Constructor and pattern synonym argument docs (#709)

* Support Haddocks on constructor arguments

This is in conjunction with https://phabricator.haskell.org/D4094.
Adds support for rendering Haddock's on (non-record) constructor
arguments, both for regular and GADT constructors.

* Support haddocks on pattern synonym arguments

It appears that GHC already parsed these - we just weren't using them.
In the process of doing this, I tried to deduplicate some code around
handling patterns.

* Update the markup guide

Add some information about the new support for commenting constructor
arguments, and mention pattern synonyms and GADT-style constructors.

* Overhaul LaTeX support for data/pattern decls

This includes at least

  * fixing several bugs that resulted in invalid LaTeX
  * fixing GADT data declaration headers
  * overhaul handling of record fields
  * overhaul handling of GADT constructors
  * overhaul handling of bundled patterns
  * add support for constructor argument docs

* Support GADT record constructors

This means changes what existing HTML docs look like.

As for LaTeX, looks like GADT records were never even supported. Now they are.

* Clean up code/comments

Made code/comments consistent between the LaTeX and XHTML backend
when possible.

* Update changelog

* Patch post-rebase regressions

* Another post-rebase change

We want return values to be documentable on record GADT constructors.

6 files changed, 595 insertions, 353 deletions

diff --git a/CHANGES.md b/CHANGES.md
index f4602e85..9ba8be07 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -22,6 +22,9 @@
  * Overhaul Haddock's rendering of kind signatures so that invisible kind
    parameters are not printed (#681) (Fixes #544)
 
+ * Overhaul handling of data declarations in XHTML and LaTeX. Adds support for
+   documenting individual arguments of constructors/patterns (#709)
+
 ## Changes in version 2.18.1
 
  * Synopsis is working again (#599)
diff --git a/doc/markup.rst b/doc/markup.rst
index d0b9392d..4d56cc83 100644
--- a/doc/markup.rst
+++ b/doc/markup.rst
@@ -39,6 +39,8 @@ the following:
 
 -  A ``data`` declaration,
 
+-  A ``pattern`` declaration,
+
 -  A ``newtype`` declaration,
 
 -  A ``type`` declaration
@@ -118,6 +120,15 @@ Constructors are documented like so: ::
 or like this: ::
 
     data T a b
+      = C1   -- ^ This is the documentation for the 'C1' constructor
+          a  -- ^ This is the documentation for the argument of type 'a'
+          b  -- ^ This is the documentation for the argument of type 'b'
+
+There is one edge case that is handled differently: only one ``-- ^``
+annotation occuring after the constructor and all its arguments is
+applied to the constructor, not its last argument: ::
+
+    data T a b
       = C1 a b  -- ^ This is the documentation for the 'C1' constructor
       | C2 a b  -- ^ This is the documentation for the 'C2' constructor
 
@@ -164,6 +175,9 @@ Individual arguments to a function may be documented like this: ::
        -> Float    -- ^ The 'Float' argument
        -> IO ()    -- ^ The return value
 
+Pattern synonyms and GADT-style data constructors also support this
+style of documentation.
+
 .. _module-description:
 
 The Module Description
diff --git a/haddock-api/src/Haddock/Backends/LaTeX.hs b/haddock-api/src/Haddock/Backends/LaTeX.hs
index 793e40d8..51e183c7 100644
--- a/haddock-api/src/Haddock/Backends/LaTeX.hs
+++ b/haddock-api/src/Haddock/Backends/LaTeX.hs
@@ -179,13 +179,14 @@ string_txt (PStr s1) s2 = unpackFS s1 ++ s2
 string_txt (ZStr s1) s2 = zString s1 ++ s2
 string_txt (LStr s1 _) s2 = unpackLitString s1 ++ s2
 
-
+-- | Prints out an entry in a module export list.
 exportListItem :: ExportItem DocNameI -> LaTeX
 exportListItem ExportDecl { expItemDecl = decl, expItemSubDocs = subdocs }
-  = sep (punctuate comma . map ppDocBinder $ declNames decl) <>
-     case subdocs of
-       [] -> empty
-       _  -> parens (sep (punctuate comma (map (ppDocBinder . fst) subdocs)))
+  = let (leader, names) = declNames decl
+    in sep (punctuate comma [ leader <+> ppDocBinder name | name <- names ]) <>
+         case subdocs of
+           [] -> empty
+           _  -> parens (sep (punctuate comma (map (ppDocBinder . fst) subdocs)))
 exportListItem (ExportNoDecl y [])
   = ppDocBinder y
 exportListItem (ExportNoDecl y subs)
@@ -249,13 +250,17 @@ ppDocGroup lev doc = sec lev <> braces doc
         sec _ = text "\\paragraph"
 
 
-declNames :: LHsDecl DocNameI -> [DocName]
+-- | Given a declaration, extract out the names being declared
+declNames :: LHsDecl DocNameI
+          -> ( LaTeX           -- ^ to print before each name in an export list
+             , [DocName]       -- ^ names being declared
+             )
 declNames (L _ decl) = case decl of
-  TyClD d  -> [tcdName d]
-  SigD (TypeSig lnames _ ) -> map unLoc lnames
-  SigD (PatSynSig lnames _) -> map unLoc lnames
-  ForD (ForeignImport (L _ n) _ _ _) -> [n]
-  ForD (ForeignExport (L _ n) _ _ _) -> [n]
+  TyClD d  -> (empty, [tcdName d])
+  SigD (TypeSig lnames _ ) -> (empty, map unLoc lnames)
+  SigD (PatSynSig lnames _) -> (text "pattern", map unLoc lnames)
+  ForD (ForeignImport (L _ n) _ _ _) -> (empty, [n])
+  ForD (ForeignExport (L _ n) _ _ _) -> (empty, [n])
   _ -> error "declaration not supported by declNames"
 
 
@@ -278,47 +283,44 @@ moduleBasename mdl = map (\c -> if c == '.' then '-' else c)
 -- * Decls
 -------------------------------------------------------------------------------
 
-
-ppDecl :: LHsDecl DocNameI
-       -> [(HsDecl DocNameI, DocForDecl DocName)]
-       -> DocForDecl DocName
-       -> [DocInstance DocNameI]
-       -> [(DocName, DocForDecl DocName)]
-       -> [(DocName, Fixity)]
+-- | Pretty print a declaration
+ppDecl :: LHsDecl DocNameI                         -- ^ decl to print
+       -> [(HsDecl DocNameI, DocForDecl DocName)]  -- ^ all pattern decls
+       -> DocForDecl DocName                       -- ^ documentation for decl
+       -> [DocInstance DocNameI]                   -- ^ all instances
+       -> [(DocName, DocForDecl DocName)]          -- ^ all subdocs
+       -> [(DocName, Fixity)]                      -- ^ all fixities
        -> LaTeX
 
-ppDecl (L loc decl) pats (doc, fnArgsDoc) instances subdocs _fixities = case decl of
-  TyClD d@(FamDecl {})          -> ppTyFam False loc doc d unicode
-  TyClD d@(DataDecl {})
-                                -> ppDataDecl pats instances subdocs loc (Just doc) d unicode
-  TyClD d@(SynDecl {})          -> ppTySyn loc (doc, fnArgsDoc) d unicode
+ppDecl decl pats (doc, fnArgsDoc) instances subdocs _fxts = case unLoc decl of
+  TyClD d@FamDecl {}         -> ppTyFam False doc d unicode
+  TyClD d@DataDecl {}        -> ppDataDecl pats instances subdocs (Just doc) d unicode
+  TyClD d@SynDecl {}         -> ppTySyn (doc, fnArgsDoc) d unicode
 -- Family instances happen via FamInst now
---  TyClD d@(TySynonym {})
+--  TyClD d@TySynonym{}
 --    | Just _  <- tcdTyPats d    -> ppTyInst False loc doc d unicode
 -- Family instances happen via FamInst now
-  TyClD d@(ClassDecl {})    -> ppClassDecl instances loc doc subdocs d unicode
-  SigD (TypeSig lnames t)   -> ppFunSig loc (doc, fnArgsDoc) (map unLoc lnames)
-                                        (hsSigWcType t) unicode
-  SigD (PatSynSig lnames ty) ->
-      ppLPatSig loc (doc, fnArgsDoc) (map unLoc lnames) ty unicode
-  ForD d                         -> ppFor loc (doc, fnArgsDoc) d unicode
-  InstD _                        -> empty
-  DerivD _                       -> empty
-  _                              -> error "declaration not supported by ppDecl"
+  TyClD d@ClassDecl{}        -> ppClassDecl instances doc subdocs d unicode
+  SigD (TypeSig lnames ty)   -> ppFunSig (doc, fnArgsDoc) (map unLoc lnames) (hsSigWcType ty) unicode
+  SigD (PatSynSig lnames ty) -> ppLPatSig (doc, fnArgsDoc) (map unLoc lnames) ty unicode
+  ForD d                     -> ppFor (doc, fnArgsDoc) d unicode
+  InstD _                    -> empty
+  DerivD _                   -> empty
+  _                          -> error "declaration not supported by ppDecl"
   where
     unicode = False
 
