Using Literal Union Enums

Part of the Introduction To Luau section of Coddy's Lua journey — lesson 54 of 73.

Literal unions earn their keep as function parameters. Type a parameter with the union and the checker guarantees the function is only ever called with a valid member:

type Direction = "up" | "down" | "left" | "right"

local function move(direction: Direction)
    print(`Moving {direction}...`)
end

move("left")  -- ✓
move("north") -- ✗ type error: not a Direction

Inside the function, handle the members exhaustively with an if/elseif chain. Since the type limits the input to four values, checking three and letting else take the last one covers everything:

local function describe(direction: Direction): string
    if direction == "up" then
        return "Going upward"
    elseif direction == "down" then
        return "Going downward"
    elseif direction == "left" then
        return "Turning left"
    else
        return "Turning right"
    end
end

One wrinkle: strings arriving from outside — like io.read() — are typed as plain string, because the checker can't know what a user will type. The :: cast you learned earlier tells it to treat the value as your enum type:

local direction = io.read() :: Direction

A cast is a promise, not a check — at runtime nothing verifies the input. Production code would validate first; here it lets typed functions accept user input cleanly.

Challenge

Easy

Create the literal union type Direction with members "up", "down", "left" and "right".

Create a function move that takes direction: Direction and prints Moving [direction]...

Create a function getMovementDescription that takes direction: Direction, returns a string, and handles every member with if/elseif/else:

"up" → Going upward
"down" → Going downward
"left" → Turning left
"right" → Turning right

Read one line from the user with io.read() and cast it to Direction with ::. Then:

call move with the direction
print the result of getMovementDescription for the same direction

Try it yourself

-- Write code here
-- 1) define type Direction
-- 2) write move and getMovementDescription (if/elseif/else)
-- 3) read the input: local direction = io.read() :: Direction
-- 4) call move, then print the description

Test yourself

This lesson includes a short quiz. Start the lesson to answer it and track your progress.

Using Literal Union Enums

All lessons in Introduction To Luau