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

[Thomas van Noort]

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