diff --git a/CHANGELOG.md b/CHANGELOG.md
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,3 +1,8 @@
+# 0.2.1.0 - December 2023
+
+* Add `findIndexL`, `findIndexR`, `findIndicesL`, `findIndicesR`
+* Fix bug in `|>` & `<|` ([#10](https://github.com/konsumlamm/rrb-vector/issues/10))
+
 # 0.2.0.1 - October 2023
 
 * Support `primitive-0.9` and `deepseq-1.5`
diff --git a/rrb-vector.cabal b/rrb-vector.cabal
--- a/rrb-vector.cabal
+++ b/rrb-vector.cabal
@@ -1,5 +1,5 @@
 name:               rrb-vector
-version:            0.2.0.1
+version:            0.2.1.0
 synopsis:           Efficient RRB-Vectors
 description:
   An RRB-Vector is an efficient sequence data structure.
@@ -29,8 +29,9 @@
   GHC == 8.10.7
   GHC == 9.0.2
   GHC == 9.2.8
-  GHC == 9.4.7
+  GHC == 9.4.8
   GHC == 9.6.3
+  GHC == 9.8.1
 
 source-repository head
   type:     git
@@ -59,7 +60,7 @@
     Strictness
   type:                 exitcode-stdio-1.0
   ghc-options:          -Wall -Wno-orphans -Wno-type-defaults
-  build-depends:        base, deepseq, quickcheck-classes-base, rrb-vector, tasty, tasty-quickcheck
+  build-depends:        base, containers, deepseq, quickcheck-classes-base, rrb-vector, tasty, tasty-quickcheck
   if impl(ghc >= 8.6)
     build-depends:      nothunks
   default-language:     Haskell2010
diff --git a/src/Data/RRBVector.hs b/src/Data/RRBVector.hs
--- a/src/Data/RRBVector.hs
+++ b/src/Data/RRBVector.hs
@@ -33,6 +33,7 @@
     , adjust, adjust'
     , take, drop, splitAt
     , insertAt, deleteAt
+    , findIndexL, findIndexR, findIndicesL, findIndicesR
     -- * With Index
     --
     -- | Reexported from [indexed-traversable](https://hackage.haskell.org/package/indexed-traversable).
diff --git a/src/Data/RRBVector/Internal.hs b/src/Data/RRBVector/Internal.hs
--- a/src/Data/RRBVector/Internal.hs
+++ b/src/Data/RRBVector/Internal.hs
@@ -24,6 +24,7 @@
     , adjust, adjust'
     , take, drop, splitAt
     , insertAt, deleteAt
+    , findIndexL, findIndexR, findIndicesL, findIndicesR
     -- * Transformations
     , map, map', reverse
     -- * Zipping and unzipping
@@ -650,6 +651,22 @@
 -- If the index is out of range, return the original vector.
 deleteAt :: Int -> Vector a -> Vector a
 deleteAt i v = let (left, right) = splitAt (i + 1) v in take i left >< right
+
+-- | \(O(n)\). Find the first index from the left that satisfies the predicate.
+findIndexL :: (a -> Bool) -> Vector a -> Maybe Int
+findIndexL f = ifoldr (\i x acc -> if f x then Just i else acc) Nothing
+
+-- | \(O(n)\). Find the first index from the right that satisfies the predicate.
+findIndexR :: (a -> Bool) -> Vector a -> Maybe Int
+findIndexR f = ifoldl (\i acc x -> if f x then Just i else acc) Nothing
+
+-- | \(O(n)\). Find the indices that satisfy the predicate, starting from the left.
+findIndicesL :: (a -> Bool) -> Vector a -> [Int]
+findIndicesL f = ifoldr (\i x acc -> if f x then i : acc else acc) []
+
+-- | \(O(n)\). Find the indices that satisfy the predicate, starting from the right.
+findIndicesR :: (a -> Bool) -> Vector a -> [Int]
+findIndicesR f = ifoldl (\i acc x -> if f x then i : acc else acc) []
 
