Use Semigroup's (<>) for Doc/SDocwip/semigroup-sdoc

Before this patch, Outputable.hs defined its own (<>) which caused
conflicts with (Data.Semigroup.<>) and thus led to inconvenience.

However, replacing it is not trivial due to a different fixity:

  http://www.haskell.org/pipermail/libraries/2011-November/017066.html

Nevertheless, it is possible to update the pretty-printing code to work
with (<>) of a different fixitiy, and that's what this patch implements.

Now Doc and SDoc are instances of Semigroup.

36 files changed, 75 insertions, 93 deletions

diff --git a/compiler/GHC/CmmToAsm/PPC/RegInfo.hs b/compiler/GHC/CmmToAsm/PPC/RegInfo.hs
index 58e3f44ece..59a0fd083b 100644
--- a/compiler/GHC/CmmToAsm/PPC/RegInfo.hs
+++ b/compiler/GHC/CmmToAsm/PPC/RegInfo.hs
@@ -28,7 +28,7 @@ import GHC.Cmm
 import GHC.Cmm.CLabel
 
 import GHC.Types.Unique
-import Outputable (ppr, text, Outputable, (<>))
+import Outputable (ppr, text, Outputable)
 
 data JumpDest = DestBlockId BlockId
 
diff --git a/compiler/GHC/Driver/Plugins.hs b/compiler/GHC/Driver/Plugins.hs
index d9e29d451b..972a9fd89a 100644
--- a/compiler/GHC/Driver/Plugins.hs
+++ b/compiler/GHC/Driver/Plugins.hs
@@ -63,10 +63,6 @@ import Fingerprint
 import Data.List (sort)
 import Outputable (Outputable(..), text, (<+>))
 
---Qualified import so we can define a Semigroup instance
--- but it doesn't clash with Outputable.<>
-import qualified Data.Semigroup
-
 import Control.Monad
 
 -- | Command line options gathered from the -PModule.Name:stuff syntax
diff --git a/compiler/GHC/Hs/Doc.hs b/compiler/GHC/Hs/Doc.hs
index 7da56b1524..104ce57015 100644
--- a/compiler/GHC/Hs/Doc.hs
+++ b/compiler/GHC/Hs/Doc.hs
@@ -126,7 +126,7 @@ instance Binary DeclDocMap where
 instance Outputable DeclDocMap where
   ppr (DeclDocMap m) = vcat (map pprPair (Map.toAscList m))
     where
-      pprPair (name, doc) = ppr name Outputable.<> colon $$ nest 2 (ppr doc)
+      pprPair (name, doc) = ppr name <> colon $$ nest 2 (ppr doc)
 
 emptyDeclDocMap :: DeclDocMap
 emptyDeclDocMap = DeclDocMap Map.empty
@@ -144,9 +144,9 @@ instance Outputable ArgDocMap where
   ppr (ArgDocMap m) = vcat (map pprPair (Map.toAscList m))
     where
       pprPair (name, int_map) =
-        ppr name Outputable.<> colon $$ nest 2 (pprIntMap int_map)
+        ppr name <> colon $$ nest 2 (pprIntMap int_map)
       pprIntMap im = vcat (map pprIPair (Map.toAscList im))
-      pprIPair (i, doc) = ppr i Outputable.<> colon $$ nest 2 (ppr doc)
+      pprIPair (i, doc) = ppr i <> colon $$ nest 2 (ppr doc)
 
 emptyArgDocMap :: ArgDocMap
 emptyArgDocMap = ArgDocMap Map.empty
diff --git a/compiler/GHC/HsToCore/Docs.hs b/compiler/GHC/HsToCore/Docs.hs
index 48a8ef6f20..42901e4b59 100644
--- a/compiler/GHC/HsToCore/Docs.hs
+++ b/compiler/GHC/HsToCore/Docs.hs
@@ -26,7 +26,6 @@ import Data.Bifunctor (first)
 import Data.Map (Map)
 import qualified Data.Map as M
 import Data.Maybe
-import Data.Semigroup
 
 -- | Extract docs from renamer output.
 extractDocs :: TcGblEnv
diff --git a/compiler/GHC/HsToCore/PmCheck/Oracle.hs b/compiler/GHC/HsToCore/PmCheck/Oracle.hs
index 63cc4710dd..c1cde37d7b 100644
--- a/compiler/GHC/HsToCore/PmCheck/Oracle.hs
+++ b/compiler/GHC/HsToCore/PmCheck/Oracle.hs
@@ -78,7 +78,6 @@ import Data.Foldable (foldlM, minimumBy, toList)
 import Data.List     (find)
 import qualified Data.List.NonEmpty as NonEmpty
 import Data.Ord      (comparing)
-import qualified Data.Semigroup as Semigroup
 import Data.Tuple    (swap)
 
 -- Debugging Infrastructure
diff --git a/compiler/GHC/HsToCore/PmCheck/Types.hs b/compiler/GHC/HsToCore/PmCheck/Types.hs
index 60ed0ce356..e4f01a56a0 100644
--- a/compiler/GHC/HsToCore/PmCheck/Types.hs
+++ b/compiler/GHC/HsToCore/PmCheck/Types.hs
@@ -70,7 +70,6 @@ import Numeric (fromRat)
 import Data.Foldable (find)
 import qualified Data.List.NonEmpty as NonEmpty
 import Data.Ratio
-import qualified Data.Semigroup as Semi
 
