Avoid partial functions Data.List.init and Data.List.last

This is to comply with https://github.com/haskell/core-libraries-committee/issues/292

src/Trace/Hpc/Draft.hs

@@ -2,6 +2,9 @@
 -- Module             : Trace.Hpc.Draft
 -- Description        : The subcommand @hpc draft@
 -- License            : BSD-3-Clause
+
+{-# LANGUAGE CPP #-}
+
 module Trace.Hpc.Draft (draftPlugin) where
 
 import qualified Data.Map as Map
@@ -14,6 +17,13 @@ import Trace.Hpc.Tix
 import Trace.Hpc.Util
 import Trace.Hpc.Utils
 
+#if MIN_VERSION_base(4,19,0)
+import Data.List (unsnoc)
+#else
+unsnoc :: [a] -> Maybe ([a], a)
+unsnoc = foldr (\x -> Just . maybe ([], x) (\(~(a, b)) -> (x : a, b))) Nothing
+#endif
+
 ------------------------------------------------------------------------------
 
 draftOptions :: FlagOptSeq
@@ -89,7 +99,7 @@ makeDraft hpcflags tix = do
       showPleaseTick d (TickFun str pos) =
         spaces d
           ++ "tick function \""
-          ++ last str
+          ++ maybe "" snd (unsnoc str)
           ++ "\" "
           ++ "on line "
           ++ show (firstLine pos)

src/Trace/Hpc/Utils.hs

@@ -7,6 +7,9 @@
 module Trace.Hpc.Utils where
 
 import Data.Char (isControl)
+import Data.List (intercalate)
+import Data.List.NonEmpty (NonEmpty(..))
+import qualified Data.List.NonEmpty as NE
 import qualified Data.Map as Map
 import qualified Data.Set as Set
 import Numeric (showHex)
@@ -30,9 +33,11 @@ grabHpcPos hsMap srcspan =
     [ln] ->
       take ((c2 - c1) + 1) $ drop (c1 - 1) ln
     hd : tl ->
-      let lns1 = drop (c1 - 1) hd : tl
-          lns2 = init lns1 ++ [take (c2 + 1) (last lns1)]
-       in foldl1 (\xs ys -> xs ++ "\n" ++ ys) lns2
+      let lns1 :: NonEmpty String
+          lns1 = drop (c1 - 1) hd :| tl
+          lns2 :: [String]
+          lns2 = NE.init lns1 ++ [take (c2 + 1) (NE.last lns1)]
+       in intercalate "\n" lns2
   where
     (l1, c1, l2, c2) = fromHpcPos srcspan
     lns =