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[[Documentation]] *Guards **Rules with a Guard The full syntax of a rule is: ( '''Head''' :- ['''Guard''' | ] '''Body''' ) where '''Guard''' is a multiset of ''type constraints'' of the form: &math(p(\$p_1,\ldots,\$p_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. ***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 In reality, 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 if $p and $q are determined. The same abbreviation scheme as defined for atoms applies to type constraints when a process context name &math(\$p_i); occurs exactly two times in the rule. For example, p($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 as follows: ( Res = gen(N) :- N > 0 | Res = [N|gen(N-1)] ), p(gen(10)) **Guard Library Currently, the following type constraints can be written in the guard. The + specifies an input argument. '='(+Unary,-Unary) - equivalence '='(-Unary,+Unary) - equivalence '=='(+Unary,+Unary) - equivalence unary(+Unary) int(+Int) float(+Float) int(+Float,-Int) - cast float(+Int,-Float) - cast 345(-Int) - for every integer, not only with 345 '-3.14'(-Float) - for every floating, not only with -3.14 '<'(+Int,+Int) - integer comparison, as well as: > =< >= =:= =\= '+'(+Int,+Int,-Int) - integer operation, as well as: - * / mod '<.'(+Float,+Float) - floating comparison, as well as: >. =<. >=. =:=. =\=. '+.'(+Float,+Float,-Float) - floating operation, as well as: -. *. /.