Assorted changes to avoid Data.List.{head,tail}

11 files changed, 32 insertions, 31 deletions

diff --git a/compiler/GHC/CmmToAsm/BlockLayout.hs b/compiler/GHC/CmmToAsm/BlockLayout.hs
index 56afdfb668..ebbbf5bbd3 100644
--- a/compiler/GHC/CmmToAsm/BlockLayout.hs
+++ b/compiler/GHC/CmmToAsm/BlockLayout.hs
@@ -14,6 +14,7 @@ module GHC.CmmToAsm.BlockLayout
 where
 
 import GHC.Prelude hiding (head, init, last, tail)
+import qualified GHC.Prelude as Partial (head, tail)
 
 import GHC.Platform
 
@@ -41,7 +42,6 @@ import GHC.Utils.Panic.Plain
 import GHC.Utils.Misc
 
 import Data.List (sortOn, sortBy, nub)
-import qualified Data.List as Partial (head, tail)
 import Data.List.NonEmpty (nonEmpty)
 import qualified Data.List.NonEmpty as NE
 import Data.Foldable (toList)
diff --git a/compiler/GHC/Core/Opt/Simplify/Utils.hs b/compiler/GHC/Core/Opt/Simplify/Utils.hs
index 28b1ebc221..2c1a6e7f76 100644
--- a/compiler/GHC/Core/Opt/Simplify/Utils.hs
+++ b/compiler/GHC/Core/Opt/Simplify/Utils.hs
@@ -43,6 +43,7 @@ module GHC.Core.Opt.Simplify.Utils (
     ) where
 
 import GHC.Prelude hiding (head, init, last, tail)
+import qualified GHC.Prelude as Partial (head)
 
 import GHC.Core
 import GHC.Types.Literal ( isLitRubbish )
@@ -84,7 +85,6 @@ import GHC.Utils.Panic.Plain
 
 import Control.Monad    ( when )
 import Data.List        ( sortBy )
-import qualified Data.List as Partial ( head )
 
 {- *********************************************************************
 *                                                                      *
diff --git a/compiler/GHC/JS/Make.hs b/compiler/GHC/JS/Make.hs
index fc30d0d915..6dee4cec93 100644
--- a/compiler/GHC/JS/Make.hs
+++ b/compiler/GHC/JS/Make.hs
@@ -141,7 +141,6 @@ import Control.Arrow ((***))
 
 import Data.Array
 import qualified Data.Map as M
-import qualified Data.List as List
 
 import GHC.Utils.Outputable (Outputable (..))
 import GHC.Data.FastString
@@ -277,21 +276,21 @@ jVar f = UnsatBlock . IS $ do
 jForIn :: ToSat a => JExpr -> (JExpr -> a)  -> JStat
 jForIn e f = UnsatBlock . IS $ do
                (block, is) <- runIdentSupply $ toSat_ f []
-               let i = List.head is
+               let i = head is
                return $ decl i `mappend` ForInStat False i e block
 
 -- | As with "jForIn" but creating a \"for each in\" statement.
 jForEachIn :: ToSat a => JExpr -> (JExpr -> a) -> JStat
 jForEachIn e f = UnsatBlock . IS $ do
                (block, is) <- runIdentSupply $ toSat_ f []
-               let i = List.head is
+               let i = head is
                return $ decl i `mappend` ForInStat True i e block
 
 -- | As with "jForIn" but creating a \"for each in\" statement.
 jTryCatchFinally :: (ToSat a) => JStat -> a -> JStat -> JStat
 jTryCatchFinally s f s2 = UnsatBlock . IS $ do
                      (block, is) <- runIdentSupply $ toSat_ f []
-                     let i = List.head is
+                     let i = head is
                      return $ TryStat s i block s2
 
 -- | construct a JS variable reference
diff --git a/compiler/GHC/StgToJS/Linker/Linker.hs b/compiler/GHC/StgToJS/Linker/Linker.hs
index 99e56efbb6..9c4a077c0b 100644
--- a/compiler/GHC/StgToJS/Linker/Linker.hs
+++ b/compiler/GHC/StgToJS/Linker/Linker.hs
@@ -84,9 +84,10 @@ import Data.Function            (on)
 import Data.IntSet              (IntSet)
 import qualified Data.IntSet              as IS
 import Data.IORef
-import Data.List  ( partition, nub, intercalate, group, sort
+import Data.List  ( partition, nub, intercalate, sort
                   , groupBy, intersperse,
                   )
+import qualified Data.List.NonEmpty       as NE
 import Data.Map.Strict          (Map)
 import qualified Data.Map.Strict          as M
 import Data.Maybe
@@ -228,7 +229,7 @@ computeLinkDependencies cfg logger target unit_env units objFiles extraStaticDep
   (objDepsMap, objRequiredUnits) <- loadObjDeps objFiles
 
