Initial commit. The library includes a simple Builder, a Semigroup instance, primitives for running the builder, and a function for decimal-encoding a 64-bit word. There is also a test suite to confirm that it works.

test/Main.hs

@@ -0,0 +1,28 @@
+{-# language TypeApplications #-}
+
+import Data.ByteArray.Builder.Small
+import Data.Word
+import Data.Char (ord)
+import Data.Primitive (ByteArray)
+import Test.Tasty (defaultMain,testGroup,TestTree)
+import Test.QuickCheck ((===))
+import qualified Test.Tasty.QuickCheck as TQC
+import qualified Test.QuickCheck as QC
+import qualified GHC.Exts as Exts
+
+main :: IO ()
+main = defaultMain tests
+
+tests :: TestTree
+tests = testGroup "Tests"
+  [ TQC.testProperty "word64Dec" $ \w ->
+      run 1 (word64Dec w) === pack (show w)
+  , TQC.testProperty "word64Dec-x3" $ \x y z ->
+      run 1 (word64Dec x <> word64Dec y <> word64Dec z)
+      ===
+      pack (show x ++ show y ++ show z)
+  ]
+
+pack :: String -> ByteArray
+pack = Exts.fromList . map (fromIntegral @Int @Word8 . ord)
+