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

12 Nov 2008

      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 mailto:thomas@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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