Semigroup / Monoid instance

[phadej]

Does it make sense to have:

instance Ord a => Semigroup (Interval a) where
  (<>) = hull

instance Ord => Monoid (Interval a) wherre
  mempty = empty
  mappend = (<>)

I could make a PR, if these instances are OK

[bergey]

Things to think about:

• We'd incur a semigroups dependency
• intersection is also a semigroup / monoid

It looks like the the revdeps already depend on semigroups. I'm leaning towards a newtype for one or both instances.

[phadej]

intersection is a partial function for non-empty variant, thus I'd prefer Semigroup for it.
The newtype for normal variant could be useful indeed: newtype Intersection a = Intersection (Interval a) with mempty = Intersection whole ?

[Icelandjack]

@bergey
Data.Semigroup is now in base if that matters,

I was hoping to use this for a similar purpose to data Interval = Between Time Time from Constructing Applicative Functors.