[Haskell-cafe] existential type problem

Andrew Pimlott andrew at pimlott.net
Fri Oct 15 19:55:02 EDT 2004

I'm having trouble using an existential type, becasue of what seem to be
limitations in how I can express the type.  I want values in my
existential type to denote, for some monad, a monadic operation and a
way to run the monad.  Except, I want it mix the operation with
operations in another monad, so it use a monad transformer.

    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"

ghci gives the error

    Could not deduce (Monad (t IO)) from the context (MonadTrans t)
      arising from use of `op' at try.hs:22

Hmm....  I see that my constraints did not mean what I expected.  As the
"Probable fix" suggests, I can add Monad (t IO) to the existential
context to make this code type check, but I want it to work for all
monads, not just IO.  Something like

    data Foo = forall t. (MonadTrans t, (forall m. Monad m => Monad (t m)))
                    => Foo ...

But I haven't found anything like that.

I would also welcome other approaches to this problem.


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