   let roots    = S.fromList . filter isRootFun $ concatMap (M.keys . depsHaskellExported . fst) (M.elems objDepsMap)
-      rootMods = map (moduleNameString . moduleName . head) . group . sort . map funModule . S.toList $ roots
+      rootMods = map (moduleNameString . moduleName . NE.head) . NE.group . sort . map funModule . S.toList $ roots
       objPkgs  = map moduleUnitId $ nub (M.keys objDepsMap)
 
   when (logVerbAtLeast logger 2) $ void $ do
diff --git a/compiler/GHC/Utils/Misc.hs b/compiler/GHC/Utils/Misc.hs
index 09111ccc47..af32a679bb 100644
--- a/compiler/GHC/Utils/Misc.hs
+++ b/compiler/GHC/Utils/Misc.hs
@@ -121,6 +121,7 @@ module GHC.Utils.Misc (
     ) where
 
 import GHC.Prelude.Basic hiding ( head, init, last, tail )
+import qualified GHC.Prelude.Basic as Partial ( head )
 
 import GHC.Utils.Exception
 import GHC.Utils.Panic.Plain
@@ -129,7 +130,6 @@ import GHC.Utils.Fingerprint
 
 import Data.Data
 import qualified Data.List as List
-import qualified Data.List as Partial ( head )
 import Data.List.NonEmpty  ( NonEmpty(..), last, nonEmpty )
 import qualified Data.List.NonEmpty as NE
 
diff --git a/libraries/base/Data/Data.hs b/libraries/base/Data/Data.hs
index 1a081484a9..5fcecc867a 100644
--- a/libraries/base/Data/Data.hs
+++ b/libraries/base/Data/Data.hs
@@ -704,10 +704,9 @@ readConstr dt str =
 
     -- Traverse list of algebraic datatype constructors
     idx :: [Constr] -> Maybe Constr
-    idx cons = let fit = filter ((==) str . showConstr) cons
-                in if fit == []
-                     then Nothing
-                     else Just (head fit)
+    idx cons = case filter ((==) str . showConstr) cons of
+                [] -> Nothing
+                hd : _ -> Just hd
 
     ffloat :: Double -> Constr
     ffloat =  mkPrimCon dt str . FloatConstr . toRational
@@ -850,17 +849,17 @@ isNorepType dt = case datarep dt of
 -- drop *.*.*... before name
 --
 tyconUQname :: String -> String
-tyconUQname x = let x' = dropWhile (not . (==) '.') x
-                 in if x' == [] then x else tyconUQname (tail x')
+tyconUQname x = case dropWhile (not . (==) '.') x of
+                  [] -> x
+                  _ : tl -> tyconUQname tl
 
