fix: correctly convert variable character access to letter_of block

- In variables.js, only handle '[]' if the receiver is a list block.
- In operators.js, allow 'variable' type in '[]' handler to convert to 'operator_letter_of'.
- This fixes a bug where $my_variable[0] was incorrectly converted to a list item block.

Fixes #464
Co-Authored-By: Gemini <noreply@google.com>

Author: takaokouji

src/lib/ruby-to-blocks-converter/operators.js

@@ -15,7 +15,7 @@ const OperatorsConverter = {
             return converter._changeBlock(args[0], 'operator_random', 'value');
         });
 
-        converter.registerOnSend(['string', 'block'], '[]', 1, params => {
+        converter.registerOnSend(['string', 'block', 'variable'], '[]', 1, params => {
             const {receiver, args} = params;
             if (!converter._isNumberOrBlock(args[0])) return null;
 

src/lib/ruby-to-blocks-converter/variables.js

@@ -160,9 +160,13 @@ const VariablesConverter = {
             const {receiver, args} = params;
             if (!converter._isNumberOrBlock(args[0])) return null;
 
-            const block = converter._changeBlock(receiver, 'data_itemoflist', 'value');
-            converter._addNumberInput(block, 'INDEX', 'math_integer', args[0], 1);
-            return block;
+            if (converter._isBlock(receiver) && converter.isListBlock(receiver)) {
+                const block = converter._changeBlock(receiver, 'data_itemoflist', 'value');
+                converter._addNumberInput(block, 'INDEX', 'math_integer', args[0], 1);
+                return block;
+            }
+
+            return null;
         });
 
         converter.registerOnSend('variable', 'index', 1, params => {

test/unit/lib/ruby-to-blocks-converter/variables.test.js

@@ -44,6 +44,35 @@ describe('RubyToBlocksConverter/Variables', () => {
                 convertAndExpectToEqualBlocks(converter, target, code, expected);
             });
 
+            test('operator_letter_of', () => {
+                const code = `${varName}[0]`;
+                const expected = [
+                    {
+                        opcode: 'operator_letter_of',
+                        inputs: [
+                            {
+                                name: 'STRING',
+                                block: {
+                                    opcode: 'data_variable',
+                                    fields: [
+                                        {
+                                            name: 'VARIABLE',
+                                            variable: varName
+                                        }
+                                    ]
+                                },
+                                shadow: expectedInfo.makeText('apple')
+                            },
+                            {
+                                name: 'LETTER',
+                                block: expectedInfo.makeNumber(1)
+                            }
+                        ]
+                    }
+                ];
+                convertAndExpectToEqualBlocks(converter, target, code, expected);
+            });
+
             test('data_setvariableto', () => {
                 let code;
                 let expected;