On the Generality and Convenience of Etypes

The Ethos operating system provides a number of features which aid programmers as they craft robust computer programs. One such feature of Ethos is its distributed, mandatory type system -- Etypes. Etypes provides three key properties: (1) every Ethos object (e.g., A file or network connection) has a declared type, (2) Ethos forbids programs from writing ill-formed data to an object, and (3) Ethos forbids programs from reading ill-formed data from an object. In any case, programmers declare ahead of time the permitted data types, and Ethos' application of operating-system-level recognition simplifies their programs. This paper first investigates the generality of Etypes. Toward this end, we describe how to convert a grammar in Chomsky normal form into an Ethos type capable of expressing exactly the set of syntax trees which are valid vis-a-vis the grammar. Next, the paper addresses the convenience of Etypes. If Etypes does not make it easier to craft programs, then programmers will avoid the facilities it provides, for example by declaring string types which in fact serve to encode other types (here Etypes would check the string but not the encoded type). Finally, we present a sample distributed program for Ethos which makes use of the techniques we describe.