 -- | Literals (simple and overloaded ones) for pattern match checking.
 --
diff --git a/compiler/GHC/Iface/Ext/Types.hs b/compiler/GHC/Iface/Ext/Types.hs
index edd6540e80..a859188257 100644
--- a/compiler/GHC/Iface/Ext/Types.hs
+++ b/compiler/GHC/Iface/Ext/Types.hs
@@ -20,17 +20,15 @@ import FastString                 ( FastString )
 import GHC.Iface.Type
 import GHC.Types.Module           ( ModuleName, Module )
 import GHC.Types.Name             ( Name )
-import Outputable hiding ( (<>) )
+import Outputable
 import GHC.Types.SrcLoc           ( RealSrcSpan )
 import GHC.Types.Avail
-import qualified Outputable as O ( (<>) )
 
 import qualified Data.Array as A
 import qualified Data.Map as M
 import qualified Data.Set as S
 import Data.ByteString            ( ByteString )
 import Data.Data                  ( Typeable, Data )
-import Data.Semigroup             ( Semigroup(..) )
 import Data.Word                  ( Word8 )
 import Control.Applicative        ( (<|>) )
 
@@ -217,7 +215,7 @@ instance Outputable a => Outputable (HieASTs a) where
   ppr (HieASTs asts) = M.foldrWithKey go "" asts
     where
       go k a rest = vcat $
-        [ "File: " O.<> ppr k
+        [ "File: " <> ppr k
         , ppr a
         , rest
         ]
@@ -244,7 +242,7 @@ instance Binary (HieAST TypeIndex) where
 instance Outputable a => Outputable (HieAST a) where
   ppr (Node ni sp ch) = hang header 2 rest
     where
-      header = text "Node@" O.<> ppr sp O.<> ":" <+> ppr ni
+      header = text "Node@" <> ppr sp <> ":" <+> ppr ni
       rest = vcat (map ppr ch)
 
 -- | The information stored in one AST node.
@@ -569,7 +567,7 @@ instance Outputable TyVarScope where
   ppr (ResolvedScopes xs) =
     text "type variable scopes:" <+> hsep (punctuate ", " $ map ppr xs)
   ppr (UnresolvedScope ns sp) =
-    text "unresolved type variable scope for name" O.<> plural ns
+    text "unresolved type variable scope for" <+> plural "name" ns
       <+> pprBindSpan sp
 
 instance Binary TyVarScope where
diff --git a/compiler/GHC/Iface/Recomp.hs b/compiler/GHC/Iface/Recomp.hs
index 57809a6d59..a1f7e6d0ee 100644
--- a/compiler/GHC/Iface/Recomp.hs
+++ b/compiler/GHC/Iface/Recomp.hs
@@ -49,10 +49,6 @@ import qualified Data.Map as Map
 import qualified Data.Set as Set
 import GHC.Driver.Plugins ( PluginRecompile(..), PluginWithArgs(..), pluginRecompile', plugins )
 
---Qualified import so we can define a Semigroup instance
--- but it doesn't clash with Outputable.<>
-import qualified Data.Semigroup
-
 {-
   -----------------------------------------------
           Recompilation checking
diff --git a/compiler/GHC/Parser/PostProcess.hs b/compiler/GHC/Parser/PostProcess.hs
index 7ce2f4fb9a..47fa0df602 100644
--- a/compiler/GHC/Parser/PostProcess.hs
+++ b/compiler/GHC/Parser/PostProcess.hs
@@ -1261,8 +1261,8 @@ checkDoAndIfThenElse guardExpr semiThen thenExpr semiElse elseExpr
  | otherwise            = return ()
     where pprOptSemi True  = semi
           pprOptSemi False = empty
-          expr = text "if"   <+> ppr guardExpr <> pprOptSemi semiThen <+>
-                 text "then" <+> ppr thenExpr  <> pprOptSemi semiElse <+>
+          expr = text "if"   <+> (ppr guardExpr <> pprOptSemi semiThen) <+>
+                 text "then" <+> (ppr thenExpr  <> pprOptSemi semiElse) <+>
                  text "else" <+> ppr elseExpr
 
 isFunLhs :: Located (PatBuilder GhcPs)
diff --git a/compiler/GHC/Rename/Module.hs b/compiler/GHC/Rename/Module.hs
index bc2c7d3d5d..8ac2d1a485 100644
--- a/compiler/GHC/Rename/Module.hs
+++ b/compiler/GHC/Rename/Module.hs
@@ -1987,7 +1987,7 @@ rnInjectivityAnn tvBndrs (L _ (TyVarSig _ resTv))
    ; when (noRnErrors && not (Set.null rhsValid)) $
       do { let errorVars = Set.toList rhsValid
          ; addErrAt srcSpan $ ( hsep
-                        [ text "Unknown type variable" <> plural errorVars
+                        [ text "Unknown type" <+> plural "variable" errorVars
                         , text "on the RHS of injectivity condition:"
                         , interpp'SP errorVars ] ) }
 
