Operator overload not working

Question

When reading Programming in Lua, I tried this example in the book for operator overloading

Set = {} mt = {} mt.__add = Set.union --create a new set with the values of the given list function Set.new (l) local set = {} setmetatable (set, mt) for _, v in ipairs (l) do set [v] = true end return set end function Set.union (a, b) local result = Set.new {} for k in pairs (a) do result [k] = true end for k in pairs (b) do result [k] = true end return result end function Set.intersection (a, b) local result = Set.new {} for k in pairs (a) do result [k] = b[k] end return result end function Set.tostring (set) local l = {} for e in pairs (set) do l[#l + 1] = e end return "{" .. table.concat (l, ", ") .. "}" end function Set.print (s) print (Set.tostring (s)) end s1 = Set.new {10, 20, 30, 50} s2 = Set.new {30, 1} Set.print (s1) Set.print (s2) s3 = s1 + s2 Set.print (s3) 

But with the latest lua for windows I get the following error

 lua: C:\meta.lua:47: attempt to perform arithmetic on global 's1' (a table value) stack traceback: C:\meta.lua:47: in main chunk [C]: ? {30, 10, 20, 50} {1, 30} 

Answer 1

You are doing this task too early:

 mt.__add = Set.union 

since Set.union is not yet initialized.

Move it below Set.union and it will work.

For the same reason, if you assigned mt.__mul , it should be below Set.intersection

Answer 2

You need to define mt as a suitable metatet:

 mt = { __add = Set.union, __mul = Set.intersection, __tostring = Set.tostring }