[commit: ghc] master: Allow deriving Typeable for more tycons (92191a3)

[Simon Peyton-Jones]

Oh, it's easy.  If you know what TyCons are defined in this file (and you do, in derivTyDecl), then find the TyCon, and then do
          [ promoted_dc | Just promoted_dc <- map promoteDataCon_maybe (tyConDataCons tc) ]


Simon

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From: josepedromagalhaes at gmail.com [mailto:josepedromagalhaes at gmail.com] On Behalf Of José Pedro Magalhães
Sent: 16 May 2013 10:34
To: Simon Peyton-Jones
Cc: ghc-devs at haskell.org
Subject: Re: [commit: ghc] master: Allow deriving Typeable for more tycons (92191a3)

Hi Simon,

Yes. I still need to do some more work, though, because one thing is not yet working as I would like it
to. Right now, if you define a datatype with -XDataKinds, you can derive Typeable for the promoted
constructors:
data N = Z | S N deriving Typeable -- for N
deriving instance Typeable Z
deriving instance Typeable S

Note that the last two Typeable instances can only be given by standalone deriving. However, in case
the user specifies -XAutoDeriveTypeable, I would expect all three instances to be unnecessary. Right
now, however, we do not automatically derive Typeable instances for promoted constructors. This is
because they are not tycl_decls (as in makeDerivSpecs in TcDeriv). What's the best way to easily
get hold of all the promoted tycons in a module?


Thanks,
Pedro
On Wed, May 8, 2013 at 10:37 AM, Simon Peyton-Jones <simonpj at microsoft.com<mailto:simonpj at microsoft.com>> wrote:
Pedro

Surely should be some user-manual change to go with this?

Simon

| -----Original Message-----
| From: ghc-commits-bounces at haskell.org<mailto:ghc-commits-bounces at haskell.org> [mailto:ghc-commits-<mailto:ghc-commits->
| bounces at haskell.org<mailto:bounces at haskell.org>] On Behalf Of José Pedro Magalhães
| Sent: 07 May 2013 13:27
| To: ghc-commits at haskell.org<mailto:ghc-commits at haskell.org>
| Subject: [commit: ghc] master: Allow deriving Typeable for more tycons
| (92191a3)
|
| Repository : http://darcs.haskell.org/ghc.git/
|
| On branch  : master
|
| https://github.com/ghc/ghc/commit/92191a39a59a036fde4de926e2e322a2c50c84
| cf
|
| >---------------------------------------------------------------
|
| commit 92191a39a59a036fde4de926e2e322a2c50c84cf
| Author: Jose Pedro Magalhaes <jpm at cs.ox.ac.uk<mailto:jpm at cs.ox.ac.uk>>
| Date:   Tue May 7 09:16:29 2013 +0100
|
|     Allow deriving Typeable for more tycons
|
| >---------------------------------------------------------------
|
|  compiler/typecheck/TcDeriv.lhs | 3 ++-
|  1 file changed, 2 insertions(+), 1 deletion(-)
|
| diff --git a/compiler/typecheck/TcDeriv.lhs
| b/compiler/typecheck/TcDeriv.lhs index 7da30d19b..9b82ed6 100644
| --- a/compiler/typecheck/TcDeriv.lhs
| +++ b/compiler/typecheck/TcDeriv.lhs
| @@ -683,7 +683,8 @@ mkEqnHelp :: CtOrigin -> [TyVar] -> Class -> [Type]
| -> Type
|
|  mkEqnHelp orig tvs cls cls_tys tc_app mtheta
|    | Just (tycon, tc_args) <- tcSplitTyConApp_maybe tc_app
| -  , isAlgTyCon tycon    -- Check for functions, primitive types etc
| +  , className cls == typeableClassName || isAlgTyCon tycon
| +  -- Avoid functions, primitive types, etc, unless it's Typeable
|    = mk_alg_eqn tycon tc_args
|    | otherwise
|    = failWithTc (derivingThingErr False cls cls_tys tc_app
|
|
|
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