How to Create Your Own Programming Language

A programming language is a system of communication and computation used to control a computer. Languages allow us to write instructions in a format that computers can understand and execute.

While many popular languages like Python, Java, and JavaScript already exist, you may want to create your own for learning purposes or to solve a specific problem. Here is an overview of the key steps involved in designing a custom programming language.

1. Define the Language Goals and Scope

First, you need to decide what you want your language to do. Consider the following:

• Type of language: Will it be imperative (focused on statements that change state), functional (focused on evaluating expressions and avoiding state change), procedural, object-oriented, or something else?

• Target platform: Will your language compile to bytecode for a virtual machine, to native machine code, or interpret code directly?

• Primary domain: Is your language optimized for a specific purpose like web programming, scientific computing, or system scripting?

• Unique features: Does your language have any special features or capabilities not found in other languages? These could include non-standard control flow, unusual data types, etc.

• Simplicity vs power: Find a balance between simplicity for beginners and expressive power for advanced capabilities.

Your language does not have to be completely unique - it can borrow features from existing languages. But have a clear vision in mind.

Here is some sample code to show defining a simple imperative language:

// Language goals:
// - Imperative style
// - Statically typed
// - Compiles to bytecode
// - Functions and basic data types 
// - Simple syntax suitable for beginners

2. Define the Language Syntax

Syntax refers to the structure and format of the code. It determines what is considered valid vs invalid in your language.

Most language syntax can be expressed in Backus-Naur Form (BNF) - a notation for context-free grammars. For example:

// Program = {Statement}
Statement = IfStatement | ForStatement | PrintStatement
IfStatement = \"if\" Condition \"then\" Statement \"end\"
Condition = Expression (\"==\" | \"!=\" | \"<\" | \">\") Expression 
ForStatement = \"for\" Identifier \"in\" Expression \"..\" Expression Statement \"end\" 
PrintStatement = \"print\" Expression
Expression = Identifier | Number | Expression BinaryOp Expression
BinaryOp = \"+\" | \"-\" | \"*\" | \"/\"
Identifier = letter{letter | digit} 
Number = digit{digit}

This shows the syntax for a simple language with if statements, for loops, print statement, arithmetic expressions, and basic data types.

Use BNF to fully specify the syntax for your language’s features.

3. Define the Language Semantics

Semantics refer to what statements mean in your language. This gives meaning to the syntax.

You need to document the behavior of each statement and operator. For example:

• If statement executes the body if the condition is true
• For loop iterates the body for each value in the range
• Arithmetic operators add, subtract, multiply, divide
• Objects have properties that can be accessed via dot notation
• Function calls pass control to the function body

Semantics can be described informally in text and examples. But for full formality, you can use formal semantics which describes program meaning using mathematical notation.

Here is one way to define simple expression semantics:

// Environment (E) maps identifiers to values

[[x]]E = E(x) // variable value
[[5]]E = 5 // number value 
[[true]]E = true // boolean value

[[E1 + E2]]E = [[E1]]E + [[E2]]E // addition
[[E1 - E2]]E = [[E1]]E - [[E2]]E // subtraction
// etc for other operators

[[print E]]E = output [[E]]E  // print semantics

Fully document your language’s semantics to avoid ambiguity.

4. Define a Lexer and Parser

To process code in your language, you need:

• Lexer: Breaks input code into tokens (keywords, operators, identifiers, etc)
• Parser: Validates syntax and builds an abstract syntax tree

You can write these by hand, but tools like lex/yacc, ANTLR, and ply can automate the process.

For example, in ANTLR:

grammar MyLanguage;

// Lexer rules
PRINT: 'print';
IF: 'if'; 
ID: [a-zA-Z_] [a-zA-Z_0-9]*;
INT: [0-9]+;

// Parser rules
statement: IF expr THEN statement # IfStatement
         | FOR ID IN INT '..' INT statement # ForStatement
         | PRINT expr # PrintStatement;

expr: ID # Variable
     | INT # Number
     | expr op=(ADD | SUB) expr # BinaryExpression;

This defines tokens and grammar rules for a lexer/parser. The tools will generate lexer and parser code from this definition.

You can output the abstract syntax tree for further processing.

5. Define the Semantic Analysis

After parsing valid syntax, you need to enforce language semantics:

• Static type checking: Verify types are correct
• Scope resolution: Link variable references to definitions
• Error checking: Detect invalid operations or values

For this analysis, you traverse and annotate the AST from the parser.

For example:

class TypeChecker:
  def visit(self, node):
    if isinstance(node, BinaryExpression):
      self.check_binary_op(node)
  
  def check_binary_op(self, node):
    left_type = node.left.type 
    right_type = node.right.type
    
    if node.op == ADD and left_type != right_type:
      raise TypeError(\"Invalid types for +\")
    
    # Insert code to check and set type of node

This traverses the AST, enforcing semantics like type checking.

You can emit errors or annotate the AST with semantic information to use later.

6. Define Code Generation

To execute programs, you need to translate them to a target format:

• Bytecode: Generate bytecode for a stack-based virtual machine
• Native code: Generate assembly or machine code to run natively
• Interpretation: Directly execute the AST without prior translation

For example, a simple bytecode generator:

class CodeGenerator:
  def visit(self, node):
    if isinstance(node, BinaryExpression):
      self.visit(node.left)
      self.visit(node.right)
      self.emit(ADD_OPCODE) # Emit bytecode

This recursively walks the AST, emitting bytecode instructions for each node.

You define the instruction set and generate sequences of bytecode for your language.

For native code, you would generate assembly instructions instead.

7. Implement the Runtime

To execute the generated code, you need a runtime:

• Virtual machine: Executes bytecode instructions
• Garbage collection: Automatically frees unused memory
• Standard library: Provides built-in functions

For a VM, you implement each bytecode instruction:

switch(opcode) {
  case ADD_OPCODE:
    push(pop() + pop()); 
    break;

  case PRINT_OPCODE:
    printf(\"%d\", pop());
    break;
    
  // Other opcodes
}

This interprets the bytecode by manipulating a stack.

You also need to implement garbage collection, standard library functions, and any other runtime behavior needed by your language.

8. Putting It All Together

Here are the key steps again:

1. Define language goals and scope
2. Design language syntax
3. Specify detailed semantics
4. Implement lexer and parser
5. Define semantic analysis
6. Generate bytecode or machine code
7. Build virtual machine or runtime

Follow these steps to create a custom programming language for your specific use case!

While non-trivial, you can start small and iteratively add features. The same general principles apply whether creating simple teaching languages or large-scale languages like Java or C++.

With the key phases and examples above, you now have an overview of language implementation.