fix(repl): don't echo an evaluator's empty result

src/bundles/repl/src/__tests__/stringify_value.test.ts

@@ -1,7 +1,7 @@
 import { DataType, type TypedValue } from '@sourceacademy/conductor/types';
 import { TestDataHandler, booleanValue, emptyListValue, numberValue, stringValue, voidValue } from '@sourceacademy/modules-testplugin';
 import { describe, expect, test } from 'vitest';
-import { stringifyReplValue } from '../stringify_value';
+import { hasNothingToDisplay, stringifyReplValue } from '../stringify_value';
 
 describe(stringifyReplValue, () => {
   test('VOID stringifies as "undefined"', async () => {
@@ -103,3 +103,18 @@ describe(stringifyReplValue, () => {
     expect(result).toContain('...');
   });
 });
+
+describe(hasNothingToDisplay, () => {
+  test('VOID and EMPTY_LIST (a Python None) have nothing to display', () => {
+    expect(hasNothingToDisplay(voidValue())).toBe(true);
+    expect(hasNothingToDisplay(emptyListValue())).toBe(true);
+  });
+
+  test.each([
+    ['BOOLEAN', booleanValue(false)],
+    ['NUMBER', numberValue(0)],
+    ['CONST_STRING', stringValue('')]
+  ])('a %s value is displayed, even when falsy', (_, value) => {
+    expect(hasNothingToDisplay(value)).toBe(false);
+  });
+});

src/bundles/repl/src/index.ts

@@ -54,7 +54,7 @@ import {
   type ReplSetProgramTextMessage
 } from './protocol';
 import { processRichDisplayContent } from './rich_display';
-import { stringifyReplValue } from './stringify_value';
+import { hasNothingToDisplay, stringifyReplValue } from './stringify_value';
 
 type ReplTabLoader = {
   tabs: string[];
@@ -368,10 +368,15 @@ export default class ReplModulePlugin extends BaseModulePlugin {
       }
       const result = step.value;
 
-      const content = result.type === DataType.CONST_STRING
-        ? result.value
-        : await stringifyReplValue(this.evaluator, result);
-      this.__displayOutput({ content, color: COLOR_RUN_CODE_RESULT, outputMethod: 'plaintext' });
+      // An evaluator without a result (a Python function that returns None crosses the boundary
+      // as EMPTY_LIST, see #877) has nothing to echo; showing it as text would print a stray
+      // "null" after everything the evaluator displayed itself.
+      if (!hasNothingToDisplay(result)) {
+        const content = result.type === DataType.CONST_STRING
+          ? result.value
+          : await stringifyReplValue(this.evaluator, result);
+        this.__displayOutput({ content, color: COLOR_RUN_CODE_RESULT, outputMethod: 'plaintext' });
+      }
     } catch (error: unknown) {
       const message = error instanceof Error ? error.message : String(error);
       this.__displayOutput({ content: message, color: COLOR_ERROR_MESSAGE, outputMethod: 'plaintext' });

src/bundles/repl/src/stringify_value.ts

@@ -26,6 +26,17 @@ const MAX_NODES = 10_000;
 
 type Budget = { remaining: number };
 
+/**
+ * Whether an evaluator's result is "no value" and the Repl tab should show nothing for the run,
+ * like Python's REPL does for `None`. A Python `None` crosses the Conductor boundary as
+ * DataType.EMPTY_LIST (it has no type of its own - see issue #877), and a function without a
+ * result as DataType.VOID; rendering either as text would show a bare `null` or `undefined` after
+ * every run of an evaluator that, say, displays its own output.
+ */
+export function hasNothingToDisplay(value: TypedValue<DataType>): boolean {
+  return value.type === DataType.VOID || value.type === DataType.EMPTY_LIST;
+}
+
 export async function stringifyReplValue(evaluator: IDataHandler, value: TypedValue<DataType>): Promise<string> {
   return stringifyReplValueDepth(evaluator, value, 0, { remaining: MAX_NODES });
 }