9. Enums and better `match`¶

Enums let you represent “one of several shapes” as a real type, and match lets you handle it exhaustively.

A small command enum¶

enum Command:
    Add(int)
    Remove(int)
    List

Coming from Python?

Python’s Enum is mostly “named constants”. If you want variants that carry different data, Python often uses: strings, dataclass hierarchies, or Union types.

In Incan, enum variants can carry data (like Add(int)), and match gives you an exhaustive, compiler-checked way to handle every case.

Matching on enums¶

def run(cmd: Command) -> None:
    match cmd:
        Add(x) => println(f"add {x}")
        Remove(x) => println(f"remove {x}")
        List => println("list")

def main() -> None:
    run(Command.Add(1))     # outputs: add 1
    run(Command.Remove(1))  # outputs: remove 1
    run(Command.List)       # outputs: list

Why this matters¶

Unlike a stringly-typed “command name”, enums:

prevent typos
make invalid states unrepresentable
give you exhaustive match checking

Try it¶

Add a Clear variant and update the match accordingly.
Add a Rename(str) variant and print the new name.

One possible solution

enum Command:
    Add(int)
    Remove(int)
    Rename(str)
    Clear
    List

def run(cmd: Command) -> None:
    match cmd:
        Add(x) => println(f"add {x}")
        Remove(x) => println(f"remove {x}")
        Rename(name) => println(f"rename {name}")
        Clear => println("clear")
        List => println("list")

Where to learn more¶

Enums deep dive: Enums

Next¶

Back: 8. Collections and iteration

Next chapter: 10. Models vs classes

9. Enums and better match¶