Thank you. I think I need to understand the different means of forall when it at different positions.
data Sealed where
Sealed :: forall a. SomeClass a => TVar a -> Sealed
or
data Sealed = exists a . SomeClass a=> Sealed (TVar a)
In the former notation the emphasis is on Sealed being used to construct a value, whereas in the second case the emphasis is more on its role as a constituent of a pattern.
_______________________________________________
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
2014-08-21 8:36 GMT+02:00 Magicloud Magiclouds <magicloud.magiclouds@gmail.com>:
mkSealed :: (SomeClass a) => a -> IO Sealed
mkSealed = fmap Sealed . newTVarIO
> _______________________________________________> 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?
>
> --
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