diff --git a/compiler/GHC/Rename/Names.hs b/compiler/GHC/Rename/Names.hs
index ed08087899..1fe8522040 100644
--- a/compiler/GHC/Rename/Names.hs
+++ b/compiler/GHC/Rename/Names.hs
@@ -1701,8 +1701,8 @@ badImportItemErrDataCon :: OccName -> ModIface -> ImpDeclSpec -> IE GhcPs
                         -> SDoc
 badImportItemErrDataCon dataType_occ iface decl_spec ie
   = vcat [ text "In module"
-             <+> quotes (ppr (is_mod decl_spec))
-             <+> source_import <> colon
+             <+> (quotes (ppr (is_mod decl_spec)) <+> source_import)
+             <> colon
          , nest 2 $ quotes datacon
              <+> text "is a data constructor of"
              <+> quotes dataType
diff --git a/compiler/GHC/Rename/Utils.hs b/compiler/GHC/Rename/Utils.hs
index 3c4f5d065f..b7811b9987 100644
--- a/compiler/GHC/Rename/Utils.hs
+++ b/compiler/GHC/Rename/Utils.hs
@@ -408,7 +408,7 @@ addNameClashErrRn rdr_name gres
 shadowedNameWarn :: OccName -> [SDoc] -> SDoc
 shadowedNameWarn occ shadowed_locs
   = sep [text "This binding for" <+> quotes (ppr occ)
-            <+> text "shadows the existing binding" <> plural shadowed_locs,
+            <+> text "shadows the existing" <+>  plural "binding" shadowed_locs,
          nest 2 (vcat shadowed_locs)]
 
 
diff --git a/compiler/GHC/SysTools/Ar.hs b/compiler/GHC/SysTools/Ar.hs
index 200b652049..8c2fdb88f9 100644
--- a/compiler/GHC/SysTools/Ar.hs
+++ b/compiler/GHC/SysTools/Ar.hs
@@ -35,7 +35,6 @@ module GHC.SysTools.Ar
 import GhcPrelude
 
 import Data.List (mapAccumL, isPrefixOf)
-import Data.Monoid ((<>))
 import Data.Binary.Get
 import Data.Binary.Put
 import Control.Monad
diff --git a/compiler/GHC/Tc/Errors.hs b/compiler/GHC/Tc/Errors.hs
index ae08f78443..be6524d1ba 100644
--- a/compiler/GHC/Tc/Errors.hs
+++ b/compiler/GHC/Tc/Errors.hs
@@ -477,7 +477,7 @@ warnRedundantConstraints ctxt env info ev_vars
  = do { msg <- mkErrorReport ctxt env (important doc)
       ; reportWarning (Reason Opt_WarnRedundantConstraints) msg }
  where
-   doc = text "Redundant constraint" <> plural redundant_evs <> colon
+   doc = text "Redundant" <+> plural "constraint" redundant_evs <> colon
          <+> pprEvVarTheta redundant_evs
 
    redundant_evs =
@@ -1249,7 +1249,7 @@ mkIPErr ctxt cts
              givens  = getUserGivens ctxt
              msg | null givens
                  = addArising orig $
-                   sep [ text "Unbound implicit parameter" <> plural cts
+                   sep [ text "Unbound implicit" <+> plural "parameter" cts
                        , nest 2 (pprParendTheta preds) ]
                  | otherwise
                  = couldNotDeduce givens (preds, orig)
@@ -1552,7 +1552,7 @@ mkTyVarEqErr' dflags ctxt report ct oriented tv1 ty2
   , let esc_skols = filter (`elemVarSet` (tyCoVarsOfType ty2)) skols
   , not (null esc_skols)
   = do { let msg = important $ misMatchMsg ct oriented ty1 ty2
-             esc_doc = sep [ text "because" <+> what <+> text "variable" <> plural esc_skols
+             esc_doc = sep [ text "because" <+> what <+> plural "variable" esc_skols
                              <+> pprQuotedList esc_skols
                            , text "would escape" <+>
                              if isSingleton esc_skols then text "its scope"
@@ -2361,7 +2361,7 @@ mk_dict_err ctxt@(CEC {cec_encl = implics}) (ct, (matches, unifiers, unsafe_over
           = vcat [ ppWhen lead_with_ambig $
                      text "Probable fix: use a type annotation to specify what"
                      <+> pprQuotedList ambig_tvs <+> text "should be."
-                 , text "These potential instance" <> plural unifiers
+                 , text "These potential" <+>  plural "instance" unifiers
                    <+> text "exist:"]
 
         mb_patsyn_prov :: Maybe SDoc
@@ -2737,8 +2737,8 @@ mkAmbigMsg prepend_msg ct
 
     msg |  any isRuntimeUnkSkol ambig_kvs  -- See Note [Runtime skolems]
         || any isRuntimeUnkSkol ambig_tvs
-        = vcat [ text "Cannot resolve unknown runtime type"
-                 <> plural ambig_tvs <+> pprQuotedList ambig_tvs
+        = vcat [ text "Cannot resolve unknown runtime"
+                 <+> plural "type" ambig_tvs <+> pprQuotedList ambig_tvs
                , text "Use :print or :force to determine these types"]
 
