Wearing the hair shirt Wearing the hair shirt - Microsoft Research

Type classes are useful
Type classes have proved extraordinarily
convenient in practice
Equality, ordering, serialisation, numerical
operations, and not just the built-in ones
(e.g. pretty-printing, time-varying values)
Monadic operations
class Monad m where
return :: a -> m a
(>>=) :: m a -> (a -> m b) -> m b
fail :: String -> m a
Note the higher-kinded type variable, m