The Teal Programming Language

Union types

The language supports a basic form of union types. You can register a type that is a logical "or" of multiple types: it will accept values from multiple types, and you can discriminate them at runtime.

You can declare union types like this:

local a: string | number | MyRecord
local b: {boolean} | MyEnum
local c: number | {string:number}

To use a value of this type, you need to discriminate the variable, using the is operator, which takes a variable of a union type and one of its types:

local a: string | number | MyRecord

if a is string then
   print("Hello, " .. a)
elseif a is number then
   print(a + 10)
else
   print(a.my_record_field)
end

As you can see in the example above, each use of the is operator causes the type of the variable to be properly narrowed to the type tested in its respective block.

The flow analysis of is also takes effect within expressions:

local a: string | number

local x: number = a is number and a + 1 or 0