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*6 (2004-10-22 (Fri) 09:52:20)*- 7 (2006-01-08 (Sun) 15:18:09)
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- 14 (2017-03-02 (Thu) 03:32:49)

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:

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.

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.

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: -. *. /.