I suppose we want an instance
forall k. instance Typeable (k -> *) (Proxy k)

Yes, that looks right.

So the universally quantified kind variables of the tycon should end up universally
quantified in the instance as well. How does this translate to the ds_tvs, ds_tys, and
ds_tc_args arguments of a DerivSpec

My instinct is that the ds_tvs include both type and kind variables, kind variables first.  The kinds of the type variables can mention the kind variables.

The ds_tys field shoud include kind arguments as well as type arguments.

Does that work?

Simon

From: josepedromagalhaes@gmail.com [mailto:josepedromagalhaes@gmail.com] On Behalf Of José Pedro Magalhães
Sent: 19 February 2013 10:04
To: ghc-devs@haskell.org; Simon Peyton-Jones
Subject: Fwd: [GHC] #7704: Phantom types break new polykind Typeable with either StandaloneDeriving or AutoDeriveTypeable

Hi Simon,

How should an instance of Typeable for a polykinded tycon look like? E.g. if we have

data Proxy (a :: k) = Proxy
-- Proxy :: forall (k :: BOX). k -> *

I suppose we want an instance

forall k. instance Typeable (k -> *) (Proxy k)

Right? So the universally quantified kind variables of the tycon should end up universally
quantified in the instance as well. How does this translate to the ds_tvs, ds_tys, and
ds_tc_args arguments of a DerivSpec?...


Thanks,
Pedro

---------- Forwarded message ----------
From: GHC <cvs-ghc@haskell.org>
Date: Mon, Feb 18, 2013 at 8:42 PM
Subject: [GHC] #7704: Phantom types break new polykind Typeable with either StandaloneDeriving or AutoDeriveTypeable
To:
Cc: ghc-tickets@haskell.org


#7704: Phantom types break new polykind Typeable with either StandaloneDeriving or
AutoDeriveTypeable
-----------------------------+----------------------------------------------
Reporter:  nwf               |          Owner:
    Type:  bug               |         Status:  new
Priority:  normal            |      Component:  Compiler
 Version:  7.7               |       Keywords:
      Os:  Unknown/Multiple  |   Architecture:  Unknown/Multiple
 Failure:  None/Unknown      |      Blockedby:
Blocking:                    |        Related:
-----------------------------+----------------------------------------------
 This test program, reduced from an attempt to bring ekmett's tagged
 package to HEAD,
 {{{
 {-# LANGUAGE DeriveDataTypeable #-}
 {-# LANGUAGE PolyKinds #-}
 {-# LANGUAGE AutoDeriveTypeable #-}

 newtype Tagged s b = Tagged { unTagged :: b }
 }}}
 results in
 {{{
 GHCi, version 7.7.20130218: http://www.haskell.org/ghc/  :? for help
 Loading package ghc-prim ... linking ... done.
 Loading package integer-gmp ... linking ... done.
 Loading package base ... linking ... done.
 [1 of 1] Compiling Main             ( typeable.hs, interpreted )

 typeable.hs:5:1:
     Derived typeable instance must be of form (Typeable Tagged)
     In the stand-alone deriving instance for
       `Data.Typeable.Internal.Typeable Tagged'
 }}}

 This message is also emitted if I replace `AutoDeriveTypeable`
 `StandaloneDeriving` and `deriving instance Typeable Tagged`.

 However, a "deriving (Typeable)" clause on the newtype definition works
 just fine.

--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7704>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler

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