Concatenating Objects

Part of the Object Oriented Programming section of Coddy's Lua journey — lesson 28 of 70.

The .. operator in Lua concatenates strings, but what if you want to combine an object with a string? The __concat metamethod lets you define exactly how this should work.

When Lua encounters a .. b and one operand has a __concat metamethod, it calls that function with both values. This is useful when you want objects to blend naturally into string expressions.

local Name = {}
Name.__index = Name

function Name:new(first, last)
    local obj = {first = first, last = last}
    setmetatable(obj, Name)
    return obj
end

function Name.__concat(a, b)
    if type(a) == "string" then
        return a .. b.first .. " " .. b.last
    else
        return a.first .. " " .. a.last .. b
    end
end

Notice the function checks which operand is the string. Since .. can be used as "Hello " .. obj or obj .. "!", your metamethod should handle both cases.

local person = Name:new("Ada", "Lovelace")

print("Dr. " .. person)      -- Output: Dr. Ada Lovelace
print(person .. " PhD")      -- Output: Ada Lovelace PhD

Like other operator metamethods, __concat uses dot syntax and receives both operands as arguments. The key difference from arithmetic metamethods is that you're typically working with mixed types—your object and a string.

Challenge

Easy

Let's build a Title class that can blend naturally into string expressions using the .. operator. This is useful when you want to combine objects with strings in a readable, intuitive way—like adding honorifics or suffixes to a person's title.

You'll organize your code across two files:

Title.lua: Define your Title class with a text field that stores the title string. Include a :new(text) constructor and implement the __concat metamethod. Your metamethod should handle both cases: when a string comes before the object ("prefix" .. obj) and when a string comes after (obj .. "suffix"). The result should combine the string with the title's text appropriately.
main.lua: Require your Title module, create a title instance, and demonstrate concatenation from both directions.

You will receive three inputs:

The title text (e.g., "Engineer")
A prefix string to place before the title (e.g., "Senior ")
A suffix string to place after the title (e.g., " III")

In your main file, create a Title with the given text. Then perform two concatenations and print each result on a separate line:

Concatenate the prefix string with the title object
Concatenate the title object with the suffix string

For example, if the inputs are Engineer, Senior , and III, the output should be:

Senior Engineer
Engineer III

The first line shows the prefix combined with the title, and the second line shows the title combined with the suffix. Your __concat metamethod makes this natural string blending possible!

Try it yourself

-- Read inputs
local titleText = io.read()
local prefix = io.read()
local suffix = io.read()

-- Require the Title module
local Title = require('Title')

-- TODO: Create a Title instance with the given titleText

-- TODO: Concatenate prefix with the title object and print the result

-- TODO: Concatenate the title object with suffix and print the result