Proposal: add isEmpty to Monoid

[Henning Thielemann]

On Sat, 25 May 2019, Dr.Koster wrote:

> It's often useful to have a generalized null/unit test, e.g. to check if a monadic stream `m a` reaches EOF.  A
> workaround is to use `Eq a, Monoid a` constraint and use `== mempty` test. But this is no only inefficient for
> some type, but also impossible for some monoids, such as Builders.
> I propose add this test `isEmpty :: a -> Bool` to `Monoid`, to provide the ability to check if a value is the
> unit value.

This will not work. You cannot always inspect what you have constructed 
via Monoid. E.g. you will not be able to implement isEmpty for the Endo 
monoid. The Eq constraint imposed by (== mempty) is really required. If 
you want isEmpty as method, you need a Monoid sub-class.