Exercise 6: Keep a collection

Aim: index a vector and observe a persistent update. Save ex06.fib.

Type and predict

(ns main (:use fib.core fib.seq fib.coll))
(defun main () -> i64
  (let ((original [10 20 30])
        (extended (conj original 40)))
    (do
      (println (count original))
      (println (nth original 0))
      (println original)
      (println extended)
      0)))

Square brackets construct a vector. Indexing starts at zero. conj returns a vector with the new item at the end; original still names its old value. The :use declaration introduces the exported core/sequence/collection names.

Check your prediction

3
10
[10 20 30]
[10 20 30 40]
0

Change it

1. Print index 2, then print the last index as (- (count original) 1).
2. Make a second extension of extended. Print all three vectors to show that each earlier holder still sees its earlier value.
3. Replace the numbers with strings and predict the displayed vector.

Break and repair

Ask for index 3 in the original vector. It has no such element. This complete case records the intended runtime failure:

;; spec: docs/hardway/06-vectors.md
;; expect: trap
;; trap: nth: index out of range
(defun main () -> i64 (nth [10 20 30] 3))

Run a copy without those comments if you want to observe the trap directly. Repair the index, then explain why a compiling program can still fail at runtime.

Checkpoint

Write every valid index for a five-element vector. Explain why changing the name extended cannot itself make original contain a fourth item.

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