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

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 &math(\\$p_i); 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 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. For inscance, the above example can be written as:

`waitint(X) :- int(X) | ok.`

### Guard Library

Currently, the following type constraints can be written in the guard. The + specifies an input argument.

```'='(+U1,-U2)               - make sure that U1 and U2 are (connected to)
unary atoms with the same name
'='(-U1,+U2)               - same as above
'=='(+U1,+U2)              - check if U1 and U2 are (connected to) unary
atoms with the same name
unary(+U)                  - check if U is (connected to) a unary atom
ground(+G)                 - check if G is (connected to) a connected graph
with exactly one free link (which is G)
int(+I)                    - check if I is (connected to) an integer
float(+F)                  - check if F is (connected to) 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: -. *. /.```