The principle (and sole) type.
We can think of this set-theoretically as the carrier set of the algebraic structure or category-theoretically as the single object in the category, with each element being a morphism
t -> t.
val unit : t
mconcat xs is the concatenation of all elements in
xs into a single value, derived by use of
This is equivalent to
List.fold_right op xs unit.
The initial m marks this as monoidal concatenation rather than the
concat function inherited from the underlying Semigroup. Since
mconcat can use the
unit to prime the fold, it can work over a normal list, in contrast to