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+# Next Steps: Immutable Real-World Programming Features
+
+## Overview
+
+This document outlines the plan for extending the Simple Scripting Language to support real-world programming scenarios while maintaining its immutable, functional design philosophy.
+
+## Core Principles
+
+- **Immutability First**: All data structures are immutable
+- **Transformation Over Mutation**: Operations return new data rather than modifying existing data
+- **Functional Composition**: Complex operations built from simple, composable functions
+- **Type Safety**: Enhanced pattern matching and type checking
+- **Performance**: Efficient persistent data structures
+
+## Phase 1: String Operations and Type System
+
+### 1.1 String Operations
+**Goal**: Add essential string manipulation capabilities
+
+**New Functions**:
+```javascript
+// String operations as functions
+length : string.length "hello";           // 5
+contains : string.contains "hello" "ll";  // true
+startsWith : string.startsWith "hello" "he"; // true
+endsWith : string.endsWith "hello" "lo";  // true
+substring : string.substring "hello" 1 3; // "el"
+
+// String concatenation
+result : string.concat "Hello" " " "World"; // "Hello World"
+
+// String transformation
+uppercase : string.upper "hello";         // "HELLO"
+lowercase : string.lower "HELLO";         // "hello"
+trimmed : string.trim "  hello  ";        // "hello"
+```
+
+**Implementation**:
+- Add `string.` namespace for string operations
+- Add string operation functions to standard library
+- No new syntax - uses existing function call patterns
+- Extend existing string literal support
+
+### 1.2 Runtime Type Checking with `is` Keyword
+**Goal**: Add explicit, optional type checking mechanism using the `is` keyword
+
+**Design Philosophy**:
+- Type checking is purely additive - no breaking changes to existing code
+- Works seamlessly with existing case expression syntax
+- Returns boolean values, fitting the functional style
+- Simple implementation with clear semantics
+
+**New Features**:
+```javascript
+// Basic type checking
+isNumber : x -> x is number;
+isString : x -> x is string;
+isTable : x -> x is table;
+isFunction : x -> x is function;
+
+// Type-safe operations in case expressions
+safeStringOp : x -> case x of
+    x is string : string.length x
+    _ : "not a string";
+
+// Complex type validation
+validateUser : user -> case user of
+    user.name is string and user.age is number : true
+    _ : false;
+
+// Type guards in pattern matching
+processData : data -> case data of
+    data is table : "processing table"
+    data is string : "processing string"
+    data is number : "processing number"
+    _ : "unknown type";
+```
+
+**Supported Types**:
+- `number` (both integers and floats)
+- `string` 
+- `boolean`
+- `table` (objects and arrays)
+- `function`
+
+**Implementation**:
+- Add `IS` token type and type-specific tokens (`NUMBER_TYPE`, `STRING_TYPE`, etc.)
+- Update lexer to recognize `is` keyword and type names
+- Extend parser to handle type checking expressions in case patterns
+- Add type checking logic in interpreter that returns boolean values
+- No changes to existing syntax or semantics
+
+## Phase 2: Persistent Data Structures
+
+### 2.1 Immutable Tables with Transformations
+**Goal**: Replace mutable table operations with immutable transformations
+
+**Current Problem**:
+```javascript
+// This won't work (mutable)
+cache[key] = value;
+```
+
+**Solution - Functional Immutable Transformations**:
+```javascript
+// Functional transformations using table namespace
+cache : {};
+cache1 : table.set cache "user.1" "Alice";
+cache2 : table.set cache1 "user.2" "Bob";
+value : table.get cache2 "user.1";  // "Alice"
+cache3 : table.delete cache2 "user.1";
+
+// Nested updates with dot notation
+user : {name: "Alice", profile: {age: 25}};
+updatedUser : table.set user "profile.age" 26;
+
+// Complex transformations using composition
+cache1 : pipe
+    (table.set "user.1" "Alice")
+    (table.set "user.2" "Bob")
+    (table.set "user.3" "Charlie")
+    cache;
+
+// Table merging
+combined : table.merge table1 table2;
+```
+
+**Implementation**:
+- Add `table.set`, `table.get`, `table.delete`, `table.merge` functions
+- Implement efficient structural sharing for immutable updates
+- Add nested key support (e.g., "profile.age")
+- All functions return new tables, never modify originals
+- Use existing function composition patterns
+
+### 2.2 APL-Inspired Table Primitives
+**Goal**: Enhance tables with APL-style operations for ergonomic data manipulation
+
+**Design Philosophy**:
+- Use existing table syntax for array-like behavior
+- Add APL-inspired primitives for vectorized operations
+- Keep immutable transformations
+- Provide concise, expressive data manipulation
+
+**New Features**:
+```javascript
