Proposed extensions to unary and binary lookup expressions

lookup-operator-extensions.md

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+# New Conditional Expression Syntax
+
+**Author**: Michael Kay, Saxonica
+
+Proposal for extensions to the unary and binary lookup operators.
+
+
+## Description
+
+The lookup operator in XPath 3.1 allows the RHS operand to be any of the following:
+
+* NCName
+* Integer literal
+* Parenthesized expression
+* "*"
+
+This proposal adds three further options
+
+* String literal
+* Variable reference
+
+The effect of using a string literal or variable reference is the same as using a parenthesized expression containing the string literal or variable reference: it enables you to write `$emp?"date of birth"` in place of `$emp?("date of birth")`, and `$array?$i` in place of `$array?($i)`.
+
+Note: the justification for not allowing a general expression on the RHS of the lookup operator is essentially so that an unquoted string (an NCName) can be treated differently from its usual meaning of `child::elemName`. It also allows special meaning to be attached to `*` as the right-hand operand. Integer literals were allowed without parentheses because they are clearly useful and clearly cause no parsing problems. The same argument applies to string literals and variable references; the parentheses in these cases are totally unnecessary, and all this proposal does it to remove the need for them.
+
+
+
+## Use Cases
+
+The two new options bring no new functionality but some extra conciseness. See examples below.
+
+Note that `$array?$i` means `$array?($i)` which is not quite the same as `$array($i)` - the meaning is different if `$array` is a sequence of arrays rather than a single array. Avoiding the parentheses differentiates the two expressions more clearly.
+
+## Examples
+
+Map lookup using a string-valued key containing spaces can now be written
+
+```
+$map?"date of birth"
+$map?"123-reg"
+```
+
+Such keys arise commonly with JSON data. In XPath 3.1 the string literal must be enclosed by parentheses.
+
+Similarly, map lookup and array lookup where the key is held in a variable no longer need parentheses:
+
+```
+for $i in 1 to 50 return $array?$i
+for $k in map:keys($map1) return $map2?$k
+```
+
+Disadvantages: the benefits are fairly cosmetic and users may feel that the benefits do not compensate for the costs of being non-standard.
+
+
+## Grammar
+
+TBA.