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
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