# REPL Test-Driven Development Report

## Executive Summary

Successfully implemented TDD approach for Ruchy REPL with functional tests based on specification and real-world demos from `../ruchy-repl-demos`.

## Test Coverage Status

### Functional Tests Created
- **31 specification-based tests** in `tests/repl_functional_spec_tests.rs`
- **48 comprehensive unit tests** in `tests/repl_comprehensive_tests.rs`
- **Total**: 79 REPL-specific tests

### Test Results
- ✅ **18/31 functional spec tests passing** (58% pass rate)
- ✅ **31/48 comprehensive tests passing** (65% pass rate)
- ✅ **374 library tests passing** (100% pass rate)

## Working Features (Proven by Tests and Examples)

### ✅ Fully Working
1. **Basic Arithmetic**: `+`, `-`, `*`, `/`, `%`, `**` (power)
2. **Variables**: `let x = value`, immutable by default
3. **Functions**: `fn name(params) { body }`, recursion support
4. **Lambda Functions**: `fn(x) { body }`, `x => expr`
5. **Control Flow**: `if-else`, `match` expressions
6. **Boolean Operations**: `&&`, `||`, `!`
7. **Comparison**: `>`, `<`, `>=`, `<=`, `==`, `!=`
8. **Arrays**: `[1, 2, 3]`, indexing with `arr[0]`
9. **Objects**: `{ x: 10, y: 20 }`, field access with `obj.x`
10. **Loops**: `for i in collection`, `while condition`
11. **Enums**: `enum Color { Red, Green, Blue }`
12. **Structs**: `struct Point { x, y }`
13. **Pattern Matching**: `match value { patterns }`
14. **Pipe Operator**: `value |> function`
15. **Range**: `1..5`
16. **String Concatenation**: `"Hello" + " World"`
17. **Destructuring**: `let [a, b] = [1, 2]`

### 🚧 Partially Working
1. **String Methods**: `.length()` works, others need implementation
2. **Array Methods**: `.length()` works, `.map()`, `.filter()` need work
3. **Error Handling**: Basic try-catch parsing exists

### ❌ Not Yet Implemented (Failed Tests)
1. **Spread Operator**: `...array`
2. **String Interpolation**: `f"Hello {name}"`
3. **Optional Chaining**: `obj?.field`
4. **Null Coalescing**: `value ?? default`
5. **Async/Await**: `async fn`, `await`
6. **Generics**: `fn identity<T>(x: T)`
7. **Higher-order functions**: `.map()`, `.filter()`, `.reduce()`
8. **Tuple Operations**: `(1, 2, 3)`
9. **While loops with mutations**: Variable mutation in loops

## Examples Created

### Working Examples (cargo run --example <name>)
1. **repl_basic_arithmetic** - Demonstrates all arithmetic operations
2. **repl_variables_and_functions** - Shows variables, functions, recursion
3. **repl_control_flow** - If-else, loops, pattern matching
4. **repl_comprehensive_demo** - All working features in one demo

### Example Output
```
=== Basic Arithmetic Demo ===

Addition:
  > 2 + 2
  4

Multiplication:
  > 10 * 5
  50

...
```

## Code Quality Improvements

### Lint Status
- Fixed format string warnings in repl.rs
- Fixed unused variable warnings
- Fixed redundant clone warnings
- **59 clippy warnings remain** (to be addressed)

### Code Coverage
- REPL module: ~10% coverage (268/2714 lines)
- Target: 80% coverage
- Strategy: Add more unit tests for internal methods

## Technical Debt Identified

### High Priority
1. **Mutation in loops not working** - Core interpreter issue
2. **String/Array methods incomplete** - Missing standard library
3. **Error recovery system** - Implemented but not fully integrated

### Medium Priority
1. **Memory tracking** - Implemented but not exposed
2. **Progressive modes** - Structure exists, needs refinement
3. **REPL commands** - Basic commands work, need expansion

### Low Priority
1. **Async/await** - Parser support exists, runtime needed
2. **Generics** - Type system enhancement required
3. **Spread operator** - Parser and evaluator updates needed

## Recommendations

### Immediate Actions
1. **Fix mutation in loops** - Critical for basic programming
2. **Implement string/array methods** - Essential for usability
3. **Increase test coverage** - Add unit tests for internal methods

### Next Sprint
1. **Complete failing features** - Focus on high-value features
2. **Achieve 80% coverage** - Comprehensive unit testing
3. **Zero clippy warnings** - Clean codebase

### Long Term
1. **Full specification compliance** - Implement all language features
2. **Performance optimization** - Profile and optimize hot paths
3. **Developer experience** - Better error messages, debugging tools

## Conclusion

The TDD approach successfully identified working vs broken features. The REPL has solid fundamentals (arithmetic, functions, control flow) but needs work on advanced features (mutations, methods, error handling). The test suite provides clear targets for improvement and regression prevention.