# s(CASP): Goal directed Constraint Answer Set Programming s(CASP) is related to ASP. Unlike ASP, which is traditionally solved using _grounding_ and a SAT solver, s(CASP) is solved using top-down goal directed search without grounding. This allows s(CASP) to solve problems that cannot be grounded, while the generated proof tree is a good basis for giving a justification for the answer. s(CASP) supports both negation as failure (NAF) and classical negation. These features make s(CASP) particularly suitable for commonsense reasoning tasks that require a justification of the answer. For more information we refer to [Gopal Gupta's home page](https://personal.utdallas.edu/~gupta/) which provides many pointers to related resources. The original source by Joaquin Arias is [here](https://gitlab.software.imdea.org/ciao-lang/sCASP) ## S(CASP) and SWI-Prolog s(CASP) traditionally comes as an executable `scasp` which takes an s(CASP) program, usually with an embedded query, and prints the result at different levels of detail. The SWI-Prolog port primarily aims at __embedding s(CASP) into Prolog__. Because s(CASP) programs are close to Prolog predicates, embedded s(CASP) does a few program transformations while loading Prolog code (deal with classical notation and global constraints) and provides a _meta_ goal scasp/2 to evaluate the program under s(CASP) semantics. The scasp/2 predicate collects the reachable call tree, all _global constraints_ whose call tree _overlaps_ with the reachable call tree, prepares this program for execution usig the s(CASP) solver and solves the query. Note that unlike default s(CASP) this implies that global constraints that are completely unrelated from the query are not considered. This is the implementation currently embedded into SWISH. ### Differences from Prolog As s(CASP) has a few extensions when compared to normal Prolog we need to establish some program transformation: - After loading library(scasp), literals subject to _classical negation_ are prefixed using the unary `-` operator, as in `-p(X)`. The term- and goal expansion include the `-` into the functor, thus generating e.g., `'-p'(X)`. User predicates whose name start with `-` are considered negative literals. - Normally, _global constraints_ are written with an "empty head", i.e., using the Prolog syntax for _directives_. To avoid this conflict, _global constraints_ are written as rules for the predicate false/0. For example, the following rule defines that something cannot be a `p` and a `q` at the same time. false :- p(X), q(X). - Normal assert/1, etc. to modify the database cannot be used as it has to perform the above transformations. For this reason we provide scasp_assert/1. ## s(CASP) in SWISH All the normal conventions of SWISH apply to using s(CASP) programs. Every program or notebook that wishes to use s(CASP) must load library(scasp). Every query must be inside scasp/1 or its shorthand ?/1. ## Status of s(CASP) in SWI-Prolog and SWISH s(CASP) for SWI-Prolog and SWISH is very much __work in progress__. We intend to make s(CASP) a stable standard library for SWI-Prolog. For discussion please go to the [SWI-Prolog forum](https://swi-prolog.discourse.group/). For now, the s(CASP) port for SWI-Prolog is at [GitHub](https://github.com/JanWielemaker/sCASP). ## Examples First we need to include the library!

### Opera from http://platon.etsii.urjc.es/~jarias/slides/gde21-tutorial.pdf slide 6

So our subject can go to the opera on any day but Monday and Tuesday. Great! Now typing scasp(Goal) is a lot of typing. We can use the _prefix operator_ ?/1 as __shorthand__.

### Cyclic programs with an even number of negations