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First Program

Let's write your first Desi program - the classic "Hello, World!"

Hello, World!

Create a new file called hello.desi:

# hello.desi
def main() -> int:
    print("Namaste, World!")
    0

Understanding the Code

Let's break down each part:

def main():    # (1)!
    print("Namaste, World!")  # (2)!
  1. Every Desi program needs a main function. The return type is optional!
  2. print() is a built-in function that outputs to the console

Run It

The simplest way — build and run in one step:

desic run hello.desi

Output:

Namaste, World!

Or build an executable you can distribute:

desic build hello.desi -o hello
./hello

🎉 Congratulations! You've just run your first Desi program!

A More Interesting Example

Let's create something more substantial - a greeting program:

# greet.desi

def greet(name: str) -> str:
    return "Namaste, " + name + "!"

def main() -> int:
    let message = greet("Desi Developer")
    print(message)

    # String interpolation with f-strings
    let x = 10
    let y = 20
    print(f"Sum: {x + y}")
    0

Run it:

desic run greet.desi

Output:

Namaste, Desi Developer!
Sum: 30

Key Concepts

Variables

Use let for immutable variables:

let name = "Desi"       # Type inferred as str
let count: int = 42     # Explicit type annotation

Use var for mutable variables:

let mut counter = 0
counter := counter + 1   # OK - counter is mutable

Functions

Define functions with def:

def add(a: int, b: int) -> int:
    return a + b

Or use the implicit return (last expression):

def add(a: int, b: int) -> int:
    a + b  # No return keyword needed!

Comments

# This is a single-line comment

# Multi-line comments use
# multiple single-line comments

Return Types

# Return int for exit codes
def main() -> int:
    print("Hello")
    0  # Exit code 0 = success

String Interpolation

let name = "world"
let count = 42
print(f"Hello {name}, count is {count}")

Creating a Project

For larger programs, use desic init to create a project with a desi.mod manifest:

desic init myapp
cd myapp
desic run          # runs src/main.desi
desic build        # builds to build/output/myapp

This creates: 

myapp/
├── desi.mod          # Project manifest
├── src/
│   └── main.desi     # Entry point
└── tests/            # Test files

What's Next?

Now that you can write basic programs, continue with: