lux/docs/STRESS_TEST_FINDINGS.md

# Lux Stress Test Findings

This document summarizes issues found during systematic stress testing of the Lux parser, typechecker, and REPL.

## Critical Bugs (All Fixed)

### 1. ~~Record Equality Returns False for Equal Records~~ (FIXED)
```lux
let r1 = { x: 1, y: 2 }
let r2 = { x: 1, y: 2 }
r1 == r2  // Now returns true
```
**Fix**: Added Record case to `Interpreter::values_equal` in `src/interpreter.rs`

### 2. ~~Invalid Escape Sequences Silently Accepted~~ (FIXED)
```lux
"\z"  // Now produces error: "Invalid escape sequence: \z"
```
**Fix**: Modified lexer in `src/lexer.rs` to error on unknown escape sequences

### 3. ~~Unknown Effects Silently Accepted in Declarations~~ (FIXED)
```lux
fn test(): Unit with {CompletelyFakeEffect} = ()  // Now produces error with suggestions
```
**Fix**: Added effect validation in `TypeChecker::check_function` in `src/typechecker.rs`

### 4. ~~No Forward References for Mutual Recursion~~ (FIXED/DOCUMENTED)
```lux
fn isEven(n: Int): Bool = if n == 0 then true else isOdd(n - 1)
fn isOdd(n: Int): Bool = if n == 0 then false else isEven(n - 1)
// Works in files (two-pass type checking), REPL limitation documented
```
**Note**: Works when loaded from files due to two-pass type checking. REPL processes line-by-line, so forward references don't work there.

### 5. ~~Circular Type Definitions Silently Fail~~ (FIXED)
```lux
type A = B
type B = A
// Now produces error: "Circular type definition detected: A -> B -> A"
```
**Fix**: Added `check_type_cycles` method with DFS cycle detection in `src/typechecker.rs`. Also fixed parser to require `|` for enum variants, allowing `type A = B` to be parsed as a type alias.

## Parser Issues

### Missing Features

| Feature | Status | Notes |
|---------|--------|-------|
| Hex literals (`0x10`) | Not supported | Generic parse error |
| Binary literals (`0b101`) | Not supported | Generic parse error |
| Octal literals (`0o77`) | Not supported | Generic parse error |
| Scientific notation (`1e10`) | Not supported | Generic parse error |
| Leading dot floats (`.5`) | Not supported | Generic parse error |
| Trailing dot floats (`1.`) | Not supported | Generic parse error |
| Underscore separators (`1_000_000`) | **Supported** | Works correctly |
| List patterns in match (`[a, b, c]`) | Not supported | Parse error |
| Guard clauses (`x if x > 0 =>`) | Not working | Shows redundant pattern error |

### Poor Error Messages

| Input | Current Error | Better Error |
|-------|---------------|--------------|
| `[1 2 3]` | "Expected declaration" | "Missing comma between list elements" |
| `{ x 1 }` | "Undefined variable: x" | "Missing colon in record field" |
| `funtion foo()` | "Expected declaration" | "Unknown keyword 'funtion'. Did you mean 'fn'?" |
| `(1 + 2` | No error | "Unclosed parenthesis" |
| `{ x: 1` | No error | "Unclosed brace" |
| `[1,, 2]` | No output | "Unexpected comma" |

### Identifier Issues
- Double underscore identifiers (`__name`) not allowed
- Unicode identifiers (`café`) partially work but may have issues

## Type Checker Issues

### Good Error Messages
- Type mismatches: Clear "Cannot unify X with Y"
- Undefined variables: Includes "Did you mean..." suggestions
- Wrong argument count: Clear arity mismatch message
- Non-exhaustive match: Lists uncovered patterns
- Effect mismatches: Shows expected vs actual effects
- Undefined record field: Clear "Record has no field"
- Calling non-function: Clear type mismatch

### Issues
- `:type 42` shows "(type checking passed)" but not the actual type
- No type inference for function parameters - must be annotated

## Runtime Issues

### Division Handling
- Integer `/0`: Proper "Division by zero" error
- Integer `%0`: Proper "Modulo by zero" error
- Float `/0.0`: Returns `inf` (IEEE 754 compliant)

### Recursion
- Deep recursion (10,000 calls): Works! Good tail call optimization
- Infinite recursion: Eventually times out (expected)

### Missing Stdlib Functions
- `String.get(s, i)` - not implemented
- `String.charAt(s, i)` - not implemented

## REPL Issues

### Working Well
- State persistence between lines
- Function redefinition
- Multi-line input
- `:help`, `:quit`, `:load` commands
- Tab completion (noted in help)

### Issues
- `:type expr` doesn't show the type, just "(type checking passed)"
- No `:ast` or `:tokens` debugging commands
- String interpolation syntax is `{x}` not `${x}` (unexpected)
  - `"Hello {name}"` works
  - `"Hello ${name}"` outputs `"Hello $<value>"` (keeps literal `$`)

## Comparison & Equality

| Type | `==` Works? |
|------|-------------|
| Int | Yes |
| Float | Yes |
| Bool | Yes |
| String | Yes |
| List | Yes |
| Tuple | Yes |
| Option/Result | Yes |
| **Record** | **NO (BUG)** |
| Function | No (expected) |

## Short-Circuit Evaluation

Works correctly:
- `true || crash()` - doesn't call `crash()`
- `false && crash()` - doesn't call `crash()`
- `if true then 1 else crash()` - doesn't call `crash()`

## Recommendations

### High Priority
1. Fix record equality
2. Error on invalid escape sequences
3. Validate effect names at declaration time

### Medium Priority
4. Add forward reference support for mutual recursion
5. Improve parser error messages with context
6. Add list pattern matching support
7. Error on circular type definitions

### Low Priority
8. Add hex/binary/octal literal support
9. Add scientific notation support
10. Fix `:type` command to show actual type
11. Document actual string interpolation syntax

## Test Commands Used

```bash
# Run stress tests
./target/release/lux test examples/test_math.lux

# Test specific edge case
echo 'let r1 = {x: 1}; let r2 = {x: 1}; r1 == r2' | ./target/release/lux
```