feat(syntax): parse sectioned api blocks

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

packages/syntax/src/api-sections.ts

@@ -0,0 +1,79 @@
+/**
+ * Parse the sectioned form of an `api` block body.
+ *
+ * Returns null when no section keyword is present, which is how the legacy
+ * bare-body form stays valid: the caller keeps treating the body as the
+ * response expression.
+ */
+export interface ApiFieldDecl {
+  name: string;
+  optional: boolean;
+  type: string;
+}
+
+export interface ApiSections {
+  parameters: ApiFieldDecl[];
+  body: ApiFieldDecl[];
+  response: string;
+  error: string;
+}
+
+const SECTION_NAMES = ["request", "response", "error"] as const;
+
+/** Slice the balanced `{ ... }` that follows `keyword`, or null when absent. */
+function sectionBody(source: string, keyword: string): string | null {
+  const match = new RegExp(`(^|[^A-Za-z0-9_$])${keyword}\\s*\\{`).exec(source);
+  if (!match) return null;
+
+  const open = source.indexOf("{", match.index + match[1]!.length);
+  let depth = 0;
+
+  for (let index = open; index < source.length; index++) {
+    const character = source[index];
+    if (character === "{") depth++;
+    else if (character === "}") {
+      depth--;
+      if (depth === 0) return source.slice(open + 1, index);
+    }
+  }
+
+  throw new Error(`Unclosed "${keyword}" section in an api block`);
+}
+
+/** `name?: string` -> { name, optional, type }. Blank lines and comments are skipped. */
+function parseFields(source: string): ApiFieldDecl[] {
+  const fields: ApiFieldDecl[] = [];
+
+  for (const rawLine of source.split("\n")) {
+    const line = rawLine.trim().replace(/,$/, "");
+    if (!line || line.startsWith("//")) continue;
+
+    const match = /^([A-Za-z_$][A-Za-z0-9_$]*)(\?)?\s*:\s*(.+)$/.exec(line);
+    if (!match) {
+      throw new Error(`Expected "name: type" in an api request section, got "${line}"`);
+    }
+
+    fields.push({ name: match[1]!, optional: match[2] === "?", type: match[3]!.trim() });
+  }
+
+  return fields;
+}
+
+export function parseApiSections(source: string): ApiSections | null {
+  const present = SECTION_NAMES.some((name) => sectionBody(source, name) !== null);
+  if (!present) return null;
+
+  const request = sectionBody(source, "request");
+
+  return {
+    parameters: request ? parseFields(sectionBody(request, "parameters") ?? "") : [],
+    body: request ? parseFields(sectionBody(request, "body") ?? "") : [],
+    response: sectionBody(source, "response") ?? "",
+    error: sectionBody(source, "error") ?? "",
+  };
+}
+
+/** True when the block declares a `request` section. */
+export function hasRequestSection(source: string): boolean {
+  return sectionBody(source, "request") !== null;
+}