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
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
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