Add functions for recovering length from ByteArrayN

Author: andrewthad

src/Data/Bytes/Indexed.hs

@@ -1,14 +1,35 @@
 {-# language DataKinds #-}
+{-# language MagicHash #-}
 {-# language TypeOperators #-}
 
 module Data.Bytes.Indexed
   ( ByteArrayN
   , append
+  , length
+  , length#
   ) where
 
+import Prelude hiding (length)
+
+import Data.Primitive (ByteArray(ByteArray))
 import Data.Bytes.Types (ByteArrayN(ByteArrayN))
 import GHC.TypeNats (type (+))
+import Arithmetic.Types (Nat, Nat#)
+
+import qualified Data.Primitive as PM
+import qualified Arithmetic.Unsafe as Unsafe
+import qualified GHC.Exts as Exts
 
 append :: ByteArrayN m -> ByteArrayN n -> ByteArrayN (m + n)
 {-# inline append #-}
 append (ByteArrayN x) (ByteArrayN y) = ByteArrayN (x <> y)
+
+-- | Recover a witness of the length.
+length :: ByteArrayN n -> Nat n
+{-# inline length #-}
+length (ByteArrayN x) = Unsafe.Nat (PM.sizeofByteArray x)
+
+-- | Recover an unboxed witness of the length.
+length# :: ByteArrayN n -> Nat# n
+{-# inline length# #-}
+length# (ByteArrayN (ByteArray x)) = Unsafe.Nat# (Exts.sizeofByteArray# x)