silicon/docs/automatic-semicolon-insertion.html at main · natescode/silicon · GitHub

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<!doctype html>
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  <title>Automatic Semicolon Insertion for Silicon</title>
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<body>
  <main>
    <header>
      <p class="eyebrow">Accepted · ADR 0026</p>
      <h1>Automatic Semicolon Insertion for Silicon</h1>
      <p class="lede">
        Silicon should let a newline terminate a complete statement without
        turning newline into expression syntax. ASI is accepted only where the
        parser already expects a statement terminator.
      </p>
      <div class="notice">
        This page documents the accepted ADR 0026 behavior implemented by the
        handwritten lexer/parser.
      </div>
    </header>

    <section>
      <h2>Motivation</h2>
      <p>
        ADR 0020 moved Silicon toward a bare, line-readable surface:
        definitions are ordinary names, calls are parenthesized, and explicit
        sigils are reserved for language forms. Requiring semicolons after every
        complete item keeps the grammar simple, but it makes ordinary examples
        look heavier than the language intends.
      </p>
      <p>
        The goal is not JavaScript-style recovery. The goal is a narrow rule:
        at statement boundaries, a newline may do the same job as a semicolon.
      </p>
    </section>

    <section>
      <h2>Final Insertion Rules</h2>
      <ol>
        <li>Explicit <code>;</code> stays valid everywhere it is valid today.</li>
        <li>At the top level, a newline or EOF may terminate a complete item.</li>
        <li>Inside a block, a newline may terminate a complete item only when the next token starts another item.</li>
        <li>Do not insert before <code>}</code> by default; a trailing block expression remains the block value.</li>
        <li>Do not insert after incomplete tokens: <code>:=</code>, <code>=</code>, binary operators, commas, <code>::</code>, open delimiters, <code>\\</code>, or signature modifiers.</li>
        <li>Do not insert before continuation tokens: binary operators, commas, <code>.</code>, <code>::</code>, <code>)</code>, or <code>]</code>.</li>
        <li>A call's <code>(</code> must stay on the same logical line as the callee.</li>
        <li>Normal <code>\\</code> signature lines attach to the next definition. Bodyless <code>\\ @extern ...</code> signatures may terminate at line end.</li>
      </ol>
    </section>

    <section>
      <h2>Examples</h2>
      <h3>Top-level items</h3>
      <pre><code class="language-silicon">\\ add (Int, Int) -> Int
@fn add a, b := a + b

answer := add(20, 22)
print_int(answer)</code></pre>

      <h3>Block values</h3>
      <pre><code class="language-silicon">\\ score (Int) -> Int
@fn score n := {
    doubled := n * 2
    capped := @if(doubled &gt; 100, { 100 }, { doubled })
    capped
}</code></pre>
      <p>
        No semicolon is inserted before <code>}</code>, so <code>capped</code>
        remains the block's value.
      </p>

      <h3>Multiline calls</h3>
      <pre><code class="language-silicon">total := add(
    subtotal,
    tax
)

message := format_invoice(
    customer,
    total
)</code></pre>

      <h3>Binary continuations</h3>
      <pre><code class="language-silicon">total := subtotal
    + tax
    + shipping

ready := subtotal
    + tax
    + shipping</code></pre>

      <h3>Signature lines</h3>
      <pre><code class="language-silicon">\\ distance (Point, Point) -> Int
@fn distance a, b := abs(a.x - b.x) + abs(a.y - b.y)

\\ @extern puts (String) -> Int
puts("hello")</code></pre>
    </section>

    <section>
      <h2>Non-examples</h2>
      <div class="grid">
        <div class="card">
          <h3>Call opener moved to the next line</h3>
          <pre><code class="language-silicon">print
("hello")</code></pre>
          <p>The callee and call <code>(</code> are not on the same logical line.</p>
        </div>
        <div class="card">
          <h3>Assignment split after <code>:=</code></h3>
          <pre><code class="language-silicon">value :=
    compute()</code></pre>
          <p>No terminator is inserted after <code>:=</code>; the expression is incomplete.</p>
        </div>
        <div class="card">
          <h3>Comma as a continuation</h3>
          <pre><code class="language-silicon">pair := make_pair(first,
    second)</code></pre>
          <p>No terminator is inserted after a comma.</p>
        </div>
        <div class="card">
          <h3>Namespace continuation</h3>
          <pre><code class="language-silicon">value := math::
    abs(n)</code></pre>
          <p>No terminator is inserted after a namespace separator.</p>
        </div>
      </div>
    </section>

    <section>
      <h2>Parser Implementation Shape</h2>
      <p>
        ASI should live at parser statement boundaries. A helper like
        <code>consumeTerminator()</code> can accept either an explicit semicolon
        or a virtual terminator when newline metadata says the current item is
        complete and the following token is a valid follow token.
      </p>
      <pre><code>parseElement():
    if next_is_signature_line():
        parseSignatureLine()
        consumeSignatureTerminatorIfExtern()
        return

    parseItem()
    consumeTerminator(topLevelFollow)

parseBlockTail():
    parseItem()
    if next_token_is("}"):
        keep_item_as_trailing_block_value()
    else:
        consumeTerminator(blockItemFollow)</code></pre>
      <p>
        The lexer can expose newline tokens or attach newline metadata to tokens.
        It should not globally rewrite newlines into semicolons before the parser
        knows whether it is at an <code>Element</code> or <code>BlockTail</code>
        follow position.
      </p>
    </section>

    <section>
      <h2>Test Matrix</h2>
      <table>
        <thead>
          <tr>
            <th>Case</th>
            <th>Expected result</th>
          </tr>
        </thead>
        <tbody>
          <tr>
            <td>Two top-level definitions separated by newline</td>
            <td>Parses as two items</td>
          </tr>
          <tr>
            <td>EOF after a complete top-level item</td>
            <td>Parses without a trailing semicolon</td>
          </tr>
          <tr>
            <td>Block with final expression before <code>}</code></td>
            <td>No virtual semicolon before <code>}</code>; expression is block value</td>
          </tr>
          <tr>
            <td>Block with multiple newline-separated bindings</td>
            <td>Virtual semicolons between item starts</td>
          </tr>
          <tr>
            <td>Newline after binary operator, comma, <code>:=</code>, <code>=</code>, <code>::</code>, or open delimiter</td>
            <td>No virtual semicolon</td>
          </tr>
          <tr>
            <td>Newline before binary operator, comma, <code>.</code>, <code>::</code>, <code>)</code>, or <code>]</code></td>
            <td>No virtual semicolon</td>
          </tr>
          <tr>
            <td>Callee newline before call <code>(</code></td>
            <td>Parse error</td>
          </tr>
          <tr>
            <td>Normal <code>\\</code> signature followed by definition</td>
            <td>Signature attaches to definition</td>
          </tr>
          <tr>
            <td>Bodyless <code>\\ @extern ...</code> at line end</td>
            <td>Signature line terminates without a following body</td>
          </tr>
        </tbody>
      </table>
    </section>
  </main>
</body>
</html>