Added a new Set monoid

[BlackCapCoder]

Here's a Monoid I've re-implemented a bunch of times, and I thought it might fit in here!

My use-case is when you have some sort of Config datatype:

data Config = Config
  { foo, bar, baz :: Set Bool
  , ...
  }

unit = Config Unset Unset Unset

Unlike maybe, we can specify only some of the fields of our datatype and combine them with (<>) without inadvertently clearing all the stuff we left out

ex0 = unit { foo = Clear }
ex1 = unit { bar = Set True }
ex2 = unit { baz = Set False }

ex3 = ex0 <> ex1 <> ex2

For a real world example, consider rendering in an ANSI terminal. Each "pixel" can have a bunch of styling (Bold, Blink, Underline, Foreground and background color, an actual character..). Set let us do the rendering in parts by composing a bunch of Array Pixels together. Clear let us only remove- say- the background color, but leave everything else as-is, and an explicit clear means that we have to update the terminal, unlike Unset.

Set a is equivalent to Maybe (Data.Semigroup.Last (Maybe a)), but exactly nobody is going to use that over just re-implementing Set for the fifteenth time!

[BlackCapCoder]

https://hackage.haskell.org/package/focus-1.0.3.2/docs/Focus.html#t:Change

I actually looked for this library prior to posting, but I couldn't remember the name!

[byorgey]

Thanks for the PR! I've actually re-implemented something similar to this myself, in https://github.com/swarm-game/swarm/blob/main/src/swarm-util/Swarm/Util/Erasable.hs (except in that version 'set' values are combined with an underlying Semigroup rather than having Last be built-in). So it's nice to know the version in focus exists, I didn't know about that before.

I think this could actually make a nice addition to monoid-extras except I definitely wouldn't want to name it Set because that naming conflicts with Data.Set (yes, you can use qualified imports, but it's just confusing).