[Haskell-cafe] Confusion with GeneralizedNewtypeDeriving + MultiParamTypeClasses

[Nikita Karetnikov]

> Does anyone have any advice for how to make this work? It's quite tempting
> just to make `type Neuron a = Vector a`, but I'd rather do a proper wrapper
> type.

Here's my attempt (no, I didn't succeed):

{-# LANGUAGE GeneralizedNewtypeDeriving #-}
{-# LANGUAGE StandaloneDeriving #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE DeriveAnyClass #-}
{-# LANGUAGE ExistentialQuantification #-}

import qualified Data.Vector.Generic as Generic
import Data.Vector

newtype Neuron a = Neuron (Vector a)

--deriving instance (Generic.Vector Neuron a)
-- Can't make a derived instance of ‘Generic.Vector Neuron a’:
--   The last argument of the instance must be a data or newtype application
-- In the stand-alone deriving instance for ‘Generic.Vector Neuron a’

newtype Any a = Any a
--deriving instance (Generic.Vector Neuron (Any a))
{-
Var/Type length mismatch: 
  [a_aGM]
  []
Var/Type length mismatch: 
  [a_aGM]
  []
Var/Type length mismatch: 
  [a_aGM]
  []
Var/Type length mismatch: 
  [a_aGM]
  []
Var/Type length mismatch: 
  [a_aGM]
  []
Var/Type length mismatch: 
  [a_aGM]
  []
Var/Type length mismatch: 
  [a_aGM]
  []
Var/Type length mismatch: 
  [a_aGM]
  []
Var/Type length mismatch: 
  [a_aGM]
  []
ghc: panic! (the 'impossible' happened)
  (GHC version 7.10.1 for x86_64-unknown-linux):
	tcTyVarDetails a_aGM

Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug
-}

-- data Foo = Foo
-- deriving instance (Generic.Vector Neuron Foo)
-- Ditto.

-- data Bar = forall a. Bar a
-- deriving instance (Generic.Vector Neuron Bar)
-- Same error.
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