Proposal for Extended Element and Attribute Tests

extended-element-attribute-tests.md

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+# Extended Element and Attribute Tests
+
+**Author**: Reece H. Dunn. 67 Bricks.
+
+Allows element and attribute tests to support union and wildcard name tests.
+
+
+## Description
+
+### Element and Attribute Unions
+
+The XPath and XQuery grammar allows element and attribute unions in a path expression. For example:
+
+    let $list := $current/(ul|ol)
+    let $label := $anchor/@(title|name|alt)
+    ...
+
+However, it does not allow the precise value of these to be specified by the user (e.g. as parameters or return types of functions).
+
+With a union item type proposal, these types could be specified verbosely as:
+
+    (element(ul) | element(ol))
+    (attribute(title) | attribute(name) | attribute(alt))
+
+This proposal defines the following concise syntax for the `ElementNameOrWildcard` part of an `ElementTest`, and the `AttributeNameOrWildcard` part of an `AttributeTest`:
+
+    element(ul|ol)
+    attribute(title|name|alt)
+
+These would be equivalent to the more verbose item type union.
+
+The wildcard `*` cannot be combined in a union type as it would be redundant.
+
+### Wildcard Names
+
+The XPath and XQuery grammar allows element and attribute path expressions to have wildcard-based node tests of the following forms:
+1.  `*` / `@*` -- match any element/attribute
+1.  `prefix:*` / `@prefix:*` -- match any element/attribute in the in-scope namespace prefix
+1.  `*:local-name` / `@*:local-name` -- match any element/attribute with the given local name, ignoring the namespace uri
+1.  `Q{namespace-uri}*` / `@Q{namespace-uri}*` -- match any element/attribute in the specified namespace uri
+
+Like element/attribute unions, a user cannot currently specify the precise type for these expressions except for the general `*` case.
+
+Wildcard type tests would exclude the `*` form -- that is only allowed on its own (i.e. not as part of a union type). The other wildcard forms (prefix, local-name, braced uri literal) are allowed in a union expression, or in a single name. For example:
+
+    element(xh:*)
+    attribute(xh:* | Q{}*)
+
+### Type Names
+
+In all of the above extensions a `TypeName` can be optionally specified, like can be done in the current `ElementTest` and `AttributeTest` syntax. For example:
+
+    element(ol|ul, html-list)
+    attribute(price|cost, currency)
+
+## Use Cases
+
+A user can write an element path expression over a union of elements, but cannot define a precise type for the result.
+
+A user can write an element path expression over a union of attributes, but cannot define a precise type for the result.
+
+A user can write a wildcard node test on element path expression, but can only define a type for `*` wildcards.
+
+A user can write a wildcard node test on attribute path expression, but can only define a type for `*` wildcards.
+
+## Examples
+
+### Matching different list types
+
+    declare function print-list($list as element(ul|ol)) external;
+
+    print-list($item/ul)
+
+### Handling HTML and XHTML in the same expression
+
+    declare function save-html(
+        $html as element(xh:html|Q{}html),
+        $path as xs:string
+    ) external;
+
+### Any XHTML (or namespaced) element
+
+    declare function is-inline-element($e as element(xh:*)) external;
+
+    (: union and wildcard: :)
+    declare function is-block-element($e as element(xh:* | Q{}*)) external;
+
+### Checking if an element is one of a set of element names
+
+    let $e := ...
+    return $e instance of element(a|span|em|i|strong|b)
+
+## Grammar
+
+TBD.