Deconstructing an existential type

Question

I am using an existential type as a wrapper. At a point in my code where I know the enclosed type, I want to do something with it that is specific to the enclosed type. This

Answer 1

Use Data.Dynamic.

import Data.Dynamic

class Typeable a => Agent a where
  agentId :: a -> String
  -- no need for speciesId

fromAgentBox :: Agent a => AgentBox -> Maybe a
fromAgentBox (AgentBox inner) = fromDynamic (toDyn inner)

instance Agent Bug where
  agentId (Bug name) = name
  -- no need for speciesId

doSomethingWith :: AgentBox -> IO ()
doSomethingWith a = do
  case fromAgentBox a of
    Just bug -> do
      -- Now the compiler knows it's a bug, and I can do something bug-specific
      doBugStuff2 bug
      return ()
    Nothing -> return ()

Alternatively, consider declaring doSomethingWith in the Agent class, perhaps with a default definition.

class Agent a where
  agentId :: a -> String
  -- still don't need speciesId
  doSomethingWith :: a -> IO ()
  doSomethingWith _ = return ()

instance Agent Bug where
  agentId (Bug name) = name
  -- still don't need speciesId
  doSomethingWith bug = do
    -- Now the compiler knows it's a bug, and I can do something bug-specific
    doBugStuff2 bug
    return ()

Finally, I should point out that your AgentBox type is an example of the existential typeclass anti-pattern, so you should perhaps ignore what I've written above and redesign your Agent class as an ordinary datatype.