all tasks

Task/Undefined-values/Haskell/undefined-values-5.hs

@@ -0,0 +1,16 @@
+import Control.Exception (catch, evaluate, ErrorCall)
+import System.IO.Unsafe (unsafePerformIO)
+import Prelude hiding (catch)
+import Control.DeepSeq (NFData, deepseq)
+
+scoopError :: (NFData a) => a -> Either String a
+scoopError x = unsafePerformIO $ catch right left
+  where right = deepseq x $ return $ Right x
+        left e = return $ Left $ show (e :: ErrorCall)
+
+safeHead :: (NFData a) => [a] -> Either String a
+safeHead = scoopError . head
+
+main = do
+  print $ safeHead ([] :: String)
+  print $ safeHead ["str"]