Always allow merge key in mapping (#313)

Author: fisker

src/options.test.ts

@@ -1,12 +1,22 @@
 import { parse } from "./parse.ts";
 
+// `<<` is a special key in YAML 1.1
+// https://github.com/eemeli/yaml/blob/main/src/schema/yaml-1.1/merge.ts
+test(`duplicate '<<' keys should always be allowed`, () => {
+  const text = "<<: 1\n<<: 2";
+  expect(parse(text)).toBeDefined();
+  expect(parse(text, { uniqueKeys: false })).toBeDefined();
+  const ast = parse(text, { uniqueKeys: true });
+  expect(ast).toBeDefined();
+  const node = ast.children[0].children[1].children[0];
+  expect(node?.type).toBe("mapping");
+});
+
 for (const { type, text } of [
   { type: "mapping", text: "a: 1\na: 2" },
-  // `<<` is a special key in YAML 1.1
-  // https://github.com/eemeli/yaml/blob/main/src/schema/yaml-1.1/merge.ts
-  // { type: "mapping", text: "<<: 1\n<<: 2" },
   { type: "flowMapping", text: `{"a":1,"a":2}` },
   { type: "flowMapping", text: `{"a":1,'a':2}` },
+  { type: "flowMapping", text: `{"<<":1,"<<":2}` },
 ]) {
   test(`(${type}): duplicate keys in ${text}`, () => {
     expect(() => parse(text)).toThrowError(`Map keys must be unique`);

src/parse.ts

@@ -18,6 +18,7 @@ export function parse(text: string, options?: ParseOptions): Root {
     // https://eemeli.org/yaml/#options
     uniqueKeys: options?.uniqueKeys,
     lineCounter,
+    merge: true,
   });
   const parsedDocuments: YAML.Document.Parsed[] = [];
   const cstTokens = [...parser.parse(text)];