From dab92250d3070c120d76ad0afdd523ea8a5353d4 Mon Sep 17 00:00:00 2001
From: Ignat Insarov <kindaro@gmail.com>
Date: Thu, 25 Jan 2024 16:58:21 +0300
Subject: [PATCH 1/2] Add `vectorToMaybe`.

---
 vector/src/Data/Vector/Generic.hs | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/vector/src/Data/Vector/Generic.hs b/vector/src/Data/Vector/Generic.hs
index ec847c20..d146aa76 100644
--- a/vector/src/Data/Vector/Generic.hs
+++ b/vector/src/Data/Vector/Generic.hs
@@ -244,10 +244,21 @@ v !? i | i `inRange` length v = case basicUnsafeIndexM v i of Box a -> Just a
 
 
 -- | /O(1)/ First element.
+--
+-- Partial.
+-- Undefined at @fromList [ ]@.
+-- Completion is 'vectorToMaybe'
 head :: Vector v a => v a -> a
 {-# INLINE_FUSED head #-}
 head v = v ! 0
 
+-- | /O(1)/ Just the first element — or nothing when there are none.
+--
+-- Completes 'head'.
+vectorToMaybe :: Vector v a => v a -> Maybe a
+{-# INLINE_FUSED vectorToMaybe #-}
+vectorToMaybe v = v !? 0
+
 -- | /O(1)/ Last element.
 last :: Vector v a => v a -> a
 {-# INLINE_FUSED last #-}

From b7d2f4d97e219c56db122c6e316dfdb9c0843e8a Mon Sep 17 00:00:00 2001
From: Ignat Insarov <kindaro@gmail.com>
Date: Thu, 25 Jan 2024 17:21:28 +0300
Subject: [PATCH 2/2] Add property check for `vectorToMaybe`.

---
 vector/src/Data/Vector/Generic.hs     | 2 +-
 vector/tests/Tests/Vector/Property.hs | 3 ++-
 2 files changed, 3 insertions(+), 2 deletions(-)

diff --git a/vector/src/Data/Vector/Generic.hs b/vector/src/Data/Vector/Generic.hs
index d146aa76..ebdffe0f 100644
--- a/vector/src/Data/Vector/Generic.hs
+++ b/vector/src/Data/Vector/Generic.hs
@@ -29,7 +29,7 @@ module Data.Vector.Generic (
   length, null,
 
   -- ** Indexing
-  (!), (!?), head, last,
+  (!), (!?), head, vectorToMaybe, last,
   unsafeIndex, unsafeHead, unsafeLast,
 
   -- ** Monadic indexing
diff --git a/vector/tests/Tests/Vector/Property.hs b/vector/tests/Tests/Vector/Property.hs
index dc8f0b63..c033ece9 100644
--- a/vector/tests/Tests/Vector/Property.hs
+++ b/vector/tests/Tests/Vector/Property.hs
@@ -98,7 +98,7 @@ testPolymorphicFunctions _ = $(testProperties [
         'prop_length, 'prop_null,
 
         -- Indexing
-        'prop_index, 'prop_safeIndex, 'prop_head, 'prop_last,
+        'prop_index, 'prop_safeIndex, 'prop_head, 'prop_vectorToMaybe, 'prop_last,
         'prop_unsafeIndex, 'prop_unsafeHead, 'prop_unsafeLast,
 
         -- Monadic indexing (FIXME)
@@ -241,6 +241,7 @@ testPolymorphicFunctions _ = $(testProperties [
     prop_createT = (\v -> V.createT (T.mapM V.thaw v)) `eq` id
 
     prop_head      :: P (v a -> a) = not . V.null ===> V.head `eq` head
+    prop_vectorToMaybe :: P (v a -> Maybe a) = not . V.null ===> V.vectorToMaybe `eq` (Just . head)
     prop_last      :: P (v a -> a) = not . V.null ===> V.last `eq` last
     prop_index        = \xs ->
                         not (V.null xs) ==>