Abstract: We introduce basic concepts from object-oriented programming into dependent type theory based on the idea of modelling objects as interactive programs. We consider methods, interfaces, and the interaction between a fixed number of objects, including self-referential method calls. We introduce a monad like syntax for developing objects in dependent type theory. 1.1

Abstract We develop rules for coalgebras in type theory, and give meaning explanations for them. We show that elements of coalgebras are determined by their elimination rules, whereas the introduction rules can be considered as derived. This is in contrast with algebraic data types, for which the opposite is true: elements are determined by their introduction rules, and the elimination rules can be considered as derived. In this sense, the function type from the logical framework is more like a coalgebraic data type, the elements of which are determined by the elimination rule. We illustrate why the simplest form of guarded recursion is nothing but the introduction rule originating from the formulation of coalgebras in category theory. We discuss restrictions needed in order to preserve decidability of equality. Dedicated to Per Martin-Löf on the occasion of his retirement. 1.1