PL researchers have a profound understanding of automata theory but fail to grasp the subtleties and nuances of the type systems used in modern programming languages. My research pursues new insights into the computational power of type systems by connecting them with well-founded classes of automata through type automata---machines that employ program types as control and storage. In addition, I demonstrate advanced type-level metaprogramming applications of type automata that coerce the compiler into performing computations at compile time.

Fluent API is an object-oriented pattern for elegant APIs and embedded DSLs. A smart fluent API can enforce the API protocol or DSL syntax at compile time. Since fluent API implementations typically rely on overloading function names, they are hard to realize in functional programming languages. This work shows how functional fluent APIs can be implemented in the absence of name overloading, by relying on parametric polymorphism and Hindley-Milner type inference. The implementation supports fluent API protocols in the regular- and deterministic context-free language classes, and even beyond.

This is a study of the computing power of the subtyping machine behind Kennedy and Pierce's nominal subtyping with variance. We depict the lattice of fragments of Kennedy and Pierce's type system and characterize their computing power in terms of regular, context-free, deterministic, and non-deterministic tree languages. Based on the theory, we present Treetop---a generator of C# implementations of subtyping machines. The software artifact constitutes the first feasible (yet POC) fluent API generator to support context-free API protocols in a decidable type system fragment.

We present the first general and practical solution of the fluent API problem - an algorithm, that given a deterministic language (equivalently, LR(k), k >= 0 language) encodes it in an unbounded parametric polymorphism type system employing only a polynomial number of types. The theoretical result is accompanied by an actual tool Fling - a fluent API compiler-compiler in the venue of YACC, tailored for embedding DSLs in Java.

Care and great e.ort are often taken to dress program code of libraries, just as model implementations, in its most presentable form, which includes adherence to strict coding standards, careful selection of identifiers, avoiding unnecessary constructs, etc. However, a presentable dress is not a janitor's uniform and is often inferior to the more lax working outfit.The spartanizer is a tool that brings Java code into a canonical, short form. Trying to say the most with the fewest words. In contrast, the athenizer is a tool that expands the code, placing it in a more maintainable form, using plenty of auxiliary variables, many potential locations for breakpoints and for change.The tool reported on here allows developers to interactively use their joystick and its buttons for code navigation, and in particular for zooming-in into the code (athenizing) and zooming-out of it (spartanizing).