9. The CEL environment

Contents of DESL 0.2

9.1 Base language

DESL uses CEL as specified at https://github.com/google/cel-spec, with these standard extensions REQUIRED: strings (lowerAscii, upperAscii, trim, split, join, replace, indexOf, substring, format), lists (distinct, flatten, range, slice, sort, sortBy), optional types (?., .?, orValue) and the cel.bind macro. The macros all, exists, exists_one, map, filter and has are available as in core CEL. Strings order by Unicode code point, and sorts are stable.

9.2 Types

CEL type Members

Element id (string), type (string), every attribute as a field (typed by the metamodel; a reference resolves to an Element or null), isA(typeName) → bool, parent → Element? (null for the document's own elements), slot → string (the slot it is kept in, '' without one), children → list(Element) (in order), childrenOfType(t) → list(Element), ancestors() → list(Element) (the parent first), ancestorsOfType(t) → list(Element) (in the same order), positionIn(list) → int (9.4), label() → string (the value of its labelAttribute).

Document elements → list(Element) (every element, in document order), elementsOfType(t) → list(Element), elementById(id) → Element?, and the document's attributes as fields.

CEL type	Members
`Element`	`id` (string), `type` (string), every attribute as a field (typed by the metamodel; a reference resolves to an `Element` or `null`), `isA(typeName) → bool`, `parent → Element?` (`null` for the document's own elements), `slot → string` (the slot it is kept in, `''` without one), `children → list(Element)` (in order), `childrenOfType(t) → list(Element)`, `ancestors() → list(Element)` (the parent first), `ancestorsOfType(t) → list(Element)` (in the same order), `positionIn(list) → int` (9.4), `label() → string` (the value of its `labelAttribute`).
`Document`	`elements → list(Element)` (every element, in document order), `elementsOfType(t) → list(Element)`, `elementById(id) → Element?`, and the document's attributes as fields.

Attribute types map to CEL types as section 4.2 says; enumeration values are strings, their keys; many attributes are lists.

9.3 Contexts

The context of an expression determines its variables. A validator type-checks each expression in exactly one context.

Context	Used by	Variables
`constraint`	a constraint's `when`, `rule`, `severity`, `message` and `target`	`self` (an element, or the document for `scope: "document"`), `document`
`identity`	the `expression` and `ephemeral` of an id rule (5.3.2)	`self`, `document`; `self.id` is not readable, and the ids readable are those of `self`'s ancestors
`operation`	an operation's `label`, `unavailable`, `confirm` and actions	`self` (for `for` naming types) or `selection` (for `"selection"`), `document`, `p` (not in `label` and `unavailable`), `count` in a confirmation, and the names `create … as`, `let` and `forEach … as` bind
`function`	a function's body	its parameters, and `document` when its `uses` lists it

9.4 Function library

Besides the members of 9.2, these are available in every context:

Function	Description
`e.positionIn(l) → int`	The zero-based position of element `e` in list `l`, comparing elements by identity, never by id, or −1. Lists of elements are in reading order (10.2), so `positionIn` works while ids are being computed and when ids are duplicated: `self.positionIn(group) == 0` holds for the first of a group only.
`uniqueName(name, names) → string`	`name` when no string of `names` equals it ignoring case; otherwise `name + ' ' + string(n)` for the smallest `n` ≥ 2 that no string of `names` equals ignoring case.
`l.isUnique() → bool`	All items of the list differ.

Implementations MUST provide cost estimates for these functions.

9.5 Determinism

Expressions in the constraint and identity contexts MUST be deterministic given the document: they have no source of time, randomness or viewer state. Lists of elements are in document order, so results are the same in every host.