Split rule expressions

Split rules are @cornerstonejs/metadata rule data. Most of a rule is a map of name to test — "Modality": "CT", "isMultiFrame": false, "Rows": { "greaterThan": 256 } — and where a test is not enough, a context entry is an expression, and the rule reads the entry by its name:

"context": { "isLargeCt": "Modality == 'CT' && Rows >= 256" },
"matches": { "isLargeCt": true }

The expression language is the safe functions of @cornerstonejs/metadata. OHIF has no expression language of its own, so the same rule data compiles in the viewer and on a server. The language reference is in the Cornerstone docs:

• Safe functions: the syntax, the operators, the helper functions, and the meaning of a missing value.
• Display sets: the rule fields, the context, and the displaySet. prefix.

defined is a literal

defined is not a helper function. It is a literal for == and != only: X == defined is true when X has a value. X != null is the same test.

Where an expression goes

Place	Form
a context entry (the metadataContext customization, or the context of a rule)	a string
an attribute value of attributes	{ "expression": "..." }

{ "template": "{SeriesDescription} b={DiffusionBValue}" } is the simpler form for text, in a context entry and in attributes.

A bare name always reads the series value. In sortBy and attributes, the displaySet. prefix reads the value of the display set, for example {displaySet.sliceSpacing} in a template.

Errors

The metadata compiler parses every expression, and checks every name, when it compiles the rule. A name that is not a DICOM keyword and not a context name is a compile error, for example unknown name 'modality'; did you mean 'Modality'?. A private tag or a vendor attribute needs a declaration in the context, for example "myValue": { "attribute": "00191010" }.

OHIF does not drop a rule that does not compile: the viewer creates no display sets, and shows an error notification that names the rule (see Rule errors). A mistake in a rule must stop the display, because a grouping without that rule can look correct and still be wrong.

An expression does not fail at run time. Only a function that code supplies, for example a code entry of the context, can throw at run time. That error stops display set creation for the study, and the notification names the rule, the field and the series.

A keyword that the instance does not carry gives no value, and does not throw. So a rule that reads an attribute that the data source does not supply compiles, and then matches nothing. When an expression matches nothing, check that the instance carries the attributes that it reads. collectIdentifiers from @cornerstonejs/metadata reports the names that an expression reads.