my first attempt at implementing Lisp

lisp.lua

#!/usr/bin/env lua
local eval

local global = {
    ["+"] = function(args)
        local acc = 0
        for i = 1, #args do
            acc = acc + args[i]
        end
        return acc
    end,
    ["*"] = function(args)
        local acc = 1
        for i = 1, #args do
            acc = acc * args[i]
        end
        return acc
    end,
    define = function(args, env)
        local name, val = unpack(args)
        env[name] = val
        return val
    end,
    lambda = function(args, env)
        local arg_names = args[1]
        local env_mt = { __index = env }
        return function(arg_list)
            local _local = setmetatable({}, env_mt)
            for i, val in ipairs(arg_list) do
                _local[arg_names[i]] = val
            end
            local res
            for i = 2, #args do
                res = eval(args[i], _local)
            end
            return res
        end
    end,
    map = function(args, env)
        local fn, list = unpack(args)
        local res = {}
        for i = 1, #list do
            res[i] = fn({ list[i] }, env)
        end
        return res
    end,
}

function eval(code, env)
    env = env or global
    if type(code) == "string" then
        return env[code] or code    --FIXME: strings are identifiers AND strings
    elseif type(code) ~= "table" then
        return code
    elseif code.quote then
        code.quote = nil
        return code
    end
    local list = {}
    local no_eval = code[1] == "lambda"
    for i, item in ipairs(code) do
        if no_eval and i > 1 then
            list[i] = item
        else
            list[i] = eval(item, env)
        end
    end
    local fn = table.remove(list, 1)
    return fn(list, env)
end

local function quote(list)
    list.quote = true
    return list
end


eval{ "define", "double", { "lambda", { "x" }, { "*", "x", 2 } } }
print(eval{ "double", { "*", { "+", 1, 2 }, 4 } })

eval{ "define", "i", "6" }
eval{ "define", "test", { "lambda", {},
        { "define", "i", 33 },
        { "lambda", {}, "i" }
    }
}
eval{ "define", "fn", { "test" } }
print(eval{ "fn" })
print(eval("i"))

eval{ "define", "stuff", { "map", "double", quote{ 1, 2, 3, 4, 5 } }}
print(unpack(eval("stuff")))