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I just stumbled on one of Tony Morris' blog-posts about Java and a fundamental problem with the language: that of defining a bespoke equality-relation for a collection. This is something that I think is a big deal and wondered whether there was some scala solution. The classic issue manifests itself in thinking about, say, a trade. Let's say I make two trades of +100 vodafone shares @150p. The two trades are equal, yes? Except they are not the same trade . In the case of a normal real-world system, with persistence or serialization, I cannot rely on identity to tell me whether two references are to the same trade ! So what I want is to be able to create a collection which I can pass an Equality-relation to: val as = CleverSet[Trade](IdEquality) val bs = CleverSet[Trade](EconomicsEquality) How would I implement my set in an efficient manner (unless the EqualityRelation also defines a hash mechanism)? trait EqualityRelation[T] { def equal(t1: T, t2: T) : Boolean def hash(t: T) : Int } So the questions are: Is there a library which provides this ability? Is there some way of doing this neatly in Scala? It seems that with implicits, it would be quite an easy thing to add to the existing scala Set type.

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