Re: Restricted Data Types: A reformulation

John Meacham wrote:

newtype Eq a => Set a = Set (List a) singleton :: Eq a => a -> Set a class Monad m where return :: a -> m a

instance Monad Set where return x = singleton x

okay, our goal is to make this typesafe.

You shouldn't be able to, should you? Monad makes a promise that Set can't keep. In particular: returnid :: (Monad m) => m (a -> a) returnid = return id bad :: Set (a -> a) bad = returnid