Fast serialization for Lua

local marshal = require "marshal"

Provides:

• s = marshal.encode(v[, constants]) - serializes a value to a byte stream
• t = marshal.decode(s[, constants]) - deserializes a byte stream to a value
• t = marshal.clone(orig[, constants]) - deep clone a value (deep for tables and functions)

Features:

Serializes tables, which may contain cycles, Lua functions with upvalues and basic data types.

All functions take an optional constants table which, if encountered during serialization, are simply referenced from the constants table passed during deserialization. For example:

local orig = { answer = 42, print = print }
local pack = marshal.encode(orig, { print })
local copy = marshal.decode(pack, { print })
assert(copy.print == print)

Hooks

A hook is provided for influencing serializing behaviour via the __persist metamethod. The __persist metamethod is expected to return a closure which is called during deserialization. The return value of the closure is taken as the final decoded result.

This is useful for serializing both userdata and for use with object-oriented Lua, since metatables are not serialized.

For example:

local Point = { }
function Point:new(x, y)
   self.__index = self
   return setmetatable({ x = x, y = y }, self)
end
function Point:move(x, y)
   self.x = x
   self.y = y
end
function Point:__persist()
   local x = self.x
   local y = self.y
   return function()
      -- do NOT refer to self in this scope
      return setmetatable({ x = x, y = y }, Point)
   end
end

The above shows a way to persist an "instance" of Point (if you're thinking in OO terms). In this case Point itself will be included in the encoded chunk because it's referenced as an upvalue of the returned closure.

The __persist hook may NOT refer to the receiver (i.e. self in the example) because this will cause deep recursion when upvalues are serialized.

Limitations:

Coroutines are not serialized. Userdata doesn't serialize either however support for userdata the __persist metatable hook can be used.

Metatables and function environments are not serialized.

Attempt to serialize C functions, threads and userdata without a __persist hook raises an exception.

Serialized code is not portable.