Ich verwende einen existenziellen Typ als Wrapper. An einem Punkt in meinem Code, an dem ich den eingeschlossenen Typ kenne, möchte ich etwas damit tun, das spezifisch für den eingeschlossenen Typ ist. Dies ist der nächste, den ich bekommen kann:

 {-# LANGUAGE ExistentialQuantification #-}

class Agent a where
  agentId :: a -> String
  speciesId :: a -> String
  -- plus other functions that all agents support

-- | A wrapper allowing my daemon to read and write agents of any species.
--   (Agents are stored in files that contain a tag so I know which function
--   to call to read the agent.)
data AgentBox = forall a. Agent a => AgentBox { unbox :: a }

instance Agent AgentBox where
  agentId (AgentBox a) = agentId a
  speciesId (AgentBox a) = speciesId a
  -- plus other functions that all agents support

bugTag :: String
bugTag = "Bug"

data Bug = Bug String

instance Agent Bug where
  agentId (Bug name) = name
  speciesId _ = bugTag

doSomethingWith :: AgentBox -> IO ()
doSomethingWith a = do
  if speciesId a == bugTag
    then do
      -- Now I know it's a bug, and I want to do something bug-specific
      doBugStuff2 a
      return ()
    else return ()

doBugStuff :: Bug -> IO ()
doBugStuff a = putStrLn $ agentId a ++ " does bug stuff"

doBugStuff2 AgentBox{unbox=a} = doBugStuff (a `asTypeOf` model) -- line 39
  where model = undefined :: Bug

Der Fehler, den ich erhalte, ist:

Amy30.hs:39:45:
    Could not deduce (a ~ Bug)
    from the context (Agent a)
      bound by a pattern with constructor
                 AgentBox :: forall a. Agent a => a -> AgentBox,
               in an equation for `doBugStuff2'
      at Amy30.hs:39:13-29
      `a' is a rigid type variable bound by
          a pattern with constructor
            AgentBox :: forall a. Agent a => a -> AgentBox,
          in an equation for `doBugStuff2'
          at Amy30.hs:39:13
    In the first argument of `asTypeOf', namely `a'
    In the first argument of `doBugStuff', namely
      `(a `asTypeOf` model)'
    In the expression: doBugStuff (a `asTypeOf` model)
Failed, modules loaded: none.

Wie kann ich das erreichen? Vielen Dank im Voraus für alle Vorschläge.