[Template-haskell] Re: [Haskell-cafe] Proposal for associated type synonyms in Template Haskell

[Sean Leather]

It looks like Manuel Chakravarty is making some progress on this.

http://hackage.haskell.org/trac/ghc/ticket/1673#comment:11

Sean

On Thu, Jan 15, 2009 at 21:57, Nicolas Frisby wrote:

> Any movement on this?
>
> (I am actually just looking forward to generating kind ascriptions and
> having access to the kinds when processing TH.Dec, TH.Type, and such.)
>
> 2008/11/27 Simon Peyton-Jones <simonpj at microsoft.com>:
> > I've been away.  I hope others will reply to this thread too; whatever
> you
> > decide will end up in TH indefinitely.  I know that Roman is interested
> in
> > this.
> >
> >
> >
> > ·         You focus just on type families in class declarations (which is
> > indeed where associated types started).  But I suggest you also allow
> them
> > at top level, as GHC does using the syntax
> >
> > type family T a :: *
> >
> > Indeed, since you propose to add to Dec, that'll happen automatically.
>  But
> > perhaps "AssocTySynKindD" is not a good name. Perhaps "TySynFamilyD"?
> >
> >
> >
> > ·         GHC uses
> >
> > type instance T [a] = Tree a
> >
> > as the way to add an equation to the definition of T.  So perhaps
> > "TySynInstance" rather than "AssocTySynD"?
> >
> >
> >
> > ·         I agree that it'd be good to do data type/newtype families at
> the
> > same time.  Roman needs this.
> >
> >
> >
> > ·         Your proposal for kinds looks fine.
> >
> >
> >
> > Simon
> >
> >
> >
> > From: haskell-cafe-bounces at haskell.org
> > [mailto:haskell-cafe-bounces at haskell.org] On Behalf Of José Pedro
> Magalhães
> > Sent: 11 November 2008 11:11
> > To: Haskell Cafe
> > Subject: Re: [Haskell-cafe] Proposal for associated type synonyms in
> > Template Haskell
> >
> >
> >
> > Hello Thomas,
> >
> > I see this is a proposal for a partial implementation of #1673
> > (http://hackage.haskell.org/trac/ghc/ticket/1673). Maybe it would be
> good if
> > the remaining syntax (associated datatypes and type families) would also
> be
> > defined and implemented in TH. Or maybe there isn't much demand for
> this?...
> >
> >
> > Cheers,
> > Pedro
> >
> > On Wed, Nov 5, 2008 at 15:57, Thomas van Noort <thomas at cs.ru.nl> wrote:
> >
> > Hello,
> >
> > Recently, we released a library on Hackage for generic rewriting (package
> > "rewriting" if you are curious). The user of the library is expected to
> > define type class instances to enable rewriting on his or her own
> datatypes.
> > As these instances follow the datatype declarations closely, we tried to
> > generate the instances using Template Haskell. Unfortunately, associated
> > type synonyms are not yet supported by TH.
> >
> > After a presentation at the WGP'08, Simon encouraged us to write a
> proposal
> > about adding associated type synonyms to TH, so that it can be added to
> GHC.
> > So, here is our proposal.
> >
> > The TH AST must allow 1) kind declarations of associated type synonyms
> > in class declarations and 2) their definitions in instance declarations.
> For
> > example,
> >
> > class Foo a where
> >  type Bar a :: *
> >
> > instance Foo Int where
> >  type Bar Int = String
> >
> > The TH library defines a datatype Dec which contains a constructor for
> class
> > declarations and instance declarations:
> >
> > data Dec
> > = ...
> > | ClassD Cxt Name [Name] [FunDep] [Dec]
> > | InstanceD Cxt Type [Dec]
> >  ...
> >
> > 1) Associated type synonym kind declarations
> >
> > We suggest to add a constructor to the Dec type:
> >
> >  ...
> > | AssocTySynKindD Name [Name] (Maybe Kind)
> >  ...
> >
> > assocTySynKindD :: Name -> [Name] -> Maybe KindQ -> DecQ
> >
> > The first field is the name of the associated type synonym, the second
> field
> > is a list of type variables, and the third field is an optional kind.
> Since
> > kinds are not yet defined in TH, we have to add some kind of kind
> definition
> > (pun intended):
> >
> > data Kind
> > = StarK
> > | ArrowK Kind Kind
> >
> > type KindQ = Q Kind
> > starK :: KindQ
> > arrowK :: KindQ -> KindQ -> KindQ
> >
> > We explicitly choose not to reuse the Type type to define kinds (i.e.,
> type
> > Kind = Type as in GHC) since we think a separation between the two worlds
> is
> > much clearer to the users of TH.
> >
> > 2) Associated type synonym definitions
> >
> > We suggest to add another constructor to the Dec type:
> >
> >  ...
> > | AssocTySynD Name [Type] Type
> >  ...
> >
> > assocTySynD :: Name -> [TypeQ] -> TypeQ -> DecQ
> >
> > The first field is the name of the type synonym, the second field is a
> list
> > of type arguments, and the third field is the body of the type synonym.
> >
> > We would like to hear your comments to this proposal.
> >
> > Regards,
> > Thomas
>
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