#5612: Better support for kinds in Template Haskell
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Reporter: guest | Owner:
Type: feature request | Status: patch
Priority: low | Milestone: 7.6.1
Component: Compiler | Version: 7.3
Keywords: PolyKinds, TemplateHaskell | Os: Unknown/Multiple
Architecture: Unknown/Multiple | Failure: Other
Difficulty: Unknown | Testcase:
Blockedby: | Blocking:
Related: |
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Comment(by simonpj):
Thank you for doing this, goldfire. I'm sorry I've been playing dead. I'm
surfacing slowly.
As you say, the implementation is straightforward, but there are design
choices to make.
* The existing `Type` data type uses `TupleT 3` plus `AppT`s for
3-tuples, and it would really be inconsistent not to continue to do this.
Even arrows work like this! So I think I strongly prefer your "simpler
types" alternative. Indeed that was the way you went for kinds!
* If I have `data Foo = Foo`, then I can use both `Foo` and `'Foo` as a
type constructor, and they will have different `Name`s. Yet I think it is
useful to be able to distinguish the two. So I'm inclined to suggest
{{{
data Type = ...
| VarT Name -- Type variable
| ConT Name -- Ordinary type constructor
| PromotedT Name -- Promoted data constructor
...
}}}
* In GHC we combine `Type` and `Kind` into one data type (called `Type`).
In TH up to now it's been separate, but as you have found you have to add
most of the machinery of `Type` to `Kind`, so the two are almost
duplicates! (Indeed I think you have left `Forall` out of kinds, but it's
needed too.) So I wonder about collapsing the two into one in TH as well.
{{{
type Kind = Type
}}}
I don't have a firm view, but it looks attractive.
* I think you will need `ForallT` in kinds.
Does this make sense?
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/5612#comment:14>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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