# Stack Language Specification v0.1

## 1. Overview

A statically-typed, stack-based language with postfix notation combining the execution model of HP's RPL, the type system of C and Rust, and modern array operations from Uiua.

### Design Principles
- All operations are postfix (RPN)
- Stack-based execution (no local variables)
- Static typing with type inference
- Manual heap memory management
- C-like syntax style adapted to postfix notation
- Functions manipulate the stack directly

## 2. Lexical Structure

### 2.1 Comments
```
// Single-line comment
```

### 2.2 Identifiers
- Start with letter or underscore: `[a-zA-Z_][a-zA-Z0-9_]*`
- Case-sensitive

### 2.3 Literals

**Integer Literals**
```
42          // i32 (default)
42i64       // i64
0xFF        // hexadecimal
0b1010      // binary
```

**Floating Point Literals**
```
3.14        // f64 (default)
3.14f32     // f32
```

**String Literals**
```
"hello world"
"escape sequences: \n \t \\ \""
```

**Boolean Literals**
```
true
false
```

**Array Literals**
```
[1 2 3 4 5]              // array of i32
[1.0 2.0 3.0]            // array of f64
[[1 2] [3 4]]            // 2D array
```

## 3. Type System

### 3.1 Primitive Types
- `i8`, `i16`, `i32`, `i64` - Signed integers
- `u8`, `u16`, `u32`, `u64` - Unsigned integers
- `f32`, `f64` - Floating point
- `bool` - Boolean
- `char` - Single character (UTF-8)
- `ptr<T>` - Raw pointer to type T

### 3.2 Compound Types

**Arrays**
```
[T; N]      // Fixed-size array
[T]         // Dynamic array (heap-allocated)
```

**Structs**
```
struct Point {
    x: f64,
    y: f64,
}
```

**Tagged Unions**
```
union Result<T, E> {
    Ok(T),
    Err(E),
}
```

### 3.3 Type Inference
Types are inferred where possible but can be explicitly annotated:
```
42 :i64            // Annotate literal
x get :f32         // Annotate stack value
```

## 4. Stack Operations

### 4.1 Stack Manipulation
```
dup         // ( a -- a a ) Duplicate top
drop        // ( a -- ) Remove top
swap        // ( a b -- b a ) Swap top two
over        // ( a b -- a b a ) Copy second to top
rot         // ( a b c -- b c a ) Rotate three items
-rot        // ( a b c -- c a b ) Rotate three items reverse
2dup        // ( a b -- a b a b ) Duplicate top two
2drop       // ( a b -- ) Drop top two
nip         // ( a b -- b ) Drop second
tuck        // ( a b -- b a b ) Copy top below second
```

### 4.2 Stack Inspection
```
depth       // ( -- n ) Push stack depth
pick        // ( n -- x ) Copy nth item to top (0 = top)
roll        // ( n -- ) Move nth item to top
```

## 5. Operators (Postfix)

### 5.1 Arithmetic
```
3 4 +       // ( a b -- result ) Addition
10 3 -      // Subtraction
5 6 *       // Multiplication
20 4 /      // Division
17 5 %      // Modulo
2 8 **      // Exponentiation
```

### 5.2 Comparison
```
5 3 >       // Greater than
5 3 >=      // Greater or equal
5 3 <       // Less than
5 3 <=      // Less or equal
5 5 ==      // Equal
5 3 !=      // Not equal
```

### 5.3 Logical
```
true false &&    // Logical AND
true false ||    // Logical OR
true !           // Logical NOT
```

### 5.4 Bitwise
```
0xFF 0x0F &      // Bitwise AND
0xFF 0x0F |      // Bitwise OR
0xFF 0x0F ^      // Bitwise XOR
0xFF ~           // Bitwise NOT
8 2 <<           // Left shift
8 2 >>           // Right shift
```

## 6. Functions

Functions consume arguments from the stack and push results to the stack.

