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

[Dominique Devriese]

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>:
> 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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>
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>
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