Bernd Grobauer

Cost recurrences for DML programs

A cost recurrence describes an upper bound for the running time of a program in terms of the size of its input. Finding cost recurrences is a frequent intermediate step in complexity analysis, and this step requires an abstraction from data to data size. In this article, we use information contained in dependent types to achieve such an abstraction: Dependent ML (DML), a conservative extension of ML, provides dependent types that can be used to associate data with size information, thus describing a possible abstraction. We automatically extract cost recurrences from first-order DML programs, guiding the abstraction from data to data size with information contained in DML type derivations.

A unifying approach to goal-directed evaluation

Goal-directed evaluation, as embodied in Icon and Snobol, is built on the notions of backtracking and of generating successive results, and therefore it has always been something of a challenge to specify and implement. In this article, we address this challenge using computational monads and partial evaluation.We consider a subset of Icon and we specify it with a monadic semantics and a list monad. We then consider a spectrum of monads that also fit the bill, and we relate them to each other. For example, we derive a continuation monad as a Church encoding of the list monad. The resulting semantics coincides with Gudeman’s continuation semantics of Icon.We then compile Icon programs by specializing their interpreter (i.e., by using the first Futamura projection), using type-directed partial evaluation. Through various back ends, including a run-time code generator, we generate ML code, C code, and OCaml byte code. Binding-time analysis and partial evaluation of the continuation-based interpreter automatically give rise to C programs that coincide with the result of Proebsting’s optimized compiler.

From I/O Automata to Timed I/O Automata

The model of timed I/O automata represents an extension of the model of I/O automata with the aim of reasoning about real-time systems. A number of case studies using timed I/O automata has been carried out, among them a treatment of the so-called Generalized Railroad Crossing (GRC). An already existing formalization of the meta-theory of I/O automata within Isabelle/HOLCF allows for fully formal tool-supported Verification using I/O automata. We present a modification of this formalization which accomodates for reasoning about timed I/O automata. The guiding principle in choosing the parts of the meta-theory of timed I/O automata to formalize has been to provide all the theory necessary for formalizing the solution to the GRC. This leads to a formalization of the GRC, in which not only the correctness proof itself has been formalized, but also the underlying meta-theory of timed I/O automata, on which the correctness proof is based.