/************************************************************************* * * * YAP Prolog * * * * Yap Prolog was developed at NCCUP - Universidade do Porto * * * * Copyright L.Damas, V.S.Costa and Universidade do Porto 1985-2014 * * * ************************************************************************** * * * File: boot.yap * * Last rev: 8/2/88 * * mods: * * commen ts: boot file for Prolog * * * *************************************************************************/ /** @file boot.yap @brief YAP bootstrap @addtogroup YAPControl Control Predicates @ingroup builtins @{ */ /** @pred 0:P,0:Q is iso, meta Conjunction of goals (and). The conjunction is a fundamental construct of Prolog. Example: ~~~~~~~ p(X) :- q(X), r(X). ~~~~~~~ should be read as `p( _X_) if q( _X_) and r( _X_). */ ','(X,Y) :- yap_hacks:env_choice_point(CP), '$current_module'(M), '$call'(X,CP,(X,Y),M), '$call'(Y,CP,(X,Y),M). /** @pred 0:P ; 0:Q is iso Disjunction of goals (or). Example: ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ p(X) :- q(X); r(X). ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ should be read as "p( _X_) if q( _X_) or r( _X_)". */ ';'((X->A),Y) :- !, yap_hacks:env_choice_point(CP), '$current_module'(M), ( '$execute'(X) -> '$call'(A,CP,(X->A;Y),M) ; '$call'(Y,CP,(X->A;Y),M) ). ';'((X*->A),Y) :- !, yap_hacks:env_choice_point(CP), '$current_module'(M), ( '$current_choice_point'(DCP), '$execute'(X), yap_hacks:cut_at(DCP), '$call'(A,CP,((X*->A),Y),M) ; '$call'(Y,CP,((X*->A),Y),M) ). ';'(X,Y) :- yap_hacks:env_choice_point(CP), '$current_module'(M), ( '$call'(X,CP,(X;Y),M) ; '$call'(Y,CP,(X;Y),M) ). '|'(X,Y) :- yap_hacks:env_choice_point(CP), '$current_module'(M), ( '$call'(X,CP,(X|Y),M) ; '$call'(Y,CP,(X|Y),M) ). /** @pred 0:Condition -> 0:Action is iso @short If _Condition__ has a solution, call _Action_; @long Read as "if-then-else" or "commit". This operator is similar to the conditional operator of imperative languages and can be used alone or with an else part as follows: ~~~~~ +P -> +Q ~~~~~ "if P then Q". ~~~~~ +P -> +Q; +R ~~~~~ "if P then Q else R". These two predicates could be defined respectively in Prolog as: ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ (P -> Q) :- P, !, Q. ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ and ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ (P -> Q; R) :- P, !, Q. (P -> Q; R) :- R. ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ if there were no "cuts" in _P_, _Q_ and _R_. vNote that the commit operator works by "cutting" any alternative solutions of _P_. Note also that you can use chains of commit operators like: ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ P -> Q ; R -> S ; T. ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Note that `(->)/2` does not affect the scope of cuts in its arguments. */ '->'(X,Y) :- yap_hacks:env_choice_point(CP), '$current_module'(M), ( '$call'(X,CP,(X->Y),M) -> '$call'(Y,CP,(X->Y),M) ). /** @pred 0:Condition *-> 0:Action is iso This construct implements the so-called soft-cut. The control is defined as follows: + If _Condition_ succeeds at least once, the semantics is the same as ( _Condition_, _Action_). + If _Condition_ does not succeed, the semantics is that of (\\+ _Condition_, _Else_). In other words, if _Condition_ succeeds at least once, simply behave as the conjunction of _Condition_ and _Action_, otherwise execute _Else_. The construct _A *-> B_, i.e. without an _Else_ branch, is translated as the normal conjunction _A_, _B_. */ '*->'(X,Y) :- yap_hacks:env_choice_point(CP), '$current_module'(M), ( '$call'(X,CP,(X*->Y),M), '$call'(Y,CP,(X*->Y),M) ). /** @pred ! is iso Read as "cut". Cuts any choices taken in the current procedure. When first found "cut" succeeds as a goal, but if backtracking should later return to it, the parent goal (the one which matches the head of the clause containing the "cut", causing the clause activation) will fail. This is an extra-logical predicate and cannot be explained in terms of the declarative semantics of Prolog. example: ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ member(X,[X|_]). member(X,[_|L]) :- member(X,L). ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ With the above definition ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ?- member(X,[1,2,3]). ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ will return each element of the list by backtracking. With the following definition: ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ member(X,[X|_]) :- !. member(X,[_|L]) :- member(X,L). ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ the same query would return only the first element of the list, since backtracking could not "pass through" the cut. */ ! :- yap_hacks:parent_choice_point(CP), yap_hacks:cut_at(CP). /** @pred \+ 0:P is iso, meta Negation by failure. Goal _P_ is not provable. The execution of this predicate fails if and only if the goal _P_ finitely succeeds. It is not a true logical negation, which is impossible in standard Prolog, but "negation-by-failure". This predicate might be defined as: ~~~~~~~~~~~~ \+(P) :- P, !, fail. \+(_). ~~~~~~~~~~~~ if _P_ did not include "cuts". If _P_ includes cuts, the cuts are defined to be scoped by _P_: they cannot cut over the calling prredicate. ~~~~~~~~~~~~ go(P). :- \+ P, !, fail. \+(_). ~~~~~~~~~~~~ */ \+(G) :- \+ '$execute'(G). not(G) :- \+ '$execute'(G). /** @pred repeat is iso Succeeds repeatedly. In the next example, `repeat` is used as an efficient way to implement a loop. The next example reads all terms in a file: ~~~~~~~~~~~~~{.prolog} a :- repeat, read(X), write(X), nl, X=end_of_file, !. ~~~~~~~~~~~~~ the loop is effectively terminated by the cut-goal, when the test-goal `X=end` succeeds. While the test fails, the goals `read(X)`, `write(X)`, and `nl` are executed repeatedly, because backtracking is caught by the `repeat` goal. The built-in `repeat/0` could be defined in Prolog by: ~~~~~{.prolog} repeat. repeat :- repeat. ~~~~~ The predicate between/3 can be used to iterate for a pre-defined number of steps. */ repeat :- '$repeat'. '$repeat'. '$repeat'. '$repeat'. '$repeat'. '$repeat'. '$repeat'. '$repeat'. '$repeat'. '$repeat'. '$repeat' :- '$repeat'. /** @pred + _P_ is nondet The same as `call( _P_)`. This feature has been kept to provide compatibility with C-Prolog. When compiling a goal, YAP generates a `call( _X_)` whenever a variable _X_ is found as a goal. ~~~~~{.prolog} a(X) :- X. ~~~~~ is converted to: ~~~~~{.prolog} a(X) :- call(X). ~~~~~ */ /** @pred call( 0:P ) is iso Meta-call predicate. If _P_ is instantiated to an atom or a compound term, the goal `call( _P_)` is executed as if the clause was originally written as _P_ instead as call( _P_ ), except that any "cut" occurring in _P_ only cuts alternatives in the execution of _P_. */ call(G) :- '$execute'(G). /** @pred incore( 0:P ) same as call/1. */ /** @pred once( 0 G) is iso Execute the goal _G_ only once. The predicate is defined by: ~~~~~{.prolog} once(G) :- call(G), !. ~~~~~ Note that cuts inside once/1 can only cut the other goals inside once/1. */ once(G) :- strip_module(G, M, C), '$meta_call'(C, M), !. (:- G) :- '$execute'(G), !. (?- G) :- '$execute'(G). '$$!'(CP) :- '$cut_by'(CP). [] :- true. %% @}