Logical Condition in RULE

The WHEN clause of the RULE command takes a logical expression, which is evaluated on every new record of the specified stream. If the expression evaluates to true, the process defined in the DO clause is triggered.

Comparison operators

A single comparison has two scalar operands. Chains such as 0 < measurements[0] < 10, 0 <= measurements[0] < 10, or measurements[0] = 1 = 1 are syntax errors; the parser identifies the second comparison operator. Express a range as two comparisons joined by AND:

RULE in_range ON measurements WHEN 0 < measurements[0] AND measurements[0] < 10 DO DUMP -1 TO 0

Comparison chains have been rejected since fix #555. The earlier parser accepted them and used a comparison’s result as an operand of another comparison; this did not check the intended range.

Logical connectives

Precedence in WHEN, from highest to lowest, is arithmetic, comparison, NOT, AND, OR. NOT measurements[0] < 0 negates the entire comparison measurements[0] < 0; for a non-NULL value, the condition is true when the field is non-negative. In NOT measurements[0] < 0 AND measurements[0] > 5, negation applies to the first comparison, and AND joins the two conditions. AND binds more strongly than OR.

Since fix #555, NOT belongs exclusively to the WHEN condition. It cannot be used as a field operator in SELECT or inside arithmetic, for example measurements[0] + NOT measurements[0]. Previously, NOT measurements[0] < 0 negated the field alone and then compared the result with zero, so the condition was never true.

The grammar does not support parentheses around logical conditions or comparisons. Parentheses may group arithmetic expressions on either side of a comparison, for example (measurements[0]+1) < 10. Write compound conditions according to the precedence above.

A bare STRING field is true when its value is nonempty. String comparisons and NOT over a string return INTEGER 1 or 0. AND and OR use the left operand’s type, or the right operand’s type if the left is NULL; a result based on a string is INTEGER. Thus WHEN NOT status[0] is true for an empty string, and a false string comparison does not trigger a rule through OR. NULL still follows the condition’s three-valued logic.

Expression structure

A condition is built from the fields of the stream schema specified in the ON clause. Fields are identified the same way as in SELECT - by the stream name with an index:

WHEN stream[index] operator value

Compound conditions are joined with connectives:

WHEN stream[0] > 10 AND stream[1] != 0
WHEN stream[0] = 5 OR stream[0] = 7
WHEN NOT stream[0] < 0

Examples

RULE high_alarm \
ON measurements \
WHEN measurements[0] > 100 OR measurements[0] < -100 \
DO DUMP -10 TO 10 RETENTION 50

RULE signaling \
ON status \
WHEN status[0] = 1 AND status[1] != 0 \
DO SYSTEM 'systemctl restart sensor-reader'

RULE one_time ON data WHEN NOT data[0] = 0 DO DUMP -5 TO 0

Field access

The condition refers to the fields of the stream specified in ON. The field index corresponds to its position in that stream’s schema - the same as in the SELECT clause. The condition reads the stream’s output record, so an index equal to or greater than its field count is a compilation error (see Index out of range). Aliasing works exactly as described in the chapter Aliasing.

If the stream in ON was produced by an interleave A#B, the condition must use the output stream name:

RULE valid ON result WHEN result[0] > 0 DO DUMP -1 TO 0

A reference to a named interleave component is ambiguous and causes compilation to fail:

RULE invalid ON result WHEN A[0] > 0 DO DUMP -1 TO 0

The same rule applies when # is hidden in a substrate generated for a compound FROM expression.