         | not (null ambig_tvs)
@@ -2749,11 +2749,11 @@ mkAmbigMsg prepend_msg ct
 
     pp_ambig what tkvs
       | prepend_msg -- "Ambiguous type variable 't0'"
-      = text "Ambiguous" <+> what <+> text "variable"
-        <> plural tkvs <+> pprQuotedList tkvs
+      = text "Ambiguous" <+> what <+> plural "variable" tkvs
+        <+> pprQuotedList tkvs
 
       | otherwise -- "The type variable 't0' is ambiguous"
-      = text "The" <+> what <+> text "variable" <> plural tkvs
+      = text "The" <+> what <+> plural "variable" tkvs
         <+> pprQuotedList tkvs <+> isOrAre tkvs <+> text "ambiguous"
 
 pprSkols :: ReportErrCtxt -> [TcTyVar] -> SDoc
@@ -2940,7 +2940,7 @@ warnDefaulting wanteds default_ty
              (loc, ppr_wanteds) = pprWithArising tidy_wanteds
              warn_msg =
                 hang (hsep [ text "Defaulting the following"
-                           , text "constraint" <> plural tidy_wanteds
+                           , plural "constraint" tidy_wanteds
                            , text "to type"
                            , quotes (ppr default_ty) ])
                      2
diff --git a/compiler/GHC/Tc/Errors/Hole.hs b/compiler/GHC/Tc/Errors/Hole.hs
index 771765901c..4e7b02bad0 100644
--- a/compiler/GHC/Tc/Errors/Hole.hs
+++ b/compiler/GHC/Tc/Errors/Hole.hs
@@ -492,7 +492,7 @@ pprHoleFit (HFDC sWrp sWrpVars sTy sProv sMs) (HoleFit {..}) =
                                Just (_, unfunned) -> unwrapTypeVars unfunned
                                _ -> []
              where (vars, unforalled) = splitForAllVarBndrs t
-       holeVs = sep $ map (parens . (text "_" <+> dcolon <+>) . ppr) hfMatches
+       holeVs = sep $ map (parens . ((text "_" <+> dcolon) <+>) . ppr) hfMatches
        holeDisp = if sMs then holeVs
                   else sep $ replicate (length hfMatches) $ text "_"
        occDisp = pprPrefixOcc name
diff --git a/compiler/GHC/Tc/Errors/Hole/FitTypes.hs b/compiler/GHC/Tc/Errors/Hole/FitTypes.hs
index 8aabc615ce..bade5a28f9 100644
--- a/compiler/GHC/Tc/Errors/Hole/FitTypes.hs
+++ b/compiler/GHC/Tc/Errors/Hole/FitTypes.hs
@@ -98,7 +98,7 @@ instance Outputable HoleFit where
   ppr (HoleFit _ cand ty _ _ mtchs _) =
     hang (name <+> holes) 2 (text "where" <+> name <+> dcolon <+> (ppr ty))
     where name = ppr $ getName cand
-          holes = sep $ map (parens . (text "_" <+> dcolon <+>) . ppr) mtchs
+          holes = sep $ map (parens . ((text "_" <+> dcolon) <+>) . ppr) mtchs
 
 -- We compare HoleFits by their name instead of their Id, since we don't
 -- want our tests to be affected by the non-determinism of `nonDetCmpVar`,
diff --git a/compiler/GHC/Tc/Gen/Annotation.hs b/compiler/GHC/Tc/Gen/Annotation.hs
index ef7168076f..98bf1a04bc 100644
--- a/compiler/GHC/Tc/Gen/Annotation.hs
+++ b/compiler/GHC/Tc/Gen/Annotation.hs
@@ -39,7 +39,7 @@ warnAnns :: [LAnnDecl GhcRn] -> TcM [Annotation]
 warnAnns [] = return []
 warnAnns anns@(L loc _ : _)
   = do { setSrcSpan loc $ addWarnTc NoReason $
-             (text "Ignoring ANN annotation" <> plural anns <> comma
+             (text "Ignoring ANN" <+> plural "annotation" anns <> comma
              <+> text "because this is a stage-1 compiler without -fexternal-interpreter or doesn't support GHCi")
        ; return [] }
 
diff --git a/compiler/GHC/Tc/Gen/Expr.hs b/compiler/GHC/Tc/Gen/Expr.hs
index 70201773b9..4d4e33edae 100644
--- a/compiler/GHC/Tc/Gen/Expr.hs
+++ b/compiler/GHC/Tc/Gen/Expr.hs
@@ -2657,8 +2657,8 @@ provided.
 
 badFieldTypes :: [(FieldLabelString,TcType)] -> SDoc
 badFieldTypes prs
-  = hang (text "Record update for insufficiently polymorphic field"
-                         <> plural prs <> colon)
+  = hang (text "Record update for insufficiently polymorphic"
+                         <+> plural "field" prs <> colon)
        2 (vcat [ ppr f <+> dcolon <+> ppr ty | (f,ty) <- prs ])
 
 badFieldsUpd
diff --git a/compiler/GHC/Tc/Gen/Pat.hs b/compiler/GHC/Tc/Gen/Pat.hs
index 2ae1f1cfb9..b7acc7e879 100644
--- a/compiler/GHC/Tc/Gen/Pat.hs
+++ b/compiler/GHC/Tc/Gen/Pat.hs
@@ -737,7 +737,7 @@ tcPatSig in_pat_bind sig res_ty
 
