Option

Some times values are optional, e.g. an expression like array_of_string.find(fn(s) -> s == "hallo") may or may not have a result.

To handle these there is an Option<type> wrapper.

E.g.

Option<int>            // an optional integer
Option<string>         // an optional string
Option<(int, string)>  // an optional tuple of integer and string

Creation

The simplest way to create/instantiate an optional value is to use the builtin Some and None functions:

let maybe_a : Option<int> = Some(2345)
let maybe_b : Option<int> = None()

in which case

maybe_a is an optional integer that has the value 2345 (i.e. is defined)
maybe_b is an optional integer that has no value (i.e. is not defined)

Alternative an optional value can be create as the result of an if-expression

let maybe_a = if (b < 0) 2345

in which case

if b < 0 is true maybe_a will be an optional integer with value 2345
if b < 0 is not true maybe_a will be an optional integer with no value

Accessing the value

Option has the following properties to access the value directly:

optional_value.defined a bool indicating if there is a value or not
optional_value.value the actual value, if there is one
- Be careful using this! Accessing the value of an option with defined == false will most likely lead to unexpected behaviour. At the very least wrap this in an if condition like

if (optional_value.defined) {
  optional.value  // use it in some way
}

A better way to access an optional value is to use the | operator:

let maybe_a : Option<int> = Some(2345)
let maybe_b : Option<int> = None()

maybe_a | 1234    // will be integer 2345 since maybe_a is defined
maybe_b | 1234    // will be integer 1234 since maybe_b is not defined