3. The None problem: Option

Python uses None to mean “no value here.” A function that might not find something returns None, and every caller is trusted to remember to check for it. When someone forgets, the program runs until an AttributeError surfaces far from the cause. Pyfun makes the absence part of the type. A value that might be missing has type Option, and there are exactly two shapes it can take: Some x when a value is present, and None when it is not.

Because the possibility of None is in the type, the compiler makes you handle both cases. You take an Option apart with match, which works like Python’s match statement: you list the shapes the value can have and give each one a result.

let describe s =
  match String.toInt s:
    case Some n: f"got the number {n}"
    case None: "not a number"

print (describe "41")
print (describe "hello")

let ns = [10, 20, 30]
let third = Option.withDefault 0 (List.get 2 ns)
let missing = Option.withDefault 0 (List.get 9 ns)
print third
print missing

Running this prints:

got the number 41
not a number
30
0

String.toInt is a total parse. In Python int("hello") raises, so calling code wraps it in try/except. Pyfun’s String.toInt returns Some 41 for "41" and None for "hello", and the match handles each. List.get behaves the same way: instead of an index that might raise IndexError, it returns Some when the position exists and None when it does not. When you only want a fallback and not a full match, Option.withDefault supplies one: Option.withDefault 0 (List.get 9 ns) is 0 because position 9 is past the end.

Exercise

The function below labels a parsed number, but it only handles the Some case. Run pyfun check and the compiler reports what is missing:

error: non-exhaustive match: `None` is not matched
 --> 2:3
  |
2 |   match String.toInt s:
  |   ^^^^^^^^^^^^^^^^^^^^^

Add the None case so the match is total and the program runs.

let label s =
  match String.toInt s:
    case Some n: f"number: {n}"

print (label "7")
print (label "nope")

Expected output:

number: 7
no number

Open in the playground

let label s =
  match String.toInt s:
    case Some n: f"number: {n}"
    case None: "no number"

print (label "7")
print (label "nope")