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3. Functions

Functions are named, reusable blocks of code.

Defining a function

Function parameters and return types are explicit:

def add(a: int, b: int) -> int:
    return a + b
  • Parameters use name: Type.
  • Return type uses -> Type.
  • Use -> None for “returns nothing”.

Calling a function

Program entry point: main

Most runnable programs define a main function. When you run a file with incan run ..., execution starts at:

  • def main() -> None:
  • -> None means it doesn’t return a value.

In order to run an incan file, you must define a main function:

def main() -> None:
    total = add(2, 3)
    println(f"total={total}")

Coming from Python?

In Python, a common pattern is:

if __name__ == "__main__":
    main()

In Incan, main is the program entry point when you run a file (e.g. incan run ...), so you don’t need an __name__ guard - it's implicit in Incan. It is, however, still good practice to keep “do work” code inside main, and keep other files as imported helper modules.

Docstrings

Use docstrings to describe intent (especially for public helpers):

def normalize_name(name: str) -> str:
    """
    Normalize a user name for consistent comparisons.
    """
    return name.strip().lower()

Multiple returns (with Result)

Many “can fail” functions return Result[T, E] instead of throwing exceptions:

def parse_port(s: str) -> Result[int, str]:
    if len(s.strip()) == 0:
        return Err("port must not be empty")
    return Ok(int(s))

You’ll learn the Result pattern in Chapter 6.

Try it

  1. Write def is_even(n: int) -> bool and print the result for a few values.
  2. Write def greet(name: str) -> str that trims whitespace and returns "Hello, <name>!".
  3. (Stretch) Write def safe_div(a: int, b: int) -> Result[float, str].
One possible solution 
def is_even(n: int) -> bool:
    return n % 2 == 0

def greet(name: str) -> str:
    cleaned = name.strip()
    return f"Hello, {cleaned}!"

def safe_div(a: float, b: float) -> Result[float, str]:
    if b == 0.0:
        return Err("division by zero")
    return Ok(a / b)

def main() -> None:
    println(f"is_even(2)={is_even(2)}")
    println(f"is_even(3)={is_even(3)}")
    println(greet("  Alice  "))

What to learn next

Next

Back: 2. Values, variables, and types

Next chapter: 4. Control flow