local schema = {}
function schema.new(k, default_value)
  assert(k)
  if default_value then
    local self = {
      assert_get = function(t) return assert(t.getspecific(k).get()) end,
      assert_runtime = function(t) return assert(t.getspecific(k).runtime) end,
      persist = function(t) t.persist(k, function() t.getspecific(k).save() end)
      end,
      load = function(t)
        if t.readonly then
          rawset(t, 'persist', function(...) end)
        end
        return t.register_specific(k, function()
          return default_value
        end)
      end
    }
    return self
  else
    local self = {
      assert_get = function(t) return assert(false, k) end,
      assert_runtime = function(t) return assert(t.getspecific(k).runtime) end,
      persist = function(t) return assert(false, k) end,
      load = function(t) return t.register_specific(k) end
    }
    return self
  end
end

return schema