Constraints and updates

Atomic actions can contain a guard, updates, or both:

event(args | constraints | updates)
event(args | constraints)
event(args | | updates)

Constraints

Use && to combine constraints:

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

String constraints use == and !=:

Construct	Meaning
x == y	string equality
x != y	string disequality
x == "literal"	compare with a string literal
"literal" != y	compare a literal with a variable

String literals use double quotes.

Numeric constraints use <, <=, =, <>, >=, and >:

Construct	Meaning
n < m	less than
n <= m	less than or equal
n = m	numeric equality
n <> m	numeric disequality
n >= m	greater than or equal
n > m	greater than

Numeric expressions may use identifiers, numbers, parentheses, +, and -:

amount + fee <= limit - 1

Numbers may be integers or decimals, with an optional leading -.

Updates

Use := to update global variables declared in var:

arrival(sender | sender == current_sender | count := count + 1)

Separate multiple updates with semicolons:

arrival(sender, amount | amount > 0 | saved_sender := sender; total := total + amount)

String variables can be updated from a string literal or a string-valued identifier. Number variables can be updated from a numeric expression.

Updates target global variables, not event fields declared in a signature.

Guards without updates

When an action only checks a condition, use one |:

arrival(sender | sender != "blocked@example.net")

Updates without guards

When an action only performs updates, keep the empty guard section:

arrival(sender | | saved_sender := sender)

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