Expression Language

Muninn Expression Language

To make it easy to search for products in an archive, muninn implements its own expression language. The expression language is somewhat similar to the WHERE clause in an SQL SELECT statement.

When a muninn extension includes namespace definitions, all product properties defined in these namespaces can be used in expressions.

The details of the expression language are described below. See Data Types for more information about the data types supported by muninn.

Property references

A product property x defined in namespace y is referred to using y.x. If the namespace prefix y is omitted, it defaults to core. This means that any property from the core namespace may be referenced directly.

Some examples of property references:

uuid

validity_start

core.uuid

core.validity_start

Namespace references

A namespace x is referred to using simply x. It is undesirable to create a namespace with the same name as one of the properties in core, as this may lead to ambiguities.

Parameter references

A name preceded by an at sign @ denotes the value of the parameter with that name. This is primarily useful when calling library functions that take an expression as an argument. These functions will also take a dictionary of parameters that will be used to resolved any parameter references present in the expression.

Some examples of parameter references:

@uuid

@start

Functions and operators

The supported logical operators are not, and, or, in order of decreasing precedence.

The comparison operators == (equal) and != (not equal) are supported for all types except geometry.

The comparison operators < (less than), > (greater than), <= (less than or equal), >= (greater than or equal) are supported for all types except boolean, uuid, and geometry.

The membership operators in and not in are supported for all types except boolean, uuid, timestamp and geometry. They only work with lists of literals. Example syntax:

expression in [1, 2, 3]
not text in ["text1", "text2"]

The comparison operator ~= (matches pattern) is supported only for text. The syntax is:

text ~= pattern

Any character in the pattern matches itself, except the percent sign %, the underscore _, and the backslash \.

The percent sign % matches any sequence of zero or more characters. The underscore _ matches any single characters. To match a literal percent sign or underscore, it must be preceded by a backslash \. To match a literal backslash, write four backslashes \\\\.

The result of the comparison is true only if the pattern matches the text value on the left hand side. Therefore, to match a pattern anywhere it should be preceded and followed by a percent sign.

Some examples of the ~= operator:

"foobarbaz" ~= "foobarbaz" (true)
"foobarbaz" ~= "foo" (false)
"foobarbaz" ~= "%bar%" (true)
"foobarbaz" ~= "%ba_" (true)

The unary and binary arithmetic operators + and - are supported for all numeric types. Furthermore, the binary operator - applied to a pair of timestamps returns the length of the time interval between the timestamps as a fractional number of seconds. Due to the way timestamps are represented in sqlite, time intervals are limited to millisecond precision when using the sqlite backend.

The unary function is_defined is supported for all data types and returns true if its argument is defined. This can be used to check whether optional properties or namespaces are defined or not.

The function covers(timestamp, timestamp, timestamp, timestamp) returns true if the time range formed by the pair of timestamps covers the time range formed by the second pair of timestamps. Both time ranges are closed.

The function intersects(timestamp, timestamp, timestamp, timestamp) returns true if the time range formed by the pair of timestamps intersects the time range formed by the second pair of timestamps. Both time ranges are closed.

The function covers(geometry, geometry) returns true if the first geometry covers the second geometry.

The function distance(geometry, geometry) returns the distance between the two geometries with unit degrees.

The function intersects(geometry, geometry) returns true if the first geometry intersects the second geometry.

The function is_source_of(uuid) returns true if the product under consideration is a (direct) source of the product referred to by specified uuid.

The function is_derived_from(uuid) returns true if the product under consideration is (directly) derived from the product referred to by the specified uuid.

For is_source_of and is_derived_from, instead of a uuid, it is also possible to specify a sub-expression resolving into one or multiple uuids (see below for an example).

The function has_tag(text) returns true if the product under consideration is tagged with the specified tag.

The function now() returns a timestamp that represents the current time in UTC.

Examples

is_defined(core.validity_start) and core.validity_start < now()

covers(core.validity_start, core.validity_stop, @start, @stop)

not covers(core.footprint, POINT(5.0 52.0))

distance(core.footprint, POINT(5.0 52.0)) < 5

is_derived_from(32a61528-a712-427a-b28f-8ebd5b472b16)

is_derived_from(physical_name == "filename.txt")

validity_stop - validity_start > 300 (timestamp differences are in seconds)