diff --git a/src/Data/TypeLevel/List.hs b/src/Data/TypeLevel/List.hs
--- a/src/Data/TypeLevel/List.hs
+++ b/src/Data/TypeLevel/List.hs
@@ -9,7 +9,9 @@
 
 module Data.TypeLevel.List (
 	Length(..), IsPrefixOf, InfixIndex(..),
-	module Data.TypeLevel.List.Unzip
+	module Data.TypeLevel.List.Unzip,
+
+	Append
 	) where
 
 import Prelude hiding (length, unzip)
@@ -32,3 +34,7 @@
 
 instance {-# OVERLAPPABLE #-} InfixIndex xs ys => InfixIndex xs (y ': ys) where
 	infixIndex = infixIndex @_ @xs @ys + 1
+
+type family Append xs ys where
+	'[] `Append` ys = ys
+	(x ': xs) `Append` ys = x ': (xs `Append` ys)
diff --git a/typelevel-tools-yj.cabal b/typelevel-tools-yj.cabal
--- a/typelevel-tools-yj.cabal
+++ b/typelevel-tools-yj.cabal
@@ -1,11 +1,11 @@
 cabal-version: 1.12
 
--- This file has been generated from package.yaml by hpack version 0.37.0.
+-- This file has been generated from package.yaml by hpack version 0.38.0.
 --
 -- see: https://github.com/sol/hpack
 
 name:           typelevel-tools-yj
-version:        0.1.0.8
+version:        0.1.0.9
 synopsis:       type level tools
 description:    Please see the README on GitHub at <https://github.com/YoshikuniJujo/typelevel-tools-yj#readme>
 category:       Types