 
-ppTyFam :: Bool -> SrcSpan -> Documentation DocName ->
+ppTyFam :: Bool -> Documentation DocName ->
               TyClDecl DocNameI -> Bool -> LaTeX
-ppTyFam _ _ _ _ _ =
+ppTyFam _ _ _ _ =
   error "type family declarations are currently not supported by --latex"
 
 
-ppFor :: SrcSpan -> DocForDecl DocName -> ForeignDecl DocNameI -> Bool -> LaTeX
-ppFor loc doc (ForeignImport (L _ name) typ _ _) unicode =
-  ppFunSig loc doc [name] (hsSigType typ) unicode
-ppFor _ _ _ _ = error "ppFor error in Haddock.Backends.LaTeX"
+ppFor :: DocForDecl DocName -> ForeignDecl DocNameI -> Bool -> LaTeX
+ppFor doc (ForeignImport (L _ name) typ _ _) unicode =
+  ppFunSig doc [name] (hsSigType typ) unicode
+ppFor _ _ _ = error "ppFor error in Haddock.Backends.LaTeX"
 --  error "foreign declarations are currently not supported by --latex"
 
 
@@ -328,18 +330,18 @@ ppFor _ _ _ _ = error "ppFor error in Haddock.Backends.LaTeX"
 
 
 -- we skip type patterns for now
-ppTySyn :: SrcSpan -> DocForDecl DocName -> TyClDecl DocNameI -> Bool -> LaTeX
+ppTySyn :: DocForDecl DocName -> TyClDecl DocNameI -> Bool -> LaTeX
 
-ppTySyn loc doc (SynDecl { tcdLName = L _ name, tcdTyVars = ltyvars
+ppTySyn doc (SynDecl { tcdLName = L _ name, tcdTyVars = ltyvars
                          , tcdRhs = ltype }) unicode
-  = ppTypeOrFunSig loc [name] (unLoc ltype) doc (full, hdr, char '=') unicode
+  = ppTypeOrFunSig (unLoc ltype) doc (full, hdr, char '=') unicode
   where
     hdr  = hsep (keyword "type"
                  : ppDocBinder name
                  : map ppSymName (tyvarNames ltyvars))
     full = hdr <+> char '=' <+> ppLType unicode ltype
 
-ppTySyn _ _ _ _ = error "declaration not supported by ppTySyn"
+ppTySyn _ _ _ = error "declaration not supported by ppTySyn"
 
 
 -------------------------------------------------------------------------------
@@ -347,61 +349,98 @@ ppTySyn _ _ _ _ = error "declaration not supported by ppTySyn"
 -------------------------------------------------------------------------------
 
 
-ppFunSig :: SrcSpan -> DocForDecl DocName -> [DocName] -> LHsType DocNameI
+ppFunSig :: DocForDecl DocName -> [DocName] -> LHsType DocNameI
          -> Bool -> LaTeX
-ppFunSig loc doc docnames (L _ typ) unicode =
-  ppTypeOrFunSig loc docnames typ doc
+ppFunSig doc docnames (L _ typ) unicode =
+  ppTypeOrFunSig typ doc
     ( ppTypeSig names typ False
     , hsep . punctuate comma $ map ppSymName names
-    , dcolon unicode)
+    , dcolon unicode
+    )
     unicode
  where
    names = map getName docnames
 
-ppLPatSig :: SrcSpan -> DocForDecl DocName -> [DocName]
-          -> LHsSigType DocNameI
-          -> Bool -> LaTeX
-ppLPatSig _loc (doc, _argDocs) docnames ty unicode
-  = declWithDoc pref1 (documentationToLaTeX doc)
+-- | Pretty-print a pattern synonym
+ppLPatSig :: DocForDecl DocName  -- ^ documentation
+          -> [DocName]           -- ^ pattern names in the pattern signature
+          -> LHsSigType DocNameI -- ^ type of the pattern synonym
+          -> Bool                -- ^ unicode
+          -> LaTeX
+ppLPatSig doc docnames ty unicode
+  = ppTypeOrFunSig typ doc
+      ( keyword "pattern" <+> ppTypeSig names typ False
+      , keyword "pattern" <+> (hsep . punctuate comma $ map ppSymName names)
+      , dcolon unicode
+      )
+      unicode
   where
-    pref1 = hsep [ keyword "pattern"
-                 , hsep $ punctuate comma $ map ppDocBinder docnames
-                 , dcolon unicode
-                 , ppLType unicode (hsSigType ty)
-                 ]
-
-ppTypeOrFunSig :: SrcSpan -> [DocName] -> HsType DocNameI
-               -> DocForDecl DocName -> (LaTeX, LaTeX, LaTeX)
-               -> Bool -> LaTeX
-ppTypeOrFunSig _ _ typ (doc, argDocs) (pref1, pref2, sep0)
-               unicode
-  | Map.null argDocs =
-      declWithDoc pref1 (documentationToLaTeX doc)
-  | otherwise        =
-      declWithDoc pref2 $ Just $
+    typ = unLoc (hsSigType ty)
+    names = map getName docnames
+
+-- | Pretty-print a type, adding documentation to the whole type and its
+-- arguments as needed.
+ppTypeOrFunSig :: HsType DocNameI
+               -> DocForDecl DocName  -- ^ documentation
+               -> ( LaTeX             -- ^ first-line (no-argument docs only)
+                  , LaTeX             -- ^ first-line (argument docs only)
+                  , LaTeX             -- ^ type prefix (argument docs only)
+                  )
+               -> Bool                -- ^ unicode
+               -> LaTeX
+ppTypeOrFunSig typ (doc, argDocs) (pref1, pref2, sep0) unicode
+  | Map.null argDocs = declWithDoc pref1 (documentationToLaTeX doc)
+  | otherwise        = declWithDoc pref2 $ Just $
         text "\\haddockbeginargs" $$
-        do_args 0 sep0 typ $$
+        vcat (map (uncurry (<->)) (ppSubSigLike unicode typ argDocs [] sep0)) $$
         text "\\end{tabulary}\\par" $$
         fromMaybe empty (documentationToLaTeX doc)
+
+-- This splits up a type signature along `->` and adds docs (when they exist)
+-- to the arguments. The output is a list of (leader/seperator, argument and
+-- its doc)
+ppSubSigLike :: Bool                  -- ^ unicode
+             -> HsType DocNameI       -- ^ type signature
+             -> FnArgsDoc DocName     -- ^ docs to add
+             -> [(DocName, DocForDecl DocName)] -- ^ all subdocs (useful when we have `HsRecTy`)
+             -> LaTeX                 -- ^ seperator (beginning of first line)
+             -> [(LaTeX, LaTeX)]      -- ^ arguments (leader/sep, type)
+ppSubSigLike unicode typ argDocs subdocs leader = do_args 0 leader typ
   where
-     do_largs n leader (L _ t) = do_args n leader t
-
-     arg_doc n = rDoc . fmap _doc $ Map.lookup n argDocs
-
-     do_args :: Int -> LaTeX -> HsType DocNameI -> LaTeX
-     do_args _n leader (HsForAllTy tvs ltype)
-       = decltt leader
-         <-> decltt (hsep (forallSymbol unicode : ppTyVars tvs ++ [dot]))
-         <+> ppLType unicode ltype
-     do_args n leader (HsQualTy lctxt ltype)
-       = decltt leader
-         <-> ppLContextNoArrow lctxt unicode <+> nl $$
-             do_largs n (darrow unicode) ltype
-     do_args n leader (HsFunTy lt r)
-       = decltt leader <-> decltt (ppLFunLhType unicode lt) <-> arg_doc n <+> nl $$
-         do_largs (n+1) (arrow unicode) r
-     do_args n leader t
-       = decltt leader <-> decltt (ppType unicode t) <-> arg_doc n <+> nl
+    do_largs n leader (L _ t) = do_args n leader t
+
+    arg_doc n = rDoc . fmap _doc $ Map.lookup n argDocs
+
+    do_args :: Int -> LaTeX -> HsType DocNameI -> [(LaTeX, LaTeX)]
+    do_args _n leader (HsForAllTy tvs ltype)
+      = [ ( decltt leader
+          , decltt (hsep (forallSymbol unicode : ppTyVars tvs ++ [dot]))
+              <+> ppLType unicode ltype
+          ) ]
+    do_args n leader (HsQualTy lctxt ltype)
+      = (decltt leader, ppLContextNoArrow lctxt unicode <+> nl)
+        : do_largs n (darrow unicode) ltype
+
+    do_args n leader (HsFunTy (L _ (HsRecTy fields)) r)
+      = [ (decltt ldr, latex <+> nl)
+        | (L _ field, ldr) <- zip fields (leader <+> gadtOpen : repeat gadtComma)
+        , let latex = ppSideBySideField subdocs unicode field
+        ]
+        ++ do_largs (n+1) (gadtEnd <+> arrow unicode) r
+    do_args n leader (HsFunTy lt r)
+      = (decltt leader, decltt (ppLFunLhType unicode lt) <-> arg_doc n <+> nl)
+        : do_largs (n+1) (arrow unicode) r
+    do_args n leader t
+      = [ (decltt leader, decltt (ppType unicode t) <-> arg_doc n <+> nl) ]
+
+    -- FIXME: this should be done more elegantly
+    --
+    -- We need 'gadtComma' and 'gadtEnd' to line up with the `{` from
+    -- 'gadtOpen', so we add 3 spaces to cover for `-> `/`:: ` (3 in unicode
+    -- mode since `->` and `::` are rendered as single characters.
+    gadtComma = hcat (replicate (if unicode then 3 else 4) (text "\\ ")) <> text ","
+    gadtEnd = hcat (replicate (if unicode then 3 else 4) (text "\\ ")) <> text "\\}"
+    gadtOpen = text "\\{"
 
 
 ppTypeSig :: [Name] -> HsType DocNameI  -> Bool -> LaTeX
@@ -483,10 +522,10 @@ ppFds fds unicode =
                            hsep (map (ppDocName . unLoc) vars2)
 
