Boolean, nil, and optional values

The Boolean type has two values:

local visible = true
local archived = false

Conditions in if, while, and until require a Boolean. Pop does not convert other values into conditions:

local count = 1

if count then -- error: count is Int, not Boolean
    print("one")
end

Write the actual test instead, such as if count > 0 then.

Absence with nil

nil means that no value is present. A literal nil has the exact built-in type nil:

local missing = nil

String? describes “a String or nil.” It is shorthand for the union type String | nil.

Optional values prevent accidental use of a missing value. A function that needs a String cannot be called with a String? until the program has narrowed away the nil possibility.

Array indexing demonstrates this rule:

local names: {String} = { "Ada", "Grace" }
local possibleName = names[1]

Ordinary indexing has the optional result String?, because the requested position might be outside the array. A function can preserve that result type:

function first(values: {String}): String?
    return values[1]
end

Array.get(names, 1) instead returns String and traps if the position is invalid.

The 0.1.0-rc.2 checker does not implicitly widen a standalone nil literal into an annotated optional local, so local nickname: String? = nil is currently rejected. Optional values are most useful when an operation such as array indexing already produces the complete optional type. There is also no dedicated convenience unwrapping syntax; APIs that need a guaranteed value commonly use a checked operation after proving its precondition.