### 6.1 Function Definition
```
fn add_point : (Point Point -- Point) {
    // Stack: p1 p2
    swap .x get     // p2.x
    swap .x get     // p1.x
    +               // sum_x
    
    swap .y get     // p2.y
    swap .y get     // p1.y
    +               // sum_y
    
    Point::new      // Create new Point
}
```

### 6.2 Function Signature
Format: `(input_types -- output_types)`
```
fn square : (i32 -- i32) {
    dup *
}

fn divmod : (i32 i32 -- i32 i32) {
    2dup / -rot %
}

fn no_op : (-- ) {
    // Takes nothing, returns nothing
}
```

### 6.3 Generic Functions
```
fn identity<T> : (T -- T) {
    // Simply returns the input
}

fn swap_pair<T, U> : (T U -- U T) {
    swap
}
```

## 7. Control Flow

### 7.1 Conditionals
```
// if-then
condition if {
    // Executed if true
}

// if-then-else
x 0 > if {
    // Positive
} else {
    // Non-positive
}
```

### 7.2 Loops

**While Loop**
```
// While condition is true
{ condition } while {
    // Loop body
}

// Example: sum 1 to 10
0 1                     // sum counter
{ dup 10 <= } while {
    2dup +              // Add counter to sum
    swap 1 + swap       // Increment counter
}
drop                    // Drop counter, leave sum
```

**For Loop (Range-based)**
```
1 10 for i {
    i print
}

// Equivalent to:
1 10 range each { print }
```

### 7.3 Loop Control
```
break       // Exit loop
continue    // Skip to next iteration
```

## 8. Memory Management

### 8.1 Stack vs Heap
- Stack: Automatic, fixed-size types
- Heap: Manual, dynamic allocations

### 8.2 Heap Operations
```
// Allocate
Point::new heap_alloc   // ( Point -- ptr<Point> )

// Dereference
ptr @                   // ( ptr<T> -- T )

// Store
value ptr !             // ( T ptr<T> -- )

// Free
ptr free                // ( ptr<T> -- )
```

### 8.3 Example
```
// Create heap-allocated point
3.0 4.0 Point::new heap_alloc   // ptr<Point>
dup .x get print                // Print x (3.0)
free                            // Clean up
```

## 9. Data Structures

### 9.1 Struct Definition and Usage
```
struct Rectangle {
    width: f64,
    height: f64,
}

// Constructor (auto-generated)
10.0 20.0 Rectangle::new    // Create Rectangle

// Field access (postfix)
rect .width get             // Get width
rect .width 15.0 set        // Set width

// Method-like functions
fn Rectangle::area : (Rectangle -- f64) {
    dup .width get
    swap .height get
    *
}

// Usage
rect Rectangle::area        // Calculate area
```

### 9.2 Tagged Unions
```
union Option<T> {
    Some(T),
    None,
}

// Construction
42 Option::Some             // Create Some(42)
Option::None                // Create None

// Pattern matching
value match {
    Some(x) => {
        x print
    },
    None => {
        "Nothing" print
    },
}
```

### 9.3 Enums
```
enum Status {
    Pending,
    Active,
    Complete,
}

Status::Active              // Create enum value
```

## 10. Traits

Traits define shared behavior across types.

### 10.1 Trait Definition
```
trait Drawable {
    fn draw : (Self -- );
}

trait Add<T> {
    fn add : (Self T -- Self);
}
```

### 10.2 Trait Implementation
```
impl Drawable for Rectangle {
    fn draw : (Rectangle -- ) {
        "Drawing rectangle" print
        dup .width get print
        .height get print
    }
}

impl Add<Point> for Point {
    fn add : (Point Point -- Point) {
        // Implementation from earlier
        swap .x get swap .x get + 
        swap .y get swap .y get +
        Point::new
    }
}
```

### 10.3 Trait Bounds
```
fn draw_twice<T: Drawable> : (T -- ) {
    dup draw
    draw
}
```

## 11. Array Operations (Uiua-inspired)

### 11.1 Basic Array Operations
```
// Creation
[1 2 3 4 5] range       // Create range array

// Shape operations
arr shape                // Get shape
arr [2 3] reshape        // Reshape to 2x3

// Element access
arr 2 @                  // Index access
arr [1 3] slice          // Slice array
```

### 11.2 Array Combinators
```
// Map
[1 2 3 4] { 2 * } map        // [2 4 6 8]

// Filter
[1 2 3 4 5] { 2 % 0 == } filter   // [2 4]

// Reduce
[1 2 3 4] 0 { + } reduce     // 10

// Each (apply to each element)
[[1 2] [3 4]] { sum } each   // [3 7]
```

