Pick Cookbook

This subsection of the guide is a focused, example-driven reference for Nitpick's pick construct as it stands at the close of the v0.37.x cycle. It complements the shorter overview in control_flow/pick.md by walking through every dispatch shape pick supports, the exhaustiveness rules, the fall and where arm modifiers, and the v0.37.7 expression form that yields a value.

pick is not a C switch. There is no implicit fallthrough, the catch-all arm is * (never _), arms are comma-separated, and the compiler proves the match is exhaustive at compile time.

Chapters

1. Basics — arm syntax, the * catch-all, no implicit fallthrough, and the failsafe requirement.
2. Integer dispatch & jump tables — literal integer arms, ranges, and when the backend lowers a pick to an LLVM jump table.
3. Enums & exhaustiveness — variant arms, tagged payloads, and how covering every variant makes * optional.
4. Strings — string-literal arms and the npk_string_equals dispatch chain.
5. Balanced ternary (tbb) — the finite trit domain, the ERR sentinel, and sticky-ERR propagation.
6. Fall — explicit labelled fallthrough with fall <label>; and the ARIA-PICK-004 label diagnostics.
7. Nested picks — picks inside arm bodies, tagged-payload inner picks, and how fall resolves to the nearest enclosing pick.
8. Guards (where) — the v0.37.6 where arm modifier, why guarded arms are excluded from exhaustiveness, and binding-scope guard expressions.
9. Expression pick (give) — the v0.37.7 form: pick used as a value, the give <expr>; yield, arm-type unification, and mandatory exhaustiveness.

Each chapter is short and self-contained — read whichever one matches the problem in front of you.

Conventions used in these chapters

• Examples are minimal func:main programs that compile and run with npkc.
• Every program includes a func:failsafe — it is the mandatory error endpoint.
• The catch-all arm is written (*). _ is not a pick wildcard (ARIA-PICK-001); it is the discard binding used elsewhere in the language.
• Diagnostics are quoted as the compiler emits them, with the ARIA-PICK-NNN code.
• Where K semantics is referenced, the corresponding tests/core/ test number is given in parentheses, e.g. (K test 031).

Validation status (v0.37.8)

• Compiler CTest: 181/181.
• K core tests: 215/215.
• Pick-related bug regressions: bug808 … bug859 (v0.37.0 … v0.37.8).
• Diagnostics: ARIA-PICK-001 (_ wildcard), ARIA-PICK-002 (duplicate arm), ARIA-PICK-003 (unreachable arm), ARIA-PICK-004 (bad fall label).