 
-ppClassDecl :: [DocInstance DocNameI] -> SrcSpan
+ppClassDecl :: [DocInstance DocNameI]
             -> Documentation DocName -> [(DocName, DocForDecl DocName)]
             -> TyClDecl DocNameI -> Bool -> LaTeX
-ppClassDecl instances loc doc subdocs
+ppClassDecl instances doc subdocs
   (ClassDecl { tcdCtxt = lctxt, tcdLName = lname, tcdTyVars = ltyvars, tcdFDs = lfds
              , tcdSigs = lsigs, tcdATs = ats, tcdATDefs = at_defs }) unicode
   = declWithDoc classheader (if null body then Nothing else Just (vcat body)) $$
@@ -508,7 +547,7 @@ ppClassDecl instances loc doc subdocs
 
     methodTable =
       text "\\haddockpremethods{}\\textbf{Methods}" $$
-      vcat  [ ppFunSig loc doc names (hsSigWcType typ) unicode
+      vcat  [ ppFunSig doc names (hsSigWcType typ) unicode
             | L _ (TypeSig lnames typ) <- lsigs
             , let doc = lookupAnySubdoc (head names) subdocs
                   names = map unLoc lnames ]
@@ -518,7 +557,7 @@ ppClassDecl instances loc doc subdocs
 
     instancesBit = ppDocInstances unicode instances
 
-ppClassDecl _ _ _ _ _ _ = error "declaration type not supported by ppShortClassDecl"
+ppClassDecl _ _ _ _ _ = error "declaration type not supported by ppShortClassDecl"
 
 ppDocInstances :: Bool -> [DocInstance DocNameI] -> LaTeX
 ppDocInstances _unicode [] = empty
@@ -567,15 +606,17 @@ lookupAnySubdoc n subdocs = case lookup n subdocs of
 -- * Data & newtype declarations
 -------------------------------------------------------------------------------
 
-
-ppDataDecl :: [(HsDecl DocNameI, DocForDecl DocName)] -> [DocInstance DocNameI] ->
-              [(DocName, DocForDecl DocName)] -> SrcSpan ->
-              Maybe (Documentation DocName) -> TyClDecl DocNameI -> Bool ->
-              LaTeX
-ppDataDecl pats instances subdocs _loc doc dataDecl unicode
-
-   =  declWithDoc (ppDataHeader dataDecl unicode <+> whereBit)
-                  (if null body then Nothing else Just (vcat body))
+-- | Pretty-print a data declaration
+ppDataDecl :: [(HsDecl DocNameI, DocForDecl DocName)] -- ^ relevant patterns
+           -> [DocInstance DocNameI]                  -- ^ relevant instances
+           -> [(DocName, DocForDecl DocName)]         -- ^ relevant decl docs
+           -> Maybe (Documentation DocName)           -- ^ this decl's docs
+           -> TyClDecl DocNameI                       -- ^ data decl to print
+           -> Bool                                    -- ^ unicode
+           -> LaTeX
+ppDataDecl pats instances subdocs doc dataDecl unicode =
+   declWithDoc (ppDataHeader dataDecl unicode <+> whereBit)
+               (if null body then Nothing else Just (vcat body))
    $$ instancesBit
 
   where
@@ -587,28 +628,26 @@ ppDataDecl pats instances subdocs _loc doc dataDecl unicode
     (whereBit, leaders)
       | null cons
       , null pats = (empty,[])
-      | null cons = (decltt (keyword "where"), repeat empty)
+      | null cons = (text "where", repeat empty)
       | otherwise = case resTy of
-        ConDeclGADT{} -> (decltt (keyword "where"), repeat empty)
+        ConDeclGADT{} -> (text "where", repeat empty)
         _             -> (empty, (decltt (text "=") : repeat (decltt (text "|"))))
 
     constrBit
       | null cons = Nothing
       | otherwise = Just $
+          text "\\enspace" <+> emph (text "Constructors") <> text "\\par" $$
           text "\\haddockbeginconstrs" $$
           vcat (zipWith (ppSideBySideConstr subdocs unicode) leaders cons) $$
           text "\\end{tabulary}\\par"
 
     patternBit
-      | null cons = Nothing
-      | otherwise = Just $
+      | null pats = Nothing
+      | otherwise = Just $ 
+          text "\\enspace" <+> emph (text "Bundled Patterns") <> text "\\par" $$
           text "\\haddockbeginconstrs" $$
-          vcat [ hsep [ keyword "pattern"
-                      , hsep $ punctuate comma $ map (ppDocBinder . unLoc) lnames
-                      , dcolon unicode
-                      , ppLType unicode (hsSigType ty)
-                      ] <-> rDoc (fmap _doc . combineDocumentation . fst $ d)
-               | (SigD (PatSynSig lnames ty),d) <- pats
+          vcat [ empty <-> ppSideBySidePat lnames typ d unicode
+               | (SigD (PatSynSig lnames typ), d) <- pats
                ] $$
           text "\\end{tabulary}\\par"
 
@@ -627,41 +666,100 @@ ppConstrHdr forall tvs ctxt unicode
       False -> empty
 
 
-ppSideBySideConstr :: [(DocName, DocForDecl DocName)] -> Bool -> LaTeX
-                   -> LConDecl DocNameI -> LaTeX
-ppSideBySideConstr subdocs unicode leader (L _ con@(ConDeclH98 {})) =
-  leader <->
-  case con_args con of
-
-    PrefixCon args ->
-      decltt (hsep ((header_ unicode <+> ppOcc) :
-                 map (ppLParendType unicode) args))
-      <-> rDoc mbDoc <+> nl
-
-    RecCon (L _ fields) ->
-      (decltt (header_ unicode <+> ppOcc)
-        <-> rDoc mbDoc <+> nl)
-      $$
-      doRecordFields fields
-
-    InfixCon arg1 arg2 ->
-      decltt (hsep [ header_ unicode <+> ppLParendType unicode arg1,
-                 ppOcc,
-                 ppLParendType unicode arg2 ])
-      <-> rDoc mbDoc <+> nl
+-- | Pretty-print a constructor
+ppSideBySideConstr :: [(DocName, DocForDecl DocName)]  -- ^ all decl docs
+                   -> Bool                             -- ^ unicode
+                   -> LaTeX                            -- ^ prefix to decl
+                   -> LConDecl DocNameI                -- ^ constructor decl
+                   -> LaTeX
+ppSideBySideConstr subdocs unicode leader (L _ con) =
+  leader <-> decltt decl <-> rDoc mbDoc <+> nl
+  $$ fieldPart
+  where
+    -- Find the name of a constructors in the decl (`getConName` always returns
+    -- a non-empty list)
+    aConName = unLoc (head (getConNames con))
+
+    occ      = map (nameOccName . getName . unLoc) $ getConNames con
+
+    ppOcc      = cat (punctuate comma (map ppBinder occ))
+    ppOccInfix = cat (punctuate comma (map ppBinderInfix occ))
+
+    -- Extract out the map of of docs corresponding to the constructors arguments
+    argDocs = maybe Map.empty snd (lookup aConName subdocs)
+    hasArgDocs = not $ Map.null argDocs
+
+    -- First line of the constructor (no doc, no fields, single-line)
+    decl = case con of
+      ConDeclH98{ con_args = det
+                , con_ex_tvs = vars
+                , con_mb_cxt = cxt
+                } -> let tyVars = map (getName . hsLTyVarName) vars
+                         context = unLoc (fromMaybe (noLoc []) cxt)
+                         forall_ = False
+                         header_ = ppConstrHdr forall_ tyVars context unicode
+                     in case det of
+        -- Prefix constructor, e.g. 'Just a'
+        PrefixCon args
+          | hasArgDocs -> header_ <+> ppOcc
+          | otherwise -> hsep [ header_
+                              , ppOcc
+                              , hsep (map (ppLParendType unicode) args)
+                              ]
+
+        -- Record constructor, e.g. 'Identity { runIdentity :: a }'
+        RecCon _ ->  header_ <+> ppOcc
+
+        -- Infix constructor, e.g. 'a :| [a]'
+        InfixCon arg1 arg2
+          | hasArgDocs -> header_ <+> ppOcc
+          | otherwise -> hsep [ header_
+                              , ppLParendType unicode arg1
+                              , ppOccInfix
+                              , ppLParendType unicode arg2
+                              ]
+
+      ConDeclGADT{}
+        | hasArgDocs || not (isEmpty fieldPart) -> ppOcc
+        | otherwise -> hsep [ ppOcc
+                            , dcolon unicode
+                            -- ++AZ++ make this prepend "{..}" when it is a record style GADT
+                            , ppLType unicode (getGADTConType con)
+                            ]
+
+    fieldPart = case (con, getConArgs con) of
+        -- Record style GADTs
+        (ConDeclGADT{}, RecCon _)            -> doConstrArgsWithDocs []
+
+        -- Regular record declarations
+        (_, RecCon (L _ fields))             -> doRecordFields fields
+
+        -- Any GADT or a regular H98 prefix data constructor
+        (_, PrefixCon args)     | hasArgDocs -> doConstrArgsWithDocs args
+
+        -- An infix H98 data constructor
+        (_, InfixCon arg1 arg2) | hasArgDocs -> doConstrArgsWithDocs [arg1,arg2]
+
+        _ -> empty
 
