Parsoid/Todo

If you would like to hack the Parsoid parser, these are the tasks we currently see ahead. Some of them are marked as especially well suited for newbies. If you have questions, try to ping gwicke on #mediawiki or send a mail to the wikitext-l mailinglist. If all that fails, you can also contact Gabriel Wicke by mail.

Tokenizer
Earlier minor syntactical changes in Tim's preprocessor rewrite:
 * Tim's Preprocessor ABNF
 * User documentation for preprocessor rewrite

Low-hanging fruit
Simple and fun tasks for somebody wishing to dive into the tokenizer.
 * magic words (the __UNDERSCORED__ variant) : extraction mostly implemented by Ori and Adam, see for example https://gerrit.wikimedia.org/r/4050. Actual behavior still needs to be implemented.
 * ISBN, RFC, PMID - implemented by Ori
 * Language variants ('-{')

Clean-up

 * Move list handler from tokenizer to sync23 phase token transformer to support list items from templates. (Stabs being made by Adam: https://gerrit.wikimedia.org/r/3729) - still some interaction with quotes (test '5 quotes, code coverage +1 line' changed output after list patch) to investigate, but landed

Round-trip info

 * Add more round-trip information using the dataAttribs object property on tokens. This is serialized as JSON into a data-mw attribute on DOM nodes.
 * HTML vs wiki syntax
 * Try hard to preserve variable whitespace: Search for uses of the space production (or equivalent) in the grammar and capture the value into round-trip info.
 * Add source offset information to most blocks
 * Source offset/range annotation for elements to support selective re-serialization of modified DOM fragments to minimize dirty diffs

Extension tags
Make sure that (potential) extension end tags are always matched, even if parsing the content causes a switch to a plain-text parsing mode. An example would be an unclosed html comment ( [Main Page]]

These cannot generally be supported without stripping comments before parsing. Even if parsed, this type of comment could not be represented in the DOM. Before deployment, we should check if this is common enough to warrant an automated conversion. Grepping a dump works well for this check.

Mis-nested parser functions
The grammar-based tokenizer assumes some degree of sane nesting. Parser functions can return full tokens or substrings of an attribute, but not the first half of a token including half an attribute. Similar to the issues above, this limitation could be largely removed by dumbing down the tokenizer and deferring actual parsing until after template exansions at the cost of performance and PEG tokenizer grammar completeness. Mis-nested parser functions are hard to figure out for humans too, and should better be avoided / removed if possible.

Example: search for 'style="}}' in Template:Navbox. There are a total of 12 templates and no articles matching that string in the English Wikipedia. (used the following statement: )

expands to font-weight: bold">Some text in the PHP parser, but expands to {{#if:foo| Some text in Parsoid. This can be fixed by modifying the source to  . Hopefully this is rare enough to allow fixing this manually. Reliable detection of this case is needed to analyze how common this is.

Token stream transforms
Generally, try to run most transformations after template expansions to retain widest possible support for constructs that only match up after expansion. Try to pick up attributes for non-template tokens as late as possible.

Parser functions and magic words
Some implementation and lots of stubs (FIXME, quite straightforward!) in ext.core.ParserFunctions.js. Many magic words in particular depend on information from the wiki. Idea for now is to fall-back to action=parse api for extensions and other unsupported constructs. Basically build a page of unsupported elements in document order with each element prefixed/postfixed with unique (non-wikisyntax) delimiters. Then extract results between delimiters. See Parsoid/Interfacing with MW API and Wikitext_parser/Environment.

Sanitizer
Filter attributes, convert non-whitelisted tags into text tokens. See ext.core.Sanitizer.js for an outline, should be a relatively straightforward port from the PHP version. Good task if you'd like to dive into the JS parser.

Internal links, categories and images
The tokenizer is still independent of configuration data, so it does not pay attention to a wiki link's namespace. This means that image parameters are not parsed differently from normal link parameters, leaving specialized treatment to the LinkHandler token stream transformer. For images, arguments need to be separated from the caption. Full rendering requires information about the image dimensions, which needs to be retrieved from the wiki using either the generic fall-back described in Parsoid/Interfacing with MW API, or a specialized Image-specific API method. For action=parse, templates and -arguments in image options need to be fully expanded using the AttributeExpander before converting options back to wikitext. The (mostly)plain-text nature of options makes this quite easy fortunately. External link tokens produced by a link= option need to map back to the plain URL.

The LinkHandler transformer also needs to handle the Help:Pipe_trick while rendering.

HTML5 microdata
Track provenance on all tokens, preserve this information while building the tree and convert it into microdata / RDFa markup.

