Proposal: add isEmpty to Monoid

[Simon Jakobi]

The monoid-subclasses packages provides a MonoidNull class that you could
use:

http://hackage.haskell.org/package/monoid-subclasses-0.4.6.1/docs/Data-Monoid-Null.html

Am Sa., 25. Mai 2019 um 07:56 Uhr schrieb Dr.Koster <drkoster at qq.com>:

> 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.
>
> Cheers
> Han Dong~
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