Type Hinting

Type-hinting looks like this:

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local var: string = "hello world"

It works with functions too:

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local function myfunc(a: string, b: string): number
    return tonumber(a) + tonumber(b)
end
print(myfunc(1, "1")) -- This will emit a warning for argument type mismatch.

There's no effect on performance. This is implemented entirely during the compilation phase.

Primitive Types

• string
• number
  • int
  • float
• boolean/bool
• function
• table
• userdata
• nil
• any

Multiple Return Values

For function return values, there is the void hint to indicate 0 return values, as well as the following syntax for 2 or more:

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local function get_status(): (bool, string)
    return true, "OK!"
end
print(get_status())

Union Types

Prefix a ? to indicate void-able types, e.g. ?string indicates that it may be absent, nil, or a string.

You can also use | to delimit alternatives, e.g. string|int indicates that it may be a string or an int.

Putting all of this together, we could also have e.g. ?string|int to indicate that it may be absent, nil, a string, or an int.

Complex Table Types

You can specify type requirements for fields inside of tables like so:

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local point: { x: number, y: number } = { x = 1, y = 2 }

Named Types

You can use $type to assign a name to a type.

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$type Point = { x: number, y: number }
local p: Point = { x = 1, y = 2 }

Complex Function Types

You can also specify type expectations for the parameters and/or return values of functions.

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$type LogCallback = function(msg: string): void
function write_env(log: LogCallback)
    log(_VERSION)
    log(_PVERSION)
end
write_env(print)

$declare

The parser can only do typechecking for what it can see, so $declare allows you to directly set the type of globals.

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$declare _VERSION: string

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$declare function tonumber(str: string, base: ?number): number

$getproptype

If you want to know what type the parser propagation has detected for a given variable, you can use $getproptype to ask it:

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local x = "hello"
print($getproptype(x)) --> string

Note that the return value of this function should not be relied upon.