How can I implement custom exceptions in Haskell?

How can I implement custom exceptions in Haskell?

In Haskell, you can create custom exceptions using the Control.Exception module. You define a new data type for your exception and then use throw and catch to handle it.

Can you show me a simple example of that?

Sure! Here's a basic example:

import Control.Exception

data MyException = MyError String deriving Show

instance Exception MyException

throwMyError :: IO ()
throwMyError = throw (MyError "This is a custom error!")

main :: IO ()
main = do
    result <- try throwMyError :: IO (Either MyException ())
    case result of
        Left err -> putStrLn $ "Caught an exception: " ++ show err
        Right _  -> putStrLn "No exception occurred."

What if I want to catch multiple types of exceptions?

You can catch multiple exceptions by using pattern matching in the catch function. You can define different handlers for each exception type.

Is there a way to create a hierarchy of exceptions?

Yes, you can create a hierarchy by defining a base exception type and then deriving other exceptions from it. This allows you to catch them at different levels.