lango

Lango Programming Language

Lango is a simple interpreted programming language implemented in Go. It supports basic arithmetic operations, variable declarations, control flow statements, and more.

Inspired by Crafting Interpreters by Robert Nystrom.

Table of Contents

1. Features
2. Project Structure
3. Usage
4. Language Syntax
5. Examples

Features

• Basic arithmetic operations (+, -, *, /, %)
• Variable declarations and assignments
• Control flow statements (if, while, for)
• Print statements
• Support for numbers, strings, and boolean values

Project Structure

The project consists of several Go files, each responsible for a specific part of the language implementation:

• main.go: Entry point of the interpreter
• scanner.go: Tokenizes the input source code
• parser.go: Parses the tokens into an Abstract Syntax Tree (AST)
• interpreter.go: Executes the parsed AST
• expr.go: Defines expression types
• stmt.go: Defines statement types
• token.go: Defines token types and structure
• environment.go: Manages variable scoping and storage
• astprinter.go: Utility for printing the AST (useful for debugging)

Usage

To run a Lango script:

go run . <script_name>.lango

To start an interactive REPL (Read-Eval-Print Loop):

go run .

Language Syntax

Variables

var x = 10;
var y = "Hello, World!";
var z = true;

Arithmetic Operations

var a = 5 + 3 * 2;
var b = 10 / 2 - 1;
var c = 7 % 3;

Control Flow

If Statement

if (x > 5) {
    print "x is greater than 5";
} else {
    print "x is not greater than 5";
}

While Loop

var i = 0;
while (i < 5) {
    print i;
    i = i + 1;
}

For Loop

for (var j = 0; j < 5; j = j + 1) {
    print j;
}

Print Statement

print "Hello, World!";
print 42;
print true;

Examples

Here are some examples demonstrating the features of Lango:

Example 1: Basic Arithmetic and Variables

var x = 10;
var y = 5;
print x + y;
print x * y;
print x / y;
print x % y;

Output:

15
50
2
0

Example 2: Control Flow

var i = 0;
while (i < 5) {
    if (i % 2 == 0) {
        print "Even";
    } else {
        print "Odd";
    }
    i = i + 1;
}

Output:

Even
Odd
Even
Odd
Even

Example 3: For Loop and String Concatenation

for (var i = 1; i <= 5; i = i + 1) {
    print "Count: " + i;
}

Output:

Count: 1
Count: 2
Count: 3
Count: 4
Count: 5

Implementation Details

Scanner (scanner.go)

The scanner tokenizes the input source code. It reads the source character by character and produces a list of tokens. Key functions include:

• ScanTokens(): Main function that scans the entire source and returns a list of tokens
• scanToken(): Scans a single token
• isDigit(), isAlpha(), isAlphaNumeric(): Helper functions for character classification

Parser (parser.go)

The parser takes the list of tokens from the scanner and builds an Abstract Syntax Tree (AST). Key functions include:

• Parse(): Main parsing function that returns a list of statements
• declaration(), statement(), expression(): Parse different language constructs
• match(), consume(): Helper functions for token matching and consumption

Interpreter (interpreter.go)

The interpreter executes the AST produced by the parser. It implements the Visitor pattern to traverse and execute each node of the AST. Key functions include:

• Interpret(): Main interpretation function
• VisitXxxExpr() and VisitXxxStmt(): Visitor methods for different expression and statement types
• execute() and evaluate(): Helper functions for statement execution and expression evaluation

Environment (environment.go)

The Environment manages variable scoping and storage. It supports nested scopes for block-level variable declarations. Key functions include:

• Define(): Defines a new variable in the current scope
• Get(): Retrieves a variable's value
• Assign(): Assigns a new value to an existing variable

Conclusion

Lango is a simple yet functional programming language that demonstrates the basics of language design and implementation. It provides a foundation for further experimentation and expansion, such as adding functions, classes, or more advanced control flow constructs.