Thanks, i tried it with your code changes but then i ll only get a type mismatch Test.hs:12:11: Couldn't match expected type `forall a. (Conf a, MonadIO m) => m a' against inferred type `forall a (m1 :: * -> *). (Conf a, MonadIO m1) => m1 a' In the expression: TestType In the definition of `testFunc': testFunc = TestType Miguel Mitrofanov schrieb:
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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