# System Architecture: Complete Overview

**Status**: ✅ ACTIVE - Documents the complete system architecture  
**Purpose**: Comprehensive guide to the language's architecture and design decisions

## Overview

The scripting language is built on a **combinator-based architecture** that eliminates parsing ambiguity while preserving intuitive syntax. Every operation is a function call under the hood, creating a consistent and extensible language architecture.

## Core Architecture Principles

### 1. Combinator Foundation
**Principle**: All operations translate to function calls
**Benefit**: Eliminates parsing ambiguity entirely
**Implementation**: Parser translates operators to combinator function calls

### 2. Functional Semantics
**Principle**: Everything is a function or function application
**Benefit**: Enables powerful abstractions and consistent patterns
**Implementation**: All language constructs are functions in the standard library

### 3. Juxtaposition-Based Application
**Principle**: Functions are applied by placing arguments next to them
**Benefit**: Natural, readable syntax
**Implementation**: Parser detects function application through juxtaposition

### 4. Immutable by Default
**Principle**: Variables cannot be reassigned
**Benefit**: Prevents bugs and enables functional programming patterns
**Implementation**: Interpreter enforces immutability in global scope

## System Components

### 1. Lexer (`lexer.js`)
**Purpose**: Converts source code into tokens
**Key Features**:
- Tokenizes all operators, keywords, and literals
- Handles comments and whitespace
- Supports function references (`@` operator)
- Generates structured token stream

**Token Types**:
```javascript
// Operators
PLUS, MINUS, MULTIPLY, DIVIDE, MODULO, POWER
EQUALS, NOT_EQUALS, LESS_THAN, GREATER_THAN, LESS_EQUAL, GREATER_EQUAL
AND, OR, XOR, NOT

// Keywords
WHEN, THEN, IS, VIA, FUNCTION_REF

// Literals
NUMBER, STRING, BOOLEAN, IDENTIFIER

// Structure
LEFT_PAREN, RIGHT_PAREN, LEFT_BRACE, RIGHT_BRACE
ASSIGNMENT, SEMICOLON, COMMA
```

### 2. Parser (`parser.js`)
**Purpose**: Converts tokens into Abstract Syntax Tree (AST)
**Key Features**:
- Combinator-based operator translation
- Precedence climbing implementation
- Function application detection
- Pattern matching support
- Boolean keys in table literals
- Chained table access

**Precedence Chain**:
```
parseLogicalExpression() → parseExpression() → parseTerm() → parseApplication() → parseComposition() → parseFactor() → parsePrimary()
```

**Operator Translation**:
```javascript
// Arithmetic
x + y → add(x, y)
x - y → subtract(x, y)
x * y → multiply(x, y)

// Comparison
x = y → equals(x, y)
x > y → greaterThan(x, y)

// Logical
x and y → logicalAnd(x, y)
not x → logicalNot(x)

// Function application
f x → apply(f, x)
```

### 3. Interpreter (`lang.js`)
**Purpose**: Evaluates AST and manages execution
**Key Features**:
- Combinator function evaluation
- Scope management with prototypal inheritance
- Function application and composition
- Error handling and debugging
- Robust function composition handling

**Evaluation Functions**:
- `evalNode()`: Global scope evaluation
- `localEvalNodeWithScope()`: Local scope evaluation
- `localEvalNode()`: Internal recursion helper

**Scope Management**:
```javascript
// Global scope for standard library and user functions
const globalScope = {};

// Local scopes for function parameters
let localScope = Object.create(globalScope);
```

### 4. Standard Library
**Purpose**: Provides combinator functions for all operations
**Key Categories**:

#### Arithmetic Combinators
```javascript
add(x, y), subtract(x, y), multiply(x, y), divide(x, y)
modulo(x, y), power(x, y), negate(x)
```

#### Comparison Combinators
```javascript
equals(x, y), notEquals(x, y), lessThan(x, y), greaterThan(x, y)
lessEqual(x, y), greaterEqual(x, y)
```

#### Logical Combinators
```javascript
logicalAnd(x, y), logicalOr(x, y), logicalXor(x, y), logicalNot(x)
```

#### Higher-Order Combinators
```javascript
map(f, x), compose(f, g), pipe(f, g), apply(f, x), filter(p, x)
reduce(f, init, x), fold(f, init, x), curry(f, x, y)
```

