Używam typu egzystencjalnego jako opakowania. W punkcie mojego kodu, w którym znam typ zamknięty, chcę zrobić coś, co jest specyficzne dla zamkniętego typu. To najbliższy, jaki mogę uzyskać:

 {-# 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

Błąd, który otrzymuję to:

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.

Jak mogę to osiągnąć? Z góry dziękuję za wszelkie sugestie.