On 2019-08-05 2:33 p.m., Jinxuan Zhu wrote:

… Bool can be monoid by either || or && operations, which would lead to ambiguity if Bool is monoid by default.

You can:

1. use Maybe Unit instead
2. (overkill) Define AndMonoid Bool newtype and use DeriveVia and coerce

Data.Monoid from base already has Any and All for this.