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Hello World#

Write and run your first Jac program in 2 minutes.

Your First Program#

Create a file named hello.jac:

with entry {
    print("Hello, World!");
}

Run it:

jac hello.jac

Output:

Hello, World!

Congratulations! You just wrote your first Jac program.

Understanding the Code#

with entry {
    print("Hello, World!");
}
Part Meaning
with entry The program's starting point (like main() in other languages)
{ } Code block (Jac uses braces, not indentation)
print() Built-in function to output text
; Statement terminator (required in Jac)

A Bit More#

Variables and Functions#

def greet(name: str) -> str {
    return f"Hello, {name}!";
}

with entry {
    message = greet("Jac");
    print(message);
}

Output: Hello, Jac!

Using Python Libraries#

Jac is a Python superset - use any Python library directly:

import math;

with entry {
    result = math.sqrt(16);
    print(f"Square root of 16 is {result}");
}

Output: Square root of 16 is 4.0

Control Flow#

with entry {
    numbers = [1, 2, 3, 4, 5];

    for n in numbers {
        if n % 2 == 0 {
            print(f"{n} is even");
        } else {
            print(f"{n} is odd");
        }
    }
}

Key Syntax Differences from Python#

Python Jac
Indentation-based blocks { } braces
No semicolons ; required
def func(): def func() { }
if x: if x { }
elif elif (same)
for x in y: for x in y { }

Quick AI Example#

If you have byllm installed, try this:

import from byllm.lib { Model }

glob llm = Model(model_name="gpt-4o-mini");

"""Translate the given text to French."""
def translate(text: str) -> str by llm();

with entry {
    result = translate("Hello, World!");
    print(result);
}

Output: Bonjour, le monde!

(Requires OPENAI_API_KEY environment variable)

Next Steps#

Ready for something more substantial?