Re: [Haskell-cafe] Problem with forall type in type declaration

On 04/05/12 09:08, Magicloud Magiclouds wrote:

Hi, Assuming this: run :: Monad IO a -> IO a data Test = Test { f }

Here I'd like to set f to run, like "Test run". Then what is the type of f? The confusing (me) part is that, the argument pass to f is not fixed on return type, like "f1 :: Monad IO ()", "f2 :: Monad IO Int". So "data Test a = Test { f :: Monad IO a -> IO a} does not work.

You need to explicitly add "forall a." to the type of f. For example: {-# LANGUAGE PolymorphicComponents #-} run :: MonadIO m => m a -> IO a newtype Test = Test { f :: forall m a. MonadIO m a => m a -> IO a } Twan