 
 -- | Gets the module of a type constructor:
 -- take *.*.*... before name
 tyconModule :: String -> String
-tyconModule x = let (a,b) = break ((==) '.') x
-                 in if b == ""
-                      then b
-                      else a ++ tyconModule' (tail b)
+tyconModule x = case break ((==) '.') x of
+                  (_, "") -> ""
+                  (a, _ : tl) -> a ++ tyconModule' tl
   where
     tyconModule' y = let y' = tyconModule y
                       in if y' == "" then "" else ('.':y')
diff --git a/utils/check-exact/ExactPrint.hs b/utils/check-exact/ExactPrint.hs
index be7bb54c0d..a569b803d4 100644
--- a/utils/check-exact/ExactPrint.hs
+++ b/utils/check-exact/ExactPrint.hs
@@ -3696,12 +3696,13 @@ exactVanillaDeclHead :: (Monad m, Monoid w)
 exactVanillaDeclHead thing tvs@(HsQTvs { hsq_explicit = tyvars }) fixity context = do
   let
     exact_tyvars (varl:varsr)
-      | fixity == Infix && length varsr > 1 = do
+      | hvarsr : tvarsr@(_ : _) <- varsr
+      , fixity == Infix = do
           varl' <- markAnnotated varl
           thing' <- markAnnotated thing
-          hvarsr <- markAnnotated (head varsr)
-          tvarsr <- markAnnotated (tail varsr)
-          return (thing', varl':hvarsr:tvarsr)
+          hvarsr' <- markAnnotated hvarsr
+          tvarsr' <- markAnnotated tvarsr
+          return (thing', varl':hvarsr':tvarsr')
       | fixity == Infix = do
           varl' <- markAnnotated varl
           thing' <- markAnnotated thing
diff --git a/utils/check-exact/Main.hs b/utils/check-exact/Main.hs
index c4eedde74a..74525dd5f9 100644
--- a/utils/check-exact/Main.hs
+++ b/utils/check-exact/Main.hs
@@ -577,11 +577,11 @@ changeWhereIn3b :: Changer
 changeWhereIn3b _libdir (L l p) = do
   let decls0 = hsmodDecls p
       (decls,_,w) = runTransform (balanceCommentsList decls0)
-      (de0:_:de1:d2:_) = decls
+      (de0:tdecls@(_:de1:d2:_)) = decls
       de0' = setEntryDP de0 (DifferentLine 2 0)
       de1' = setEntryDP de1 (DifferentLine 2 0)
       d2' = setEntryDP d2 (DifferentLine 2 0)
-      decls' = d2':de1':de0':(tail decls)
+      decls' = d2':de1':de0':tdecls
   debugM $ unlines w
   debugM $ "changeWhereIn3b:de1':" ++ showAst de1'
   let p2 = p { hsmodDecls = decls'}
@@ -799,8 +799,9 @@ rmDecl5 _libdir lp = do
           go :: HsExpr GhcPs -> Transform (HsExpr GhcPs)
           go (HsLet a tkLet lb tkIn expr) = do
             decs <- hsDeclsValBinds lb
+            let hdecs : _ = decs
             let dec = last decs
-            _ <- transferEntryDP (head decs) dec
+            _ <- transferEntryDP hdecs dec
             lb' <- replaceDeclsValbinds WithoutWhere lb [dec]
             return (HsLet a tkLet lb' tkIn expr)
           go x = return x
diff --git a/utils/check-exact/Preprocess.hs b/utils/check-exact/Preprocess.hs
index 756dc18984..55d84763f5 100644
--- a/utils/check-exact/Preprocess.hs
+++ b/utils/check-exact/Preprocess.hs
@@ -192,7 +192,7 @@ stripPreprocessorDirectives :: GHC.StringBuffer -> GHC.StringBuffer
 stripPreprocessorDirectives buf = buf'
   where
     srcByLine = lines $ sbufToString buf
-    noDirectivesLines = map (\line -> if line /= [] && head line == '#' then "" else line) srcByLine
+    noDirectivesLines = map (\line -> case line of '#' : _ -> ""; _ -> line) srcByLine
     buf' = GHC.stringToStringBuffer $ unlines noDirectivesLines
 
 -- ---------------------------------------------------------------------
@@ -259,7 +259,7 @@ fingerprintStrings ss = GHC.fingerprintFingerprints $ map GHC.fingerprintString
 getPreprocessorAsComments :: FilePath -> IO [(GHC.Located GHC.Token, String)]
 getPreprocessorAsComments srcFile = do
   fcontents <- readFileGhc srcFile
-  let directives = filter (\(_lineNum,line) -> line /= [] && head line == '#')
+  let directives = filter (\(_lineNum,line) -> case line of '#' : _ -> True; _ -> False)
                     $ zip [1..] (lines fcontents)
 
   let mkTok (lineNum,line) = (GHC.L l (GHC.ITlineComment line (makeBufSpan l)),line)
diff --git a/utils/check-exact/Transform.hs b/utils/check-exact/Transform.hs
index 3e3ebdcb39..45f3612a76 100644
--- a/utils/check-exact/Transform.hs
+++ b/utils/check-exact/Transform.hs
@@ -322,7 +322,7 @@ setEntryDP (L (SrcSpanAnn (EpAnn (Anchor r _) an cs) l) a) dp
                l) a
               where
                 cs'' = setPriorComments cs (L (Anchor (anchor ca) (MovedAnchor dp)) c:cs')
-                lc = head $ reverse $ (L ca c:cs')
+                lc = last $ (L ca c:cs')
                 delta = tweakDelta $ ss2delta (ss2pos $ anchor $ getLoc lc) r
                 line = getDeltaLine delta
                 col = deltaColumn delta
diff --git a/utils/check-exact/Utils.hs b/utils/check-exact/Utils.hs
index b60c989bcf..14a05a2337 100644
--- a/utils/check-exact/Utils.hs
+++ b/utils/check-exact/Utils.hs
@@ -458,8 +458,8 @@ glast  info []    = error $ "glast " ++ info ++ " []"
 glast _info h     = last h
 
 gtail :: String -> [a] -> [a]
-gtail  info []   = error $ "gtail " ++ info ++ " []"
-gtail _info h    = tail h
+gtail  info []    = error $ "gtail " ++ info ++ " []"
+gtail _info (_:t) = t
 
 gfromJust :: String -> Maybe a -> a
 gfromJust _info (Just h) = h