How does the __call metamethod in Lua 5.1 actually work?

Question

I'm trying, as an exercise, to make a set implementation in Lua. Specifically I want to take the simplistic set implementation of Pil2 11.5 and grow it up to include the ability to insert values, delete values, etc.

Now the obvious way to do this (and the way that works) is this:

Set = {}
function Set.new(l)
    local s = {}
    for _, v in ipairs(l) do
        s[v] = true
    end
    return s
end
function Set.insert(s, v)
    s[v] = true
end

ts = Set.new {1,2,3,4,5}
Set.insert(ts, 5)
Set.insert(ts, 6)

for k in pairs(ts) do
    print(k)
end

As expected I get the numbers 1 through 6 printed out. But those calls to Set.insert(s, value) are really rather ugly. I'd much rather be able to call something like ts:insert(value).

My first attempt at a solution to this looked like this:

Set = {}
function Set.new(l)
    local s = {
        insert = function(t, v)
            t[v] = true
        end
    }
    for _, v in ipairs(l) do
        s[v] = true
    end
    return s
end

ts = Set.new {1,2,3,4,5}
ts:insert(5)
ts:insert(6)

for k in pairs(ts) do
    print(k)
end

This works mostly fine until you see what comes out of it:

1
2
3
4
5
6
insert

Very obviously the insert function, which is a member of the set table, is being displayed. Not only is this even uglier than the original Set.insert(s, v) problem, it's also prone to some serious trouble (like what happens if "insert" is a valid key someone is trying to enter?). It's time to hit the books again. What happens if I try this instead?:

Set = {}
function Set.new(l)
    local s = {}
    setmetatable(s, {__call = Set.call})
    for _, v in ipairs(l) do
        s[v] = true
    end
    return s
end
function Set.call(f)
    return Set[f]
end
function Set.insert(t, v)
    t[v] = true
end

ts = Set.new {1,2,3,4,5}
ts:insert(5)
ts:insert(6)

for k in pairs(ts) do
    print(k)
end

Now the way I'm reading this code is:

• When I call ts:insert(5), the fact that insert doesn't exist to be called means that the ts metatable is going to be searched for "__call".
• The ts metatable's "__call" key returns Set.call.
• Now Set.call is called with the name insert which causes it to return the Set.insert function.
• Set.insert(ts, 5) is called.

What's really happening is this:

lua: xasm.lua:26: attempt to call method 'insert' (a nil value)
stack traceback:
        xasm.lua:26: in main chunk
        [C]: ?

And at this point I'm stumped. I have absolutely no idea where to go from here. I hacked around for an hour with varying degrees of increasingly desperate variations on this code but the end result is that I have nothing that works. What undoubtedly obvious thing am I overlooking at this point?

Answer 1

Now the way I'm reading this code is:

• When I call ts:insert(5), the fact that insert doesn't exist to be called means that the ts metatable is going to be searched for "__call".

There's your problem. The __call metamethod is consulted when the table itself is called (ie, as a function):

local ts = {}
local mt = {}

function mt.__call(...)
    print("Table called!", ...)
end

setmetatable(ts, mt)

ts() --> prints "Table called!"
ts(5) --> prints "Table called!" and 5
ts"String construct-call" --> prints "Table called!" and "String construct-call"

Object-oriented colon-calls in Lua such as this:

ts:insert(5)

are merely syntactic sugar for

ts.insert(ts,5)

which is itself syntactic sugar for

ts["insert"](ts,5)

As such, the action that is being taken on ts is not a call, but an index (the result of ts["insert"] being what is called), which is governed by the __index metamethod.

The __index metamethod can be a table for the simple case where you want indexing to "fall back" to another table (note that it is the value of the __index key in the metatable that gets indexed and not the metatable itself):

local fallback = {example = 5}
local mt = {__index = fallback}
local ts = setmetatable({}, mt)
print(ts.example) --> prints 5

The __index metamethod as a function works similarly to the signature you expected with Set.call, except that it passes the table being indexed before the key:

local ff = {}
local mt = {}

function ff.example(...)
  print("Example called!",...)
end

function mt.__index(s,k)
  print("Indexing table named:", s.name)
  return ff[k]
end

local ts = {name = "Bob"}
setmetatable(ts, mt)
ts.example(5) --> prints "Indexing table named:" and "Bob",
              --> then on the next line "Example called!" and 5

For more information on metatables, consult the manual.