Formula Language
You can compute values derived from raw tag values by writing formulae similar to JavaScript expressions. The formulae are continuously evaluated as input tag values change. Currently, Clarity supports using formulae for notification conditions, but it may support them for more uses in the future.
Examples
Test if either tag1
or tag2
is not available (null
):
Test if flow
is less than 10 (in whatever units the tag has) while pumping, assmuing that the pumping
tag is a boolean
:
Test if flow
has been above 10 (in whatever units the tag has) for at least an hour:
Test if the max of three tags is above another tag:
Language Reference
Data Types
Formulae support JavaScript number
, boolean
, and string
data types, as well as null
(used when a value isn't available for a tag or an expression on that tag). Numbers are represented as 64-bit floating point values, although when performing bitwise operations, they are first converted to 32-bit integer values, then the result is converted back to a 64-bit floating point value.
No Coercion to Number or Boolean
Unlike JavaScript, formula won't automatically coerce other data types to number
or boolean
values, and any operation on mismatched data types (except for string concatenation with +
) will be flagged as an error.
null
propagation
null
propagationUnlike JavaScript, the result of an arithmetic, bitwise, or comparison operation will be null
if one of the operands is null
. For example the expression tagValue("a") + tagValue("b")
will be null
if the value of either of the two tags is null
(not available).
No NaN
NaN
Unlike JavaScript, there is no NaN
constant; null
is returned instead, for example from 0 / 0
.
Keywords
true
, false
, and null
have the same values as in JSON.
Number Literals
As in JSON, a number may consist of decimal digits 0
-9
, optional digits after a decimal place .
, and and optional positive or negative exponent e8
or e-8
.
String Literals
As in JSON, a string literal consists of text between double quotes ("
), for example:
The backslash (\
) allows you to escape special characters:
Time Intervals
Some functions accept time intervals as arguments; a time interval is a number with a time unit suffix, for example:
Built-in Functions
tagValue(<tag>)
tagValue(<tag>)
Gets the value of the given <tag>
at the current time. <tag>
must be a string literal, for example:
tagValue(<tag>)
may evaluate to a number
, string
, boolean
, depending on the data type of the given tag, or null
if a value for the given tag is not available at the current time.
tagIsNAFor(<tag>, <interval>)
tagIsNAFor(<tag>, <interval>)
Evaluates to true
if the given <tag>
has been unavailable for at least the given amount of time. If there is or was a value available for the tag more recently than the given amount of time before the present, evaluates to false
.
<tag>
must be a string literal and <interval>
must be a time interval, for example:
maxChange(<expression>, <interval>)
maxChange(<expression>, <interval>)
Evaluates to the maximum amount the given <expression>
has changed over the given <interval>
of time since the present. Always evaluates to a positive number, the absolute difference between the minimum and maximum values of <expression>
over the past <interval>
.
<expression>
can be any expression that evaluates to a number
or null
.
<interval>
must be a time interval no greater than one day.
conditionHoldsFor(<expression>, <interval>)
conditionHoldsFor(<expression>, <interval>)
Evaluates to true
if the given <expression>
has remained true
over the given <interval>
of time since the present. Otherwise, evaluates to false
.
<expression>
can be any expression that evaluates to a boolean
or null
.
<interval>
must be a time interval no greater than one day.
Math functions
The following JavaScript Math
functions are available:
Operators
All operators are computed as in JavaScript, except that no coercion to number or boolean is supported.
When performing bitwise operations, the operands are first converted to 32-bit integer values, then the result of the operation is converted back to a 64-bit floating point value.
Equality and Strict Equality
Unlike JavaScript, the non-strict ==
and !=
operators evaluate to null
if either operand is null
. That is, whereas in JavaScript, null == null
is true
, in Clarity formulae, null == null
is null
.
If you want to test for null
, use the ===
and !==
operators; null === null
is true.
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