Interfaces and module combinators for algebraic structures
Parameter #1 Alg_structs . Semigroup . Make . S
type t

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 op : t -> t -> t

op x y is an associative operation over all elements x and y of type t. Category-theoretically, this is the composition of morphisms.