Skip to content

4. Control flow

Control flow is how you branch and loop.

if / elif / else

def describe(n: int) -> str:
    if n < 0:
        return "negative"
    elif n == 0:
        return "zero"
    else:
        return "positive"

match (pattern matching)

match is the main way to branch on enums like Result and Option:

def main() -> None:
    result = parse_port("8080")

    match result:
        case Ok(port): println(f"port={port}")
        case Err(e): println(f"error: {e}")

Coming from Rust?

Incan also supports a more Rust-like match-arm style using =>:

def main() -> None:
    match parse_port("8080"):
        Ok(port) => println(f"port={port}")
        Err(e) => println(f"error: {e}")

This is equivalent to the case ...: form; pick whichever reads best to you.

for loops

Incan supports Python-like for loops:

def main() -> None:
    items = ["Alice", "Bob", "Cara"]
    for name in items:
        println(name)

You can break early:

for name in items:
    if name == "Bob":
        break

Try it

  1. Write a function classify(n: int) -> str using if/elif/else.
  2. Use match on a Result and print either the value or the error.
  3. Loop over a list and stop early with break.
One possible solution 
# 1) classify function
def classify(n: int) -> str:
    if n < 0:
        return "negative"
    elif n == 0:
        return "zero"
    else:
        return "positive"

def main() -> None:
    println(classify(-1))  # negative
    println(classify(0))   # zero
    println(classify(2))   # positive

    # 2) match on Result
    match parse_port("8080"):
        Ok(port) => println(f"port={port}")
        Err(e) => println(f"error={e}")

    # 3) loop over a list and stop early with break
    items = ["Alice", "Bob", "Cara"]
    for name in items:
        if name == "Bob":
            break
        println(name)

Where to learn more

Next

Back: 3. Functions

Next chapter: 5. Modules and imports