add Haskell snippet for Writer monad to accumulate logs alongside computations

Author: ACR1209

snippets/haskell/monads/writer-monad.md

@@ -0,0 +1,23 @@
+--- 
+title: Writer Monad
+description: Using the Writer monad to accumulate logs or other outputs alongside a computation.
+author: ACR1209
+tags: haskell, monads, writer, logs
+---
+
+```hs
+import Control.Monad.Writer
+
+addAndLog :: Int -> Int -> Writer [String] Int
+addAndLog x y = do
+    tell ["Adding " ++ show x ++ " and " ++ show y]
+    return (x + y)
+
+main :: IO ()
+main = do
+    let (result, logs) = runWriter $ do
+            res1 <- addAndLog 3 5
+            addAndLog res1 1
+    print result  -- Output: 9
+    print logs  -- Output: ["Adding 3 and 5", "Adding 8 and 1"]
+```