ADR-132: else if Is an else Whose Block Is One if, and It Is Written Without the Braces

Status: Accepted Date: September 19, 2026 Related: ADR-301 (a condition is an ordinary expression, and the brace rule), ADR-276 (what a keyword costs), ADR-276 (a function that never returns)

1. Context

if a { … } else if b { … } else { … } was a parse error — expected {, found if — and every language a reader comes from has it. The examples worked around it with nested blocks and sequential ifs (http’s status_line is three ifs in a row that mean one), which reads as three decisions where there is one. Nothing in the language argued against the form; it was never written down.

2. Decisions

D1 — After else, an if may stand where a block would.

let grade = if score >= 90 {
    "A"
} else if score >= 80 {
    "B"
} else {
    "C"
}

An else if is an else whose block holds exactly one if expression, with the braces of that block left out. Nothing else is added: the chain is an if inside an if, so every rule of if holds at every link — the condition is an ordinary expression with the brace rule of ADR-301 D4 (an expression that starts with { is parenthesised), the branches of a chain whose value is taken agree on one type and the chain ends with an else, and a return inside a branch leaves the function (Part I 3.1).

A chain may be as long as it likes. else if is two words with whitespace between them, not a keyword: no word is added to the language, and elseif is a name like any other, which NK1117 says nothing declares.

D2 — Lowering is Rust’s own else if, and the formatter keeps the chain flat.

The emitter writes else if where the else block holds one if and nothing else, so the generated Rust reads as the source does; an else whose block holds an if and other statements stays a block. nikaia fmt writes } else if cond { on one line, and never unfolds a chain into nested blocks or folds nested blocks into a chain — the author chose one.

3. Consequences

• One decision reads as one decision.
• No keyword, no new rule about conditions or values: the chain inherits every one from if.
• What it costs: one alternative in the parser’s else rule, and the emitter’s flat form.

4. What this does not decide

• else match, or an else followed by any other expression than if. Those stay a block with the expression inside.
• An if without else as a value, which Part I 3.2 already answers.

This site is open source. Improve this page.