Continuations in Prolog

Continuations are well known in functional programming where they have been used to transform and compile programs. Some languages provide explicit manipulations of the continuation for the user: The user can catch and modify the current continuation. Continuations have also been used in the logic programming context to give a denotational semantics for Prolog, to generate Prolog compilers and to transform Prolog programs. In this paper, we propose to introduce new built-ins in a logic programming language to enable the user to explicitly replace the continuations. These built-ins allow the user to have a new control of the execution. We choose Prolog because of its higher-order syntax and implications in the goals which are necessary for the deenition and use of these built-ins. In order to deene the built-ins, we extend to Prolog the Prolog semantics based on continuations. Then, we show that an exception mechanism can be easily implemented using these new built-ins. The proposed semantics is also used to prove equivalence of goals changing the continuations.