Fix filter sometimes overwriting inner parser's expected values

bwrrp · Mar 29, 2022 · e890fb5 · e890fb5
1 parent 48854f8
commit e890fb5
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Showing 2 changed files with 10 additions and 5 deletions.
diff --git a/src/parser-combinators.ts b/src/parser-combinators.ts
@@ -197,9 +197,6 @@ export function filter<T>(
 	return (input, offset) => {
 		const res = parser(input, offset);
 		if (!res.success) {
-			if (res.offset === offset) {
-				return error(offset, expected);
-			}
 			return res;
 		}
 		if (!filter(res.value)) {

diff --git a/test/parser-combinators.tests.ts b/test/parser-combinators.tests.ts
@@ -107,7 +107,7 @@ describe('parser combinators', () => {
 			expect(res2.offset).toBe(3);
 		});
 
-		it('replaces the expected value for child errors only if the child fails at the same offset', () => {
+		it('retains the expected value for child errors if the inner parser fails', () => {
 			const parser = filter(
 				then(token('a'), token('b'), (a, b) => b + a),
 				() => true,
@@ -116,13 +116,21 @@ describe('parser combinators', () => {
 			const res1 = parser('bb', 0);
 			expect(res1.success).toBe(false);
 			expect(res1.offset).toBe(0);
-			expect((res1 as any).expected).toEqual(['expected']);
+			expect((res1 as any).expected).toEqual(['a']);
 
 			const res2 = parser('aa', 0);
 			expect(res2.success).toBe(false);
 			expect(res2.offset).toBe(1);
 			expect((res2 as any).expected).toEqual(['b']);
 		});
+
+		it('returns the given expected value if the filter rejects', () => {
+			const parser = filter(token('a'), (a) => false, ['expected']);
+			const res1 = parser('a', 0);
+			expect(res1.success).toBe(false);
+			expect(res1.offset).toBe(0);
+			expect((res1 as any).expected).toEqual(['expected']);
+		});
 	});
 
 	describe('or', () => {