[GHC] #11345: Template Haskell's handling of infix GADT constructors is broken

3 Jan 2016

      #11345: Template Haskell's handling of infix GADT constructors is broken
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           Reporter:  RyanGlScott    |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Template       |           Version:  8.1
  Haskell                            |
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Incorrect result
  Unknown/Multiple                   |  at runtime
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 There are several infelicities in the way that Template Haskell treats
 GADT constructors that are declared to be infix. To illustrate:

 {{{#!hs
 {-# 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
 [http://git.haskell.org/ghc.git/blob/04f3524f787b2cbd3f460e058c753529d3f2f7ac...
 /template-haskell/Language/Haskell/TH/Ppr.hs#l362 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:

 {{{#!hs
 {-# 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
 }}}

 [http://git.haskell.org/ghc.git/blob/04f3524f787b2cbd3f460e058c753529d3f2f7ac...
 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`).

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11345
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