Metamethods in Lua

__add and __tostring teach operators new types.

-- Metamethods teach operators new types: __add, __tostring.
local Vec = {}
Vec.__index = Vec

function Vec.new(x, y)
  return setmetatable({ x = x, y = y }, Vec)
end

Vec.__add = function(a, b)
  return Vec.new(a.x + b.x, a.y + b.y)
end

Vec.__tostring = function(v)
  return "(" .. v.x .. ", " .. v.y .. ")"
end

local step = Vec.new(1, 0) + Vec.new(0, 2)
print("Player moves " .. tostring(step))
print(tostring(step + Vec.new(3, 3)))

How it works

1. Vec.__add runs when + meets two vectors.
2. __tostring feeds print and tostring.
3. The constructor stamps each table with setmetatable.

Keywords and builtins used here

• Vec
• end
• function
• local
• new
• print
• return
• setmetatable
• tostring

The run, in numbers

Lines

19

Characters to type

430

Tokens

138

Three-star pace

65 tpm

At the three-star pace of 65 tokens a minute, this run takes about 127 seconds.

Type this snippet

Step 2 of 3 in Metatables, step 18 of 27 in Language basics.

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