Expressions

Expressions describe event patterns. An expression can be an atomic action, a named expression, a composition of smaller expressions, or a timed expression.

Atomic actions

An atomic action has the event name followed by parentheses:

arrival(sender)

Arguments are identifiers separated by commas. They are matched to the event's signature fields in declaration order: string fields first, then number fields.

An atomic action may also include a guard and updates:

arrival(sender, size | sender == "carol@example.net" && size > 0 | count := count + 1)

Use an empty guard section when an action only updates variables:

arrival(sender | | saved_sender := sender)

See Constraints for guard and update syntax.

Composition operators

Construct	Meaning
e1 ; e2	match e1 followed by e2
e1 && e2	match both expressions
e1 || e2	match either expression
( e )	group an expression

Use parentheses when mixing operators if the intended grouping is not obvious.

Repetition and optional expressions

SyMon supports postfix operators and equivalent block forms:

Postfix form	Block form	Meaning
e*	zero_or_more { e }	zero or more repetitions
e+	one_or_more { e }	one or more repetitions
e?	optional { e }	zero or one occurrence

For example:

arrival(sender | sender != "carol@example.net")*;
arrival(sender | sender == "carol@example.net")

Choices and conjunction blocks

Use one_of for alternatives. Branches after the first use or:

one_of {
    arrival(sender | sender == "alice@example.com")
} or {
    arrival(sender | sender == "carol@example.net")
}

Use all_of when all branches must match. Branches after the first use and:

all_of {
    arrival(sender | sender == "alice@example.com")
} and {
    arrival(sender | sender != "blocked@example.net")
}

Both forms require at least two branches.

Timing restrictions

Use within to restrict the duration of an expression:

within [1, 3] {
    arrival(sender);
    arrival(sender)
}

Use % for the postfix form:

(arrival(sender); arrival(sender)) % (<= 3)

Timing constraints can be intervals or half guards:

Form	Meaning
[a,b]	inclusive lower and upper bounds
(a,b)	exclusive lower and upper bounds
[a,b)	inclusive lower, exclusive upper
(a,b]	exclusive lower, inclusive upper
(< a), (<= a), (= a), (<> a), (>= a), (> a)	single timing guard

Bounds are numeric expressions. They may use numbers, timing parameters declared as param, parentheses, +, and -. Use timing parameters with parametric timing mode; see Execution modes.

Ignoring events

Use ignore to allow selected event names to occur without being consumed by the inner expression:

ignore arrival {
    arrival(sender | sender == "carol@example.net")
}

Multiple event names are separated by commas:

ignore arrival, update {
    target
}

Next: File structure, Constraints, Syntax examples, and Common mistakes.