26 Sep
2008
26 Sep
'08
6:27 p.m.
On Fri, 2008-09-26 at 15:25 -0700, Jason Dusek wrote:
Can someone explain, why is it that Set can not be a Monad?
It can't even be a functor (which all monads are). You can't implement fmap (+) $ Set.fromList [1, 2, 3] with Data.Set, because you can't order functions of type Integer -> Integer in a non-arbitrary way. So you can't have a balanced binary tree of them in a non-arbitrary way, either. Something like fmap putStrLn $ Set.fromList ["Hello", "world"] is similar. Since Data.Set is implemented in Haskell, it can only use facilities available to Haskell libraries. So it can't work for arbitrary elements; but a Functor instance requires that it does work. jcc