Generic Arrays

Part of the Introduction To Luau section of Coddy's Lua journey. Lesson 60 of 73.

Generics get really useful once tables are involved. You already write typed arrays as {number} or {string} — inside a generic function the element type can simply be T, and {T} means "an array of whatever T turns out to be".

Here's the classic example — fetching the first element of any array:

local function first<T>(items: {T}): T?
    return items[1]
end

Pass a {string} and the checker infers T = string, so the result is a string. Pass a {number} and it's a number. One function replaces a whole family of per-type copies.

Look closely at the return type: it's T?, not T. An array can be empty, and then items[1] is nil — the optional type you met earlier says so honestly: "a T, or nil". The checker will nudge callers to handle the nil case before using the result, which is exactly the bug-catching you want from a typed language.

Challenge

Easy

Create a generic function named first that:

declares a type parameter T
accepts one parameter named items of type {T}
returns the first element, with return type T? (it's nil for an empty array)

Create these typed arrays:

fruits of type {string} with "apple", "banana", "cherry"
scores of type {number} with 10, 20, 30, 40
flags of type {boolean} with false, true
empty of type {string} with no elements

Print the result of calling first with each array, in that order, each on its own line.

Try it yourself

-- Write code here
-- 1) define first<T>(items: {T}): T? returning items[1]
-- 2) create fruits: {string}, scores: {number}, flags: {boolean},
--    and an empty {string} array
-- 3) print first(...) of each array, in that order

Test yourself

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

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