Conditionals Cookbook

This subsection is the canonical v0.40.x guide for Nitpick's conditional family: the if / else if / else statement chain and the is (cond) : a : b inline ternary. It supersedes the older, scattered conditional notes in guide/control_flow/ and the if/ternary fragments embedded in guide/basics/, where those pages are shorter or stale.

The v0.40.x cycle locked the shipped behavior of every conditional surface:

• the if statement, the else if chain, and the trailing else
• strict bool conditions (no truthiness; integers/pointers/Result are not conditions on their own)
• the is (cond) : true_value : false_value inline ternary as the only value-producing conditional besides the pick expression
• short-circuit evaluation: exactly one is arm is ever evaluated
• correct interaction with the unused-variable and dead-code warnings (a variable used only inside an is arm or a single if branch is used; code after a branch that exits is dead)
• borrow checking through every is arm and every if branch
• the diagnostics ARIA-IF-001, ARIA-IF-002, ARIA-IF-003, and ARIA-IS-001

Chapters

1. Basics — the conditional mental model, statement vs expression, and when to reach for if vs is vs pick.
2. if / else if / else — the statement chain, block scope, and the order of arm evaluation.
3. Conditions and bool — strict-bool conditions, why there is no truthiness, and raw fn() for Result<bool>.
4. The is ternary — is (cond) : a : b, its type rules, and common patterns.
5. Short-circuit — exactly-one-arm evaluation in is, and how it parallels if.
6. Nested conditionals — nesting if and is, and how to keep chains readable.
7. is vs pick vs when — choosing the right construct for a value, a multi-way branch, or a loop-completion hook.
8. Diagnostics — ARIA-IF-001/002/003 and ARIA-IS-001, what triggers them, and how to fix the underlying mistake.

Quick mental model

• Reach for if (c) { ... } else { ... } to do something conditionally.
• Reach for is (c) : a : b to compute a value conditionally.
• Reach for a pick expression with give for a multi-way value.
• Conditions are strictly bool — there is no truthiness. A function returning Result<bool> must be unwrapped with raw fn() before it is a condition.
• if is a statement: it never produces a value. Writing int32:x = if (c) { ... } is rejected with ARIA-IF-003, which points you at is and pick.
• An is ternary evaluates its condition, then reduces exactly one arm; the other arm is discarded unevaluated.
• A variable used only inside an is arm — or only inside one if branch — is a genuine use, never a false [unused-variable].

Validation snapshot (v0.40.x)

• Compiler non-K CTest: 201/201 (adds bug_tests_v0401–v0406).
• K core tests: 235/235 (adds 229–235 for the is ternary).
• K proofs: unchanged.
• v0.40.x conditional bug regressions: bug951–bug990.
• Conditional diagnostics covered by this cycle: ARIA-IF-001, ARIA-IF-002, ARIA-IF-003, ARIA-IS-001.
• One compiler bug fixed mid-cycle: a mixed-width PHI-node verification failure when an is ternary's two arms had different integer widths (e.g. int16 and int32); the narrower arm is now sign-extended to the common type before the PHI is built.