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[[Documentation]] *Guards **Rules with a Guard The full syntax of a rule is: >'''Head''' :- ['''Guard''' | ] '''Body''' The syntax of a rule with a guard is: >'''Head''' :- '''Guard''' | '''Body''' where '''Guard''' is a multiset of ''type constraints'' of the form: &math(p(\$p_1,\ldots,\$p_n));. '''p'''($'''p'''&size(10){1};, ..., $'''p'''&size(10){'''n'''};). Type constraints put constraints on the shapes of processes (or the names of unary atoms) with which the process contexts specified in its arguments can match. The ''type constraint name'' &math(p); is drawn from a built-in set and specifies which kind of constraints is imposed. The ''type constraint name'' '''p''' is drawn from a built-in set and specifies which kind of constraints is imposed. A constraint of the form ''uniq''($'''p'''&size(10){1};, ..., $'''p'''&size(10){'''n'''};) is also allowed. This is a control structure rather than a type constraint and used to avoid infinite rule application (see below). ***Examples Here is an example rule with guard: waitint($p) :- int($p) | ok. This rule can be thought of as an abbreviation of the following infinite number of rules: waitint(0) :- ok. waitint(1) :- ok. waitint(-1):- ok. waitint(2) :- ok. ... The following list contains examples of some type constraints that can be written in '''Guard''': - int($p) --- specifies that $p must be an integer atom. - 4($p) --- specifies that $p must be a unary integer atom of value 4 (i.e., 4(X)). - $p < $q --- specifies that $p and $q are integer atoms such that the value of $p is less than that of $q. - $r = $p +. $q --- specifies that $p, $q, and $r are floating point number atoms such that the sum of the values of $p and $q is equal to the value of $r. ***Notes Each type constraint name (such as int or <) has its own mode of usage that specifies which of its arguments are input arguments. The effect of the constraint specified by a type constraint is enabled only after the shapes (or values) of its input arguments are all determined. For example, $r = $p + $q proceeds only when $p and $q are determined. The same abbreviation scheme as defined for atoms applies to type constraints when a process context name &math(\$p_i); applies to type constraints when a process context name $'''p'''&size(10){'''k'''}; occurs exactly twice in the rule. For example, p($n) :- $n>$z, 0($z) | ok can be abbreviated to p($n) :- $n>0 | ok. **Typed Process Contexts A process context name $p constrained in '''Guard''' is said to be ''typed'' in that rule. As a syntactic sugar, typed process context names can be written as link names. For inscance, the above example can be written as: waitint(X) :- int(X) | ok. // ( Res = gen(N) :- N > 0 | Res = [N|gen(N-1)] ), p(gen(10)) **Avoiding Infinite Rule Application A constraint of the form ''uniq''($'''p'''&size(10){1};, ..., $'''p'''&size(10){'''n'''};) succeeds if each $'''p'''&size(10){'''k'''}; is a '''ground''' structure (connected graph with exactly one free link; see below) and the rule has not been applied to the tuple $'''p'''&size(10){1};, ..., $'''p'''&size(10){'''n'''}; before. As a special case of '''n'''=0, ''uniq'' succeeds if the rule in question has not been used before. The ''uniq''() test is a general tool for avoiding infinite application of rules whose right-hand side is a super(multi)set of the left-hand side. **Guard Library Currently, the following type constraints can be written in the guard. The + specifies an input argument. '='(+U1,-U2) - make sure that U1 and U2 are (connected to) unary atoms with the same name '='(-U1,+U2) - same as above '=='(+U1,+U2) - check if U1 and U2 are (connected to) unary atoms with the same name unary(+U) - check if U is (connected to) a unary atom ground(+G) - check if G is (connected to) a connected graph with exactly one free link (which is G) int(+I) - check if I is (connected to) an integer float(+F) - check if F is (connected to) a float int(+Float,-Int) - cast float(+Int,-Float) - cast 345(-Int) - defined for every integer (not only with 345) '-3.14'(-Float) - defined for every float '<'(+Int,+Int) - integer comparison; also: > =< >= =:= =\= '+'(+Int,+Int,-Int) - integer operation; also: - * / mod '<.'(+Float,+Float) - float comparison; also: >. =<. >=. =:=. =\=. '+.'(+Float,+Float,-Float) - float operation; also: -. *. /. uniq(+G1,...,+Gn) - uniqueness constraint; checks if the rule has not been applied to the tuple G1,...,Gn (n>=0)