:sparkles: Haskell exceptions

Haskell/exceptions.org

···
+* Exceptions
+
+Pure functions are lazy by default, which means that we don't know when they will be evaluated and that it really shouldn't matter.
+However, once pure functions start throwing exceptions, it matters when they are evaluated.
+That's why we can only catch exceptions thrown from pure functions in the I/O part of our code.
+And that's bad because we want to keep the I/O part as small as possible.
+However, if we don't catch them in the I/O part of our code, our program crashes.
+The solution? Don't mix exceptions and pure code.
+Take advantage of Haskell's powerful type system and use types like *Either* and *Maybe* to represent results that may have failed.
+
+Let's make an example using *catch* to treat errors when opening a file that does not exists.
+
+[[file:files/countLinesException.hs][Example of exception handling]]

Haskell/files/countLinesException.hs

···
+import System.Environment
+import System.IO.Error
+import Control.Exception
+
+main = toTry `catch` handler
+
+toTry :: IO ()
+toTry = do (fileName:_) <- getArgs
+           contents <- readFile fileName
+           putStrLn $ "The file has " ++ (show $ length $ lines contents) ++ " lines!"
+
+handler :: IOError -> IO ()
+handler e
+  | isDoesNotExistError e =
+      case ioeGetFileName e of Just name -> putStrLn ("Whoops! File does not exist at: " ++ name)
+                               Nothing -> putStrLn "Whoops! File does not exist at unknown location!"
+  | otherwise = ioError e
+