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Rafael Gustavo da Cunha Pereira Pinto | 13 Nov 18:16
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Re: Could someone teach me why we use Data.Monoid?


"...in my humble opinion. (Which, obviously, nobody else will agree with.)"

I somewhat agree with your opinion!!

What I miss the most is practical examples:

1) A function that uses a Monoid as a container
2) A function that uses Monoid as algebra

and so on, for most of categories.

I had a hard time understanding monads, not because I didn't understand the concept of a monad, but because practical uses are missing, except on Wadler's paper!

Regards

Rafael

On Fri, Nov 13, 2009 at 14:52, Andrew Coppin <andrewcoppin <at> btinternet.com> wrote:
Stephen Tetley wrote:
2009/11/13 Rafael Gustavo da Cunha Pereira Pinto <RafaelGCPP.Linux <at> gmail.com>:
 
Monoid is the category of all types that have a empty value and an append
operation.

   

Or more generally a neutral element and an associative operation:

The multiplication monoid (1,*)

9*1*1*1 = 9

1 is neutral but you might be hard pressed to consider it _empty_.
 

This is the thing. If we had a class specifically for containers, that could be useful. If we had a class specifically for algebras, that could be useful. But a class that represents "any possible thing that can technically be considered a monoid" seems so absurdly general as to be almost useless. If you don't know what an operator *does*, being able to abstract over it isn't especially helpful...

...in my humble opinion. (Which, obviously, nobody else will agree with.)


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Rafael Gustavo da Cunha Pereira Pinto

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