Add NonEmpty variants of inits and tails (#557)

* Add basic benchmarks for inits/tails

* Add NonEmpty variants of inits and tails

  The lazy versions use new implementations:
  - Lazy tails got about 10% faster with ghc-9.2. (A happy accident!)
  - Lazy inits got much faster:
    - For the first few chunks it is about 1.5x faster, due to better list fusion.
    - When there are many chunks it is about 4x faster.

* Formatting and comments, as suggested in review

* Add link to a relevant CLC issue about NonEmpty

  - haskell/core-libraries-committee#107

Data/ByteString.hs

@@ -130,6 +130,8 @@ module Data.ByteString (
         groupBy,
         inits,
         tails,
+        initsNE,
+        tailsNE,
         stripPrefix,
         stripSuffix,
 
@@ -235,6 +237,8 @@ import Data.ByteString.Lazy.Internal (fromStrict, toStrict)
 import Data.ByteString.Unsafe
 
 import qualified Data.List as List
+import qualified Data.List.NonEmpty as NE
+import Data.List.NonEmpty (NonEmpty(..))
 
 import Data.Word                (Word8)
 
@@ -427,7 +431,7 @@ last ps@(BS x l)
                     unsafeWithForeignPtr x $ \p -> peekByteOff p (l-1)
 {-# INLINE last #-}
 
--- | /O(1)/ Return all the elements of a 'ByteString' except the last one.
+-- | /O(1)/ Returns all the elements of a 'ByteString' except the last one.
 -- An exception will be thrown in the case of an empty ByteString.
 --
 -- This is a partial function, consider using 'unsnoc' instead.
@@ -1686,17 +1690,47 @@ unzip ls = (pack (P.map fst ls), pack (P.map snd ls))
 -- ---------------------------------------------------------------------
 -- Special lists
 
--- | /O(n)/ Return all initial segments of the given 'ByteString', shortest first.
+-- | /O(n)/ Returns all initial segments of the given 'ByteString', shortest first.
 inits :: ByteString -> [ByteString]
-inits (BS x l) = [BS x n | n <- [0..l]]
+-- see Note [Avoid NonEmpty combinators]
+inits bs = NE.toList $! initsNE bs
 
--- | /O(n)/ Return all final segments of the given 'ByteString', longest first.
+-- | /O(n)/ Returns all initial segments of the given 'ByteString', shortest first.
+--
+-- @since 0.11.4.0
+initsNE :: ByteString -> NonEmpty ByteString
+-- see Note [Avoid NonEmpty combinators]
+initsNE (BS x len) = empty :| [BS x n | n <- [1..len]]
+
+-- | /O(n)/ Returns all final segments of the given 'ByteString', longest first.
 tails :: ByteString -> [ByteString]
-tails p | null p    = [empty]
-        | otherwise = p : tails (unsafeTail p)
+-- see Note [Avoid NonEmpty combinators]
+tails bs = NE.toList $! tailsNE bs
+
+-- | /O(n)/ Returns all final segments of the given 'ByteString', longest first.
+--
+-- @since 0.11.4.0
+tailsNE :: ByteString -> NonEmpty ByteString
+-- see Note [Avoid NonEmpty combinators]
+tailsNE p | null p    = empty :| []
+          | otherwise = p :| tails (unsafeTail p)
 
 -- less efficent spacewise: tails (BS x l) = [BS (plusForeignPtr x n) (l-n) | n <- [0..l]]
 
+{-
+Note [Avoid NonEmpty combinators]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+As of base-4.17, most of the NonEmpty API is surprisingly lazy.
+Using it without forcing the arguments yourself is just begging GHC
+to make your code waste time allocating useless selector thunks.
+This may change in the future. See also this CLC issue:
+  https://github.com/haskell/core-libraries-committee/issues/107
+But until then, "refactor" with care!
+-}
+
+
+
 -- ---------------------------------------------------------------------
 -- ** Ordered 'ByteString's
 