- where
     doRecordFields fields =
-        vcat (map (ppSideBySideField subdocs unicode) (map unLoc fields))
+      vcat [ empty <-> tt (text begin) <+> ppSideBySideField subdocs unicode field <+> nl
+           | (begin, L _ field) <- zip ("\\qquad \\{" : repeat "\\qquad ,") fields
+           ]
+      $$
+      empty <-> tt (text "\\qquad \\}") <+> nl
 
+    doConstrArgsWithDocs args = vcat $ map (\l -> empty <-> text "\\qquad" <+> l) $ case con of
+      ConDeclH98{} ->
+        [ decltt (ppLParendType unicode arg) <-> rDoc (fmap _doc mdoc) <+> nl
+        | (i, arg) <- zip [0..] args
+        , let mdoc = Map.lookup i argDocs
+        ]
+      ConDeclGADT{} ->
+        [ l <+> text "\\enspace" <+> r
+        | (l,r) <- ppSubSigLike unicode (unLoc (getGADTConType con)) argDocs subdocs (dcolon unicode)
+        ]
 
-    header_ = ppConstrHdr False tyVars context
-    occ     = map (nameOccName . getName . unLoc) $ getConNames con
-    ppOcc   = case occ of
-      [one] -> ppBinder one
-      _     -> cat (punctuate comma (map ppBinder occ))
-    tyVars  = map (getName . hsLTyVarName) (con_ex_tvs con)
-    context = unLoc (fromMaybe (noLoc []) (con_mb_cxt con))
 
     -- don't use "con_doc con", in case it's reconstructed from a .hi file,
     -- or also because we want Haddock to do the doc-parsing, not GHC.
@@ -670,27 +768,8 @@ ppSideBySideConstr subdocs unicode leader (L _ con@(ConDeclH98 {})) =
               (cn:_) -> lookup (unLoc cn) subdocs >>=
                         fmap _doc . combineDocumentation . fst
 
-ppSideBySideConstr subdocs unicode leader (L _ con@(ConDeclGADT {})) =
-  leader <->
-  doGADTCon (getGADTConType con)
-
- where
-    doGADTCon resTy = decltt (ppOcc <+> dcolon unicode <+>
-                               ppLType unicode resTy
-                            ) <-> rDoc mbDoc
-
-    occ     = map (nameOccName . getName . unLoc) $ getConNames con
-    ppOcc   = case occ of
-      [one] -> ppBinder one
-      _     -> cat (punctuate comma (map ppBinder occ))
-
-    -- don't use "con_doc con", in case it's reconstructed from a .hi file,
-    -- or also because we want Haddock to do the doc-parsing, not GHC.
-    mbDoc = case getConNames con of
-              [] -> panic "empty con_names"
-              (cn:_) -> lookup (unLoc cn) subdocs >>=
-                        fmap _doc . combineDocumentation . fst
 
+-- | Pretty-print a record field
 ppSideBySideField :: [(DocName, DocForDecl DocName)] -> Bool -> ConDeclField DocNameI ->  LaTeX
 ppSideBySideField subdocs unicode (ConDeclField names ltype _) =
   decltt (cat (punctuate comma (map (ppBinder . rdrNameOcc . unLoc . rdrNameFieldOcc . unLoc) names))
@@ -700,51 +779,37 @@ ppSideBySideField subdocs unicode (ConDeclField names ltype _) =
     -- Where there is more than one name, they all have the same documentation
     mbDoc = lookup (selectorFieldOcc $ unLoc $ head names) subdocs >>= fmap _doc . combineDocumentation . fst
 
--- {-
--- ppHsFullConstr :: HsConDecl -> LaTeX
--- ppHsFullConstr (HsConDecl _ nm tvs ctxt typeList doc) =
---      declWithDoc False doc (
--- 	hsep ((ppHsConstrHdr tvs ctxt +++
--- 		ppHsBinder False nm) : map ppHsBangType typeList)
---       )
--- ppHsFullConstr (HsRecDecl _ nm tvs ctxt fields doc) =
---    td << vanillaTable << (
---      case doc of
---        Nothing -> aboves [hdr, fields_html]
---        Just _  -> aboves [hdr, constr_doc, fields_html]
---    )
---
---   where hdr = declBox (ppHsConstrHdr tvs ctxt +++ ppHsBinder False nm)
---
--- 	constr_doc
--- 	  | isJust doc = docBox (docToLaTeX (fromJust doc))
--- 	  | otherwise  = LaTeX.emptyTable
---
--- 	fields_html =
--- 	   td <<
--- 	      table ! [width "100%", cellpadding 0, cellspacing 8] << (
--- 		   aboves (map ppFullField (concat (map expandField fields)))
--- 		)
--- -}
---
--- ppShortField :: Bool -> Bool -> ConDeclField DocName -> LaTeX
--- ppShortField summary unicode (ConDeclField (L _ name) ltype _)
---   = tda [theclass "recfield"] << (
---       ppBinder summary (docNameOcc name)
---       <+> dcolon unicode <+> ppLType unicode ltype
---     )
---
--- {-
--- ppFullField :: HsFieldDecl -> LaTeX
--- ppFullField (HsFieldDecl [n] ty doc)
---   = declWithDoc False doc (
--- 	ppHsBinder False n <+> dcolon <+> ppHsBangType ty
---     )
--- ppFullField _ = error "ppFullField"
---
--- expandField :: HsFieldDecl -> [HsFieldDecl]
--- expandField (HsFieldDecl ns ty doc) = [ HsFieldDecl [n] ty doc | n <- ns ]
--- -}
+
+-- | Pretty-print a bundled pattern synonym
+ppSideBySidePat :: [Located DocName]    -- ^ pattern name(s)
+                -> LHsSigType DocNameI  -- ^ type of pattern(s)
+                -> DocForDecl DocName   -- ^ doc map
+                -> Bool                 -- ^ unicode
+                -> LaTeX
+ppSideBySidePat lnames typ (doc, argDocs) unicode =
+  decltt decl <-> rDoc mDoc <+> nl
+  $$ fieldPart
+  where
+    hasArgDocs = not $ Map.null argDocs
+    ppOcc = hsep (punctuate comma (map (ppDocBinder . unLoc) lnames))
+
+    decl | hasArgDocs = keyword "pattern" <+> ppOcc
+         | otherwise = hsep [ keyword "pattern"
+                            , ppOcc
+                            , dcolon unicode
+                            , ppLType unicode (hsSigType typ)
+                            ]
+
+    fieldPart
+      | not hasArgDocs = empty
+      | otherwise = vcat
+          [ empty <-> text "\\qquad" <+> l <+> text "\\enspace" <+> r
+          | (l,r) <- ppSubSigLike unicode (unLoc patTy) argDocs [] (dcolon unicode)
+          ]
+
+    patTy = hsSigType typ
+
+    mDoc = fmap _doc $ combineDocumentation doc
 
 
 -- | Print the LHS of a data\/newtype declaration.
@@ -760,6 +825,7 @@ ppDataHeader (DataDecl { tcdLName = L _ name, tcdTyVars = tyvars
     ppAppDocNameNames False name (tyvarNames tyvars)
 ppDataHeader _ _ = error "ppDataHeader: illegal argument"
 
