Exercise 11: Follow ownership

Aim: observe a borrowed read and an escaping captured value. Save ex11.fib. You will inspect compiler decisions, not add manual retain/release calls.

Type and predict

(ns main)
(defun size (s: str) -> i64 (str-len s))

(defun greeting (name: str) -> (fn () str)
  (fn () (str "hello, " name)))

(defun main () -> i64
  (let ((name "Ada")
        (n (size name))
        (say (greeting name)))
    (do
      (println n)
      (println name)
      (println (say))
      0)))

size reads its argument. That read does not stop the caller using name again. greeting returns a closure: the captured name must remain alive after that call returns. Fibber borrows for reads and counts escaping values; the compiler chooses. The type (fn () str) describes a function taking no arguments and returning a string.

Check your prediction

3
Ada
hello, Ada
0

Inspect and change

fibc explain ex11.fib

1. Find size and greeting in the explanation. Record the decisions around reading and capturing name. The exact diagnostic layout is not a fixture.
2. Call say twice. Predict whether the first call consumes the greeting.
3. Produce two greetings with different names and call them in reverse order. Require each closure to retain its own name.

Break and repair

Replace (say) with (say 1) in a copy. The closure's function type takes no arguments. Repair the call and explain why ownership checking does not replace ordinary type/argument checking. Exercise 12 will provoke an ownership-specific rejection with two in-out uses of the same place.

Checkpoint

Explain what must survive after greeting returns, and why a borrowed read is different from a value kept by a returned closure. Consult the ownership guide if your explanation relies on garbage collection or programmer-written lifetimes.

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