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 local testing, immutable tables add a couple nanoseconds to things in the worst case scenario. In LuaJIT, there is a bigger performance discrepancy because immutable tables are about the same, but regular tables are much faster in LuaJIT. Your mileage may vary, but I'd say that immutable tables will probably never be a performance bottleneck for your program, especially if you were previously already using a constructor function and metatables. In some cases, immutable tables may also help reduce your program's memory footprint (if your program has many duplicate objects in memory) and may even improve speed (e.g. if your program uses a lot of deep equality checks). Don't trust this paragraph though! If in doubt, profile your code!

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