 patBindSigErr :: [(Name,TcTyVar)] -> SDoc
 patBindSigErr sig_tvs
-  = hang (text "You cannot bind scoped type variable" <> plural sig_tvs
+  = hang (text "You cannot bind scoped type" <+> plural "variable" sig_tvs
           <+> pprQuotedList (map fst sig_tvs))
        2 (text "in a pattern binding signature")
 
diff --git a/compiler/GHC/Tc/Instance/Family.hs b/compiler/GHC/Tc/Instance/Family.hs
index 68c894f2e4..c6d3010ff1 100644
--- a/compiler/GHC/Tc/Instance/Family.hs
+++ b/compiler/GHC/Tc/Instance/Family.hs
@@ -990,8 +990,8 @@ reportUnusedInjectiveVarsErr fam_tc tvs has_kinds undec_inst tyfamEqn
                                   (tyfamEqn :| [])
     in addErrAt loc (pprWithExplicitKindsWhen has_kinds doc)
     where
-      herald = sep [ what <+> text "variable" <>
-                  pluralVarSet tvs <+> pprVarSet tvs (pprQuotedList . scopedSort)
+      herald = sep [ what <+> pluralVarSet "variable" tvs
+                          <+> pprVarSet tvs (pprQuotedList . scopedSort)
                 , text "cannot be inferred from the right-hand side." ]
                $$ extra
 
diff --git a/compiler/GHC/Tc/Instance/FunDeps.hs b/compiler/GHC/Tc/Instance/FunDeps.hs
index 40344af9ed..357c4fc5a2 100644
--- a/compiler/GHC/Tc/Instance/FunDeps.hs
+++ b/compiler/GHC/Tc/Instance/FunDeps.hs
@@ -414,14 +414,14 @@ checkInstCoverage be_liberal clas theta inst_taus
                             <+> quotes (ppr clas)
                           , nest 2 $ text "for functional dependency:"
                             <+> quotes (pprFunDep fd) ]
-                    , sep [ text "Reason: lhs type"<>plural ls <+> pprQuotedList ls
+                    , sep [ text "Reason: lhs" <+> plural "type" ls <+> pprQuotedList ls
                           , nest 2 $
                             (if isSingleton ls
                              then text "does not"
                              else text "do not jointly")
-                            <+> text "determine rhs type"<>plural rs
+                            <+> text "determine rhs" <+> plural "type" rs
                             <+> pprQuotedList rs ]
-                    , text "Un-determined variable" <> pluralVarSet undet_set <> colon
+                    , text "Un-determined" <+> pluralVarSet "variable" undet_set <> colon
                             <+> pprVarSet undet_set (pprWithCommas ppr)
                     , ppWhen (not be_liberal &&
                               and (isEmptyVarSet <$> liberal_undet_tvs)) $
diff --git a/compiler/GHC/Tc/Solver.hs b/compiler/GHC/Tc/Solver.hs
index c060eac638..4d88056665 100644
--- a/compiler/GHC/Tc/Solver.hs
+++ b/compiler/GHC/Tc/Solver.hs
@@ -1165,8 +1165,8 @@ decideMonoTyVars infer_mode name_taus psigs candidates
 
     pp_bndrs = pprWithCommas (quotes . ppr . fst) name_taus
     mr_msg =
-         hang (sep [ text "The Monomorphism Restriction applies to the binding"
-                     <> plural name_taus
+         hang (sep [ text "The Monomorphism Restriction applies to the"
+                     <+> plural "binding" name_taus
                    , text "for" <+> pp_bndrs ])
             2 (hsep [ text "Consider giving"
                     , text (if isSingleton name_taus then "it" else "them")
diff --git a/compiler/GHC/Tc/TyCl/PatSyn.hs b/compiler/GHC/Tc/TyCl/PatSyn.hs
index 797ff2f594..02949968fe 100644
--- a/compiler/GHC/Tc/TyCl/PatSyn.hs
+++ b/compiler/GHC/Tc/TyCl/PatSyn.hs
@@ -227,8 +227,8 @@ dependentArgErr (arg, bad_cos)
          , hang (text "Pattern-bound variable")
               2 (ppr arg <+> dcolon <+> ppr (idType arg))
          , nest 2 $
-           hang (text "has a type that mentions pattern-bound coercion"
-                 <> plural bad_co_list <> colon)
+           hang (text "has a type that mentions pattern-bound"
+                 <+> plural "coercion" bad_co_list <> colon)
               2 (pprWithCommas ppr bad_co_list)
          , text "Hint: use -fprint-explicit-coercions to see the coercions"
          , text "Probable fix: add a pattern signature" ]
@@ -368,7 +368,7 @@ tcCheckPatSynDecl psb@PSB{ psb_id = lname@(L _ name), psb_args = details
        ; checkTc (null bad_tvs) $
          hang (sep [ text "The result type of the signature for" <+> quotes (ppr name) <> comma
                    , text "namely" <+> quotes (ppr pat_ty) ])
-            2 (text "mentions existential type variable" <> plural bad_tvs
+            2 (text "mentions existential type" <+> plural "variable" bad_tvs
                <+> pprQuotedList bad_tvs)
 
