# ImmuTable

This is a Lua library that allows the creation of lightweight 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:
```lua
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:
```lua
Singleton = immutable()
assert(Singleton() == Singleton())
```
Or if you want methods/class variables:
```lua
DogSingleton = immutable(0, {name="DogSingleton", bark=function(self) print("woof") end})
DogSingleton():bark()
```

## Tuples
If no field names are passed in, `immutable()` defaults to creating immutable tables that behave like Python's tuples:
```lua
local Tuple = immutable()
local t0 = Tuple()
local t1 = Tuple(1,2)
local t2 = Tuple(1,2,3,4,5)
assert(t0 == Tuple())
assert(({[t1]='yep'})[Tuple(1,2)])
assert(tostring(Tuple(1,2)) == "(1, 2)")
```

## General purpose immutable table recipe
Using tuples, you can make a function that returns an immutable version of a table with arbitrary keys, though it is a little bit hacky.
```lua
local Tuple = immutable()
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 table operations in the worst case scenario. In LuaJIT, there is a bigger performance discrepancy because regular tables are more heavily optimized 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 use them in place of code that already used 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

You can read more about the implementation details in the [implementation documentation](./implementation.md).