Haskell 98 semigroups.  In mathematics, a semigroup is an algebraic structure
consisting of a set together with an associative binary operation. A
semigroup generalizes a monoid in that there might not exist an identity
element. It also (originally) generalized a group (a monoid with all
inverses) to a type where every element did not have to have an inverse,
thus the name semigroup.

WWW: http://github.com/ekmett/semigroups/