-- given a global_db, makes an accessor that fetches named
-- tables from that global_db.
function make_global_accessor(db)
    return function(name)
        local global = rawget(db, name)
        if type(global) ~= "table" then
            global = {}
            rawset(db, name, global)
        end
        return global
    end
end

-- The sandbox list - a list of the builtin functions that gets imported
-- into world models.  This is a very weak form of sandboxing, a world
-- model can still easily use functions that are not on this list.  If you
-- want stronger sandboxing, you need to not link in certain builtins at all.
--
-- Omitted from this list: "debug", "dofile", "getfenv", "io", "jit",
-- "load", "loadfile", "loadstring", "module", "newproxy", "os", "package",
-- "pcall", "require", "xpcall"
--
local sandbox_list = { "assert", "bit", "error", "getmetatable", "inspect",
    "ipairs", "math", "next", "pairs", "print", "rawequal", "rawget", "rawset",
    "select", "setmetatable", "tonumber", "tostring", "type", "unpack" }

-- Make a world model.
--
function make_world_model()
    genv = {}
    genv._G = genv
    meta = {}
    meta.class_db = {}
    meta.global_db = {}
    meta.source_db = {}
    meta.tangible_db = {}
    meta.__newindex = function() error("Global environment is read-only") end
    -- meta.__metatable = false
    genv.class = classdb.accessor(meta.class_db)
    genv.global = make_global_accessor(meta.global_db)
    for i,name in ipairs(sandbox_list) do
        genv[name] = _G[name]
    end
    setmetatable(genv, meta)
    return genv
end