Data/ByteString/Char8.hs

@@ -129,6 +129,8 @@ module Data.ByteString.Char8 (
         groupBy,
         inits,
         tails,
+        initsNE,
+        tailsNE,
         strip,
         stripPrefix,
         stripSuffix,
@@ -261,7 +263,7 @@ import qualified Data.ByteString.Unsafe as B
 
 -- Listy functions transparently exported
 import Data.ByteString (null,length,tail,init,append
-                       ,inits,tails,reverse,transpose
+                       ,inits,tails,initsNE,tailsNE,reverse,transpose
                        ,concat,take,takeEnd,drop,dropEnd,splitAt
                        ,intercalate,sort,isPrefixOf,isSuffixOf
                        ,isInfixOf,stripPrefix,stripSuffix

Data/ByteString/Lazy.hs

@@ -145,6 +145,8 @@ module Data.ByteString.Lazy (
         groupBy,
         inits,
         tails,
+        initsNE,
+        tailsNE,
         stripPrefix,
         stripSuffix,
 
@@ -228,6 +230,8 @@ import Prelude hiding
     ,getContents,getLine,putStr,putStrLn ,zip,zipWith,unzip,notElem)
 
 import qualified Data.List              as List
+import qualified Data.List.NonEmpty     as NE
+import Data.List.NonEmpty (NonEmpty(..))
 import qualified Data.Bifunctor         as BF
 import qualified Data.ByteString        as P  (ByteString) -- type name only
 import qualified Data.ByteString        as S  -- S for strict (hmm...)
@@ -384,7 +388,7 @@ last (Chunk c0 cs0) = go c0 cs0
         go _ (Chunk c cs) = go c cs
 -- XXX Don't inline this. Something breaks with 6.8.2 (haven't investigated yet)
 
--- | /O(n\/c)/ Return all the elements of a 'ByteString' except the last one.
+-- | /O(n\/c)/ Returns all the elements of a 'ByteString' except the last one.
 --
 -- This is a partial function, consider using 'unsnoc' instead.
 init :: HasCallStack => ByteString -> ByteString
@@ -1433,19 +1437,39 @@ unzip ls = (pack (List.map fst ls), pack (List.map snd ls))
 -- ---------------------------------------------------------------------
 -- Special lists
 
--- | /O(n)/ Return all initial segments of the given 'ByteString', shortest first.
+-- | Returns all initial segments of the given 'ByteString', shortest first.
 inits :: ByteString -> [ByteString]
-inits = (Empty :) . inits'
-  where inits' Empty        = []
-        inits' (Chunk c cs) = List.map (`Chunk` Empty) (List.drop 1 (S.inits c))
-                           ++ List.map (Chunk c) (inits' cs)
+-- see Note [Avoid NonEmpty combinators] in Data.ByteString
+inits bs = NE.toList $! initsNE bs
 
--- | /O(n)/ Return all final segments of the given 'ByteString', longest first.
+-- | Returns all initial segments of the given 'ByteString', shortest first.
+--
+-- @since 0.11.4.0
+initsNE :: ByteString -> NonEmpty ByteString
+-- see Note [Avoid NonEmpty combinators] in Data.ByteString
+initsNE = (Empty :|) . inits' id
+  where
+    inits' :: (ByteString -> ByteString) -> ByteString -> [ByteString]
+    -- inits' f bs === map f (tail (inits bs))
+    inits' _ Empty = []
+    inits' f (Chunk c@(S.BS x len) cs)
+      = [f (S.BS x n `Chunk` Empty) | n <- [1..len]]
+      ++ inits' (f . Chunk c) cs
+
+-- | /O(n)/ Returns all final segments of the given 'ByteString', longest first.
 tails :: ByteString -> [ByteString]
-tails Empty         = [Empty]
-tails cs@(Chunk c cs')
-  | S.length c == 1 = cs : tails cs'
-  | otherwise       = cs : tails (Chunk (S.unsafeTail c) cs')
+-- see Note [Avoid NonEmpty combinators] in Data.ByteString
+tails bs = NE.toList $! tailsNE bs
+
+-- | /O(n)/ Returns all final segments of the given 'ByteString', longest first.
+--
+-- @since 0.11.4.0
+tailsNE :: ByteString -> NonEmpty ByteString
+-- see Note [Avoid NonEmpty combinators] in Data.ByteString
+tailsNE bs = case uncons bs of
+  Nothing -> Empty :| []
+  Just (_, tl) -> bs :| tails tl
+
 
