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.

After uninstalling, run this command to unregister the package from
the ghc package database:   ghc-pkg recache