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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.
Here is an example rule with guard:
wait($p) :- int($p) | ok.
This rule can be thought of as an abbreviation of the following infinite number of rules:
wait(0) :- ok. wait(1) :- ok. wait(-1):- ok. wait(2) :- ok. ...
The following list contains examples of some type constraints that can be written in Guard:
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.
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.
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))
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: -. *. /.