          -- See Note [The pattern-synonym signature splitting rule] in GHC.Tc.Gen.Sig
diff --git a/compiler/GHC/Tc/Types/Origin.hs b/compiler/GHC/Tc/Types/Origin.hs
index 86427853de..e86bdf4c19 100644
--- a/compiler/GHC/Tc/Types/Origin.hs
+++ b/compiler/GHC/Tc/Types/Origin.hs
@@ -243,7 +243,7 @@ pprSkolInfo :: SkolemInfo -> SDoc
 pprSkolInfo (SigSkol cx ty _) = pprSigSkolInfo cx ty
 pprSkolInfo (SigTypeSkol cx)  = pprUserTypeCtxt cx
 pprSkolInfo (ForAllSkol doc)  = quotes doc
-pprSkolInfo (IPSkol ips)      = text "the implicit-parameter binding" <> plural ips <+> text "for"
+pprSkolInfo (IPSkol ips)      = text "the implicit-parameter" <+> plural "binding" ips <+> text "for"
                                  <+> pprWithCommas ppr ips
 pprSkolInfo (DerivSkol pred)  = text "the deriving clause for" <+> quotes (ppr pred)
 pprSkolInfo InstSkol          = text "the instance declaration"
@@ -254,7 +254,7 @@ pprSkolInfo (RuleSkol name)   = text "the RULE" <+> pprRuleName name
 pprSkolInfo ArrowSkol         = text "an arrow form"
 pprSkolInfo (PatSkol cl mc)   = sep [ pprPatSkolInfo cl
                                     , text "in" <+> pprMatchContext mc ]
-pprSkolInfo (InferSkol ids)   = hang (text "the inferred type" <> plural ids <+> text "of")
+pprSkolInfo (InferSkol ids)   = hang (text "the inferred" <+> plural "type" ids <+> text "of")
                                    2 (vcat [ ppr name <+> dcolon <+> ppr ty
                                                    | (name,ty) <- ids ])
 pprSkolInfo (UnifyForAllSkol ty) = text "the type" <+> ppr ty
diff --git a/compiler/GHC/Tc/Utils/TcMType.hs b/compiler/GHC/Tc/Utils/TcMType.hs
index d37b37efe3..27460b0882 100644
--- a/compiler/GHC/Tc/Utils/TcMType.hs
+++ b/compiler/GHC/Tc/Utils/TcMType.hs
@@ -2396,7 +2396,7 @@ naughtyQuantification orig_ty tv escapees
              orig_ty'   = tidyType env orig_ty1
              ppr_tidied = pprTyVars . map (tidyTyCoVarOcc env)
              doc = pprWithExplicitKindsWhen True $
-                   vcat [ sep [ text "Cannot generalise type; skolem" <> plural escapees'
+                   vcat [ sep [ text "Cannot generalise type;" <+> plural "skolem" escapees'
                               , quotes $ ppr_tidied escapees'
                               , text "would escape" <+> itsOrTheir escapees' <+> text "scope"
                               ]
diff --git a/compiler/GHC/Tc/Validity.hs b/compiler/GHC/Tc/Validity.hs
index 6e44a6c399..b5ba5feec9 100644
--- a/compiler/GHC/Tc/Validity.hs
+++ b/compiler/GHC/Tc/Validity.hs
@@ -1424,7 +1424,7 @@ constraintSynErr env kind
 dupPredWarn :: TidyEnv -> [NE.NonEmpty PredType] -> (TidyEnv, SDoc)
 dupPredWarn env dups
   = ( env
-    , text "Duplicate constraint" <> plural primaryDups <> text ":"
+    , text "Duplicate" <+> plural "constraint" primaryDups <> text ":"
       <+> pprWithCommas (ppr_tidy env) primaryDups )
   where
     primaryDups = map NE.head dups
@@ -1955,7 +1955,7 @@ smallerMsg what inst_head
 
 noMoreMsg :: [TcTyVar] -> SDoc -> SDoc -> SDoc
 noMoreMsg tvs what inst_head
-  = vcat [ hang (text "Variable" <> plural tvs1 <+> quotes (pprWithCommas ppr tvs1)
+  = vcat [ hang (plural "Variable" tvs1 <+> quotes (pprWithCommas ppr tvs1)
                 <+> occurs <+> text "more often")
               2 (sep [ text "in the" <+> what
                      , text "than in the instance head" <+> quotes inst_head ])
@@ -2196,7 +2196,7 @@ checkFamPatBinders fam_tc qtvs pats rhs
 
     check_tvs tvs what what2
       = unless (null tvs) $ addErrAt (getSrcSpan (head tvs)) $
-        hang (text "Type variable" <> plural tvs <+> pprQuotedList tvs
+        hang (text "Type" <+> plural "variable" tvs <+> pprQuotedList tvs
               <+> isOrAre tvs <+> what <> comma)
            2 (vcat [ text "but not" <+> what2 <+> text "the family instance"
                    , mk_extra tvs ])
diff --git a/compiler/GHC/Types/Basic.hs b/compiler/GHC/Types/Basic.hs
index 103b1940a0..52b819a3b5 100644
--- a/compiler/GHC/Types/Basic.hs
+++ b/compiler/GHC/Types/Basic.hs
@@ -1487,7 +1487,7 @@ pprInline' :: Bool           -- True <=> do not display the inl_inline field
            -> SDoc
 pprInline' emptyInline (InlinePragma { inl_inline = inline, inl_act = activation
                                     , inl_rule = info, inl_sat = mb_arity })
-    = pp_inl inline <> pp_act inline activation <+> pp_sat <+> pp_info
+    = (pp_inl inline <> pp_act inline activation) <+> pp_sat <+> pp_info
     where
       pp_inl x = if emptyInline then empty else ppr x
 
