mp_magic

mp_magic.lua

-- For use with https://github.com/fperrad/lua-MessagePack
-- Usage:
--  local mp = require "MessagePack"
--  local MP = mp_magic(mp)
--  mp.pack{ this_is_a_string = MP{binary = "this_is_a_binary"} }

local mp_magic = function(mp)
    if mp.mpmagic then return mp.mpmagic end
    local mp_mt = {}
    local new = function(t) return setmetatable(t, mp_mt) end
    local orig = mp.packers.table
    local packers = setmetatable({
        map = function(buf, v)
            local n = 0
            for _ in pairs(v) do n = n + 1 end
            return mp.packers.map(buf, v, n)
        end,
        array = function(buf, v)
            return mp.packers.array(buf, v, #v)
        end,
    }, { __index = mp.packers })
    local _packer = function(buf, t)
        if getmetatable(t) == mp_mt then
            local k, v = next(t)
            if k and type(k) == "string" and packers[k] then
                return packers[k](buf, v)
            end
        end
        return orig(buf, t)
    end
    mp.packers.table = _packer
    mp.mpmagic = new
    return new
end

mp_magic_2.lua

-- For use with https://github.com/fperrad/lua-MessagePack
-- Usage:
--  local mp = require "MessagePack"
--  local MP = mp_magic(mp)
--  mp.pack {
--      a_binary = MP.binary(""),
--      a_string = MP.string(""),
--      a_map = MP.map({}),
--      an_array = MP.array({}),
--      a_float = MP.float(42),
--  }

local mp_magic = function(mp)
    if mp.mpmagic then return mp.mpmagic end
    local unknown = function(t, k)
        t[k] = { packer = mp.packers[k] }
        return t[k]
    end
    mts = setmetatable({
        map = {
            packer = function(buf, v)
                local n = 0
                for _ in pairs(v) do n = n + 1 end
                return mp.packers.map(buf, v, n)
            end
        },
        array = {
            packer = function(buf, v)
                return mp.packers.array(buf, v, #v)
            end
        },
    }, {
        __index = function(t, k)
            t[k] = { packer = mp.packers[k] }
            return t[k]
        end
    })
    mp.mpmagic = setmetatable({}, {
        __index = function(t, k)
            local mt = mts[k]
            if mt then return function(v)
                return setmetatable({v}, mt)
            end end
        end
    })
    local orig = mp.packers.table
    mp.packers.table = function(buf, t)
        local mt = getmetatable(t)
        if mt and mt.packer and type(mt.packer) == "function" then
            return mt.packer(buf, t[1])
        end
        return orig(buf, t)
    end
    return mp.mpmagic
end

Simpler and often enough, from fperrad:

local mp = require 'MessagePack'
mp.packers['function'] = function (buffer, fct)
    fct(buffer)
end

local function BINARY (str)
    return function (buffer)
        mp.packers['binary'](buffer, str)
    end
end

local function FLOAT (n)
    return function (buffer)
        mp.packers['float'](buffer, n)
    end
end

mp.pack { 'encoded_with_global_settings', BINARY'encoded_as_binary', 42, FLOAT(42) }

The way I use this:

do -- binary packer
    if not rawget(mp.packers, "function") then
        rawset(mp.packers, "function", function(buf, f) f(buf) end)
    end
    mp.BIN = function(s)
        return function(buf) mp.packers["binary"](buf, s) end
    end
end