What does it mean to 'hash cons'?

Question

When to use it and why?

My question comes from the sentence: "hash cons with some classes and compare their instances with reference equality"

Answer 1

From Odersky, Spoon and Venners (2007), Programming in Scala, Artima Press, p. 243:

You hash cons instances of a class by caching all instances you have created in a weak collection. Then, any time you want a new instance of the class, you first check the cache. If the cache already has an element equal to the one you are about to create, you can reuse the existing instance. As a result of this arrangement, any two instances that are equal with equals() are also equal with reference equality.