It is very often the case that programs require passing, maintaining, and updating some notion of state. Prolog programs often implement such stateful computations by carrying this state in predicate arguments (or, alternatively, in the internal datábase). This often causes code obfuscation, complicates code reuse, introduces dependencies on the data model, and is prone to incorrect propagation of the state information among predicate calis. To partly solve these problems, we introduce contexts as a consistent mechanism for specifying implicit arguments and its threading in clause goals. We propose a notation and an interpretation for contexts, ranging from single goals to complete programs, give an intuitive semantics, and describe a translation into standard Prolog. We also discuss a particular light-weight implementation in Ciao Prolog, and we show the usefulness of our proposals on a series of examples and applications, including code directiy using contexts, DCGs, extended DCGs, logical loops and other custom control structures