+
 --------------------------------------------------------------------------------
 -- * Type applications
 --------------------------------------------------------------------------------
@@ -911,7 +977,7 @@ ppr_mono_ty _         (HsListTy ty)       u = brackets (ppr_mono_lty pREC_TOP ty
 ppr_mono_ty _         (HsPArrTy ty)       u = pabrackets (ppr_mono_lty pREC_TOP ty u)
 ppr_mono_ty _         (HsIParamTy (L _ n) ty) u = brackets (ppIPName n <+> dcolon u <+> ppr_mono_lty pREC_TOP ty u)
 ppr_mono_ty _         (HsSpliceTy {})     _ = error "ppr_mono_ty HsSpliceTy"
-ppr_mono_ty _         (HsRecTy {})        _ = error "ppr_mono_ty HsRecTy"
+ppr_mono_ty _         (HsRecTy {})        _ = text "{..}"
 ppr_mono_ty _         (HsCoreTy {})       _ = error "ppr_mono_ty HsCoreTy"
 ppr_mono_ty _         (HsExplicitListTy Promoted _ tys) u = Pretty.quote $ brackets $ hsep $ punctuate comma $ map (ppLType u) tys
 ppr_mono_ty _         (HsExplicitListTy NotPromoted _ tys) u = brackets $ hsep $ punctuate comma $ map (ppLType u) tys
@@ -972,6 +1038,11 @@ ppBinder n
   | isInfixName n = parens $ ppOccName n
   | otherwise     = ppOccName n
 
+ppBinderInfix :: OccName -> LaTeX
+ppBinderInfix n
+  | isInfixName n = ppOccName n
+  | otherwise     = cat [ char '`', ppOccName n, char '`' ]
+
 isInfixName :: OccName -> Bool
 isInfixName n = isVarSym n || isConSym n
 
diff --git a/haddock-api/src/Haddock/Backends/Xhtml/Decl.hs b/haddock-api/src/Haddock/Backends/Xhtml/Decl.hs
index bf71fec4..fcc52a99 100644
--- a/haddock-api/src/Haddock/Backends/Xhtml/Decl.hs
+++ b/haddock-api/src/Haddock/Backends/Xhtml/Decl.hs
@@ -40,10 +40,19 @@ import Name
 import BooleanFormula
 import RdrName ( rdrNameOcc )
 
-ppDecl :: Bool -> LinksInfo -> LHsDecl DocNameI
-       -> [(HsDecl DocNameI, DocForDecl DocName)]
-       -> DocForDecl DocName ->  [DocInstance DocNameI] -> [(DocName, Fixity)]
-       -> [(DocName, DocForDecl DocName)] -> Splice -> Unicode -> Qualification -> Html
+-- | Pretty print a declaration
+ppDecl :: Bool                                     -- ^ print summary info only
+       -> LinksInfo                                -- ^ link information
+       -> LHsDecl DocNameI                         -- ^ declaration to print
+       -> [(HsDecl DocNameI, DocForDecl DocName)]  -- ^ relevant pattern synonyms
+       -> DocForDecl DocName                       -- ^ documentation for this decl
+       -> [DocInstance DocNameI]                   -- ^ relevant instances
+       -> [(DocName, Fixity)]                      -- ^ relevant fixities
+       -> [(DocName, DocForDecl DocName)]          -- ^ documentation for all decls
+       -> Splice
+       -> Unicode                                  -- ^ unicode output
+       -> Qualification
+       -> Html
 ppDecl summ links (L loc decl) pats (mbDoc, fnArgsDoc) instances fixities subdocs splice unicode qual = case decl of
   TyClD (FamDecl d)            -> ppTyFam summ False links instances fixities loc mbDoc d splice unicode qual
   TyClD d@(DataDecl {})        -> ppDataDecl summ links instances fixities subdocs loc mbDoc d pats splice unicode qual
@@ -51,8 +60,8 @@ ppDecl summ links (L loc decl) pats (mbDoc, fnArgsDoc) instances fixities subdoc
   TyClD d@(ClassDecl {})       -> ppClassDecl summ links instances fixities loc mbDoc subdocs d splice unicode qual
   SigD (TypeSig lnames lty)    -> ppLFunSig summ links loc (mbDoc, fnArgsDoc) lnames
                                          (hsSigWcType lty) fixities splice unicode qual
-  SigD (PatSynSig lnames ty)   -> ppLPatSig summ links loc (mbDoc, fnArgsDoc) lnames
-                                         ty fixities splice unicode qual
+  SigD (PatSynSig lnames lty)   -> ppLPatSig summ links loc (mbDoc, fnArgsDoc) lnames
+                                         (hsSigType lty) fixities splice unicode qual
   ForD d                       -> ppFor summ links loc (mbDoc, fnArgsDoc) d fixities splice unicode qual
   InstD _                      -> noHtml
   DerivD _                     -> noHtml
@@ -75,20 +84,18 @@ ppFunSig summary links loc doc docnames typ fixities splice unicode qual =
   where
     pp_typ = ppLType unicode qual HideEmptyContexts typ
 
-ppLPatSig :: Bool -> LinksInfo -> SrcSpan -> DocForDecl DocName ->
-             [Located DocName] -> LHsSigType DocNameI ->
-             [(DocName, Fixity)] ->
-             Splice -> Unicode -> Qualification -> Html
-ppLPatSig summary links loc (doc, _argDocs) docnames typ fixities splice unicode qual
-  | summary = pref1
-  | otherwise = topDeclElem links loc splice (map unLoc docnames) (pref1 <+> ppFixities fixities qual)
-                +++ docSection Nothing qual doc
+-- | Pretty print a pattern synonym
+ppLPatSig :: Bool -> LinksInfo -> SrcSpan -> DocForDecl DocName
+          -> [Located DocName]     -- ^ names of patterns in declaration
+          -> LHsType DocNameI      -- ^ type of patterns in declaration
+          -> [(DocName, Fixity)]
+          -> Splice -> Unicode -> Qualification -> Html
+ppLPatSig summary links loc doc lnames typ fixities splice unicode qual =
+  ppSigLike summary links loc (keyword "pattern") doc (map unLoc lnames) fixities
+            (unLoc typ, pp_typ) splice unicode qual (patSigContext typ)
   where
-    pref1 = hsep [ keyword "pattern"
-                 , hsep $ punctuate comma $ map (ppBinder summary . getOccName) docnames
-                 , dcolon unicode
-                 , ppPatSigType unicode qual (hsSigType typ)
-                 ]
+    pp_typ = ppPatSigType unicode qual typ
+
 
 ppSigLike :: Bool -> LinksInfo -> SrcSpan -> Html -> DocForDecl DocName ->
              [DocName] -> [(DocName, Fixity)] -> (HsType DocNameI, Html) ->
@@ -97,7 +104,7 @@ ppSigLike summary links loc leader doc docnames fixities (typ, pp_typ)
           splice unicode qual emptyCtxts =
   ppTypeOrFunSig summary links loc docnames typ doc
     ( addFixities $ leader <+> ppTypeSig summary occnames pp_typ unicode
-    , addFixities . concatHtml . punctuate comma $ map (ppBinder False) occnames
+    , (leader <+>) . addFixities . concatHtml . punctuate comma $ map (ppBinder False) occnames
     , dcolon unicode
     )
     splice unicode qual emptyCtxts
@@ -114,10 +121,26 @@ ppTypeOrFunSig :: Bool -> LinksInfo -> SrcSpan -> [DocName] -> HsType DocNameI
 ppTypeOrFunSig summary links loc docnames typ (doc, argDocs) (pref1, pref2, sep) splice unicode qual emptyCtxts
   | summary = pref1
   | Map.null argDocs = topDeclElem links loc splice docnames pref1 +++ docSection curName qual doc
-  | otherwise = topDeclElem links loc splice docnames pref2 +++
-      subArguments qual (do_args 0 sep typ) +++ docSection curName qual doc
+  | otherwise = topDeclElem links loc splice docnames pref2
+                  +++ subArguments qual (ppSubSigLike unicode qual typ argDocs [] sep emptyCtxts)
+                  +++ docSection curName qual doc
   where
     curName = getName <$> listToMaybe docnames
+
+
+-- This splits up a type signature along `->` and adds docs (when they exist) to
+-- the arguments.
+--
+-- If one passes in a list of the available subdocs, any top-level `HsRecTy`
+-- found will be expanded out into their fields.
+ppSubSigLike :: Unicode -> Qualification
+             -> HsType DocNameI                  -- ^ type signature
+             -> FnArgsDoc DocName                -- ^ docs to add
+             -> [(DocName, DocForDecl DocName)]  -- ^ all subdocs (useful when
+                                                 -- we expand an `HsRecTy`)
+             -> Html -> HideEmptyContexts -> [SubDecl]
+ppSubSigLike unicode qual typ argDocs subdocs sep emptyCtxts = do_args 0 sep typ
+  where
     argDoc n = Map.lookup n argDocs
 
     do_largs n leader (L _ t) = do_args n leader t
@@ -135,12 +158,32 @@ ppTypeOrFunSig summary links loc docnames typ (doc, argDocs) (pref1, pref2, sep)
       = (leader <+> ppLContextNoArrow lctxt unicode qual emptyCtxts, Nothing, [])
         : do_largs n (darrow unicode) ltype
 
