OGDL Path

Revision 2005.3, 8 March 2005
Copyright (c) 2003-2005 R. Veen
This is an open standard. See the license.

1. Objectives

This specification defines a path expression language that enables navigation thru an OGDL graph. The syntax is suitable for embedding into scripting languages.

A path expression applied to a graph allways returns a graph or null, except when types are used, in which case a scalar value of some type may be returned. The resulting graph can be a subgraph of the original graph, or it can be one produced during the evaluation of the path. For example, the [] operator creates a new temporary graph that holds intermediate results.

2. Introduction to the syntax

The basic structure of a path is a dot separated list of elements. Each element is an OGDL fragment written in expression form. An element can be as simple as a string, or as complex as a graph contained in parenthesis. Here are some examples:

(a b).c[1].d(e f,g)
a.b.c
a(c)

The syntax resembles that of path expressions used in object oriented languages such as Java, so that an OGDL path can be mapped to such an expression. But the syntax has a broader range, as elements composed only by argument lists are possible.

In the following graph:

• a.b selects the first b subnode of a.
• a.b.2 selects the subnode '2' of the first 'b' subnode of a. Note that here '2' is a string, not an index.
• a.b{1} selects the second occurrence of b below a.
• a.b{} selects all occurrences of b below a.
• a[1] selects the second subnode of a. This is the list or array view of a.
• [**].b returns all b elements in the graph
• [*].b returns all b elements one level below the root.

3. Definition

A path is a concatenation of OGDL elements written in expression format and separated by dots. A path has an equivalent representation in OGDL. In fact, each element can be interpreted as a complete OGDL graph.

Note: Strickly speaking, this definition of a path isn't a path anymore, in the sense as it is defined in graph theory. This spec asummes the incongruency. In graph theory, a path is a tree with 2 nodes of vertex degree 1 and n-2 nodes with vertex degree 2 (Eric W. Weisstein, MathWorld, a Wolfram web resource: http://mathworld.wolfram.com)

4. Grammar

[1] path ::= element ( '.' element )*

[2] element ::= (string (index|selector)?) | arglist | index | selector 

[3] index ::= '[' ( number | '*' | '**')? ']'

where [n] selects the nth subnode,
[**] means any path,
[*] means any node

[4] selector ::= '{' number? '}'

[5] arglist ::= '(' (path ( ',' path )* )?  ')'

5. Examples

path : libs

libs
   foo.so
     version 1.2
     date 2002-05-05
   bar.so
     version 3.4

path : libs.'foo.so'

libs
   foo.so
     version 1.2
     date 2002-05-05
   bar.so
     version 3.4

path : libs.'foo.so'.version

libs
   foo.so
     version 1.2
     date 2002-05-05
   bar.so
     version 3.4

path : libs.[*].version

libs
   foo.so
     version 1.2
     date 2002-05-05
   bar.so
     version 3.4

path : book.author{}.name{}

libs
  book
    title 'Introduction to XML'
    author 
      name 'Peter Perez'
    author
      name 'Monica Windmind'    

A. Changes to this document

20030614  A path is a concatenation of OGDL expressions.
20031117  Renamed to Version 1.0, without modifications.
          Later brought back to working draft.
20030902  Initial release