# cluss: simple alternative to type classes

[ bsd3, constraints, library, type-system ] [ Propose Tags ]

A cluss enables you to achieve function overloading, or ad-hoc polymorphism, without creating a new type class.

In order to give ad-hoc polymorphism to a type variable a, you simply use In with a list of "type patterns" like In [Type T, ...] a, which indicates that the type matches some of the patterns; which is analogous to a type class indicating that a type matches some of its "instances". The constraint In [Type T, ...] a is what we call a "cluss".

Clusses can easily be used in a nested way and even be recursive; therefore, they are expressive enough to imitate Haskell-98-style type classes.

Clusses, however, go beyond a mere alternative to type classes. They have closed and prioritized instances and open methods, while type classes have open and unprioritized instances and closed methods. Those properties give clusses the advantages different from type classes:

• You can judge whether a type a belongs to a cluss In as, on some level, writing Has as a, since cluss instances are closed.

• You can make cluss instances more flexibly, without causing overlapping instances or incoherent instances, since cluss instances are prioritized.

• You can create new methods for clusses anywhere in any module, since cluss methods are open.