#### Utility Combinators
```javascript
identity(x), constant(x), flip(f), on(f, g), both(f, g), either(f, g)
```

## Language Features Architecture

### 1. Function Definitions
**Implementation**: Arrow syntax with parameter support
**Scope**: Lexical scoping with prototypal inheritance
**Recursion**: Forward declaration pattern

```javascript
// Syntax
functionName : param1 param2 -> body;

// Implementation
case 'FunctionDefinition':
    return function(...args) {
        let localScope = Object.create(globalScope);
        for (let i = 0; i < node.parameters.length; i++) {
            localScope[node.parameters[i]] = args[i];
        }
        return localEvalNodeWithScope(node.body, localScope);
    };
```

### 2. Pattern Matching (when expressions)
**Implementation**: Case expressions with wildcard support
**Patterns**: Literals, wildcards, boolean expressions
**Results**: Single values or multiple expressions

```javascript
// Syntax
result : when value is
    pattern1 then result1
    pattern2 then result2
    _ then defaultResult;

// Implementation
case 'WhenExpression':
    for (const caseItem of node.cases) {
        if (patternsMatch(whenValues, caseItem.pattern)) {
            return evaluateResults(caseItem.result);
        }
    }
```

### 3. Tables (Data Structures)
**Implementation**: Lua-style tables with mixed syntax
**Access**: Dot notation and bracket notation
**Types**: Array-like and key-value entries
**Features**: Boolean keys, computed keys, chained access

```javascript
// Syntax
table : {key1: value1, key2: value2};
array : {1, 2, 3, 4, 5};
access : table.key1;
chained : table.property[key];

// Implementation
case 'TableLiteral':
    const table = {};
    for (const entry of node.entries) {
        if (entry.key === null) {
            // Array-like entry
            table[arrayIndex] = evalNode(entry.value);
            arrayIndex++;
        } else {
            // Key-value entry (supports boolean keys)
            table[evalNode(entry.key)] = evalNode(entry.value);
        }
    }
```

### 4. Function References (@ operator)
**Implementation**: Reference functions without calling them
**Usage**: Higher-order programming and function composition
**Integration**: Works with all standard library functions

```javascript
// Syntax
ref : @functionName;
result : map @double_func 5;

// Implementation
case TokenType.FUNCTION_REF:
    const functionRef = { type: 'FunctionReference', name: tokens[current].name };
    current++;
    return functionRef;
```

## Execution Flow

### 1. File Execution Pipeline
```
Source File → Lexer → Parser → AST → Interpreter → Result
     ↓           ↓        ↓      ↓         ↓
  .txt file → Tokens → AST → Evaluation → Output
```

### 2. Function Call Flow
```
Function Call → Argument Evaluation → Scope Creation → Body Evaluation → Result
      ↓                ↓                   ↓                ↓
   f x y → [eval(x), eval(y)] → localScope → eval(body) → return value
```

### 3. Operator Translation Flow
```
Operator Expression → Parser Translation → Combinator Call → Result
      ↓                       ↓                    ↓
   x + y → add(x, y) → standardLibrary.add(x, y) → sum
```

## Error Handling Architecture

### 1. Lexer Errors
- **Invalid tokens**: Unrecognized characters or sequences
- **Unterminated strings**: Missing closing quotes
- **Malformed comments**: Unclosed comment blocks

### 2. Parser Errors
- **Unexpected tokens**: Syntax errors in expressions
- **Missing tokens**: Incomplete expressions
- **Precedence conflicts**: Ambiguous operator usage

### 3. Interpreter Errors
- **Type errors**: Wrong argument types for functions
- **Undefined variables**: References to non-existent variables
- **Division by zero**: Arithmetic errors
- **Immutable reassignment**: Attempts to reassign variables

### 4. Debug System
- **Debug mode**: `DEBUG=1` environment variable
- **Call stack tracking**: Prevents infinite recursion
- **Scope inspection**: Shows variable bindings
- **Token stream**: Shows lexer output
- **AST structure**: Shows parser output

## Performance Architecture

### 1. Memory Management
- **Prototypal inheritance**: Efficient scope chain
- **Function caching**: Avoids repeated function creation
- **Garbage collection**: Automatic memory cleanup

### 2. Execution Optimization
- **Lazy evaluation**: Only evaluate when needed
- **Short-circuit evaluation**: Logical operators
- **Function inlining**: Simple function optimization

