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

[Thomas van Noort]

Hi Pedro,

You are right, it is a partial implementation. We chose not to propose 
an implementation for associated datatypes and type families because it 
is unknown if there is a demand for it.

But I don't think coming up with the TH AST modifications for associated 
type synonyms and type families is that much harder. Especially 
associated datatypes are very similar to associated type synonyms. The 
difficult part is in the GHC translation of course.

Regards,
Thomas

José Pedro Magalhães wrote:
> 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 
> <mailto: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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