4.10 Delimited continuations

The predicates reset/3 and shift/1 implement delimited continuations for Prolog. Delimited continuation for Prolog is described in Schrijvers et al., 2013. The mechanism allows for proper coroutines, two or more routines whose execution is interleaved, while they exchange data. Note that coroutines in this sense differ from coroutines realised using attributed variables as described in chapter 7.

The suspension mechanism provided by delimited continuations is suitable for the implementation of tabling Desouter et al., 2015, see library library(tabling).

reset(:Goal, ?Ball, -Continuation)

Call Goal. If Goal calls shift/1 and the argument of shift/1 can be unified with Ball,^{66The argument order described in Schrijvers et al., 2013 is reset(Goal,Continuation,Ball). We swapped the argument order for compatibility with catch/3} shift/1 causes reset/3 to return, unifying Continuation with a goal that represents the continuation after shift/1. In other words, meta-calling Continuation completes the execution where shift left it. If Goal does not call shift/1, Continuation are unified with the integer 0 (zero).^{67Note that older versions also unify Ball with 0. Testing whether or not shift happened on Ball however is always ambiguous.}

shift(+Ball)

Abandon the execution of the current goal, returning control to just after the matching reset/3 call. This is similar to throw/1 except that (1) nothing is `undone' and (2) the 3th argument of reset/3 is unified with the continuation, which allows the code calling reset/3 to resume the current goal.