### 11.3 Array Arithmetic
```
[1 2 3] [4 5 6] .+      // Element-wise add: [5 7 9]
[1 2 3] [4 5 6] .*      // Element-wise multiply: [4 10 18]
[1 2 3] 2 .*            // Scalar multiply: [2 4 6]
```

### 11.4 Array Manipulation
```
[1 2 3] [4 5 6] ++      // Concatenate: [1 2 3 4 5 6]
[1 2 3] reverse         // [3 2 1]
[[1 2] [3 4]] transpose // [[1 3] [2 4]]
[1 2 3 4] 2 window      // [[1 2] [2 3] [3 4]]
```

## 12. Eval Operator

Execute code dynamically at runtime.

```
// Evaluate string as code
"2 3 +" eval            // Pushes 5

// Build and execute code
"fn square : (i32 -- i32) { dup * }" eval
5 square                // 25

// Dynamic dispatch
operation_name " get" ++ eval  // Call function by name
```

**Security Note**: Eval should be used carefully as it can execute arbitrary code.

## 13. Standard Library Concepts

### 13.1 I/O
```
"Hello" print           // Print to stdout
"Enter name: " input    // Read from stdin
"file.txt" read         // Read file contents
"data" "file.txt" write // Write to file
```

### 13.2 String Operations
```
"hello" " world" ++     // Concatenate: "hello world"
"hello" length          // 5
"hello" 1 3 substr      // "el"
"a,b,c" "," split       // ["a" "b" "c"]
["a" "b"] "," join      // "a,b"
```

### 13.3 Type Conversion
```
42 to_f64               // Convert i32 to f64
"123" parse_i32         // Parse string to i32
3.14 to_string          // Convert to string
```

## 14. Example Programs

### 14.1 Factorial
```
fn factorial : (i32 -- i32) {
    dup 1 <= if {
        drop 1
    } else {
        dup 1 - factorial *
    }
}

5 factorial print       // 120
```

### 14.2 FizzBuzz
```
fn fizzbuzz : (i32 -- ) {
    dup 15 % 0 == if {
        drop "FizzBuzz" print
    } else {
        dup 3 % 0 == if {
            drop "Fizz" print
        } else {
            dup 5 % 0 == if {
                drop "Buzz" print
            } else {
                print
            }
        }
    }
}

1 100 for i {
    i fizzbuzz
}
```

### 14.3 Using Structs and Traits
```
struct Circle {
    radius: f64,
}

impl Drawable for Circle {
    fn draw : (Circle -- ) {
        "Circle with radius: " print
        .radius get print
    }
}

fn Circle::area : (Circle -- f64) {
    .radius get
    2.0 **
    3.14159 *
}

// Usage
5.0 Circle::new
dup draw                // Draw the circle
Circle::area print      // Print area
```

### 14.4 Array Processing
```
// Sum of squares of even numbers from 1 to 10
[1 2 3 4 5 6 7 8 9 10]
{ 2 % 0 == } filter     // Keep even numbers
{ dup * } map           // Square each
0 { + } reduce          // Sum
print                   // 220
```

## 15. Implementation Notes

### 15.1 Type Checking
- Perform static type checking with full type inference
- Track stack types at compile time
- Ensure function signatures match actual stack effects

### 15.2 Interpreter Design
- Value stack: runtime execution stack
- Type stack: compile-time type tracking
- Call stack: function call management
- Heap: global allocator

### 15.3 Future Compilation
- Compile to bytecode for interpretation
- Potential LLVM backend for native compilation
- Stack optimization for register allocation

## 16. Syntax Summary

### Stack Manipulation
```
dup drop swap over rot pick roll
```

### Arithmetic (postfix)
```
a b +  a b -  a b *  a b /  a b %  a b **
```

### Comparison
```
a b <  a b >  a b <=  a b >=  a b ==  a b !=
```

### Control Flow
```
cond if { ... }
cond if { ... } else { ... }
{ cond } while { ... }
start end for var { ... }
```

### Functions
```
fn name : (inputs -- outputs) { body }
fn name<T, U> : (T U -- U) { body }
```

### Memory
```
value heap_alloc  ptr @  value ptr !  ptr free
```

### Arrays
```
[1 2 3]
arr { fn } map
arr { fn } filter
arr init { fn } reduce
```

---

**Version**: 0.1  
**Status**: Draft Specification  
**License**: Define your license here