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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