Miscellaneous

 * Handle table foster-parenting with round-tripping by reordering and marking tokens
 * Handle dynamically generated nowiki sections: . Template arguments are already tokenized and expanded before substitution, so we need to revert this. Idea: Re-serialize tokens to original text using source position annotations and other round-trip information. Icky, but doable. Try to structure HTML DOM to WikiText serializer around SAX-like start/end handlers, so that the same handlers can serialize the token stream back to wikitext.

DOM tree builder
Generally we would like to avoid any changes to the default HTML5 tree builder algorithm, as this would allow us to use the built-in HTML parser in modern browsers, or unchanged libraries. There are however some tasks seem to be hard to solve otherwise, and require only small changes to the tree builder. These tasks all have to do with unbalanced token soup, which should be confined to the server.

Self-closing tags like meta and text are never stripped, but might be subject to foster-parenting. The relative order between text and self-closing tags is stable, and attributes on self-closing tags are preserved. document.body.innerHTML = ' '; console.log( document.body.innerHTML); -> foo 
 * Spurious end-tags are ignored by the tree builder, while (some) are displayed as text in current MediaWiki. Text display is helpful for authors. The necessary change to the html tree builder to replicate this would be small, but is not possible if a browser's built-in parser is used. The visual editor hopefully reduces the need for this kind of debugging aid in the medium term.
 * Propagate attribute information for end tag tokens (especially source information for tokens originating from templates) to matching start tag, to make sure that the full scope of template-affected subtrees is captured. This is hard to do without a modification to the tree builder. Only needs to be performed on the server side. Relatively simple modification.
 * Idea for a possible solution:

Before tree building: number all start- and self-closing tokens in an attribute if first template (or extension,..) token is not a start- or self-closing token: insert meta tag with info and unique id (can be token number) else: add info and id as attrib to first token insert meta tag with ref to first's id after last token or text add number of preceding space chars to it for foster parenting handling

On DOM: traverse while looking for custom attribs on end meta tag with ref: find matching start and propagate info up the DOM subtree to level of     highest element with counter start use token counter attribute to figure out content between start and end of template / extension block

DOM postprocessing

 * Some document model enforcement on HTML DOM to aid editor, should be able to run either on server or client.
 * Longer term fun project: move DOM building and transformations to webworker to provide fast Lua-extension-like or DOM/Tal/Genshi template functionality and multi-core support. See some ideas.

Parser web service
Create a node web service that returns the parsed HTML DOM given a page title (rest-style /parse/Title or similar). Multi-process workers using the cluster module would be nice to have, but not mandatory. For now, this could perform pretty much the same actions as in parse.js, and simply pass in. Fetching the source would be even better (see TemplateRequest), as this would handle noinclude in the page correctly. It might be worth moving that class to a separate API module.

Add saving support to the web service
Accept a posted HTML DOM, the page title and base revision from the editor, serialize it back to wikitext and save it through the API.

There is a good opportunity to collaborate with Ashish Dubey on the server portion (he is working on collaborative editing as GSoC project).

Alternative web service variant we do not use for now
Shell-out to parse.js.
 * Neil created an API wrapper for parse.js. Would need to be modified to use the HTML DOM serialization instead of JSON. IIRC there were some issues with WikiDom vs. the data model the API expects.
 * Reconstruction of the tokenizer takes a few seconds for each request, would need caching
 * parse.js and node would need to be installed on the full cluster. It is more practical at least for development to run the parse service on a labs VM that we can quickly update.

Wikitext serializer
Basic idea: ( HTML DOM -> ) tokens -> SAX-style serializer handlers -> wikitext
 * needs to use round-trip data
 * Can be based on serializer code in the editor. Mainly need to add handling of round-trip data and convert handlers to support per-token (or SAX event style) serialization. Per-token handlers allow us to use the same handlers for DOM or token stream serialization.
 * Will introduce some normalization- at the very least the tree builder has to fix up stuff when building a tree from tag soup, so the full round-trip cannot be 100% perfect for broken inputs.

Serializing only modified parts of a page

 * The round-trip info on elements contains source offsets in original wikitext (very incomplete currently)
 * The editor marks modified parts of the DOM
 * The serializer splices original source of unmodified DOM parts with serialization of modified subtrees. This avoids dirty diffs from normalization in unmodified parts of the page.

Testing
See tests/parser, in particular parserTests.js.

parserTests

 * Set up a more complete testing environment including the time, predefined images and so on (see phase3/tests/parser/parserTests.inc).

Round-trip tests
There is a dumpReader in tests/parser, which can be used to run full dumps through the parser. For round-tripping the WikiText serializers needs to be ported and extended from the client code. See.