When / Then / End Cookbook

This subsection is the canonical v0.39.6 guide for Nitpick's when loop and its two completion clauses, then and end. It complements the loops cookbook: when is a loop, so the shared block, scope, break/continue, label, and invariant rules from that section all apply unchanged here. This guide focuses on what is unique to when — the completion decision.

The v0.39.x cycle locked the shipped behavior of the construct:

• when (cond) { body } is a condition-driven loop, exactly like while.
• an optional then { ... } block runs when the loop exits cleanly — the body completed at least one full iteration and the condition then went false.
• an optional end { ... } block runs when the loop is cut short — a break fired, or the loop ran zero clean iterations.
• continue does not route to end: it re-tests the condition without marking completion, so a later clean pass still reaches then.
• then/end are independent and optional; either, both, or neither may appear, and they bind to the nearest unmatched when.
• the parser rejects malformed clauses with ARIA-WHEN-001..003.

Chapters

1. Overview — the mental model, the truth table, and a first end-to-end example.
2. then and end semantics — the completion decision in detail, including the zero-iteration case.
3. break routing — why break always lands in end, and loan release on the break path.
4. continue — why continue keeps then reachable, and the all-continue corner case.
5. Optional blocks — then-only, end-only, and bare when, and how the fall-through value is observed.
6. Nested when — nesting in body / then / end, innermost binding, and labeled escape through when.
7. Diagnostics — ARIA-WHEN-001..003, what triggers each, and how to fix them.
8. when vs while — when to reach for when over a hand-rolled while with a completion flag.

At a glance

func:failsafe = int32(tbb32:err) {
    exit 1;
};

func:main = int32() {
    int32:i = 0i32;
    int32:result = 0i32;
    when (i < 3i32) {
        i = i + 1i32;
    } then {
        result = 2i32;   // ran: loop exited cleanly
    } end {
        result = 3i32;   // skipped: no break, and iterations happened
    }
    exit result;         // 2
};