Add Semigroup and Monoid instances for Chunks

CHANGELOG.md

@@ -5,6 +5,7 @@
 * Add big-endian and little-endian parsers for `Word128`. This includes
   both the single and multiple element variants.
 * Export `reverseCommitsOntoChunks` from the `Unsafe` module.
+* Add `Semigroup` and `Monoid` instances for `Chunks`.
 
 ## 0.3.0.0 -- 2019-10-17
 

src/Data/Bytes/Chunks.hs

@@ -8,15 +8,17 @@
 module Data.Bytes.Chunks
   ( Chunks(..)
   , concat
+  , reverse
+  , reverseOnto
   ) where
 
-import Prelude hiding (length,concat)
+import Prelude hiding (length,concat,reverse)
 
 import GHC.ST (ST(..))
 import Data.Bytes.Types (Bytes(..))
 import Data.Primitive (ByteArray(..),MutableByteArray(..))
 import GHC.Exts (ByteArray#,MutableByteArray#)
-import GHC.Exts (IsList,Int#,State#,Int(I#),(+#),(-#))
+import GHC.Exts (Int#,State#,Int(I#),(+#))
 import Control.Monad.ST.Run (runByteArrayST)
 
 import qualified GHC.Exts as Exts
@@ -26,6 +28,15 @@ data Chunks
   = ChunksCons {-# UNPACK #-} !Bytes !Chunks
   | ChunksNil
 
+instance Semigroup Chunks where
+  ChunksNil <> a = a
+  cs@(ChunksCons _ _) <> ChunksNil = cs
+  as@(ChunksCons _ _) <> bs@(ChunksCons _ _) =
+    reverseOnto bs (reverse as)
+
+instance Monoid Chunks where
+  mempty = ChunksNil
+
 concat :: Chunks -> ByteArray
 concat x = ByteArray (concat# x)
 
@@ -69,6 +80,19 @@ copy# marr off (ChunksCons (Bytes{array,offset,length}) cs) s0 =
   case Exts.copyByteArray# (unBa array) (unI offset) marr off (unI length) s0 of
     s1 -> copy# marr (off +# unI length) cs s1
 
+
+-- | Reverse chunks but not the bytes within each chunk.
+reverse :: Chunks -> Chunks
+reverse = reverseOnto ChunksNil
+
+-- | Variant of 'reverse' that allows the caller to provide
+-- an initial list of chunks that the reversed chunks will
+-- be pushed onto.
+reverseOnto :: Chunks -> Chunks -> Chunks
+reverseOnto !x ChunksNil = x
+reverseOnto !x (ChunksCons y ys) =
+  reverseOnto (ChunksCons y x) ys
+
 unI :: Int -> Int#
 unI (I# i) = i