hs - tail recursion

seminar01/Factorial.hs

@@ -0,0 +1,19 @@
+module Main where
+
+factorial :: Integer -> Either String Integer
+factorial n = if n < 0 then 
+    Left "n cannot be negative" 
+    else Right $ factorial' n 1 where 
+        factorial' 0 res = res
+        factorial' n res = factorial' (n - 1) $ n * res
+
+main :: IO()
+main = do
+    s <- getLine
+    let n = (read s :: Integer) 
+    let res = factorial n
+    case res of
+        Right r -> do 
+            print r
+        Left err -> do
+            putStrLn $ "Error: " ++ err