[Haskell-cafe] RFC: explicit support for GADTs in Template Haskell

Roman Cheplyaka roma at ro-che.info
Mon Dec 21 18:30:22 UTC 2015

Hi Janek,

Shouldn't it be represented the same way as in GHC's own AST? Especially
regarding a recent thread about unifying the two (IIRC).

On 12/21/2015 07:52 PM, Jan Stolarek wrote:
> GHC Users,
> I am working on adding proper support for GADTs in Template Haskell. By proper I mean that GADTs 
> data constructors will no longer be encoded using H98 data constructors, but will be represented 
> explicity. 
> GADTs allow to declare several constructors with the same signature:
> data T where
>   MkT1, MkT2 :: T
> The question is whether to represent such constructors in TH syntax as:
>   (1) GadtC [Name] [StrictType] Name [Type]  -- or:
>   (2) GadtC  Name  [StrictType] Name [Type]
> Note the difference in first field. (1) is closer to the original syntax, as it stores the list of 
> all names in a single declaration, as was originally written in the source code. (2) requires to 
> have a separate `GadtC` for each constructor even if constructors were declared together, as in 
> the example above.
> I would like to hear from TH users which of these two representations you prefer. At the moment I 
> have implemented (1) as it directly represents source syntax. The downside of (1) is that 
> information whether several data constructors were declared together is not recoverable during 
> reification, and so reifying T will yield:
> data T where
>   MkT1 :: T
>   MkT2 :: T
> Janek
> ---
> Politechnika Łódzka
> Lodz University of Technology
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