Bytecode Interpreter

A bytecode interpreter and virtual machine for a dynamically typed, high-level, object-oriented programming language

Largely implemented/referenced from the Lox Programming Language http://www.craftinginterpreters.com/

The Program

The interpreter is composed of three main parts: the scanner/lexer, the compiler and the virtual machine

• Scanner: converts syntax into tokens
• Compiler: parses syntax tokens into bytes/opcodes
• Virtual machine: reads bytecode and executes instructions

Language Syntax

Console Output

print "Hello world";

Variable declarations and assignments

Fei supports three native data types: strings, numbers and booleans. The '=' assignment operator can be replaced with the 'assigned' keyword

var myInt;
myInt assigned value;
var myString = "string";
var myBool = true;

Comparators and Logical Operators

The '==' comparison operator can be replaced with the keywords 'is' or 'equals'

40 > 41;       // false
194.5 <= 992.2;   // true

var string assigned "isString";
string == "isString";         // true
string is "notString";        // false

false = !true;    // true

Arithmetic Operators

Fei supports the 4 main binary operators as well as the modulo operator

a + b;
24 / 12;
78 % 13;

Control Flow

If Statements

'else if' can be replaced with the 'elf' keyword.

if condition then
{
    print "then statement";
}
elf condition then
{
  print "else if statement";
}
else
{
  print "else statement";
}

While and For Loops

Fei inherits while and for loops from C.

while condition
{
  print "while statement";
}

for (var i = 0; i < 10; i = i + 1)
{
  print i;
}

Do While and Repeat Until Loops

Fei also supports do while and repeat until loops. 'do while' loops until the expression is false. 'repeat 'until' loops until the expression is true.

var i assigned 0;

do
{
  print i;
  i= i + 1;
} while i < 10;
 
repeat
{
  print i;
  i = i - 1;
} until i equals 0;

Switch Statements

Switch cases automatically breaks if the condition is met.

switch variable
{
  case 0:
  {
      // execute statment
  }
   case 1:
  {
      // execute statment
  }
  default:
    // default statmenet
}

Break and Continue Statements

All loops support break and continue statements.

while condition
{
    if breakCondition then break;
    if continueCondition then continue;
}

Functions

function product(a, b)
{
    return a * b;
}

function printHello()
{
    print "Hello";
}

var prod = product(12, 23);      // 276
printHello();                    // prints "Hello"

Classes

Fei has a Python-like class system. Constructors are initialize with the 'init' keyword and class members use the 'this' keyword. The keyword 'from ' used to declare inheritance. The 'super' keyword is used for superclasses.

class ParentClass
{
    init(name)
    {
        this.name = name;       // assign name to class member 'name'
    }
    
    printName()                 // class methods are initialized without the 'function' keyword
    {
        print this.name;        
    }
}

ParentClass parent("My name");
parent.printName();             // prints "My name"

class ChildClass from ParentClass
{
    init(name, age)
    {
        super.init(name);       // parent constructor
        this.age = age;
    }
    
    printAge()
    {
        print this.age;
    }
}

ChildClass child("Your name", 17);
child.printName();             // prints "Your name"
child.printAge();              // prints 17