Fix Template Haskell's handling of infix GADT constructors

This is the second (and hopefully last) fix needed to make TH handle
GADTs properly (after D1465). This Diff addresses some issues with infix
GADT constructors, specifically:

* Before, you could not determine if a GADT constructor was declared
  infix because TH did not give you the ability to determine if there is
  a //user-specified// fixity declaration for that constructor. The
  return type of `reifyFixity` was changed to `Maybe Fixity` so that it
  yields `Just` the fixity is there is a fixity declaration, and
  `Nothing` otherwise (indicating it has `defaultFixity`).
* `DsMeta`/`Convert` were changed so that infix GADT constructors are
  turned into `GadtC`, not `InfixC` (which should be reserved for
  Haskell98 datatype declarations).
* Some minor fixes to the TH pretty-printer so that infix GADT
  constructors will be parenthesized in GADT signatures.

Fixes #11345.

Test Plan: ./validate

Reviewers: goldfire, austin, bgamari, jstolarek

Reviewed By: jstolarek

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D1744

GHC Trac Issues: #11345

3 files changed, 28 insertions, 10 deletions

diff --git a/libraries/template-haskell/Language/Haskell/TH/Ppr.hs b/libraries/template-haskell/Language/Haskell/TH/Ppr.hs
index 899d27c38f..3f79920a0b 100644
--- a/libraries/template-haskell/Language/Haskell/TH/Ppr.hs
+++ b/libraries/template-haskell/Language/Haskell/TH/Ppr.hs
@@ -66,7 +66,7 @@ instance Ppr Info where
               case mb_d of { Nothing -> empty; Just d -> ppr d }]
 
 ppr_sig :: Name -> Type -> Doc
-ppr_sig v ty = ppr v <+> dcolon <+> ppr ty
+ppr_sig v ty = pprName' Applied v <+> dcolon <+> ppr ty
 
 pprFixity :: Name -> Fixity -> Doc
 pprFixity _ f | f == defaultFixity = empty
@@ -507,20 +507,24 @@ instance Ppr Con where
                          <+> pprBangType st2
 
     ppr (ForallC ns ctxt (GadtC c sts ty))
-        = commaSep c <+> dcolon <+> pprForall ns ctxt <+> pprGadtRHS sts ty
+        = commaSepApplied c <+> dcolon <+> pprForall ns ctxt
+      <+> pprGadtRHS sts ty
 
     ppr (ForallC ns ctxt (RecGadtC c vsts ty))
-        = commaSep c <+> dcolon <+> pprForall ns ctxt
+        = commaSepApplied c <+> dcolon <+> pprForall ns ctxt
       <+> pprRecFields vsts ty
 
     ppr (ForallC ns ctxt con)
         = pprForall ns ctxt <+> ppr con
 
     ppr (GadtC c sts ty)
-        = commaSep c <+> dcolon <+> pprGadtRHS sts ty
+        = commaSepApplied c <+> dcolon <+> pprGadtRHS sts ty
 
     ppr (RecGadtC c vsts ty)
-        = commaSep c <+> dcolon <+> pprRecFields vsts ty
+        = commaSepApplied c <+> dcolon <+> pprRecFields vsts ty
+
+commaSepApplied :: [Name] -> Doc
+commaSepApplied = commaSepWith (pprName' Applied)
 
 pprForall :: [TyVarBndr] -> Cxt -> Doc
 pprForall ns ctxt
@@ -731,7 +735,12 @@ instance Ppr Loc where
 -- Takes a list of printable things and prints them separated by commas followed
 -- by space.
 commaSep :: Ppr a => [a] -> Doc
-commaSep = sep . punctuate comma . map ppr
+commaSep = commaSepWith ppr
+
+-- Takes a list of things and prints them with the given pretty-printing
+-- function, separated by commas followed by space.
+commaSepWith :: (a -> Doc) -> [a] -> Doc
+commaSepWith pprFun = sep . punctuate comma . map pprFun
 
 -- Takes a list of printable things and prints them separated by semicolons
 -- followed by space.
diff --git a/libraries/template-haskell/Language/Haskell/TH/Syntax.hs b/libraries/template-haskell/Language/Haskell/TH/Syntax.hs
index f571d6b16f..a3284c53eb 100644
--- a/libraries/template-haskell/Language/Haskell/TH/Syntax.hs
+++ b/libraries/template-haskell/Language/Haskell/TH/Syntax.hs
@@ -67,7 +67,7 @@ class Monad m => Quasi m where
   qLookupName :: Bool -> String -> m (Maybe Name)
        -- True <=> type namespace, False <=> value namespace
   qReify          :: Name -> m Info
-  qReifyFixity    :: Name -> m Fixity
+  qReifyFixity    :: Name -> m (Maybe Fixity)
   qReifyInstances :: Name -> [Type] -> m [Dec]
        -- Is (n tys) an instance?
        -- Returns list of matching instance Decs
@@ -355,10 +355,13 @@ and to get information about @D@-the-type, use 'lookupTypeName'.
 reify :: Name -> Q Info
 reify v = Q (qReify v)
 
-{- | @reifyFixity nm@ returns the fixity of @nm@. If a fixity value cannot be
-found, 'defaultFixity' is returned.
+{- | @reifyFixity nm@ attempts to find a fixity declaration for @nm@. For
+example, if the function @foo@ has the fixity declaration @infixr 7 foo@, then
+@reifyFixity 'foo@ would return @'Just' ('Fixity' 7 'InfixR')@. If the function
+@bar@ does not have a fixity declaration, then @reifyFixity 'bar@ returns
+'Nothing', so you may assume @bar@ has 'defaultFixity'.
 -}
-reifyFixity :: Name -> Q Fixity
+reifyFixity :: Name -> Q (Maybe Fixity)
 reifyFixity nm = Q (qReifyFixity nm)
 
 {- | @reifyInstances nm tys@ returns a list of visible instances of @nm tys@. That is,
diff --git a/libraries/template-haskell/changelog.md b/libraries/template-haskell/changelog.md
index 9564e95678..1c0919a8a2 100644
--- a/libraries/template-haskell/changelog.md
+++ b/libraries/template-haskell/changelog.md
@@ -37,6 +37,12 @@
   * Add `reifyConStrictness` to query a data constructor's `DecidedStrictness`
     values for its fields (#10697)
 
+  * The `ClassOpI`, `DataConI`, and `VarI` constructors no longer have a
+    `Fixity` field. Instead, all `Fixity` information for a given `Name` is
+    now determined through the `reifyFixity` function, which returns `Just` the
+    fixity if there is an explicit fixity declaration for that `Name`, and
+    `Nothing` otherwise (#10704 and #11345)
+
   * TODO: document API changes and important bugfixes