The present volume contains the proceedings of the international conference Foundations of Software Science and Computation Structures(FOSSACS)2003, held in Warsaw, Poland, April 7-9, 2003. FOSSACS is an event of the Joint European Conferences on Theory and Practice of Software (ETAPS). The pre- ous ?ve FOSSACS conferences took place in Lisbon (1998), Amsterdam (1999), Berlin (2000), Genoa (2001), and Grenoble (2002). FOSSACS presents original papers on foundational researchwith a clear s- ni?cancetosoftwarescience.TheProgramCommitteeinvitedpapersontheories and methodsto supportthe analysis,...

A common attraction to functional programming is the ease with which proofs can be given of program properties. A common disappointment with functional programming is the difficulty of expressing input/output (I/O) while at the same time being able to verify programs. Here, the author shows how a theory of functional programming can be smoothly extended to admit both an operational semantics for functional I/O and verification of programs engaged in I/O. He obtains, for the first time, operational semantics for the three most widely implemented I/O mechanisms for lazy languages, and proves...