diff --git a/compiler/GHC/Types/Unique/FM.hs b/compiler/GHC/Types/Unique/FM.hs
index 01ab645783..bb7e966819 100644
--- a/compiler/GHC/Types/Unique/FM.hs
+++ b/compiler/GHC/Types/Unique/FM.hs
@@ -410,7 +410,7 @@ pprUFMWithKeys ufm pp = pp (nonDetUFMToList ufm)
 
 -- | Determines the pluralisation suffix appropriate for the length of a set
 -- in the same way that plural from Outputable does for lists.
-pluralUFM :: UniqFM a -> SDoc
-pluralUFM ufm
-  | sizeUFM ufm == 1 = empty
-  | otherwise = char 's'
+pluralUFM :: String -> UniqFM a -> SDoc
+pluralUFM s ufm
+  | sizeUFM ufm == 1 = text s
+  | otherwise = text (s ++ "s")
diff --git a/compiler/GHC/Types/Var/Set.hs b/compiler/GHC/Types/Var/Set.hs
index 5126988a2c..1d095a69e9 100644
--- a/compiler/GHC/Types/Var/Set.hs
+++ b/compiler/GHC/Types/Var/Set.hs
@@ -196,8 +196,8 @@ seqVarSet s = sizeVarSet s `seq` ()
 
 -- | Determines the pluralisation suffix appropriate for the length of a set
 -- in the same way that plural from Outputable does for lists.
-pluralVarSet :: VarSet -> SDoc
-pluralVarSet = pluralUFM . getUniqSet
+pluralVarSet :: String -> VarSet -> SDoc
+pluralVarSet s = pluralUFM s . getUniqSet
 
 -- | Pretty-print a non-deterministic set.
 -- The order of variables is non-deterministic and for pretty-printing that
diff --git a/compiler/utils/GhcPrelude.hs b/compiler/utils/GhcPrelude.hs
index dd78f15573..fa028b2d56 100644
--- a/compiler/utils/GhcPrelude.hs
+++ b/compiler/utils/GhcPrelude.hs
@@ -12,11 +12,7 @@
 
 module GhcPrelude (module X) where
 
--- We export the 'Semigroup' class but w/o the (<>) operator to avoid
--- clashing with the (Outputable.<>) operator which is heavily used
--- through GHC's code-base.
-
-import Prelude as X hiding ((<>))
+import Prelude as X
 import Data.Foldable as X (foldl')
 
 {-
diff --git a/compiler/utils/Outputable.hs b/compiler/utils/Outputable.hs
index d36faa4724..a65dda8f75 100644
--- a/compiler/utils/Outputable.hs
+++ b/compiler/utils/Outputable.hs
@@ -700,12 +700,14 @@ unicode unicode plain = sdocOption sdocCanUseUnicode $ \case
    True  -> unicode
    False -> plain
 
+infixr 6 <+>  -- matches that of (Semigroup.<>)
+infixl 5 $$, $+$
+
 nest :: Int -> SDoc -> SDoc
 -- ^ Indent 'SDoc' some specified amount
-(<>) :: SDoc -> SDoc -> SDoc
--- ^ Join two 'SDoc' together horizontally without a gap
 (<+>) :: SDoc -> SDoc -> SDoc
--- ^ Join two 'SDoc' together horizontally with a gap between them
+-- ^ Join two 'SDoc' together horizontally with a gap between them.
+-- Use '(<>)' to join without a gap.
 ($$) :: SDoc -> SDoc -> SDoc
 -- ^ Join two 'SDoc' together vertically; if there is
 -- no vertical overlap it "dovetails" the two onto one line
@@ -713,11 +715,14 @@ nest :: Int -> SDoc -> SDoc
 -- ^ Join two 'SDoc' together vertically
 
 nest n d    = SDoc $ Pretty.nest n . runSDoc d
-(<>) d1 d2  = SDoc $ \sty -> (Pretty.<>)  (runSDoc d1 sty) (runSDoc d2 sty)
 (<+>) d1 d2 = SDoc $ \sty -> (Pretty.<+>) (runSDoc d1 sty) (runSDoc d2 sty)
 ($$) d1 d2  = SDoc $ \sty -> (Pretty.$$)  (runSDoc d1 sty) (runSDoc d2 sty)
 ($+$) d1 d2 = SDoc $ \sty -> (Pretty.$+$) (runSDoc d1 sty) (runSDoc d2 sty)
 
