When is @uncheckedVariance needed in Scala, and why is it used in GenericTraversableTemplate?

Question

@uncheckedVariance can be used to bridge the gap between Scala\'s declaration site variance annotations and Java\'s invariant generics.

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Answer 1

In my thesis I describe a calculus, Scalina, that has bounds & variance annotations as part of the kind language (an earlier version is also available as a workshop paper). The relevance to this discussion is the next step that I want to take in developing this calculus: build another layer on top of that so that you can abstract over bounds (easy) and variance annotations (makes my head spin). Actually, you wouldn't just tack 1 extra layer on there, but rather generalise your polymorphism constructs so they work at all levels, and make your "attributes" (bounds, variance annotations, required implicit arguments,...) into regular types with special kinds, which are all subject to abstraction.

The "attributes are types" idea is explained nicely by Edsko de Vries in the context of uniqueness types.

Uniqueness Typing Simplified, Edsko de Vries, Rinus Plasmeijer, and David Abrahamson. In Olaf Chitil, Zoltán Horváth and Viktória Zsók (Eds.): IFL 2007, LNCS 5083, pp. 201-218, 2008.

Abstract: We present a uniqueness type system that is simpler than both Clean's uniqueness system and the system we proposed previously. The new type system is straightforward to implement and add to existing compilers, and can easily be extended with advanced features such as higher rank types and impredicativity. We describe our implementation in Morrow, an experimental functional language with both these features. Finally, we prove soundness of the core type system with respect to the call-by-need lambda calculus.