 -- ---------------------------------------------------------------------
 -- Low level constructors

Data/ByteString/Lazy/Char8.hs

@@ -124,6 +124,8 @@ module Data.ByteString.Lazy.Char8 (
         groupBy,
         inits,
         tails,
+        initsNE,
+        tailsNE,
         stripPrefix,
         stripSuffix,
 
@@ -233,7 +235,7 @@ import Data.ByteString.Lazy
         (fromChunks, toChunks
         ,empty,null,length,tail,init,append,reverse,transpose,cycle
         ,concat,take,takeEnd,drop,dropEnd,splitAt,intercalate
-        ,isPrefixOf,isSuffixOf,group,inits,tails,copy
+        ,isPrefixOf,isSuffixOf,group,inits,tails,initsNE,tailsNE,copy
         ,stripPrefix,stripSuffix
         ,hGetContents, hGet, hPut, getContents
         ,hGetNonBlocking, hPutNonBlocking

bench/BenchAll.hs

@@ -97,6 +97,12 @@ lazyByteStringData :: L.ByteString
 lazyByteStringData = case S.splitAt (nRepl `div` 2) byteStringData of
     (bs1, bs2) -> L.fromChunks [bs1, bs2]
 
+{-# NOINLINE smallChunksData #-}
+smallChunksData :: L.ByteString
+smallChunksData
+  = L.fromChunks [S.take sz (S.drop n byteStringData)
+                 | let sz = 48, n <- [0, sz .. S.length byteStringData]]
+
 {-# NOINLINE byteStringChunksData #-}
 byteStringChunksData :: [S.ByteString]
 byteStringChunksData = map (S.pack . replicate (4 ) . fromIntegral) intData
@@ -404,6 +410,15 @@ main = do
         , bench "balancedSlow"    $ partitionLazy (\x -> hashWord8 x < w 128)
         ]
       ]
+    , bgroup "inits"
+      [ bench "strict" $ nf S.inits byteStringData
+      , bench "lazy"   $ nf L.inits lazyByteStringData
+      , bench "lazy (small chunks)" $ nf L.inits smallChunksData
+      ]
+    , bgroup "tails"
+      [ bench "strict" $ nf S.tails byteStringData
+      , bench "lazy"   $ nf L.tails lazyByteStringData
+      ]
     , bgroup "sort" $ map (\s -> bench (S8.unpack s) $ nf S.sort s) sortInputs
     , bgroup "stimes" $ let  st = stimes :: Int -> S.ByteString -> S.ByteString
      in

tests/Properties/ByteString.hs

@@ -59,18 +59,19 @@ import qualified Data.ByteString.Lazy.Internal as B (invariant)
 #define BYTESTRING_TYPE B.ByteString
 #endif
 
-import Prelude hiding (head, tail)
 import Data.Int
 import Numeric.Natural (Natural)
 
 import Text.Read
 
 #endif
 
+import Prelude hiding (head, tail)
 import Control.Arrow
 import Data.Char
 import Data.Foldable
 import qualified Data.List as List
+import qualified Data.List.NonEmpty as NE
 import Data.Semigroup
 import Data.String
 import Data.Tuple
@@ -231,6 +232,10 @@ tests =
     \x -> map B.unpack (B.inits x) === List.inits (B.unpack x)
   , testProperty "tails" $
     \x -> map B.unpack (B.tails x) === List.tails (B.unpack x)
+  , testProperty "initsNE" $
+    \x -> NE.map B.unpack (B.initsNE x) === NE.inits (B.unpack x)
+  , testProperty "tailsNE" $
+    \x -> NE.map B.unpack (B.tailsNE x) === NE.tails (B.unpack x)
 #endif
   , testProperty "all" $
     \f x -> B.all f x === all f (B.unpack x)