[GHC] #7704: Phantom types break new polykind Typeable with either StandaloneDeriving or AutoDeriveTypeable
Simon Peyton-Jones
simonpj at microsoft.com
Mon Feb 25 17:11:09 CET 2013
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 at gmail.com [mailto:josepedromagalhaes at gmail.com] On Behalf Of José Pedro Magalhães
Sent: 19 February 2013 10:04
To: ghc-devs at 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 at haskell.org<mailto:cvs-ghc at 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 at haskell.org<mailto:ghc-tickets at 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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