How to type cast in Luau?

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#1

So, Luau, Roblox’s superset of Lua, is fantastic! As someone who comes from a primarily statically-typed background in software engineering, this makes me far more eager to engage with Lua.

I do have a problem, though. Assume I have a class designed to, I don’t know, represent a location on a Chessboard:

Try 1

--!strict

--- A class representing a square's position on a Chess board.
-- @classmod ChessPosition
local ChessPosition = {};
ChessPosition.prototype = {};
ChessPosition.prototype.__index = ChessPosition.prototype;

--- A map-like object that maps a chess square's column (file) numerical value to
--- its corresponding character.
ChessPosition.columnToCharacterMap = {
	[1] = "a",
	[2] = "b",
	[3] = "c",
	[4] = "d",
	[5] = "e",
	[6] = "f",
	[7] = "g",
	[8] = "h",
};

--- A type alias describing the shape of a ChessPosition instance.
type ChessPosition = {
	column: number;
	row: number;
	toNotation: (ChessPosition) -> string;
};

--- Construct a new ChessPosition from a numeric column and row.
-- @param column The value in the range [1, 8] assigned to the ChessPosition's column.
-- @param row The value in the range [1, 8] assigned to the ChessPosition's row.
function ChessPosition.new(column: number, row: number): ChessPosition
	local self = {};
	setmetatable(self, ChessPosition.prototype);
	self.column = column;
	self.row = row;
	return self;
end

--- Convert the ChessPosition object to standard algebraic notation (SAN).
function ChessPosition.prototype.toNotation(self: ChessPosition): string
	local columnAsCharacter = ChessPosition.columnToCharacterMap[self.column];
	local asNotation = ("%s%d"):format(columnAsCharacter, self.row);
	return asNotation;
end

local a1 = ChessPosition.new(1, 1):toNotation();
local h8 = ChessPosition.new(8, 8):toNotation();
-- Expect to print "a1, h8".
print(a1, ", ", h8);
-- Prints "a1, h8" (tested in Studio)!

This functionally works, although Luau tosses a warning on line 36 (return self, the final line within ChessPosition.new’s function body); that toNotation does not exist in self. So, I decided to commit a sin of sorts, and, well…

Try 2; lie to Luau and say it will all be okay.

It was time to do the unthinkable and cast to any (shudder).

So, I rewrote the line to return self as any;

It had enough, clearly, as the warning became an error: Expected 'end' (to close 'function' at line 31), got 'as'. A simple syntax error.

It’s very clear that type casting in the expression as type syntax is not quite what I have to do. That being the case, how do I type cast in Luau?

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#2

While we’re at it, where is the official documentation for type annotations in Luau? I only have what I can scrape from the devforums and small projects in the past Luau was built off of. Any leads?

#3

I’ve created an ugly fix, although still would love some help:

local proxy: any = self;
return proxy;
#4

By the way the x as y was removed because it was causing some issues. So yeah that seems to be the only way currently.

#5

I know it’s late but I think this may have what you we’re looking for
Find it here

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#6

This appears to just be the later documentation website; it solidifies that type assertions were removed.

#7

I was looking into type annotations myself until I stumbled upon the luau docs. If you scroll all the way down the page, you will find that you can do type casting like so:

local myVariable: A = getTypeB() :: A

or in your case:

return self :: any

So the typecasting operator is ::

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