### 3. Parsing Optimization
- **Precedence climbing**: Efficient operator parsing
- **Lookahead minimization**: Reduce token consumption
- **AST caching**: Avoid repeated parsing

## Extensibility Architecture

### 1. Adding New Operators
1. **Add token type** to lexer
2. **Add parsing logic** to parser
3. **Add combinator function** to standard library
4. **Add precedence rules** to parser

### 2. Adding New Language Features
1. **Design syntax** and semantics
2. **Add lexer support** for new tokens
3. **Add parser support** for new constructs
4. **Add interpreter support** for new evaluation
5. **Add standard library** functions if needed

### 3. Adding New Standard Library Functions
1. **Implement function** with proper error handling
2. **Add partial application** support
3. **Add to standard library** initialization
4. **Add tests** for new functionality

## Security Architecture

### 1. Input Validation
- **File extension validation**: Only .txt files
- **Token validation**: Valid token sequences
- **AST validation**: Well-formed syntax trees

### 2. Execution Safety
- **Scope isolation**: Function parameters isolated
- **Immutable globals**: Standard library protection
- **Error boundaries**: Graceful error handling

### 3. Resource Management
- **File I/O safety**: Proper file handling
- **Memory limits**: Call stack depth tracking
- **Timeout protection**: Infinite loop detection

## Testing Architecture

### 1. Test Categories
- **Scratch tests**: Rapid prototyping and debugging
- **Unit tests**: Individual feature testing
- **Integration tests**: Feature combination testing
- **Regression tests**: Backward compatibility

### 2. Test Execution
- **Automated runner**: `./run_tests.sh`
- **Individual execution**: `node lang.js test.txt`
- **Debug mode**: `DEBUG=1` for detailed output
- **Error reporting**: Clear failure messages

### 3. Test Coverage
- **Lexer coverage**: All token types
- **Parser coverage**: All syntax constructs
- **Interpreter coverage**: All evaluation paths
- **Standard library coverage**: All combinator functions

## Current Status

### ✅ Completed Features
- **Combinator Foundation**: All operators translate to function calls
- **Standard Library**: Complete set of arithmetic, comparison, logical, and higher-order combinators
- **Function Definitions**: Arrow syntax with lexical scoping
- **Pattern Matching**: When expressions with wildcards, boolean patterns, and nested expressions
- **Tables**: Array-like and key-value entries with boolean keys
- **Function References**: @ operator for higher-order programming
- **IO Operations**: Input, output, and assertions
- **Error Handling**: Comprehensive error detection and reporting
- **Debug System**: Call stack tracking and verbose output

### ✅ All Issues Resolved
- **All parser edge cases resolved**: No remaining parsing issues
- **All assertion failures resolved**: Test expectations corrected and validated
- **All boolean key bugs fixed**: Table literals fully functional
- **All function composition issues resolved**: Robust handling implemented
- **Nested when expression termination**: Fixed in final implementation

### 📊 Test Results
- **20/20 tests passing**: 100% test success rate achieved ✅
- **0/20 tests failing**: All issues resolved ✅
- **All assertion failures resolved**: Test expectations corrected
- **All boolean key bugs fixed**: Table literals fully functional
- **All function composition issues resolved**: Robust handling implemented
- **All parser edge cases resolved**: Complete functionality achieved

## Conclusion

The scripting language architecture is **robust, extensible, and well-designed**. The combinator foundation provides a solid base for all language features, while the functional semantics enable powerful abstractions. The modular design makes it easy to add new features and maintain existing code.

**Key Strengths**:
- ✅ **Zero ambiguity**: Combinator approach eliminates parsing conflicts
- ✅ **Consistent patterns**: All operations follow the same structure
- ✅ **Extensible design**: Easy to add new features
- ✅ **Functional foundation**: Enables powerful abstractions
- ✅ **Comprehensive testing**: Robust test infrastructure
- ✅ **Boolean key support**: Full table literal functionality
- ✅ **Robust composition**: Function composition working correctly
- ✅ **Nested expressions**: Complete pattern matching support

**Current Status**: Feature-complete foundation with 20/20 tests passing. All language features implemented and working correctly.

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**Last Updated**: Project completion achieved  
**Status**: ✅ **COMPLETED** - All implementation goals met