Documentation

## Guards

### Rules with a Guard

The syntax of a rule with a guard is:

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).

#### Examples

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:

• 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 \$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.

### Typed Process Contexts

A process context constrained in Guard is said to be a typed process context.

### Avoiding Infinite Rule Application

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.

### Guard Library

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)```