ImmuTable

This is a native Lua library that allows the creation of immutable tables.

Build

This code has been tested with Lua 5.1.5, Lua 5.2.3, Lua 5.3.5, and LuaJIT 2.0.5. To build, simply make or for LuaJIT: make LUA=luajit (you can also optionally specify LUA_INC=<path to the directory containing lua.h and lauxlib.h> or LUA_BIN=<path to the directory containing the lua binary>).

Usage

Here's a simple implementation of a 2-d vector using immutable tables:

immutable = require "immutable"
Vec = immutable({"x","y"}, {
    name='Vector',
    len2=function(self)
        return self.x*self.x + self.y*self.y
    end,
    class_variable = "classvar",
    __add=function(self, other)
        local cls = getmetatable(self)
        return cls(self.x+other.x, self.y+other.y)
    end,
})
v = Vec(2, 3)
assert(v.x == 2 and v.y == 3)
also_v = Vec(2, 3)
assert(v == also_v)
t = {[v]='yep'}
assert(t[also_v] == 'yep')
assert(v + Vec(0,1) == Vec(2,4))
assert(#v == 2)
assert(v:len2() == 13)
assert(v.class_variable == "classvar")
assert(tostring(v) == 'Vector(x=2, y=3)')
assert(Vec:is_instance(v) and not Vec:is_instance({x=2,y=3}))
for k, v in pairs(v) do
    assert((k == 'x' and v == 2) or (k == 'y' and v == 3))
end
for i, v in ipairs(v) do
    assert((i == 1 and v == 2) or (i == 2 and v == 3))
end
NotVec = immutable({"x","y"})
assert(NotVec(1,2) ~= Vec(1,2))

Singleton recipe

Singletons are pretty straightforward:

Singleton = immutable()
assert(Singleton() == Singleton())

Or if you want methods/class variables:

DogSingleton = immutable(0, {name="DogSingleton", bark=function(self) print("woof") end})
DogSingleton():bark()

Tuple recipe

With immutable tables, it's pretty simple to emulate Python-like tuples:

local tuple_classes = {}
Tuple = function(...)
    local n = select('#', ...)
    if not tuple_classes[n] then
        tuple_classes[n] = immutable(n)
    end
    return tuple_classes[n](...)
end
assert(Tuple(5,6,7) == Tuple(5,6,7))
assert(tostring(Tuple(8,9)) == "(8, 9)")

General purpose immutable table recipe

Using the tuple recipe above, you can make a function that returns an immutable version of a table with arbitrary keys.

local immutable_classes = {}
Immutable = function(t)
    local keys = {}
    for k,_ in pairs(t) do keys[#keys+1] = k end
    keys = Tuple(unpack(keys))
    if not immutable_classes[keys] then
        immutable_classes[keys] = immutable(keys, {name="Immutable"})
    end
    return immutable_classes[keys]:from_table(t)
end
assert(Immutable{x=1} == Immutable{x=1})
assert(Immutable{a=1,b=2,c=3} == Immutable{a=1,b=2,c=3})

Performance

This library is pretty dang fast, but it's still slower than native Lua tables. Based on my benchmarks, in Lua 5.3.3, creating a new 2-member immutable tables takes about 2.83 nanoseconds (compared to 0.47 for a lua table), but creating a new 2-member immutable table that matches one already in memory takes about the same time as making a lua table (0.46 nanoseconds). Accessing members takes about 0.171 nanoseconds on the immutable table, compared to about 0.016 nanoseconds on a lua table. And accessing class members takes about 0.16 nanoseconds on an immutable table, compared to about 0.038 nanoseconds accessing the __index of a lua table's metatable. The numbers on LuaJIT are a bit hard to suss out because LuaJIT does a lot of optimizations on table access, but roughly speaking, immutable tables are faster when running on LuaJIT, but there is a greater discrepancy

Implementation details

Under the hood, immutable tables are implemented in C as a userdata (that stores a hash value) with a metatable. That metatable has a weak-keyed mapping from hash -> userdata -> associated data. Immutable tables are garbage collected, so if you no longer have any references to the userdata, the userdata will get garbage collected, which will result in the entry being removed from the metatable's mapping. When new instances are created, a hash value is computed, and all the data in the associated hash bucket is scanned to look for duplicates. If a match is found, that existing instance is returned, otherwise a new one is created and added to the hash bucket. The following lua pseudocode approximates the C implementation's behavior:

function immutable(fields, class_fields)
    local cls = {
        __index = function(self, key)
            local cls = getmetatable(self)
            if cls.indices[key] then
                local data = cls.__instances[extract_hash(self)]
                return data[cls.indices[key]]
            else
                return cls[key]
            end
        end,
        __gc = function(self)
            local __instances = getmetatable(self).__instances
            local hash = extract_hash(self)
            __instances[hash][self] = nil
            if not next(__instances[hash]) then __instances[hash] = nil end
        end,
        -- also: __len, __pairs, __ipairs, __tostring, from_table, is_instance
    }
    for k,v in pairs(class_fields) do cls[k] = v end
    cls.__fields, cls.__indices = fields, {}
    for i,f in ipairs(fields) do cls.__indices[f] = i end
    cls.__instances = setmetatable({}, {__index=function(self, key)
        local bucket = setmetatable({}, {__mode='k'})
        self[key] = bucket
        return bucket
    end})
    return setmetatable(cls, {
        __call=function(cls, ...)
            local hash = make_hash(...)
            local bucket = cls.__instances[hash]
            for userdata, data in pairs(bucket) do
                local match = true
                for i=1,select('#',...) do
                    if data[i] ~= select(i, ...) then
                        match = false
                        break
                    end
                end
                if match then return userdata end
            end
            local userdata = setmetatable(new_userdata(hash), cls)
            bucket[userdata] = {...}
            return userdata
        end
    })
end