RFC: explicit support for GADTs in Template Haskell
jan.stolarek at p.lodz.pl
Mon Dec 21 17:52:34 UTC 2015
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
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
Lodz University of Technology
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