Mihaela Mihaljevic
d83e5a88ce
Compile and evaluate full XPath 1.0 expressions, not just location paths. Add the four-type Value (node-set, boolean, number, string) with the spec's coercion rules and canonical number formatting, a recursive-descent parser for the operator grammar (or/and, equality, relational, additive, multiplicative, unary minus, union) whose grammatical position disambiguates * and the operator keywords, function calls, variable references, and filter expressions. Predicates are now arbitrary expressions (a numeric predicate is a position test). Comparison follows the node-set quantification rules. Query gains value/number/string/boolean accessors and a $variable binding map; the core functions last, position, count, not, true, false, boolean, number, and string ship with it. Closes #42
59 lines
2.3 KiB
Swift
59 lines
2.3 KiB
Swift
extension PureXML.XPath {
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/// One XPath function: it receives its already-evaluated argument values and
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/// the evaluation context, and returns a value.
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typealias FunctionImplementation = @Sendable (_ arguments: [Value], _ context: EvaluationContext) throws -> Value
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/// The evaluation context an expression is evaluated against: the context
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/// node, its position and size within the current node-set, the variable
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/// bindings, and the function library. Predicates and the `position()` and
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/// `last()` functions read the position and size.
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struct EvaluationContext {
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var node: Node
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var position: Int
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var size: Int
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var variables: [String: Value]
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var functions: FunctionTable
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/// A copy positioned on `node` at one-based `position` within a node-set of
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/// `size`, keeping the same variables and functions.
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func focused(on node: Node, position: Int, size: Int) -> EvaluationContext {
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EvaluationContext(
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node: node,
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position: position,
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size: size,
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variables: variables,
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functions: functions,
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)
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}
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}
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/// The XPath function library: a name-to-implementation table. The core set is
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/// built in; further functions extend it.
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struct FunctionTable: Sendable {
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private var table: [String: FunctionImplementation]
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init(_ table: [String: FunctionImplementation] = [:]) {
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self.table = table
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}
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/// Returns a table with `implementation` registered under `name`.
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func adding(_ name: String, _ implementation: @escaping FunctionImplementation) -> FunctionTable {
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var copy = table
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copy[name] = implementation
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return FunctionTable(copy)
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}
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/// Merges another table's entries over this one's.
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func merging(_ other: FunctionTable) -> FunctionTable {
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FunctionTable(table.merging(other.table) { _, new in new })
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}
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func call(_ name: String, _ arguments: [Value], _ context: EvaluationContext) throws -> Value {
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guard let implementation = table[name] else {
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throw QueryError.unknownFunction(name)
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}
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return try implementation(arguments, context)
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}
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}
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}
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