+-- | Join two 'SDoc' together horizontally without a gap
+instance Semigroup SDoc where
+  d1 <> d2 = SDoc $ \sty -> runSDoc d1 sty <> runSDoc d2 sty
+
 hcat :: [SDoc] -> SDoc
 -- ^ Concatenate 'SDoc' horizontally
 hsep :: [SDoc] -> SDoc
@@ -790,8 +795,8 @@ coloured col sdoc = sdocOption sdocShouldUseColor $ \case
       ctx@SDC{ sdocLastColour = lastCol, sdocStyle = PprUser _ _ Coloured } ->
          let ctx' = ctx{ sdocLastColour = lastCol `mappend` col } in
          Pretty.zeroWidthText (Col.renderColour col)
-           Pretty.<> runSDoc sdoc ctx'
-           Pretty.<> Pretty.zeroWidthText (Col.renderColourAfresh lastCol)
+           <> runSDoc sdoc ctx'
+           <> Pretty.zeroWidthText (Col.renderColourAfresh lastCol)
       ctx -> runSDoc sdoc ctx
    False -> sdoc
 
@@ -1179,9 +1184,9 @@ speakNOf n d = speakN n <+> d <> char 's'               -- E.g. "three arguments
 -- > plural [] = char 's'
 -- > plural ["Hello"] = empty
 -- > plural ["Hello", "World"] = char 's'
-plural :: [a] -> SDoc
-plural [_] = empty  -- a bit frightening, but there you are
-plural _   = char 's'
+plural :: String -> [a] -> SDoc
+plural s [_] = text s  -- a bit frightening, but there you are
+plural s _   = text (s ++ "s")
 
 -- | Determines the form of to be appropriate for the length of a list:
 --
diff --git a/compiler/utils/Pretty.hs b/compiler/utils/Pretty.hs
index 5adfdd7699..6e8686a62d 100644
--- a/compiler/utils/Pretty.hs
+++ b/compiler/utils/Pretty.hs
@@ -201,8 +201,7 @@ But it doesn't work, for if x=empty, we would have
 -- ---------------------------------------------------------------------------
 -- Operator fixity
 
-infixl 6 <>
-infixl 6 <+>
+infixr 6 <+>  -- matches that of (Semigroup.<>)
 infixl 5 $$, $+$
 
 
@@ -659,14 +658,10 @@ nilAboveNest g k q           | not g && k > 0      -- No newline if no overlap
 -- ---------------------------------------------------------------------------
 -- Horizontal composition @<>@
 
--- We intentionally avoid Data.Monoid.(<>) here due to interactions of
--- Data.Monoid.(<>) and (<+>).  See
--- http://www.haskell.org/pipermail/libraries/2011-November/017066.html
-
 -- | Beside.
 -- '<>' is associative, with identity 'empty'.
-(<>) :: Doc -> Doc -> Doc
-p <>  q = beside_ p False q
+instance Semigroup Doc where
+  p <> q = beside_ p False q
 
 -- | Beside, separated by space, unless one of the arguments is 'empty'.
 -- '<+>' is associative, with identity 'empty'.
diff --git a/ghc/GHCi/UI.hs b/ghc/GHCi/UI.hs
index 87826438e3..13a220298c 100644
--- a/ghc/GHCi/UI.hs
+++ b/ghc/GHCi/UI.hs
@@ -110,7 +110,7 @@ import qualified Data.IntMap.Strict as IntMap
 import Data.Time.LocalTime ( getZonedTime )
 import Data.Time.Format ( formatTime, defaultTimeLocale )
 import Data.Version ( showVersion )
-import Prelude hiding ((<>))
+import Prelude
 
 import Exception hiding (catch)
 import Foreign hiding (void)
diff --git a/ghc/GHCi/UI/Info.hs b/ghc/GHCi/UI/Info.hs
index f5df1edc38..9c93f17ed7 100644
--- a/ghc/GHCi/UI/Info.hs
+++ b/ghc/GHCi/UI/Info.hs
@@ -28,7 +28,7 @@ import           Data.Map.Strict   (Map)
 import qualified Data.Map.Strict   as M
 import           Data.Maybe
 import           Data.Time
-import           Prelude           hiding (mod,(<>))
+import           Prelude           hiding (mod)
 import           System.Directory
 
 import qualified GHC.Core.Utils
diff --git a/ghc/GHCi/UI/Monad.hs b/ghc/GHCi/UI/Monad.hs
index 27e31b6cf6..bc7e56e48c 100644
--- a/ghc/GHCi/UI/Monad.hs
+++ b/ghc/GHCi/UI/Monad.hs
@@ -62,7 +62,7 @@ import Data.Time
 import System.Environment
 import System.IO
 import Control.Monad
-import Prelude hiding ((<>))
+import Prelude
 
 import System.Console.Haskeline (CompletionFunc, InputT)
 import Control.Monad.Catch
diff --git a/ghc/Main.hs b/ghc/Main.hs
index 7a356b920a..0699e8d075 100644
--- a/ghc/Main.hs
+++ b/ghc/Main.hs
@@ -862,7 +862,7 @@ dumpFastStringStats dflags = do
         -- the "z-encoded" total.
   putMsg dflags msg
   where
-   x `pcntOf` y = int ((x * 100) `quot` y) Outputable.<> char '%'
+   x `pcntOf` y = int ((x * 100) `quot` y) <> char '%'
 
 showPackages, dumpPackages, dumpPackagesSimple :: DynFlags -> IO ()
 showPackages       dflags = putStrLn (showSDoc dflags (pprPackages (pkgState dflags)))