Disclaimer: I don't really know all about category theory, so some definitions might not be absolutely correct. Monoid is the category of all types that have a empty value and an append operation. The best example is a list. instance Monoid [a] where mempty = [] mappend = (++) Why do I need it? Well, you can think of a function where you need to incrementally store data. Storing them to a Monoid, you can start with a list and then change to a Set, without changing the function itself, because it would be defined based on the Monoid operations. instance Ord a => Monoid (Set a) where mempty = empty mappend = union mconcat = unions Hope I have helped! Regards, Rafael On Fri, Nov 13, 2009 at 14:14, Magicloud Magiclouds < magicloud.magiclouds@gmail.com> wrote:
Hi, I have looked the concept of monoid and something related, but still, I do not know why we use it?
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