[Haskell-cafe] Multi-Class monadic type?

[Miguel Mitrofanov]

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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