![[PukiWiki]](lmntal_220x80.png "[PukiWiki]")
The syntax of a rule with a guard is:
Head :- Guard | Body
where Guard is a multiset of type constraints of the form: c($p1, ..., $pn).
Type constraints constrains the shapes of processes (or the names of unary atoms) received by the process contexts $p1, ..., $pn. The type constraint name c is drawn from a built-in set and specifies which kind of constraints is imposed.
A constraint of the form uniq($p1, ..., $pn) is also allowed. This is a control structure rather than a type constraint and used to avoid infinite rule application (see below).
Here is an example rule with guard:
waitint(X), $p[X] :- int($p) | ok.
This can be abbreviated to
waitint($p) :- int($p) | ok.
and can be thought of representing the following infinite number of rules:
waitint(0) :- ok. waitint(1) :- ok. waitint(-1) :- ok. waitint(2) :- ok. waitint(-2) :- ok. ...
The following list contains examples of some type constraints that can be written in Guard:
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 $pk occurs exactly twice in the rule. For example, p($n) :- $n>$z, 0($z) | ok can be abbreviated to p($n) :- $n>0 | ok.
A process context constrained in Guard is said to be a typed process context.
A constraint of the form uniq($p1, ..., $pn) succeeds if each $pk is a ground structure (connected graph with exactly one free link; see below) and the rule has not been applied to the tuple $p1, ..., $pn before.
As a special case of n=0, uniq succeeds if the rule in question has not been used before. The uniq() test is a general tool for avoiding infinite application of rules whose right-hand side is a super(multi)set of the left-hand side.
The following type constraints can be used in guards. The + (input) sign preceding a process context name means that the name should appear in the head, while the - (output) sign means that the name should not appear in the head.
'='(+$u,-$v) - make sure that $u[X] and $v[Y] are unary atoms
with the same name
'='(-$u,+$v) - same as above
'=='(+$u,+$v) - check if $u[X] and $v[Y] are unary atoms with
the same name
unary(+$u) - check if $u[X] is a unary atom
ground(+$g) - check if $g[X1,...,Xn] (n>0) is a connected
graph whose free links are exactly X1,...,Xn
int(+$i) - check if $i[X] is an integer
float(+$f) - check if $f[X] is a float
int(+$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[X1], ..., $gn[Xn] (n>=0)