+    do_args n leader (HsFunTy (L _ (HsRecTy fields)) r)
+      = [ (ldr <+> html, mdoc, subs)
+        | (L _ field, ldr) <- zip fields (leader <+> gadtOpen : repeat gadtComma)
+        , let (html, mdoc, subs) = ppSideBySideField subdocs unicode qual field
+        ]
+        ++ do_largs (n+1) (gadtEnd <+> arrow unicode) r
+
     do_args n leader (HsFunTy lt r)
       = (leader <+> ppLFunLhType unicode qual emptyCtxts lt, argDoc n, [])
         : do_largs (n+1) (arrow unicode) r
+
     do_args n leader t
       = [(leader <+> ppType unicode qual emptyCtxts t, argDoc n, [])]
 
+
+    -- FIXME: this should be done more elegantly
+    --
+    -- We need 'gadtComma' and 'gadtEnd' to line up with the `{` from
+    -- 'gadtOpen', so we add 3 spaces to cover for `-> `/`:: ` (3 in unicode
+    -- mode since `->` and `::` are rendered as single characters.
+    gadtComma = concatHtml (replicate (if unicode then 2 else 3) spaceHtml) <> toHtml ","
+    gadtEnd = concatHtml (replicate (if unicode then 2 else 3) spaceHtml) <> toHtml "}"
+    gadtOpen = toHtml "{"
+
+
+
 ppForAll :: [LHsTyVarBndr DocNameI] -> Unicode -> Qualification -> Html
 ppForAll tvs unicode qual =
   case [ppKTv n k | L _ (KindedTyVar (L _ n) k) <- tvs] of
@@ -707,11 +750,16 @@ ppShortDataDecl summary dataInst dataDecl pats unicode qual
             ]
 
 
-ppDataDecl :: Bool -> LinksInfo -> [DocInstance DocNameI] -> [(DocName, Fixity)] ->
-              [(DocName, DocForDecl DocName)] ->
-              SrcSpan -> Documentation DocName -> TyClDecl DocNameI ->
-              [(HsDecl DocNameI, DocForDecl DocName)] ->
-              Splice -> Unicode -> Qualification -> Html
+-- | Pretty-print a data declaration
+ppDataDecl :: Bool -> LinksInfo
+           -> [DocInstance DocNameI]                  -- ^ relevant instances
+           -> [(DocName, Fixity)]                     -- ^ relevant fixities
+           -> [(DocName, DocForDecl DocName)]         -- ^ all decl documentation
+           -> SrcSpan
+           -> Documentation DocName                   -- ^ this decl's documentation
+           -> TyClDecl DocNameI                       -- ^ this decl
+           -> [(HsDecl DocNameI, DocForDecl DocName)] -- ^ relevant patterns
+           -> Splice -> Unicode -> Qualification -> Html
 ppDataDecl summary links instances fixities subdocs loc doc dataDecl pats
            splice unicode qual
 
@@ -740,25 +788,20 @@ ppDataDecl summary links instances fixities subdocs loc doc dataDecl pats
       [ ppSideBySideConstr subdocs subfixs unicode qual c
       | c <- cons
       , let subfixs = filter (\(n,_) -> any (\cn -> cn == n)
-                                     (map unLoc (getConNames (unLoc c)))) fixities
+                                            (map unLoc (getConNames (unLoc c)))) fixities
       ]
 
     patternBit = subPatterns qual
-      [ (hsep [ keyword "pattern"
-              , hsep $ punctuate comma $ map (ppBinder summary . getOccName) lnames
-              , dcolon unicode
-              , ppPatSigType unicode qual (hsSigType typ)
-              ] <+> ppFixities subfixs qual
-        ,combineDocumentation (fst d), [])
-      | (SigD (PatSynSig lnames typ),d) <- pats
-      , let subfixs = filter (\(n,_) -> any (\cn -> cn == n) (map unLoc lnames)) fixities
+      [ ppSideBySidePat subfixs unicode qual lnames typ d
+      | (SigD (PatSynSig lnames typ), d) <- pats
+      , let subfixs = filter (\(n,_) -> any (\cn -> cn == n)
+                                            (map unLoc lnames)) fixities
       ]
 
     instancesBit = ppInstances links (OriginData docname) instances
         splice unicode qual
 
 
-
 ppShortConstr :: Bool -> ConDecl DocNameI -> Unicode -> Qualification -> Html
 ppShortConstr summary con unicode qual = cHead <+> cBody <+> cFoot
   where
@@ -768,121 +811,180 @@ ppShortConstr summary con unicode qual = cHead <+> cBody <+> cFoot
 -- returns three pieces: header, body, footer so that header & footer can be
 -- incorporated into the declaration
 ppShortConstrParts :: Bool -> Bool -> ConDecl DocNameI -> Unicode -> Qualification -> (Html, Html, Html)
-ppShortConstrParts summary dataInst con unicode qual = case con of
-  ConDeclH98{} -> case con_args con of
-    PrefixCon args ->
-      (header_ unicode qual +++ hsep (ppOcc
-            : map (ppLParendType unicode qual HideEmptyContexts) args), noHtml, noHtml)
-    RecCon (L _ fields) ->
-      (header_ unicode qual +++ ppOcc <+> char '{',
-       doRecordFields fields,
-       char '}')
-    InfixCon arg1 arg2 ->
-      (header_ unicode qual +++ hsep [ppLParendType unicode qual HideEmptyContexts arg1,
-            ppOccInfix, ppLParendType unicode qual HideEmptyContexts arg2],
-       noHtml, noHtml)
-
-  ConDeclGADT {} -> (ppOcc <+> dcolon unicode
-                           <+> ppLType unicode qual HideEmptyContexts (getGADTConType con)
-                    , noHtml, noHtml)
-
-  where
-    doRecordFields fields = shortSubDecls dataInst $
-                            map (ppShortField summary unicode qual) (map unLoc fields)
-
-    header_  = ppConstrHdr forall_ tyVars context
-    occ      = map (nameOccName . getName . unLoc) $ getConNames con
-
-    ppOcc    = case occ of
-      [one] -> ppBinder summary one
-      _     -> hsep (punctuate comma (map (ppBinder summary) occ))
+ppShortConstrParts summary dataInst con unicode qual
+  = case con of
+      ConDeclH98{ con_args = det
+                , con_ex_tvs = vars
+                , con_mb_cxt = cxt
+                } -> let tyVars = map (getName . hsLTyVarName) vars
+                         context = unLoc (fromMaybe (noLoc []) cxt)
+                         forall_ = False
+                         header_ = ppConstrHdr forall_ tyVars context unicode qual
+                     in case det of
+
+        -- Prefix constructor, e.g. 'Just a'
+        PrefixCon args ->
+          ( header_ +++ hsep (ppOcc : map (ppLParendType unicode qual HideEmptyContexts) args)
+          , noHtml
+          , noHtml
+          )
 
-    ppOccInfix = case occ of
-      [one] -> ppBinderInfix summary one
-      _     -> hsep (punctuate comma (map (ppBinderInfix summary) occ))
+        -- Record constructor, e.g. 'Identity { runIdentity :: a }'
+        RecCon (L _ fields) ->
+          ( header_ +++ ppOcc <+> char '{'
+          , shortSubDecls dataInst [ ppShortField summary unicode qual field
+                                   | L _ field <- fields
+                                   ]
+          , char '}'
+          )
 
-    -- Used for H98 syntax only
-    tyVars   = map (getName . hsLTyVarName) (con_ex_tvs con)
-    lcontext = fromMaybe (noLoc []) (con_mb_cxt con)
-    context  = unLoc lcontext
-    forall_  = False
+        -- Infix constructor, e.g. 'a :| [a]'
+        InfixCon arg1 arg2 ->
+          ( header_ +++ hsep [ ppLParendType unicode qual HideEmptyContexts arg1
+                             , ppOccInfix
+                             , ppLParendType unicode qual HideEmptyContexts arg2
+                             ]
+          , noHtml
+          , noHtml
+          )
 
+      -- GADT constructor, e.g. 'Foo :: Int -> Foo'
+      ConDeclGADT {} ->
+          ( hsep [ ppOcc, dcolon unicode, ppLType unicode qual HideEmptyContexts (getGADTConType con) ]
+          , noHtml
+          , noHtml
+          )
 
--- ppConstrHdr is for (non-GADT) existentials constructors' syntax
-ppConstrHdr :: Bool -> [Name] -> HsContext DocNameI -> Unicode
-            -> Qualification -> Html
-ppConstrHdr forall_ tvs ctxt unicode qual
- = (if null tvs then noHtml else ppForall)
-   +++
-   (if null ctxt then noHtml
-    else ppContextNoArrow ctxt unicode qual HideEmptyContexts
-         <+> darrow unicode +++ toHtml " ")
   where
-    ppForall | forall_   = forallSymbol unicode <+> hsep (map (ppName Prefix) tvs)
-                           <+> toHtml ". "
-             | otherwise = noHtml
+    occ        = map (nameOccName . getName . unLoc) $ getConNames con
+    ppOcc      = hsep (punctuate comma (map (ppBinder summary) occ))
+    ppOccInfix = hsep (punctuate comma (map (ppBinderInfix summary) occ))
+
 
