My first Mojo program

Now we have set up everything (finally). In this chapter, let's write our first Mojo program and run it.

• Hello, world
• What has happened?
  • The main() function
  • Behind pixi run mojo file.mojo
• Next step

Hello, world

Let's create a folder in our project directory called src. Then we create a sub-folder start. Within this folder, we create a file called hello.mojo. Your project directory should look like this:

my-first-mojo-project
├── pixi.toml
└── src
    └── start
        └── hello.mojo

We open the hello.mojo file and type the following code:

def main():
    print("Hello, world!")

You may be surprised (or even a little bit disappointed) to see that the code is 100% identical to Python code. But yes, it is just the design philosophy of Mojo. It is designed to similar to Python, so that a Python user, like you, can learn it easily.

Now, let's run this code. You can type the following command in your terminal:

pixi run mojo src/start/hello.mojo

And you will see the output:

Hello, World!

Congratulations! You have successfully run your first Mojo program. 🎉

What has happened?

You may ask have many questions in your mind: Why I have to define a function called main()? What happens when I run pixi run mojo src/start/hello.mojo? What is the difference between pixi run mojo and python?

Here are the answers to your questions.

The main() function

This might be the first difference you notice between Mojo and Python. In Python, you can run code in a file without defining a main function. However, in Mojo, you have to define a function called main() to run your code. By putting the code in the main() function, you tell Mojo that this is the entry point of your program. The mojo compiler will execute the code from the main() function.

We will come back to this in Section The main function in Chapter Functions.

main() function in Python

Some Python users also define a main() function in their Python code. But they have to write main() so that Python interpreter will run the code within the main() function. For example,

def main():
    print("Hello, world!")
main()  # Run the main function

In Mojo, you do not need to write the last line.

Behind pixi run mojo file.mojo

pixi run mojo file.mojo is a short form of pixi run mojo run file.mojo. There are two steps:

1. pixi run mojo: You tells pixi to run the Mojo compiler that is installed in your current environment (.pixi/envs).
2. mojo file.mojo You asks Mojo compiler to run the file.mojo file.

You may find the second step similar to python file.py, where you ask Python interpreter to run the file.py file. The output of running the hello.mojo file with Mojo is also the same as the output of running the hello.py file with Python.

However, there are some key differences between mojo file.mojo and python file.py. In Python, the software "Python" (we call it an "interpreter") reads the file.py file and executes the code in it. The python interpreter has to be kept running in the background in order to generate the output "hello, world!".

On the contrary, in Mojo, the process is a bit different:

• The Mojo compiler first reads the file.mojo file and compiles it into a binary executable file. After compilation, the Mojo compiler is no longer needed. This step can also be done by running pixi run mojo build file.mojo.
• The binary executable file is executed alone. It generates the output "hello, world!". This step can also be done by running ./file.

Two commands

pixi run mojo file.mojo is actually a short cut for the following two commands. You can try yourself:

pixi run mojo build src/hello.mojo  # Compile the Mojo file
./hello                             # Run the compiled binary file

The first command complies the hello.mojo file into a binary executable file called hello in the root directory. The second command runs the hello file to get the output "hello, world!". You can test it in your terminal.

You can copy this "hello" file to another computer (same OS) and run it without installing Mojo. This is a feature that Python does not have.

Next step

Now you have successfully run your first Mojo program. I believe that you cannot wait to write another mojo script. For example, something like this:

def main():
    a = 1
    b = 2.0
    print(a + b)

Then, let's go to the next Part of the Miji, where we try two find out a quick path for you to migrate from the Pythonic world to the Magician world.