Ensuring Streams Flow

It is our aim to develop an elementary strong functional programming (ESFP) system. To be useful, ESFP should include structures such as streams which can be computationally unwound innnitely often. We describe a syntactic analysis to ensure that innnitely proceeding structures, which we shall term codata, are productive. This analysis is an extension of the check for guardedness that has been used with deenitions over coinductive types in Martin-LL of's type theory and in the calculus of constructions. Our analysis is presented as a form of abstract interpretation that allows a wider syntactic class of corecursive deenitions to be recognised as productive than in previous work. Thus programmers will have fewer restrictions on their use of innnite streams within a strongly normalizing functional language.