But we can do more than enumerations. How about we do some error handling? Let's say we want a function to return an \code{Int}, but in case something went horribly wrong, we don't just want to return a 0 or some magic number, but a proper error message. Here we go:
\pause
\begin{haskellcode}
data MaybeInt = NoError Int
              | Error String
\end{haskellcode}
\pause
So constructors are just \emph{functions}! And they can have arguments, just like functions. Let's check their types:
\begin{haskellcode}
> :t NoError
NoError :: Int -> MaybeInt
> :t Error
Error :: String -> MaybeInt
\end{haskellcode}
\pause
And now we can do sanity checks:
\begin{haskellcode}
calcSomething :: Int -> MaybeInt
calcSomething x
  | x < 100   = NoError (x * 5)
  | otherwise = Error "Int out of range!"
\end{haskellcode}