Penguin is a subset of Apple's Swift programming language, that aims to provide developers with more support for static verification that what Swift's compiler is able to do. This is achieved by the means of a stricter type system that aims to support typestates [1].
[1] Strom, Robert E., and Yemini, Shaula. "Typestate: A programming language concept for enhancing software reliability." IEEE Transactions on Software Engineering 1 (1986): 157-171.