+// Table creation (array-like using existing syntax)
+numbers : {1, 2, 3, 4, 5};
+mixed : {1, "hello", true, {key: "value"}};
+
+// All table operations namespaced for consistency
+first : table.first numbers;              // First element
+last : table.last numbers;                // Last element
+length : table.length numbers;            // Size/shape
+
+// Less common operations (namespaced)
+rest : table.drop 1 numbers;        // Drop first element  
+slice : table.slice numbers 2 3;    // Elements 2-3
+
+// Vectorized operations
+doubled : table.add numbers numbers;      // Element-wise addition
+squared : table.multiply numbers numbers; // Element-wise multiplication
+incremented : table.add numbers 1;        // Scalar addition to each element
+
+// Reductions (all namespaced)
+sum : table.sum numbers;                 // Sum reduction
+max : table.max numbers;                 // Maximum reduction
+min : table.min numbers;                 // Minimum reduction
+product : table.product numbers;   // Product reduction
+average : table.average numbers;   // Average reduction
+
+// Scan operations (namespaced)
+runningSum : table.scan table.sum numbers;        // Running sum scan
+runningProduct : table.scan table.product numbers; // Running product scan
+
+// Table metadata operations
+keys : table.keys table;           // Get all keys
+values : table.values table;       // Get all values
+reversed : table.reverse table;    // Reverse order
+sorted : table.sort table;         // Sort
+unique : table.unique table;       // Remove duplicates
+```
+
+**Implementation**:
+- Add `table.` namespace for all table operations
+- Extend lexer to recognize table operation keywords
+- Add vectorized operation logic in interpreter
+- Implement reduction and scan operations
+- Add table metadata operations
+
+## Phase 3: Higher-Order Functions for Collections
+
+### 3.1 Enhanced Standard Library
+**Goal**: Add collection processing functions
+
+**New Functions**:
+```javascript
+// Table processing (using namespaced primitives)
+numbers : {1, 2, 3, 4, 5};
+doubled : map @double numbers;                // {2, 4, 6, 8, 10}
+evens : filter @isEven numbers;               // {2, 4}
+sum : table.sum numbers;                      // 15 (sum reduction)
+
+// Table metadata operations
+table : {a: 1, b: 2, c: 3};
+keys : table.keys table;                      // {a, b, c}
+values : table.values table;                  // {1, 2, 3}
+pairs : table.pairs table;                    // {{a, 1}, {b, 2}, {c, 3}}
+
+// Advanced operations with tables
+nested : {{1, 2}, {3, 4}, {5}};
+flattened : table.flatten nested;             // {1, 2, 3, 4, 5}
+grouped : table.groupBy @isEven numbers;     // {true: {2, 4}, false: {1, 3, 5}}
+
+// Table operations
+reversed : table.reverse numbers;             // {5, 4, 3, 2, 1}
+sorted : table.sort numbers;                  // {1, 2, 3, 4, 5}
+unique : table.unique {1, 2, 2, 3, 3, 4};   // {1, 2, 3, 4}
+```
+
+**Implementation**:
+- Extend existing `map`, `filter`, `reduce` to work with tables
+- Implement vectorized operations for tables
+- Add reduction and scan operations
+- Implement table metadata operations
+
+### 3.2 Table Generation Helpers
+**Goal**: Add convenient table creation functions
+
+**New Functions**:
+```javascript
+// Table generation helpers
+range : table.range 1 5;           // {1, 2, 3, 4, 5}
+repeated : table.repeat "hello" 3; // {"hello", "hello", "hello"}
+
+// Use existing map/filter instead of comprehensions
+squares : map (x -> x * x) {1, 2, 3, 4, 5};
+evens : filter @isEven {1, 2, 3, 4, 5};
+```
+
+## Phase 4: Error Handling with Error Type
+
+### 4.1 Error Type
+**Goal**: Provide consistent error handling without complex monads
+
+**Implementation**:
+```javascript
+// Simple error type
+error : message -> {type: "error", message: message};
+
+// Safe operations with error handling
+safeDivide : x y -> case y of
+    0 : error "division by zero"
+    _ : x / y;
+
+safeParseNumber : str -> case str of
+    str is number : str
+    _ : error "invalid number";
+
+// Error checking
+isError : value -> value is error;
+getErrorMessage : error -> case error of
+    {type: "error", message: m} : m;
+```
+
+## Phase 5: Real-World Scenario Support
+
+### 5.1 User Management System
+**Immutable Implementation**:
+```javascript
+// User validation
+isValidEmail : email -> case email of
+    email contains "@" : true
+    _ : false;
+
+createUser : name email age -> case (isValidEmail email) and (isValidAge age) of
+    true : {name: name, email: email, age: age, status: "active"}
+    false : error "invalid user data";
+
+// User management
+users : {};
+user1 : createUser "Alice" "alice@example.com" 25;
+users1 : table.set users "user.1" user1;
+user2 : createUser "Bob" "bob@example.com" 30;
+users2 : table.set users1 "user.2" user2;
+
+// Safe user lookup
