Messing with lua metatables

9 min read

Some programming languages allow users to extend built-in data types with the help of constructs that are built into the language. In Rust, you can extend a struct (opens in a new tab) by implementing various traits like Add (opens in a new tab), Sub (opens in a new tab), Mul (opens in a new tab) etc,. to extend its capabilities. Let's see what Lua has to offer!

NOTE: We are only going to use Lua 5.1 as this is the version that is supported by LuaJIT (opens in a new tab).

Metatables

In Lua, we have setmetatable (opens in a new tab) which allows us to extend/change lua table (opens in a new tab) behavior. We can use metatable to add support for operator overloading and some other crazy stuff into your tables.

From Official Docs:

Metatables allow us to change the behavior of a table. For instance, using metatables, we can define how Lua computes the expression a+b, where a and b are tables. Whenever Lua tries to add two tables, it checks whether either of them has a metatable and whether that metatable has an __add field. If Lua finds this field, it calls the corresponding value (the so-called metamethod, which should be a function) to compute the sum.

Barebones example:

metatable.lua
local a = setmetatable({ x = 10 }, {})
 
print(a.x) -- prints 10
print(getmetatable(a)) -- returns a's metatable

setmetatable takes 2 parameters; the first is the initial table and the second is another table with the metamethods.

Metamethods

So, how do we use setmetatable? Well, here comes metamethods. These are set of methods that define how your table should behave when a certain operation is performed. Let's see how we can add two tables together:

__add.lua
local mt = {}
 
-- Called when using `+`
-- Here `self` is right-hand operand and `that` is left-hand operand
function mt:__add(that)
    return self.x + that.x
end
 
-- Another syntax to define methods is using `.`
-- In this you get the `self` as the first argument; we named it `this`
--
-- function mt.__add(this, that)
--     return this.x + that.x
-- end
 
local a = setmetatable({ x = 10 }, mt)
local b = setmetatable({ x = 5 }, mt)
 
print(a + b) -- prints 15

Going with the above example, we can also add support for subtraction (-), multiplication *, less-than (<), less-than-equal (<=), equality (==), concatenation (..) and so on.

metatable.lua
local mt = {}
 
-- Called when using `+`
function mt:__add(that)
    return self.x + that.x
end
 
-- Called when using `-`
function mt:__sub(that)
    return self.x - that.x
end
 
-- Called when using `*`
function mt:__mul(that)
    return self.x * that.x
end
 
-- Called when using `<`
function mt:__lt(that)
    return self.x < that.x
end
 
-- Called when using `<=`
function mt:__lte(that)
    return self.x <= that.x
end
 
-- Called when using `==`
function mt:__eq(that)
    return self.x == that.x
end
 
-- Called when using `..`
function mt:__concat(that)
    return { x = self.x + that.x }
end
 
local a = setmetatable({ x = 10 }, mt)
local b = setmetatable({ x = 5 }, mt)
 
print(a + b) -- prints 15
print(a - b) -- prints 5
print(a * b) -- prints 50
print(a < b) -- prints false
print(a > b) -- prints true
print(a == b) -- prints false
print((a .. b).x) -- prints 15

NOTE: There is no __gt (greater-than >), __gte (greater-than-equal >=) and __ne (not-equal ~=) because they are just inverse of __lt, __lte and __eq respectively.

Check out http://lua-users.org/wiki/MetatableEvents to know the list of the supported metamethods and their signature.

Taming Superpowers

I've used metatables extensively in some of neovim plugins like Comment.nvim (opens in a new tab), Navigator.nvim (opens in a new tab), and FTerm.nvim (opens in a new tab) and believe me there are certain metamethods that are really like giving tables superpowers.

__index

This controls prototype inheritance. Whenever we access a field, lua checks whether that field is present or not in the initial table, if not, then __index is used to return the field. We can either give it a function or another table. Let's see how it works

__index.lua
local mt = {}
 
mt.__index = { y = 5 }
 
local a = setmetatable({ x = 10 }, mt)
 
print(a.x) -- prints 10
print(a.y) -- prints 5
print(a.z) -- prints nil

As you can see, a.y doesn't exists on the initial table so __index is indexed for the field and the value will be returned.

__index.lua
local mt = {}
 
function mt.__index(this, key)
    return this.x + #key
end
 
-- We can also use `:` syntax
--
-- function mt:__index(key)
--     return self.x + #key
-- end
 
local a = setmetatable({ x = 10 }, mt)
 
print(a.x) -- prints 10
print(a.hello) -- prints 15

In the example, a.x is present, but a.hello is missing, hence __index is called with the table as the first and hello as the second argument, and we are returning the sum of x and length of the key.

We can also use __index method to:

Track property usage
track.lua
local state = { hello = "world" }
local count = {}
 
local proxy = setmetatable({}, {
    __index = function(_, key)
        count[key] = count[key] + 1
        return state[key]
    end,
})
 
print(proxy.hello)
Proxy Getter
proxy.lua
local cstrs = {
    c = { '//%s', '/*%s*/' },
}
 
local strings = setmetatable({}, {
    __index = function(_, key)
        return function(ctype)
            if type(ctype) == 'number' then
                return cstrs[key][ctype]
            end
            return unpack(cstrs[key])
        end
    end,
})
 
print(strings.c()) -- prints "//%s" "/*%s*/"
print(strings.c(1)) -- prints "//%s"
print(strings.c(2)) -- prints "/*%s*/"
Default value
default_value.lua
local num = setmetatable({}, {
    __index = function()
        return 0
    end,
})
 
print(num.one) -- prints 0
print(num.two) -- prints 0
Class-like Inheritance
inheritence.lua
---@class Animal
---@field private type string
local Animal = {}
 
