Add unsafeCast to Primitive vectors

vector/changelog.md

@@ -43,6 +43,7 @@
    resulted in an error. This change was introduced in:
    [#382](https://github.com/haskell/vector/pull/382)
 * Remove redundant `Storable` constraints on to/from `ForeignPtr` conversions
+* Add `unsafeCast` to `Primitive` vectors
 
 # Changes in version 0.12.3.0
 

vector/src/Data/Vector/Primitive.hs

@@ -145,7 +145,7 @@ module Data.Vector.Primitive (
   toList, fromList, fromListN,
 
   -- ** Other vector types
-  G.convert,
+  G.convert, unsafeCast,
   unsafeCoerceVector,
 
   -- ** Mutable vectors
@@ -157,6 +157,7 @@ module Data.Vector.Primitive (
 
 import qualified Data.Vector.Generic           as G
 import           Data.Vector.Primitive.Mutable ( MVector(..) )
+import           Data.Vector.Internal.Check
 import qualified Data.Vector.Fusion.Bundle as Bundle
 import           Data.Primitive.ByteArray
 import           Data.Primitive ( Prim, sizeOf )
@@ -203,7 +204,7 @@ type role Vector nominal
 -- This is marginally safer than 'unsafeCast', since this function imposes an
 -- extra 'Coercible' constraint. This function is still not safe, however,
 -- since it cannot guarantee that the two types have memory-compatible
--- 'Storable' instances.
+-- 'Prim' instances.
 --
 -- Note that this function is unsafe. The @Coercible@ constraint guarantees
 -- that the element types are representationally equal. It however cannot
@@ -1769,6 +1770,22 @@ fromListN :: Prim a => Int -> [a] -> Vector a
 {-# INLINE fromListN #-}
 fromListN = G.fromListN
 
+-- Conversions - Unsafe casts
+-- --------------------------
+
+-- | /O(1)/ Unsafely cast a vector from one element type to another.
+-- This operation just changes the type of the vector and does not
+-- modify the elements.
+--
+-- This function will throw an error if elements are of mismatching sizes.
+--
+-- | @since 0.13.0.0
+unsafeCast :: forall a b. (HasCallStack, Prim a, Prim b) => Vector a -> Vector b
+{-# INLINE unsafeCast #-}
+unsafeCast (Vector o n ba)
+  | sizeOf (undefined :: a) == sizeOf (undefined :: b) = Vector o n ba
+  | otherwise = error "Element size mismatch"
+
 -- Conversions - Mutable vectors
 -- -----------------------------
 

vector/src/Data/Vector/Primitive/Mutable.hs

@@ -61,14 +61,15 @@ module Data.Vector.Primitive.Mutable (
   set, copy, move, unsafeCopy, unsafeMove,
 
   -- * Unsafe conversions
-  unsafeCoerceMVector,
+  unsafeCoerceMVector, unsafeCast,
   -- * Re-exports
   Prim, PrimMonad, PrimState, RealWorld
 ) where
 
 import qualified Data.Vector.Generic.Mutable as G
 import           Data.Primitive.ByteArray
 import           Data.Primitive ( Prim, sizeOf )
+import           Data.Vector.Internal.Check
 import           Data.Word ( Word8 )
 import           Control.Monad.Primitive
 import           Control.Monad ( liftM )
@@ -656,3 +657,20 @@ ifoldrM = G.ifoldrM
 ifoldrM' :: (PrimMonad m, Prim a) => (Int -> a -> b -> m b) -> b -> MVector (PrimState m) a -> m b
 {-# INLINE ifoldrM' #-}
 ifoldrM' = G.ifoldrM'
+
+
+-- Unsafe conversions
+-- ------------------
+
+-- | /O(1)/ Unsafely cast a vector from one element type to another.
+-- This operation just changes the type of the vector and does not
+-- modify the elements.
+--
+-- This function will throw an error if elements are of mismatching sizes.
+--
+-- | @since 0.13.0.0
+unsafeCast :: forall a b s. (HasCallStack, Prim a, Prim b) => MVector s a -> MVector s b
+{-# INLINE unsafeCast #-}
+unsafeCast (MVector o n ba)
+  | sizeOf (undefined :: a) == sizeOf (undefined :: b) = MVector o n ba
+  | otherwise = error "Element size mismatch"