hakorune/docs/reference/language/statements.md

Statement Separation and Semicolons

Status: Adopted for Phase 15.3+; parser implementation is staged.

Policy

• Newline as primary statement separator.
• Semicolons are optional and only needed when multiple statements appear on one physical line.
• Minimal ASI (auto semicolon insertion) rules to avoid surprises.

Rules (minimal and predictable)

• Newline ends a statement when:
  • Parenthesis/brace/bracket depth is 0, and
  • The line does not end with a continuation token (+ - * / . , etc.).
• Newline does NOT end a statement when:
  • Inside any open grouping (...), [...], {...}; or
  • The previous token is a continuation token.
• return/break/continue end the statement at newline unless the value is on the same line or grouped via parentheses.
• if/else (and similar paired constructs): do not insert a semicolon between a block and a following else.
• One‑line multi‑statements are allowed with semicolons: x = 1; y = 2; print(y).
• Method chains can break across lines after a dot: obj\n .method() (newline treated as whitespace).

Style guidance

• Prefer newline separation (no semicolons) for readability.
• Use semicolons only when placing multiple statements on a single line.

Examples

// Preferred (no semicolons)
local x = 5
x = x + 1
print(x)

// One line with multiple statements (use semicolons)
local x = 5; x = x + 1; print(x)

// Line continuation by operator
local n = 1 +
          2 +
          3

// Grouping across lines
return (
  1 + 2 + 3
)

// if / else on separate lines without inserting a semicolon
if cond {
  x = x - 1
}
else {
  print(x)
}

// Dot chain across lines
local v = obj
  .methodA()
  .methodB(42)

// Grouped assignment as expression (Stage‑3)
local y = (x = x + 1)    # (x = x + 1) が 1 を返す式として扱われる
if (x = next()) != null {
  print(x)
}

Implementation notes (parser)

• Tokenizer keeps track of grouping depth.
• At newline, attempt ASI only when depth==0 and previous token is not a continuation.
• Error messages should suggest adding a continuation token or grouping when a newline unexpectedly ends a statement.

Parser dev notes (Stage‑1/2)

• return + newline: treat bare return as statement end. To return an expression on the next line, require grouping with parentheses.
• if/else: never insert a semicolon between a closed block and else (ASI禁止箇所)。
• Dot chains: treat . followed by newline as whitespace (line continuation)。
• One‑line multi‑statements: accept ; as statement separator, but formatter should prefer newlines.
• Unary minus: disambiguate from binary minus; implement after Stage‑1（当面は括弧で回避）。

Assignment Expressions（Stage‑3 追加仕様）

Stage‑3 では、制御構造や短絡評価との相性を良くするために「括弧付き代入を式として扱う」最小拡張を導入するよ。

Rules

• x = expr は従来通り 代入文（statement） として扱う。
• '(x = expr)' のように 括弧で囲まれた代入 だけを、値を返す式（expression）として扱う。
  • 値と型は右辺 expr と同じになる（(x = 1) の値は 1）。
• この拡張は Stage‑3 パーサーのみで有効（Rust: NYASH_FEATURES=stage3 / selfhost: --stage3/NYASH_NY_COMPILER_STAGE3=1）。

Examples

# 依然として「文」
x = x + 1

# こちらは「値を返す式」
local y = (x = x + 1)   # y と x の両方が 1 増える

if (x = next()) != null {
  print("got: " + x)
}

return (counter = counter + 1)

Notes（実装指針）

• EBNF 上は assignment_expr := IDENT '=' expr を定義し、factor に '(' assignment_expr ')' を追加する形で表現する。
• lowering では「代入命令 + その結果値を表す SSA 値」を 1 セットで生成し、その ValueId を式の値として返す。
• 仕様を広げすぎないため、当面は「括弧付き代入のみ式扱い」とし、裸の x = expr を expression 文に自動昇格するような拡張は行わない。