Recently, we have started looking into a task of creating an interactive parser. A generic one.

Yes, we know there are plenty of them all around, however the goals we have defined made us to construct the new implementation.

The goals:

• Parser must be incremental.
  You should direct what to parse, and when to stop.
  This virtually demands rather "pull" than conventional "push" implementation.
• Parser must be able to synchronize a tree with text.
  Whenever the underlying text is changed, a limited part of a tree should to be updated.
• Parser should be able to recover from errors, and continue parsing.
• Parser should be manageable.
  This is a goal of every program, really.
• Parser must be fast.
• A low memory footprint is desired.

What's implemented (VS2008, C#) and put at SourceForge, is called an Incremental Parser.

These are parser's insights:

• Bookmarks are objects to track text points. We use a binary tree (see Bare binary tree algorithms) to adjust positions of bookmarks when text is changed.
• Ranges define parsed tree elements. Each range is defined by two bookmarks, and a grammar annotation.
• There are grammar primitives, which can be composed into a grammar graph.
• A grammar graph along with ranges form a state machine.
• Grammar chains are cached, turning parsing into a series of probes of literal tokens and transitions between grammar chains. This caching is done on demand, which results in warming-up effect.
• Parser itself includes a random access tokenizer, and a queue of ranges pending to be parsed.
• Parsing is conducted as a cycle of pulling and parsing of pending ranges.
• Whenever text is changed a closest range is queued for the reparsing.
• A balance between amount of parsing and memory consumption can be achieved through a detalization of grammar annotation for a range. An active text fragment can be fully annotated, while for other text parts a coarse range can be stored.

We have defined xpath like grammar to test our ideas. See printed parsed trees to get understanding of what information can be seen from ranges.