 -- concatenation
 
diff --git a/src/Data/RRBVector/Internal/Array.hs b/src/Data/RRBVector/Internal/Array.hs
--- a/src/Data/RRBVector/Internal/Array.hs
+++ b/src/Data/RRBVector/Internal/Array.hs
@@ -173,17 +173,17 @@
 last arr = index arr (length arr - 1)
 
 snoc :: Array a -> a -> Array a
-snoc (Array _ len arr) x = Array 0 len' $ runSmallArray $ do
+snoc (Array start len arr) x = Array 0 len' $ runSmallArray $ do
     sma <- newSmallArray len' x
-    copySmallArray sma 0 arr 0 len
+    copySmallArray sma 0 arr start len
     pure sma
   where
     !len' = len + 1
 
 cons :: Array a -> a -> Array a
-cons (Array _ len arr) x = Array 0 len' $ runSmallArray $ do
+cons (Array start len arr) x = Array 0 len' $ runSmallArray $ do
     sma <- newSmallArray len' x
-    copySmallArray sma 1 arr 0 len
+    copySmallArray sma 1 arr start len
     pure sma
   where
     !len' = len + 1
diff --git a/test/Properties.hs b/test/Properties.hs
--- a/test/Properties.hs
+++ b/test/Properties.hs
@@ -1,5 +1,7 @@
 {-# LANGUAGE CPP #-}
 
+{-# OPTIONS_GHC -Wno-incomplete-uni-patterns #-}
+
 module Properties
     ( properties
     ) where
@@ -12,6 +14,7 @@
 import Data.Proxy (Proxy(..))
 import Prelude hiding ((==)) -- use @===@ instead
 
+import qualified Data.Sequence as Seq
 import qualified Data.RRBVector as V
 import Test.QuickCheck.Classes.Base
 import Test.Tasty
@@ -135,6 +138,22 @@
         , testProperty "satisfies `deleteAt 0 v = drop 1 v`" $ \v -> V.deleteAt 0 v === V.drop 1 v
         , testProperty "satisfies `deleteAt (length v - 1) v = take (length v - 1) v`" $ \v -> V.deleteAt (length v - 1) v === V.take (length v - 1) v
         ]
+    , testGroup "findIndexL"
+        [ testProperty "finds the first index" $ \v (Fn f) -> V.findIndexL f v === Seq.findIndexL f (Seq.fromList (toList v))
+        , testProperty "returns Nothing for the empty vector" $ \(Fn f) -> V.findIndexL f V.empty === Nothing
+        ]
+    , testGroup "findIndexR"
+        [ testProperty "finds the last index" $ \v (Fn f) -> V.findIndexR f v === Seq.findIndexR f (Seq.fromList (toList v))
+        , testProperty "returns Nothing for the empty vector" $ \(Fn f) -> V.findIndexR f V.empty === Nothing
+        ]
+    , localOption (QuickCheckMaxSize 1000) $ testGroup "findIndicesL"
+        [ testProperty "finds the indices starting from the left" $ \v (Fn f) -> V.findIndicesL f v === Seq.findIndicesL f (Seq.fromList (toList v))
+        , testProperty "returns [] for the empty vector" $ \(Fn f) -> V.findIndicesL f V.empty === []
+        ]
+    , localOption (QuickCheckMaxSize 1000) $ testGroup "findIndicesR"
+        [ testProperty "finds the indices starting from the right" $ \v (Fn f) -> V.findIndicesR f v === Seq.findIndicesR f (Seq.fromList (toList v))
+        , testProperty "returns [] for the empty vector" $ \(Fn f) -> V.findIndicesR f V.empty === []
+        ]
     , testGroup "reverse"
         [ testProperty "reverses the vector" $ \v -> toList (V.reverse v) === reverse (toList v)
         ]
@@ -150,6 +169,7 @@
         ]
     , instances
     , laws
+    , issues
     ]
 
 instances :: TestTree
@@ -218,4 +238,13 @@
     , testLaws $ monadPlusLaws proxyV
     , localOption (QuickCheckTests 500) . localOption (QuickCheckMaxSize 5000) . testLaws $ monadZipLaws proxyV
     , localOption (QuickCheckMaxSize 100) . testLaws $ traversableLaws proxyV
+    ]
+
+-- old issues, to avoid regressions
+issues :: TestTree
+issues = testGroup "issues"
+    -- https://github.com/konsumlamm/rrb-vector/issues/10
+    [ testProperty "#10" $ \x v -> case V.viewl v of
+            Nothing -> property True
+            Just (_, v') -> x V.<| v' === V.update 0 x v
     ]