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 <[EMAIL PROTECTED]
<mailto:[EMAIL PROTECTED]>> 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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