On Thu, Jul 23, 2009 at 08:17:34PM +0100, Iain Barnett wrote: > [..] > against the empty list it's not really a problem to have it there. I didn't > realise I could use Maybe in the constructor because it's a monad, but > that's good because I was wondering about the best way to make a nullable > value.
Actually, this has nothing to do with Maybe being a monad. The reason you can do this is because "Maybe" itself is not a type, but a (unary) type constructor(It has kind * -> *), so you need to apply it to another type. Doing something like "test :: Maybe" would be an error. > That Data.Tree module looks interesting too! It does seem to be a > naturally recursive type, but I'm still trying to become easy with that sort > of thought :) > [..] A list is also recursively defined, so it is not really more difficult to use a Tree instead. E.g. one could define a list type like this: data List a = Nil | Cons a (List a) - Daniel
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