Interpreter Needs restarted if the user starts with a Closing Parenthesis ())

[TeamSPoon]

The metta+> interpreter encounters a critical issue when the user begins an input with a closing parenthesis ()). Instead of producing a syntax error or handling the input gracefully, the interpreter becomes unresponsive and does not process further inputs.

---

Steps to Reproduce

1. Start the metta+> interpreter.
2. Enter a valid input to ensure the interpreter is functioning:

   (+ 1 1)

   • The interpreter processes the input correctly.
3. Enter an input that begins with a closing parenthesis:

   ) (+ 1 1)

4. Observe the behavior.

---

Expected Behavior

• When encountering an input that starts with a closing parenthesis ()), the interpreter should:
  1. Display a descriptive error message, e.g., "Syntax Error: Input starts with an unmatched closing parenthesis."
  2. Continue to function normally, allowing further inputs.

---

Actual Behavior

• The interpreter does not handle this scenario gracefully:
• No error message is displayed. And the interpeter never works again until its restarted.

---

Suggested Fix

1. Error Detection:
   • Check if an input starts with a closing parenthesis during parsing.
   • Display a meaningful error message, e.g., "Invalid input: Input cannot start with a closing parenthesis."
2. Recovery Mechanism:
   • Ensure the interpreter resets to a functional state after detecting this error.
3. Testing:
   • Add test cases for inputs starting with ) to verify proper error handling and system recovery.

---

Additional Notes

This issue significantly affects usability, particularly for new users. Ensuring robust error handling for this edge case will improve the interpreter's reliability and user experience.

[TeamSPoon]

https://github.com/trueagi-io/metta-wam/blob/main/src/canary/metta_repl.pl#L557