add assertions

Author: pdubroy

packages/compiler/test/test-cstReader.js

@@ -178,6 +178,29 @@ test('withChildren, tupleArity, forEachTuple, and isPresent', async t => {
   });
 });
 
+test('type-specific helpers assert on the wrong handle kind', async t => {
+  const g = await compileAndLoad('G { Start = ("a" "b"?)* }');
+  g.match('ab').use(mr => {
+    const reader = createReader(mr);
+    let list;
+    reader.forEachChild(reader.root, child => {
+      list = child;
+    });
+
+    let terminal;
+    let opt;
+    reader.forEachTuple(list, (a, b) => {
+      terminal = a;
+      opt = b;
+    });
+
+    t.throws(() => reader.ruleId(list), {message: 'Not a nonterminal'});
+    t.throws(() => reader.tupleArity(reader.root), {message: 'Not a list'});
+    t.throws(() => reader.isPresent(terminal), {message: 'Not an opt'});
+    t.true(reader.isPresent(opt));
+  });
+});
+
 // --- unparse via walk ---
 
 test('unparse: simple terminals', async t => {

packages/runtime/src/cstReader.ts

@@ -8,6 +8,7 @@ import {
   MatchRecordType,
   rawMatchRecordType,
 } from './miniohm.ts';
+import {assert} from './assert.ts';
 
 import type {MatchContext, SucceededMatchResult} from './miniohm.ts';
 
@@ -156,16 +157,19 @@ export class CstReader {
 
   /** Rule ID for a nonterminal node. */
   ruleId(handle: number): number {
+    assert(this.type(handle) === CstNodeType.NONTERMINAL, 'Not a nonterminal');
     return this.details(handle);
   }
 
   /** Children per tuple for a list node. */
   tupleArity(handle: number): number {
+    assert(this.type(handle) === CstNodeType.LIST, 'Not a list');
     return this.details(handle);
   }
 
   /** Whether an optional node has a child. */
   isPresent(handle: number): boolean {
+    assert(this.type(handle) === CstNodeType.OPT, 'Not an opt');
     return this.childCount(handle) > 0;
   }