The Teal Programming Language

Metamethods

Lua supports metamethods to provide some advanced features such as operator overloading. Like Lua tables, records support metamethods. To use metamethods in records you need to do two things:

• declare the metamethods in the record type using the metamethod word to benefit from static type checking;
• and assign the metatable with setmetatable as you would normally do in Lua to get the dynamic metatable behavior.

Here is a complete example, showing the metamethod declarations in the record block and the setmetatable declarations attaching the metatable.

local type Rec = record
   x: number
   metamethod __call: function(Rec, string, number): string
   metamethod __add: function(Rec, Rec): Rec
end

local rec_mt: metatable<Rec>
rec_mt = {
   __call = function(self: Rec, s: string, n: number): string
      return tostring(self.x * n) .. s
   end,
   __add = function(a: Rec, b: Rec): Rec
      local res: Rec = setmetatable({}, rec_mt)
      res.x = a.x + b.x
      return res
   end,
}

local r: Rec = setmetatable({ x = 10 }, rec_mt)
local s: Rec = setmetatable({ x = 20 }, rec_mt)

r.x = 12
print(r("!!!", 1000)) -- prints 12000!!!
print((r + s).x)      -- prints 32

Note that we explicitly declare variables as Rec when initializing the declaration with setmetatable. The Teal standard library definition of setmetatable is function<T>(T, metatable<T>): T, so declaring the correct record type in the declaration assigns the record type to the type variable T in the return value of the function call, causing it to propagate to the argument types, matching the correct table and metatable types.

Operator metamethods for integer division // and bitwise operators are supported even when Teal runs on top of Lua versions that do not support them natively, such as Lua 5.1.