How can I create custom error types in Haskell?

How can I create custom error types in Haskell?

In Haskell, you can create custom error types using data types. For example, you can define a custom error type like this:

data MyError = NotFound | InvalidInput String | UnknownError

How do I use this custom error type in a function?

You can use the custom error type in a function by returning it as part of a result type. For example:

type Result a = Either MyError a

myFunction :: Int -> Result String
myFunction x
  | x < 0 = Left (InvalidInput "Negative number")
  | otherwise = Right ("Valid input: " ++ show x)

What about handling these errors?

You can handle these errors using pattern matching. For example:

case myFunction (-1) of
  Left err -> putStrLn ("Error: " ++ show err)
  Right value -> putStrLn value