In my own opinion, the reason why we use the concept of a monoid or a monad is in order to build libraries around the concepts. For example, the "do" construct could have been designed just for doing IO, but because it works for *any* monad you can also use the same syntax sugar to conveniently work with a calculation *) that manipulates a mutable state (State) *) that has an environment (Reader) *) that writes out a log (Writer) Likewise, the nice thing about monoid is that it lets us generalize libraries. So for example, the Writer monad could have just been designed to work by concatenating strings or lists since this is what one might typically think of for a "log". But because it is designed to work with an arbitrary *monoid*, you could use it to keep track of a running total as well, since numbers under addition is also a monoid. So the way I figure it, the important thing is to understand just enough of these patterns (monads, monoids) that you can recognize them when they come up in your own work so that you can be aware of what pre-existing libraries and/or syntax sugar you can leverage to work with them. Cheers, Greg On Nov 13, 2009, at 8:14 AM, Magicloud Magiclouds 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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