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Languages & runtimes · #prolog
Learn how to diagnose and fix linear search performance issues in Prolog by identifying missing or unsupported indexing on predicates.
Context SWI-Prolog 8.5 introduced use_module/2 with the reload(true) option, which replaces predicate definitions atomically within a single thread. This capability is documented as a mechanism for updating logic without restarting the runtime. Constraint The documentation and ISO/IEC 13211 do not specify behavior when multiple threads concurrently execute p
Dynamic Predicate Recovery In ISO Prolog, managing the state of a dynamic database involves exporting current facts as Prolog terms and subsequently re-inserting them using assert/1 . This process is essential for maintaining state across sessions or recovering from runtime changes. Ordering Constraints While retract/1 and assert/1 allow for the modification
Pruning Choice Points in Depth-First Search Prolog utilizes a depth-first search strategy to satisfy goals, relying on choice points to backtrack when a path fails. The cut operator ( ! ) is designed to discard these choice points, effectively pruning the search space to optimize execution or implement specific logical defaults. Constraint of Red Cuts While
Goal Use once/1 to guarantee that a write operation (e.g., assert/1 ) executes only once even when the surrounding predicate is retried via backtracking. Constraints and uncertainty The ISO core defines once/1 as a cut that removes choice points created inside the wrapped goal. However, implementations differ on whether that cut also removes choice points fr
Pruning Backtracking in Logic Programming In Prolog, the cut operator ( ! ) is used to prune the search space by preventing the engine from backtracking to previous choice points. While red cuts are often employed to implement if-then-else logic, they fundamentally alter the declarative meaning of a program and can prevent predicates from being used in multi
SWI-Prolog's SLG tabling memoizes answers for predicates declared with table/1 , suspending incomplete subgoals until results are available. Because answers are shared and reused across multiple derivations rather than regenerated on failure, standard backtracking changes. The cut operator inside a tabled predicate is scoped to the current derivation and doe
A design is needed to retry a write goal that can fail transiently while guaranteeing the dynamic fact is present exactly once and external side effects are not duplicated. The constraint is SWI-Prolog database semantics. transaction/1 runs a goal atomically and rolls back assert/retract changes on failure, so a retry loop around transaction/1 cannot leave p
A SWI-Prolog codebase loads and runs correctly on development machines, but on the production host the same files fail with syntax errors or produce silently wrong atoms. The working hypothesis is character encoding: the files are saved as UTF-8, while the server locale differs from the workstation locale, so the byte stream is misread before parsing and the