[Haskell-cafe] The container problem
jonathanccast at fastmail.fm
Fri Sep 26 18:27:03 EDT 2008
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"]
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.
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