+findUser : email users -> case users of
+    {} : error "user not found"
+    _ : case (table.first users).email = email of
+        true : table.first users
+        false : findUser email (table.drop 1 users);
+```
+
+### 5.2 Shopping Cart System
+**Immutable Implementation**:
+```javascript
+// Cart operations
+emptyCart : {items: {}, total: 0};
+addItem : cart item -> {
+    items: table.set cart.items (table.length cart.items + 1) item,
+    total: cart.total + item.price
+};
+removeItem : cart itemId -> {
+    items: filter (item -> item.id != itemId) cart.items,
+    total: calculateTotal (filter (item -> item.id != itemId) cart.items)
+};
+
+// Discount application
+applyDiscount : cart discountPercent -> {
+    items: cart.items,
+    total: cart.total * (1 - discountPercent / 100)
+};
+```
+
+### 5.3 Data Processing Pipeline
+**Immutable Implementation**:
+```javascript
+// Data processing
+salesData : {
+    {month: "Jan", sales: 1000, region: "North"},
+    {month: "Feb", sales: 1200, region: "North"},
+    {month: "Mar", sales: 800, region: "South"}
+};
+
+// Filter by region
+filterByRegion : data region -> filter (item -> item.region = region) data;
+
+// Calculate totals using sum reduction
+sumSales : data -> table.sum (map (item -> item.sales) data);
+
+// Process pipeline
+northData : filterByRegion salesData "North";
+northTotal : sumSales northData;
+```
+
+## Implementation Timeline
+
+### Week 1-2: String Operations and Runtime Type Checking
+- [ ] String concatenation operator
+- [ ] String method implementations
+- [ ] `is` keyword and type checking tokens
+- [ ] Type checking in case expressions
+- [ ] Type validation functions in standard library
+
+### Week 3-4: Table Primitives with Namespacing
+- [ ] `table.` namespace for all table operations
+- [ ] Vectorized operations for tables
+- [ ] Reduction and scan operations
+- [ ] Table metadata operations
+- [ ] Performance optimization
+
+### Week 5-6: Higher-Order Functions
+- [ ] Enhanced standard library
+- [ ] Collection processing functions
+- [ ] Table-specific operations
+- [ ] Utility functions
+
+### Week 7-8: Error Handling
+- [ ] Error type implementation
+- [ ] Error handling patterns
+- [ ] Error checking functions
+- [ ] Integration with existing operations
+
+### Week 9-10: Real-World Scenarios
+- [ ] User management system
+- [ ] Shopping cart system
+- [ ] Data processing pipeline
+- [ ] Integration testing
+
+## Benefits of Runtime Type Checking Approach
+
+### Simplicity
+- **Minimal Implementation**: Only need to add `is` keyword and type checking logic
+- **No Breaking Changes**: Existing code continues to work unchanged
+- **Clear Semantics**: `x is number` is obviously a boolean expression
+- **Consistent Syntax**: Works seamlessly with existing case expressions
+
+### Functional Design
+- **Boolean Results**: Type checking returns true/false, fitting functional style
+- **Composable**: Can combine with logical operators (`and`, `or`, `not`)
+- **Pattern Matching**: Integrates naturally with case expressions
+- **No Side Effects**: Pure functions for type validation
+
+### Extensibility
+- **Easy to Add Types**: Simple to extend for new types (arrays, tuples, etc.)
+- **Custom Types**: Can implement custom type checking via functions
+- **Performance**: Runtime overhead only when explicitly used
+- **Optional**: No requirement to use type checking in existing code
+
+## Testing Strategy
+
+### Unit Tests
+- String operation tests
+- Runtime type checking tests (`is` keyword)
+- Type validation function tests
+- Data structure transformation tests
+- Higher-order function tests
+- Error handling tests
+
+### Integration Tests
+- Real-world scenario tests
+- Performance tests
+- Edge case tests
+- Error handling tests
+
+### Performance Benchmarks
+- Data structure operation performance
+- Memory usage analysis
+- Transformation efficiency
+- Scalability testing
+
+## Success Metrics
+
+- [ ] All real-world scenarios in `tests/17_real_world_scenarios.txt` pass
+- [ ] Performance within acceptable bounds (no more than 2x slower than current)
+- [ ] Memory usage remains reasonable
+- [ ] Code remains readable and maintainable
+- [ ] Backward compatibility maintained
+
+## Future Considerations
+
+### Advanced Features (Post-v1.0)
+- Lazy evaluation for large collections
+- Parallel processing capabilities
+- Advanced type system with generics
+- Macro system for code generation
+- Module system for code organization
+
+### Performance Optimizations
+- Structural sharing for immutable data structures
+- Compile-time optimizations
+- JIT compilation for hot paths
+- Memory pooling for temporary objects
+
+This plan maintains the language's functional, immutable design while adding the capabilities needed for real-world programming scenarios. 
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