[Haskell-cafe] Why this existential type could not work?

[Swierstra, S.D.]

On 21 Aug 2014, at 9:08 , Magicloud Magiclouds <magicloud.magiclouds at gmail.com<mailto:magicloud.magiclouds at gmail.com>> wrote:

Thank you. I think I need to understand the different means of forall when it at different positions.

The forall in this case should be "pronounced" as "exists"; reusing the keyword forall was clearly an unfortunate decision.

When should either write:


  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.

    Doaitse




On Thu, Aug 21, 2014 at 2:47 PM, Dominique Devriese <dominique.devriese at cs.kuleuven.be<mailto:dominique.devriese at 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 at gmail.com<mailto:magicloud.magiclouds at 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<http://gmail.com/>.
>
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--
竹密岂妨流水过
山高哪阻野云飞

And for G+, please use magiclouds#gmail.com<http://gmail.com/>.
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