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Table of Contents
Node pattern
T within a query is a pattern which matches instances of X. A query variable is declared implicitly, the built-in pattern matching algorithm of the XL run- time system then tries to bind nodes of the current graph to such a variable in order to match the associated patterns
Type
Query nodes from their type. E.g. the predicate Sphere within a query is a pattern that matches instances of Sphere.
Parametrized
Parametrized pattern enables to bound query variables to parameters of the node pattern. E.g. Sphere(5) Sphere(r) both 5 and r are declared query variables, yet they are used differently:
- Sphere(5) matches Sphere where the value of the parameter is exactly 5.
- Sphere(r) matches all Spheres and bound the parameter to the query variable
r, making it available to use.
Expression
Expression patterns provide a way to fix the value which has to be bound to the pattern. The expression can either:
- return a single Node
- be a Node generator
The most common expression pattern is the root operator ^, which return the root of the current data graph.
Unary
An unary predicate enables the use of boolean methods as patterns. The methods needs to:
- return a boolean
- have at least one argument, and its first argument is of type Node (or extend Node)
Wrapper
Wrapper nodes can be included in query. When used as predicate, the wrapper node can either be used to fetch the wrapped value, or the wrapper node.
Array
The XL query syntax uses the square brackets ([ and ]) as symbol for sub trees in the path pattern. Thus, there is a conflict between the syntax for element access in an array, and sub-tree predicate.