Optional values

In the section Optional types we briefly covered optional types and values. We'll cover these in greater detail below.

When using an optional type ?T, you can pass both an instance of a T and Nil to it. For example:

let number: ?Integer = Nil

Here number is of type ?Integer, so we can assign both Nil and a Integer to it.

Sending messages

When sending a message to an optional type ?T, only messages supported by both instances of T and Nil are available. Nil supports receiving any message, and will return another Nil if there is no method for the message sent. For example:

Nil.foo # => Nil

If Nil does implement a method for a message it receives, that method gets called instead of returning Nil. For example:

Nil.to_integer # => 0
Nil.to_string  # => ''

A more realistic example is that of accessing a value from a nested array. Let's say our array looks like this:

let numbers = Array.new(Array.new(Array.new(1, 2, 3)))

When sending [] to an Array, the return value is Nil if the index is out of bounds. Nil in turn doesn't implement the [] method, meaning that sending [] to Nil produces another Nil. Thus, we can access a value from the above array like so:

numbers[0][0][1] # => 2

In other languages, you'd have to write something like this:

numbers = [[[1, 2, 3]]]

if numbers[0]
  if numbers[0][0]
    numbers[0][0][1]
  end
end

Inko's approach means that we don't have to explicitly check for a lack of a value every time an optional value is produced. Instead, we can do so at the end.

Converting optional types

An optional type ?T can be converted to a T in one of two ways:

1. Using the postfix ! operator
2. Using an explicit type cast

Using the above array example, we can convert our ?Integer to a Integer using the postfix operator like so:

numbers[0][0][1]!

Using an explicit type cast, it looks like this:

numbers[0][0][1] as Integer

Inko doesn't perform runtime checks when casting types like this, so you should use this with caution.