diff --git a/2023/9/Main1.hs b/2023/9/Main1.hs
new file mode 100644
index 0000000..c5f2075
--- /dev/null
+++ b/2023/9/Main1.hs
@@ -0,0 +1,12 @@
+import Data.List.Split (splitOn)
+
+solve :: [Int] -> Int
+solve [] = 0
+solve xs = head xs + solve (getDifferences xs)
+  where
+    getDifferences = zipWith (-) <*> tail
+
+main :: IO ()
+main = print . sum . map solveLine . lines =<< readFile "data.txt"
+  where
+    solveLine = solve . reverse . map read . splitOn " "
diff --git a/2023/9/Main2.hs b/2023/9/Main2.hs
new file mode 100644
index 0000000..b9e41c3
--- /dev/null
+++ b/2023/9/Main2.hs
@@ -0,0 +1,12 @@
+import Data.List.Split (splitOn)
+
+solve :: [Int] -> Int
+solve [] = 0
+solve xs = head xs + solve (getDifferences xs)
+  where
+    getDifferences = zipWith (-) <*> tail
+
+main :: IO ()
+main = print . sum . map solveLine . lines =<< readFile "test-data.txt"
+  where
+    solveLine = solve . map read . splitOn " "
diff --git a/flake.nix b/flake.nix
index e036f0e..456b248 100644
--- a/flake.nix
+++ b/flake.nix
@@ -32,6 +32,7 @@
             (pkgs.ghc.withPackages (p:
               with p; [
                 containers
+                extra
                 regex-compat
                 split
                 unordered-containers