+-- | Pretty print an expanded constructor
 ppSideBySideConstr :: [(DocName, DocForDecl DocName)] -> [(DocName, Fixity)]
-                   -> Unicode -> Qualification -> LConDecl DocNameI -> SubDecl
+                   -> Unicode -> Qualification
+                   -> LConDecl DocNameI -- ^ constructor declaration to print
+                   -> SubDecl
 ppSideBySideConstr subdocs fixities unicode qual (L _ con)
- = (decl, mbDoc, fieldPart)
+ = ( decl       -- Constructor header (name, fixity)
+   , mbDoc      -- Docs on the whole constructor
+   , fieldPart  -- Information on the fields (or arguments, if they have docs)
+   )
  where
-    decl = case con of
-      ConDeclH98{} -> case con_args con of
-        PrefixCon args ->
-          hsep ((header_ +++ ppOcc)
-            : map (ppLParendType unicode qual HideEmptyContexts) args)
-          <+> fixity
+    -- Find the name of a constructors in the decl (`getConName` always returns a non-empty list)
+    aConName = unLoc (head (getConNames con))
 
-        RecCon _ -> header_ +++ ppOcc <+> fixity
+    fixity   = ppFixities fixities qual
+    occ      = map (nameOccName . getName . unLoc) $ getConNames con
 
-        InfixCon arg1 arg2 ->
-          hsep [header_ +++ ppLParendType unicode qual HideEmptyContexts arg1,
-            ppOccInfix,
-            ppLParendType unicode qual HideEmptyContexts arg2]
-          <+> fixity
+    ppOcc      = hsep (punctuate comma (map (ppBinder False) occ))
+    ppOccInfix = hsep (punctuate comma (map (ppBinderInfix False) occ))
+
+    -- Extract out the map of of docs corresponding to the constructors arguments
+    argDocs = maybe Map.empty snd (lookup aConName subdocs)
+    hasArgDocs = not $ Map.null argDocs
+
+    decl = case con of
+      ConDeclH98{ con_args = det
+                , con_ex_tvs = vars
+                , con_mb_cxt = cxt
+                } -> let tyVars = map (getName . hsLTyVarName) vars
+                         context = unLoc (fromMaybe (noLoc []) cxt)
+                         forall_ = False
+                         header_ = ppConstrHdr forall_ tyVars context unicode qual
+                     in case det of
+        -- Prefix constructor, e.g. 'Just a'
+        PrefixCon args
+          | hasArgDocs -> header_ +++ ppOcc <+> fixity
+          | otherwise -> hsep [ header_ +++ ppOcc
+                              , hsep (map (ppLParendType unicode qual HideEmptyContexts) args)
+                              , fixity
+                              ]
+
+        -- Record constructor, e.g. 'Identity { runIdentity :: a }'
+        RecCon _ -> header_ +++ ppOcc <+> fixity
 
-      ConDeclGADT{} -> doGADTCon (getGADTConType con)
+        -- Infix constructor, e.g. 'a :| [a]'
+        InfixCon arg1 arg2
+          | hasArgDocs -> header_ +++ ppOcc <+> fixity
+          | otherwise -> hsep [ header_ +++ ppLParendType unicode qual HideEmptyContexts arg1
+                              , ppOccInfix
+                              , ppLParendType unicode qual HideEmptyContexts arg2
+                              , fixity
+                              ]
+
+      -- GADT constructor, e.g. 'Foo :: Int -> Foo'
+      ConDeclGADT{}
+          | hasArgDocs || not (null fieldPart) -> ppOcc <+> fixity
+          | otherwise -> hsep [ ppOcc
+                              , dcolon unicode
+                              -- ++AZ++ make this prepend "{..}" when it is a record style GADT
+                              , ppLType unicode qual HideEmptyContexts (getGADTConType con)
+                              , fixity
+                              ]
+
+    fieldPart = case (con, getConArgs con) of
+        -- Record style GADTs
+        (ConDeclGADT{}, RecCon _)            -> [ doConstrArgsWithDocs [] ]
+
+        -- Regular record declarations
+        (_, RecCon (L _ fields))             -> [ doRecordFields fields ]
+
+        -- Any GADT or a regular H98 prefix data constructor
+        (_, PrefixCon args)     | hasArgDocs -> [ doConstrArgsWithDocs args ]
+
+        -- An infix H98 data constructor
+        (_, InfixCon arg1 arg2) | hasArgDocs -> [ doConstrArgsWithDocs [arg1,arg2] ]
 
-    fieldPart = case getConArgs con of
-        RecCon (L _ fields) -> [doRecordFields fields]
         _ -> []
 
     doRecordFields fields = subFields qual
       (map (ppSideBySideField subdocs unicode qual) (map unLoc fields))
 
-    doGADTCon :: Located (HsType DocNameI) -> Html
-    doGADTCon ty = ppOcc <+> dcolon unicode
-        -- ++AZ++ make this prepend "{..}" when it is a record style GADT
-        <+> ppLType unicode qual HideEmptyContexts ty
-        <+> fixity
+    doConstrArgsWithDocs args = subFields qual $ case con of
+      ConDeclH98{} ->
+        [ (ppLParendType unicode qual HideEmptyContexts arg, mdoc, [])
+        | (i, arg) <- zip [0..] args
+        , let mdoc = Map.lookup i argDocs
+        ]
+      ConDeclGADT{} ->
+        ppSubSigLike unicode qual (unLoc (getGADTConType con))
+                     argDocs subdocs (dcolon unicode) HideEmptyContexts
 
-    fixity  = ppFixities fixities qual
-    header_ = ppConstrHdr forall_ tyVars context unicode qual
-    occ     = map (nameOccName . getName . unLoc) $ getConNames con
-
-    ppOcc   = case occ of
-      [one] -> ppBinder False one
-      _     -> hsep (punctuate comma (map (ppBinder False) occ))
-
-    ppOccInfix = case occ of
-      [one] -> ppBinderInfix False one
-      _     -> hsep (punctuate comma (map (ppBinderInfix False) occ))
-
-    -- Used for H98 syntax only
-    tyVars  = map (getName . hsLTyVarName) (con_ex_tvs con)
-    context = unLoc (fromMaybe (noLoc []) (con_mb_cxt con))
-    forall_ = False
     -- don't use "con_doc con", in case it's reconstructed from a .hi file,
     -- or also because we want Haddock to do the doc-parsing, not GHC.
     mbDoc = lookup (unLoc $ head $ getConNames con) subdocs >>=
             combineDocumentation . fst
 
 
+-- ppConstrHdr is for (non-GADT) existentials constructors' syntax
+ppConstrHdr :: Bool               -- ^ print explicit foralls
+            -> [Name]             -- ^ type variables
+            -> HsContext DocNameI -- ^ context
+            -> Unicode -> Qualification -> Html
+ppConstrHdr forall_ tvs ctxt unicode qual = ppForall +++ ppCtxt
+  where
+    ppForall
+      | null tvs || not forall_ = noHtml
+      | otherwise = forallSymbol unicode
+                      <+> hsep (map (ppName Prefix) tvs)
+                      <+> toHtml ". "
+
+    ppCtxt
+      | null ctxt = noHtml
+      | otherwise = ppContextNoArrow ctxt unicode qual HideEmptyContexts
+                      <+> darrow unicode +++ toHtml " "
+
+
+-- | Pretty-print a record field
 ppSideBySideField :: [(DocName, DocForDecl DocName)] -> Unicode -> Qualification
                   -> ConDeclField DocNameI -> SubDecl
 ppSideBySideField subdocs unicode qual (ConDeclField names ltype _) =
-  ( hsep (punctuate comma (map ((ppBinder False) . rdrNameOcc . unLoc . rdrNameFieldOcc . unLoc) names))
+  ( hsep (punctuate comma [ ppBinder False (rdrNameOcc field)
+                          | L _ name <- names
+                          , let field = (unLoc . rdrNameFieldOcc) name
+                          ])
       <+> dcolon unicode
       <+> ppLType unicode qual HideEmptyContexts ltype
   , mbDoc
@@ -900,6 +1002,40 @@ ppShortField summary unicode qual (ConDeclField names ltype _)
     <+> dcolon unicode <+> ppLType unicode qual HideEmptyContexts ltype
 
