Re: [Haskell-cafe] existential type problem

16 Oct 2004

      On Sat, Oct 16, 2004 at 10:19:14AM +0900, Koji Nakahara wrote:
...
On Fri, 15 Oct 2004 19:55:02 -0400
Andrew Pimlott  wrote:
...
data Foo = forall t. (MonadTrans t) => Foo
                    ((Monad m, Monad (t m)) => t m a -> m a)    -- run
                    ((Monad m, Monad (t m)) => t m Int)         -- op
prog :: Foo -> IO Int
    prog (Foo run op) = run $ do
      lift $ putStrLn "Running prog"
      op
ghci gives the error
Could not deduce (Monad (t IO)) from the context (MonadTrans t)
      arising from use of `op' at try.hs:22
Your prog leaks m (= IO) out of Foo.  I guess you mean:
...
data Foo m = forall t. (MonadTrans t, Monad (t m)) =>
      Foo (forall a. t m a -> m a) (t m Int)
prog :: Foo IO -> IO Int
prog (Foo run op) = run $ do
      lift $ putStrLn "Running prog"
      op
test = prog (Foo (flip evalStateT 0) get)
But my implementation may look like

    myFoo :: Int -> Foo
    myFoo i = Foo run op where
      run :: Monad m => StateT Int m a -> m a
      run prog  = do  (a, s) <- runStateT prog i
                      return a
      op :: Monad m => StateT Int m Int
      op        = get

which works for all monads, not just IO (that is the idea of using a
tranformer).  So I really don't want to encode that type in Foo.

Andrew