Using impurity to create declarative interfaces in Mercury

The logi /fun tional language Mer ury allows the programmer to annotate predi ates and fun tions to mark impure predi ates, allowing impure ode to be safely integrated into a de larative language. By using purity de larations with the foreign language interfa e, programmers an take advantage of many of the features of a high level programming language while writing imperative ode to interfa e with existing imperative libraries. This paper outlines the purity system in Mer ury and how it a e ts operational semanti s, ompares this purity system with other approa hes to de laring impurity in a pure language, and gives an extended example of how impurity and foreign language interfa es an work together to simplify the hore of writing de larative interfa es to libraries.