按类型将消息与处理程序匹配

Question

我有一个大的打开的消息类型集和处理程序Message -> IO ()列表，每个处理程序都期望某种类型的消息而忽略所有其他消息。

需要一个函数anymessage -> [Message -> IO ()] -> IO ()。我可以用type Message = Dynamic甚至type Message = String(和show/read)来做这个工作。

有更清洁的解决方案吗?

Answer 1

这听起来像通常的可扩展异常/消息处理程序。看到:

• /An Extensible dynamic - typed Hierarchy of Exceptions/， Simon Marlow, 2006.

参见XMonad的Message类:

--
-- User-extensible messages must be a member of this class.
--
class Typeable a => Message a
-- |
-- A wrapped value of some type in the 'Message' class.
--
data SomeMessage = forall a. Message a => SomeMessage a
-- |
-- And now, unwrap a given, unknown 'Message' type, performing a (dynamic)
-- type check on the result.
--
fromMessage :: Message m => SomeMessage -> Maybe m
fromMessage (SomeMessage m) = cast m
-- X Events are valid Messages.
instance Message Event

等等。