Correctly implement fromArray for strict vector

fromArray reduces all elements of array to WHNF but that changes complexity from
O(1) to O(n). To compensate we provide lazy variant which pass all
responsibility to user

Author: Shimuuar

vector/src/Data/Vector/Strict.hs

@@ -7,7 +7,7 @@
 {-# LANGUAGE TypeFamilies #-}
 
 -- |
--- Module      : Data.Vector
+-- Module      : Data.Vector.Strict
 -- Copyright   : (c) Roman Leshchinskiy 2008-2010
 --                   Alexey Kuleshevich 2020-2022
 --                   Aleksey Khudyakov 2020-2022
@@ -166,7 +166,7 @@ module Data.Vector.Strict (
   toList, Data.Vector.Strict.fromList, Data.Vector.Strict.fromListN,
 
   -- ** Arrays
-  toArray, fromArray, toArraySlice, unsafeFromArraySlice,
+  toArray, fromArray, lazyFromArray, toArraySlice, unsafeFromArraySlice, unsafeLazyFromArraySlice,
 
   -- ** Other vector types
   G.convert,
@@ -2196,12 +2196,23 @@ fromListN = G.fromListN
 -- Conversions - Arrays
 -- -----------------------------
 
--- | /O(1)/ Convert an array to a vector.
+-- | /O(n)/ Convert an array to a vector and reduce each element to WHNF.
 --
--- @since 0.12.2.0
+-- @since NEXT
 fromArray :: Array a -> Vector a
 {-# INLINE fromArray #-}
-fromArray arr = Vector 0 (sizeofArray arr) arr
+fromArray arr = liftRnfV (`seq` ()) vec `seq` vec
+  where
+    vec = lazyFromArray arr
+
+-- | /O(1)/ Convert an array to a vector. This function does not touch
+-- content of array so resulting vector may contain bottoms.
+--
+-- @since NEXT
+lazyFromArray :: Array a -> Vector a
+{-# INLINE lazyFromArray #-}
+lazyFromArray arr = Vector 0 (sizeofArray arr) arr
+
 
 -- | /O(n)/ Convert a vector to an array.
 --
@@ -2224,9 +2235,11 @@ toArraySlice :: Vector a -> (Array a, Int, Int)
 toArraySlice (Vector offset len arr) = (arr, offset, len)
 
 
--- | /O(1)/ Convert an array slice to a vector. This function is very unsafe,
--- because constructing an invalid vector can yield almost all other safe
--- functions in this module unsafe. These are equivalent:
+-- | /O(n)/ Convert an array slice to a vector and reduce each element to WHNF.
+--
+-- This function is very unsafe, because constructing an invalid
+-- vector can yield almost all other safe functions in this module
+-- unsafe. These are equivalent:
 --
 -- > unsafeFromArraySlice len offset === unsafeTake len . unsafeDrop offset . fromArray
 --
@@ -2237,7 +2250,27 @@ unsafeFromArraySlice ::
   -> Int -- ^ Length
   -> Vector a
 {-# INLINE unsafeFromArraySlice #-}
-unsafeFromArraySlice arr offset len = Vector offset len arr
+unsafeFromArraySlice arr offset len = liftRnfV (`seq` ()) vec `seq` vec
+  where vec = unsafeLazyFromArraySlice arr offset len
+
+-- | /O(1)/ Convert an array slice to a vector. This function does not touch
+-- content of array so resulting vector may contain bottoms.
+--
+-- This function is very unsafe, because constructing an invalid
+-- vector can yield almost all other safe functions in this module
+-- unsafe. These are equivalent:
+--
+-- > unsafeFromArraySlice len offset === unsafeTake len . unsafeDrop offset . fromArray
+--
+-- @since 0.13.0.0
+unsafeLazyFromArraySlice ::
+     Array a -- ^ Immutable boxed array.
+  -> Int -- ^ Offset
+  -> Int -- ^ Length
+  -> Vector a
+{-# INLINE unsafeLazyFromArraySlice #-}
+unsafeLazyFromArraySlice arr offset len = Vector offset len arr
+
 
 -- Conversions - Mutable vectors
 -- -----------------------------