Thank you. I think I need to understand the different means of forall when it at different positions. On Thu, Aug 21, 2014 at 2:47 PM, Dominique Devriese < dominique.devriese@cs.kuleuven.be> wrote:
Dear Magicloud,
What you're writing is not an existential type. The syntax you used meant that Sealed wraps a value of type "forall a. SomeClass a => TVar a", while you wanted to say that it should contain a value of type "TVar a" for some a that satisfies SomeClass, i.e. an existential type. Below is what I think you want:
Regards, Dominique
{-# LANGUAGE RankNTypes, GADTs #-}
module Test where
import GHC.Conc
class SomeClass a
data Sealed where Sealed :: forall a. SomeClass a => TVar a -> Sealed
mkSealed :: (SomeClass a) => a -> IO Sealed mkSealed = fmap Sealed . newTVarIO
2014-08-21 8:36 GMT+02:00 Magicloud Magiclouds < magicloud.magiclouds@gmail.com>:
Hi,
For example, code like this:
newtype Sealed = Sealed { unSealed :: forall a. SomeClass a => TVar a }
mkSealed :: (SomeClass a) => a -> IO Sealed mkSealed = fmap Sealed . newTVarIO
When compiling, I would get:
Expected type: a -> IO (forall a1. SomeClass a1 => TVar a1) Actual type: a -> IO (TVar a)
How to correctly restrict type parameter a here?
-- 竹密岂妨流水过 山高哪阻野云飞
And for G+, please use magiclouds#gmail.com.
_______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
-- 竹密岂妨流水过 山高哪阻野云飞 And for G+, please use magiclouds#gmail.com.