---Creates an animal
---@param type? string Type of animal
---@return Animal
function Animal:new(type)
    local state = { type = type }
    self.__index = self
    return setmetatable(state, self)
end
 
function Animal:explain()
    print('Hello! I am a', self.type)
end
 
---@class Dog : Animal
local Dog = Animal:new('dog')
 
function Dog.make_sound()
    print('Woof Woof!')
end
 
---@class Cat:Animal
---@field private type string
local Cat = Animal:new('cat')
 
---Makes cat sound
function Cat.make_sound()
    print('Meeooowww!')
end
 
function Cat:want_food()
    print('Yes!', self.type, 'wants food.')
end
 
------------------- USAGE -------------------
 
-- Base Animal
local dino = Animal:new('dino')
dino:explain()
 
-- Base dog class
Dog:explain()
Dog:make_sound()
 
-- Dog class with different type
local dalmation = Dog:new("dalmation")
dalmation:explain()
dalmation:make_sound()
 
-- Base cat class
local cat = Cat:new()
cat:explain()
cat:make_sound()
cat:want_food()
As for myself, I used these techniques to add (opens in a new tab) and simplify (opens in a new tab) features in Comment.nvim and added inheritence (opens in a new tab) in Navigator.nvim

__newindex

This metamethod controls the assignment and it's called whenever we assign a value to a new property, which doesn't exist in the initial table.

__newindex.lua
local proxy = {}
 
local mt = {}
 
function mt.__newindex(this, key, val)
    proxy[key] = string.format("%s %s", key, val)
end
 
local a = setmetatable({ x = 10 }, mt)
 
a.hello = "world"
a.foo = "bar"
a.x = 15
 
print(proxy.hello) -- prints "hello world"
print(proxy.foo) -- prints "foo bar"
print(proxy.x) -- prints nil
print(a.x) -- prints 15

This method will be called for a.hello and a.foo but not for a.x as it already exists in the initial table. It receives the initial table, the key, and the value as the arguments. In this example, we are using a as the setter for the proxy table but you can do much more, for example:

Read-only Tables
read_only.lua
local a = setmetatable({ x = 10 }, {
    __newindex = function(_, key, val)
        assert(false, string.format("Read-only Table! Cannot assign %q to %q", val, key))
    end
})
 
-- This won't throw any error
a.x = 5
 
-- This will throw error as `y` doesn't exist
a.y = 100

__call

This metamethod makes a table callable like a normal function and the returned value of the function is returned at the call site. Although you can still access properties of the table as usual.

__call.lua
local mt = {}
 
function mt.__call(this, arg)
    return this.x + arg
end
 
local a = setmetatable({ x = 10 }, mt)
 
print(a(5)) -- prints 15
print(a.x) -- prints 10

By providing __call metamethod, we can now call our table a as a function and still have access to its property i.e., a.x. We'll later see how we can combine this with __index to make a powerful abstraction.

__tostring

Using this metamethod, we can have a string representation of a table.

__tostring.lua
local mt = {}
local icon = { dir = "D", file = "F" }
 
function mt.__tostring(this)
    return string.format("[%s] %s", icon[this.type], this.name)
end
 
local node1 = setmetatable({ type = "dir", name = "node_modules" }, mt)
local node2 = setmetatable({ type = "file", name = "package.json" }, mt)
 
tostring(node1) -- return "[D] node_modules"
tostring(node2) -- returns "[F] package.json"
 
print(node1) -- prints "[D] node_modules"
print(node2) -- prints "[F] package.json"

In the example, we are using built-in tostring(table) which calls the __tostring methods, and the returned value is used as the string representation. The string representation can also be used with assert(..., node1) and print(table)

Combining Everything

I hope you got a basic understanding of metatable and how it works. Now let's see how we can combine everything and make something similar to neovim's vim.cmd interface.

nvim_cmd.lua
local nvim = {}
 
nvim.cmd = setmetatable({}, {
    __index = function(_, cmd)
        return function(arg)
            local args = type(arg) == "table" and arg.args or { arg }
            return vim.api.nvim_cmd(
                { cmd = cmd, args = args },
                { output = true }
            )
        end
    end,
    __newindex = function(this, cmd, arg)
        this[cmd](arg)
    end,
    __call = function(this, cmd)
        if type(cmd) == "table" then
            return this[cmd.cmd](cmd)
        end
        return vim.api.nvim_command(cmd)
    end,
    __tostring = function(_)
        return "<CMD>"
    end,
})
 
return nvim

Here's how you can use it

-- Runs __index
nvim.cmd.echo({ args = { "3" } })
nvim.cmd.echo("4")
 
--- Runs __newindex
nvim.cmd.echo = "5"
nvim.cmd.echo = { args = { "6" } }
 
-- Runs __call
nvim.cmd('echo "1"')
nvim.cmd({ cmd = "echo", args = { "2" } })
 
--- Runs __tostring
local _ = tostring(nvim.cmd) -- returns "<CMD>"
print(nvim.cmd) -- prints "<CMD>"

Couple of things to note here:


I hope now that you understand how we can use metatables to cook up some elegant user-facing APIs. If you want to deep dive and learn more then you should check out this excellent post: all-you-need-to-know-about-metatables-and-metamethods (opens in a new tab).