 
+-- | Pretty print an expanded pattern (for bundled patterns)
+ppSideBySidePat :: [(DocName, Fixity)] -> Unicode -> Qualification
+                   -> [Located DocName]    -- ^ pattern name(s)
+                   -> LHsSigType DocNameI  -- ^ type of pattern(s)
+                   -> DocForDecl DocName   -- ^ doc map
+                   -> SubDecl
+ppSideBySidePat fixities unicode qual lnames typ (doc, argDocs) =
+  ( decl
+  , combineDocumentation doc
+  , fieldPart
+  )
+  where
+    hasArgDocs = not $ Map.null argDocs
+    fixity = ppFixities fixities qual
+    ppOcc = hsep (punctuate comma (map (ppBinder False . getOccName) lnames))
+
+    decl | hasArgDocs = keyword "pattern" <+> ppOcc <+> fixity
+         | otherwise = hsep [ keyword "pattern"
+                            , ppOcc
+                            , dcolon unicode
+                            , ppPatSigType unicode qual (hsSigType typ)
+                            , fixity
+                            ]
+
+    fieldPart
+      | not hasArgDocs = []
+      | otherwise = [ subFields qual (ppSubSigLike unicode qual (unLoc patTy)
+                                                        argDocs [] (dcolon unicode)
+                                                        emptyCtxt) ]
+
+    patTy = hsSigType typ
+    emptyCtxt = patSigContext patTy
+
+
 -- | Print the LHS of a data\/newtype declaration.
 -- Currently doesn't handle 'data instance' decls or kind signatures
 ppDataHeader :: Bool -> TyClDecl DocNameI -> Unicode -> Qualification -> Html
@@ -990,13 +1126,9 @@ ppLKind unicode qual y = ppKind unicode qual (unLoc y)
 ppKind :: Unicode -> Qualification -> HsKind DocNameI -> Html
 ppKind unicode qual ki = ppr_mono_ty pREC_TOP ki unicode qual HideEmptyContexts
 
-ppPatSigType :: Unicode -> Qualification -> LHsType DocNameI -> Html
-ppPatSigType unicode qual typ =
-  let emptyCtxts =
-        if hasNonEmptyContext typ && isFirstContextEmpty typ
-          then ShowEmptyToplevelContexts
-          else HideEmptyContexts
-  in ppLType unicode qual emptyCtxts typ
+patSigContext :: LHsType name -> HideEmptyContexts
+patSigContext typ | hasNonEmptyContext typ && isFirstContextEmpty typ =  ShowEmptyToplevelContexts
+                  | otherwise = HideEmptyContexts
   where
     hasNonEmptyContext :: LHsType name -> Bool
     hasNonEmptyContext t =
@@ -1013,6 +1145,13 @@ ppPatSigType unicode qual typ =
         HsFunTy _ s -> isFirstContextEmpty s
         _ -> False
 
+
+-- | Pretty-print a pattern signature (all this does over 'ppLType' is slot in
+-- the right 'HideEmptyContext' value)
+ppPatSigType :: Unicode -> Qualification -> LHsType DocNameI -> Html
+ppPatSigType unicode qual typ =
+  let emptyCtxts = patSigContext typ in ppLType unicode qual emptyCtxts typ
+
 ppForAllPart :: Unicode -> Qualification -> [LHsTyVarBndr DocNameI] -> Html
 ppForAllPart unicode qual tvs = hsep (forallSymbol unicode : ppTyVars unicode qual tvs) +++ dot
 
diff --git a/haddock-api/src/Haddock/Backends/Xhtml/Utils.hs b/haddock-api/src/Haddock/Backends/Xhtml/Utils.hs
index a75c4b9a..7fbaec6d 100644
--- a/haddock-api/src/Haddock/Backends/Xhtml/Utils.hs
+++ b/haddock-api/src/Haddock/Backends/Xhtml/Utils.hs
@@ -110,7 +110,7 @@ renderToString debug html
 
 hsep :: [Html] -> Html
 hsep [] = noHtml
-hsep htmls = foldr1 (\a b -> a+++" "+++b) htmls
+hsep htmls = foldr1 (<+>) htmls
 
 -- | Concatenate a series of 'Html' values vertically, with linebreaks in between.
 vcat :: [Html] -> Html
@@ -232,4 +232,4 @@ collapseToggle id_ classes = [ theclass cs, strAttr "data-details-id" id_ ]
 -- and displays a control.
 collapseControl :: String -> String -> [HtmlAttr]
 collapseControl id_ classes = collapseToggle id_ cs
-  where cs = unwords (words classes ++ ["details-toggle-control"])
\ No newline at end of file
+  where cs = unwords (words classes ++ ["details-toggle-control"])
diff --git a/haddock-api/src/Haddock/Interface/Create.hs b/haddock-api/src/Haddock/Interface/Create.hs
index 52a983a8..f673e23b 100644
--- a/haddock-api/src/Haddock/Interface/Create.hs
+++ b/haddock-api/src/Haddock/Interface/Create.hs
@@ -380,7 +380,7 @@ mkMaps dflags gre instances decls = do
             m'   <- traverse (processDocStringParas dflags gre) m
             pure (doc', m')
 
-      (doc, args) <- declDoc docStrs (typeDocs decl)
+      (doc, args) <- declDoc docStrs (declTypeDocs decl)
 
       let
           subs :: [(Name, [HsDocString], Map Int HsDocString)]
@@ -445,14 +445,14 @@ subordinates instMap decl = case decl of
           | isDataDecl  d -> dataSubs (tcdDataDefn d)
   _ -> []
   where
-    classSubs dd = [ (name, doc, typeDocs d) | (L _ d, doc) <- classDecls dd
+    classSubs dd = [ (name, doc, declTypeDocs d) | (L _ d, doc) <- classDecls dd
                    , name <- getMainDeclBinder d, not (isValD d)
                    ]
     dataSubs :: HsDataDefn GhcRn -> [(Name, [HsDocString], Map Int HsDocString)]
     dataSubs dd = constrs ++ fields ++ derivs
       where
         cons = map unL $ (dd_cons dd)
-        constrs = [ (unL cname, maybeToList $ fmap unL $ con_doc c, M.empty)
+        constrs = [ (unL cname, maybeToList $ fmap unL $ con_doc c, conArgDocs c)
                   | c <- cons, cname <- getConNames c ]
         fields  = [ (selectorFieldOcc n, maybeToList $ fmap unL doc, M.empty)
                   | RecCon flds <- map getConArgs cons
@@ -464,17 +464,33 @@ subordinates instMap decl = case decl of
                            unLoc $ dd_derivs dd
                   , Just instName <- [M.lookup l instMap] ]
 
+-- | Extract constructor argument docs from inside constructor decls.
+conArgDocs :: ConDecl GhcRn -> Map Int HsDocString
+conArgDocs con = case getConArgs con of
+                   PrefixCon args -> go 0 (map unLoc args ++ ret)
+                   InfixCon arg1 arg2 -> go 0 ([unLoc arg1, unLoc arg2] ++ ret)
+                   RecCon _ -> go 1 ret
+  where
+    go n (HsDocTy _ (L _ ds) : tys) = M.insert n ds $ go (n+1) tys
+    go n (_ : tys) = go (n+1) tys
+    go _ [] = M.empty
+
+    ret = case con of
+            ConDeclGADT { con_res_ty = res_ty } -> [ unLoc res_ty ]
+            _ -> []
+
+-- | Extract function argument docs from inside top-level decls.
+declTypeDocs :: HsDecl GhcRn -> Map Int HsDocString
+declTypeDocs (SigD (TypeSig _ ty))      = typeDocs (unLoc (hsSigWcType ty))
+declTypeDocs (SigD (ClassOpSig _ _ ty)) = typeDocs (unLoc (hsSigType ty))
+declTypeDocs (SigD (PatSynSig _ ty))    = typeDocs (unLoc (hsSigType ty))
+declTypeDocs (ForD (ForeignImport _ ty _ _))   = typeDocs (unLoc (hsSigType ty))
+declTypeDocs (TyClD (SynDecl { tcdRhs = ty })) = typeDocs (unLoc ty)
+declTypeDocs _ = M.empty
+
 -- | Extract function argument docs from inside types.
-typeDocs :: HsDecl GhcRn -> Map Int HsDocString
-typeDocs d =
-  let docs = go 0 in
-  case d of
-    SigD (TypeSig _ ty)      -> docs (unLoc (hsSigWcType ty))
-    SigD (ClassOpSig _ _ ty) -> docs (unLoc (hsSigType ty))
-    SigD (PatSynSig _ ty)    -> docs (unLoc (hsSigType ty))
-    ForD (ForeignImport _ ty _ _)   -> docs (unLoc (hsSigType ty))
-    TyClD (SynDecl { tcdRhs = ty }) -> docs (unLoc ty)
-    _ -> M.empty
+typeDocs :: HsType GhcRn -> Map Int HsDocString
+typeDocs = go 0
   where
     go n (HsForAllTy { hst_body = ty }) = go n (unLoc ty)
     go n (HsQualTy   { hst_body = ty }) = go n (unLoc ty)
@@ -483,7 +499,6 @@ typeDocs d =
     go n (HsDocTy _ (L _ doc)) = M.singleton n doc
     go _ _ = M.empty
 
-
 -- | All the sub declarations of a class (that we handle), ordered by
 -- source location, with documentation attached if it exists.
 classDecls :: TyClDecl GhcRn -> [(LHsDecl GhcRn, [HsDocString])]