Multi-Class monadic type?
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
On 3 February 2010 13:10, Alexander Treptow
I need a data type that creates a record with a member that has no fixed type, because its not known at compile-time.
A type unknown at compile-type pretty much defines a dynamic type, so how about Data.Dynamic? Although Data.Dynamic uses unsafePerformIO there are other ways of encoding dynamic types that don't. Best wishes Stephen
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
_______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
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
_______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Edit: it is running, testFunc :: (forall a m. (Conf a, MonadIO m) => m a) -> TestType is the solution ;) ---------- 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 />>/ />>/ _______________________________________________ />>/ Haskell-Cafe mailing list />>/ Haskell-Cafe at haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe />>/ http://www.haskell.org/mailman/listinfo/haskell-cafe />>/ />/ /
On Wed, Feb 3, 2010 at 1:10 PM, Alexander Treptow
testFunc :: (forall a. Conf a, MonadIO m => m a) -> TestType
At a guess, this function takes a tuple containing a forall a. Conf a and a MonadIO m => m a, which is not what you meant. As Miguel says, more parentheses are the answer.
participants (4)
-
Alexander Treptow -
Ben Millwood -
Miguel Mitrofanov -
Stephen Tetley