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

[Nicolas Frisby]

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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