Programs

Handling missing values

Option makes missing values explicit, so the compiler helps you handle both outcomes.

def describe (input : String) : String :=
  match input.toNat? with
  | some n => s!"You entered {n}"
  | none => "That is not a natural number"

def main : IO Unit := do
  IO.println (describe "42")
  IO.println (describe "hello")

Explanation

`Option Nat`

Either some n containing a number, or none representing a missing value.

`toNat?`

Attempts to parse a natural number from a string. The question mark is part of the name.

`some / none`

The two constructors of Option. Pattern matching forces us to consider both.

Expected output

Reference only. Run the file to check its actual result.

You entered 42
That is not a natural number

Exercise

Try "-4" and "007" as inputs.