failure even though the original function returns zero for both.
2. Write describe-quantity returning str: return "quantity: N" for a parsed integer and "invalid quantity" otherwise. Test both branches.
3. Define (defenum Availability (InStock quantity: i64) (SoldOut)). Write a description function with a match branch for each variant.

Break and repair

Misspell a field name in a copy. Observe the diagnostic and restore it. Then change the nil branch of quantity-or-zero to a string. Explain why both branches must satisfy the function's declared result type.

Checkpoint

Explain why an Option carries more information than a fallback integer. Draw Order's fields and the two possible shapes of the parse result. Reference: functions and types.

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