Template Haskell's handling of infix GADT constructors is broken

There are several infelicities in the way that Template Haskell treats GADT constructors that are declared to be infix. To illustrate:

{-# LANGUAGE GADTs #-}
{-# LANGUAGE TemplateHaskell #-}
module Main (main) where

import Language.Haskell.TH

infixr 7 :***:
data GADT a where
  Prefix  :: Int -> Int -> GADT Int
  (:***:) :: Int -> Int -> GADT Int

$(return [])

main :: IO ()
main = do
  putStrLn $(reify ''GADT >>= stringE . pprint)
  putStrLn ""
  putStrLn $(reify ''GADT >>= stringE . show)

This doesn't print out quite what you'd expect:

data Main.GADT (a_0 :: *)
    = Main.Prefix :: GHC.Types.Int ->
                     GHC.Types.Int -> Main.GADT GHC.Types.Int
    | GHC.Types.Int Main.:***: GHC.Types.Int

TyConI (DataD [] Main.GADT [KindedTV a_1627394505 StarT] Nothing [GadtC [Main.Prefix] [(Bang NoSourceUnpackedness NoSourceStrictness,ConT GHC.Types.Int),(Bang NoSourceUnpackedness NoSourceStrictness,ConT GHC.Types.Int)] Main.GADT [ConT GHC.Types.Int],InfixC (Bang NoSourceUnpackedness NoSourceStrictness,ConT GHC.Types.Int) Main.:***: (Bang NoSourceUnpackedness NoSourceStrictness,ConT GHC.Types.Int)] [])

TH thinks that GADT is a Haskell98 data declaration when pprint-ing it because (:***:) is converted to an InfixC (see here for the relevant code). This causes the output to be a strange hodgepodge of Haskell98 and GADT syntax.

Another issue is that even though I can reify GADT, I can't splice it back in! Compiling this:

{-# LANGUAGE GADTs #-}
{-# LANGUAGE TemplateHaskell #-}
module Main (main) where

import Language.Haskell.TH

$(do gadtName   <- newName "GADT"
     prefixName <- newName "Prefix"
     infixName  <- newName ":***:"
     a          <- newName "a"
     return [DataD [] gadtName [KindedTV a StarT] Nothing [GadtC [prefixName] [(Bang NoSourceUnpackedness NoSourceStrictness,ConT ''Int),(Bang NoSourceUnpackedness NoSourceStrictness,ConT ''Int)] gadtName [ConT ''Int],InfixC (Bang NoSourceUnpackedness NoSourceStrictness,ConT ''Int) infixName (Bang NoSourceUnpackedness NoSourceStrictness,ConT ''Int)] []])

$(return [])

main :: IO ()
main = do
  putStrLn $(reify ''GADT >>= stringE . pprint)
  putStrLn ""
  putStrLn $(reify ''GADT >>= stringE . show)

Results in an error:

InfixGADT.hs:12:3: error:
    Cannot mix GADT constructors with Haskell 98 constructors
    When splicing a TH declaration:
      data GADT_0 (a_1 :: *)
    = Prefix_2 :: GHC.Types.Int ->
                  GHC.Types.Int -> GADT_0 GHC.Types.Int
    | GHC.Types.Int :***:_3 GHC.Types.Int

This code is responsible. We have an issue where InfixC can be either Haskell98 or GADT syntax depending on the context, but in that particular context, there's not a good way to determine it.

I can think of three solutions:

1. Add an InfixGadtC constructor. This adds more clutter to Con, but is the most straightforward fix.
2. Subsume infix GADT constructors under GadtC/RecGadtC (depending on if it has records), and treat InfixC as always being Haskell98. This wouldn't require any API changes, but it does leave a bit of asymmetry between the Haskell98 and GADT constructors, since there would be three of the former but two of the latter.
3. A radical approach (which subsumes option 2) would be to deprecate InfixC, subsume it under NormalC/RecC, and add an InfixC pattern synonym for compatibility. InfixC does seem extraneous anyway since you can just use reifyFixity to determine if a constructor is infix. That way, you have two Haskell98 and two GADT constructors (but you'd also have to deprecate InfixC).

Trac metadata

Trac field	Value
Version	8.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Template Haskell
Test case
Differential revisions
BlockedBy
Related
Blocking
CC	goldfire, jstolarek
Operating system
Architecture