Mojo quickstart

This page quickly teaches you Mojo's syntax by focusing on code.

For more explanation of the language features, see the Mojo get started tutorial or Mojo language basics.

System requirements

You can use Mac, Linux, or Windows with WSL. For details, see the system requirements.

Project setup

You can install Mojo using any Python or Conda package manager, but we recommend either pixi or uv.

Use the following command to install the nightly buildTo get the stable build, change the version in the website header.:

uv

1. If needed, install uv:

   curl -LsSf https://astral.sh/uv/install.sh | sh

2. Install Mojo:

   uv pip install mojo \
        --index https://whl.modular.com/nightly/simple/ \
        --prerelease allow

   Or create a project and install Mojo:

   uv init temperature-analyzer
   cd temperature-analyzer
   uv add mojo \
       --index https://whl.modular.com/nightly/simple/ \
       --prerelease allow

These commands install the nightly build, which isn't complete and might have new bugs.

pixi

1. If needed, install pixi:

   curl -fsSL https://pixi.sh/install.sh | sh

2. Create a project and install Mojo:

   pixi init temperature-analyzer \
       -c https://conda.modular.com/max-nightly/ \
       -c conda-forge
   cd temperature-analyzer
   pixi add mojo
   pixi shell

Hello Mojo

Create analyzer.mojo in your favorite IDE or editor.

Add this to analyzer.mojo:

def main():
    print("Temperature Analyzer")

Run it:

mojo analyzer.mojo

Insights:

• If you see "Temperature Analyzer", your setup works.
• All Mojo executables use main() as their entry point.

Variables and data

Update your file to add temperature data:

def main():
    print("Temperature Analyzer")

    # [Float64] tells Mojo the `List` type at compile time
    var temps: List[Float64] = [20.5, 22.3, 19.8, 25.1]

    print("Recorded", len(temps), "temperatures")

Loops

Print each temperature. Add to main() under the print statement:

def main():
    # ... existing code ...

    for index in range(len(temps)):  # The range is [0, len(temps))
        print(t"  Day {index + 1}: {temps[index]}°C")

Insights:

• The day is offset by 1 because the range uses zero-based indexing.
• The t"..." prefix creates a template string: braces {} interpolate expressions directly into the output. This is useful because it avoids memory allocations for the intermediate values.

Functions

Add this function above main() to calculate the average temperature:

def calculate_average(temps: List[Float64]) -> Float64:
    # The 0.0 floating point literal defaults to Float64
    var total = 0.0
    for temp in temps:
        total += temp
    return total / Float64(len(temps))

def main():
    # ... existing code ...

Add to the end of main() to call the function:

    var avg = calculate_average(temps)
    print(t"Average: {round(avg, 2)}°C")

This code uses round() to limit the average to two decimal places.

Conditionals

Classify the average temperature. Add to the end of main():

    if avg > 25.0:
        print("Status: Hot week")
    elif avg > 20.0:
        print("Status: Comfortable week")
    else:
        print("Status: Cool week")

Raise errors

Handle empty data by updating calculate_average. Now the function can raise an error.

def calculate_average(temps: List[Float64]) raises -> Float64:
    if len(temps) == 0:  # Empty list of temperatures
        raise Error("No temperature data")

    var total = 0.0
    for temp in temps:
        total += temp
    return total / Float64(len(temps))

What changed:

• You add raises before the return arrow.
• You add the check for an empty list.
• You raise an Error if it's empty.

After this change, your app won't compile yet. Because calculate_average() can now raise, the call to it in main() must handle the error. You add that handling in the next step.

Handle errors

In main(), wrap your code in try-except for error handling:

    try:
        var avg = calculate_average(temps)
        print(t"Average: {round(avg, 2)}°C")

        if avg > 25.0:
            print("Status: Hot week")
        elif avg > 20.0:
            print("Status: Comfortable week")
        else:
            print("Status: Cool week")
    except e:
        print("Error:", e)

To test the error, replace temps with []. Confirm that your app errors with "No temperature data".

Python integration

Add statistics with Python's numpy. First, install it:

uv

uv pip install numpy

pixi

pixi add numpy

Then, add the following imports at the top of your file:

from std.python import Python, PythonObject
from std.python.numpy import copy_to_numpy_array

Now, calculate Python stats at the end of the try block in main():

var np = Python.import_module("numpy")
var pytemps = copy_to_numpy_array(temps)
var std_dev = np.std(pytemps)
print("Temperature standard deviation:", std_dev)

Final code

Your complete analyzer.mojo:

from std.python import Python, PythonObject
from std.python.numpy import copy_to_numpy_array


def calculate_average(temps: List[Float64]) raises -> Float64:
    if len(temps) == 0:
        raise Error("No temperature data")

    var total = 0.0
    for temp in temps:
        total += temp
    return total / Float64(len(temps))


def main():
    print("Temperature Analyzer")
    var temps: List[Float64] = [20.5, 22.3, 19.8, 25.1]
    print("Recorded", len(temps), "temperatures")

    for index in range(len(temps)):
        print(t"  Day {index + 1}: {temps[index]}°C")

    try:
        var avg = calculate_average(temps)
        print(t"Average: {round(avg, 2)}°C")

        if avg > 25.0:
            print("Status: Hot week")
        elif avg > 20.0:
            print("Status: Comfortable week")
        else:
            print("Status: Cool week")

        var np = Python.import_module("numpy")
        var pytemps = copy_to_numpy_array(temps)
        var std_dev = np.std(pytemps)
        print("Temperature standard deviation:", std_dev)
    except e:
        print("Error:", e)

What you touched

Mojo variables, lists, loops, functions, conditionals, error handling, and Python integration, all in one working program.

Use Mojo with AI coding assistants

If you use AI coding assistants, the mojo-syntax skill keeps you aligned with the latest nightly language releases. Stay up to date with Mojo's rapid development:

npx skills add modular/skills

This installs all four Mojo agent skills, including mojo-syntax.

Keep going

Build something bigger: Tutorial: Game of Life Language guide: Mojo Manual API docs: Standard Library