Error message suggests that you've used "Conf" improperly.
testFunc :: (forall a. Conf a, MonadIO m => m a) -> TestType
is illegal, as I recall, you should use another pair of brackets:
testFunc :: (forall a. (Conf a, MonadIO m) => m a) -> TestType
Alexander Treptow wrote:
Hi,
i got a little problem and don't know how to solve that. Hope you can
help me.
code:
----------
module Test where
{-# LANGUAGE Rank2Types, RankNTypes #-}
import Control.Monad.Trans
data TestType = TestType
{tst :: (Conf a, MonadIO m) => m a}
class Conf a where
get :: MonadIO m => m a
testFunc :: (forall a. Conf a, MonadIO m => m a) -> TestType
testFunc = TestType
--------------
error:
--------------
Test.hs:11:23
Class `Conf' used as a type
In the type signature for `testFunc':
testFunc :: (forall a. Conf a, (MonadIO m) => m a) -> TestType
--------------
explanation:
I need a data type that creates a record with a member that has no fixed
type, because its not known at compile-time. The Rank2Types language
extension fits that need, but i figured out that i ll need to make that
time monadic to avoid the use of unsafePerformIO in the program that
uses this lib.
Thanks and greetings,
Alex
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