Markup language language servers

✍️ Written on 2021-02-21 in 1303 words. Part of cs software-development


Microsoft pushed the idea of a language server protocols (Wikipedia). Traditionally, every IDE/editor implemented features like auto complete, go-to definitions, workspace symbol search, documentation on hover, etc. The language server protocol defines a uniform interface. Network servers can be implemented to provide the functionality for a particular language. Every IDE/editor can then use the network server provided without reinventing the wheel. I familiarized myself with language servers because of rust’s first tier support. And I like the idea of language servers these days; even though it has some reasonable deficiacy: syntax highlighting is not part of the protocol (summarized: It’s difficult; or long version).

Language servers target programming languages and a list of languages implemented is maintained by Microsoft.

But what about markup languages? They certainly have some different characteristics and might require different features. Also, the word “markup language” might be vague. So, let us dive into this topic and discuss the status quo!

Markup language (ML)

Without doubt, I don’t have a proper definition for ML here. Sometimes, the notions of lightweight MLs, generalized MLs and document MLs are used. Maybe the document ML comes closest, but I don’t see a need to establish a definition here. Simply, I would love to see the following languages to be supported by some languages servers:

{AsciiDoc, Wolfram CDF, Creole, GML, HTML, Lout, Markdown/MultiMarkdown/CommonMark, MediaWiki, org-mode, Plain Old Documentation, PmWiki, reStructuredText, Rich Text Format, SCRIPT, Setext, Textile, Texy!, Conteχt/LaTeχ/Teχ, troff man/mdoc/me/mm/ms, txt2tags, UDO, XML}

JSON, YAML, and XML are (to some extent) data serialization formats and are not considered here. Here OpenDocument and Office Open XML are excluded, because they are instances of XML files. Teχ is included as LaTeχ and ConTeχ, because LaTeχ’s macros allow it to use the syntax as some kind of generalized ML.

Language server features

The features of languages servers can be found by looking into the protocol specification (for a less technical documentation see the Language Server Extension Guide). I consider version 3.16.0 here:

  1. auto completion items at a given cursor position

  2. hover information at a given text document position

  3. signature information at a given cursor position

  4. declaration/definition/type definition location of a symbol at a given text document position

  5. implementation location of a symbol at a given text document position

  6. project-wide references for the symbol denoted by the given text document position

  7. document highlights for a given text document position. For programming languages this usually highlights all references to the symbol scoped to this file

  8. (flat or hierarchic) list of all symbols found in a given text document

  9. actions like code fixes, to fix problems or to beautify/refactor code

  10. code lenses which is a Visual Studio feature to provide buttons like “Impact” (where is this function referenced), “test” (run unit test for this function) and “latest” (revision information for this block of lines)

  11. location of links in a document

  12. list all color references found in a given text document

  13. list of presentations for a color value at a given location

  14. auto-format a whole document

  15. workspace-wide rename of a symbol

  16. folding ranges found in a given text document

  17. suggested selection ranges at an array of given positions

  18. call hierarchy for the language element of given text document positions

  19. resolve semantic tokens (e.g. ‘event’, ‘class’, or ‘type’) for a given file. Semantic tokens are used to add additional color information to a file that depends on language specific symbol information

  20. return the range of the symbol at the given position of a given document and all ranges that have the same content

  21. Language Server Index Format (LSIF) introduced the concept of symbol monikers to help associate symbols across different indexes. Thus for a given text document position, one can provide the same symbol moniker information

Applicability to MLs

Consider e.g. XML:

  1. auto-completion: return “log>” given “</” due to the opening elements

  2. hover information is useful for XML namespaces and XML entities

  3. definition locations are useful for XML entities

  4. document highlights are useful element occurences with the same tag name

  5. the list of all symbols can refer to identical names

  6. auto-format allows quick adjustment of indentation size neglecting CDATA sections

  7. workspace-wide rename of a symbol should work for XML namespaces, tagnames, and attributes

  8. code lenses could copy the XPath of an element into the clipboard

  9. folding follows immediate by XML hierarchy where each element should be foldable

  10. call hierarchy could correspond to the element hierarchy, but I am not sure this violates the original semantics of “call hierarchy”

  11. the current list of semantic tokens is inapplicable to XML

Unlike XML, SGML/HTML should not allow whitespace auto-format, because HTML is often used in combination with CSS where CSS defines which elements are white-space [in]sensitive. The same applies to all variants of Markdown as any HTML is allowed in Markdown documents.

But in general, XML is peculiarly easy. It becomes more difficult to wrap your head around MLs like AsciiDoc. Let’s go:

  1. auto completion would make sense for inter-document link targets or filepaths like images

  2. hover information could help you to distinguish the different kinds of delimited blocks (e.g. ==== shows ‘example’) but also replacements

  3. document highlights are only useful for semantically equivalent text

  4. auto-format would establish a certain default style to write AsciiDoc documents. But this is very limited. I can think of multiple whitespaces merged to a single one.

  5. code lenses could be used to generate reference IDs for the element to be clicked on.

  6. folding would totally make sense for the section hierarchy or any kind of blocks

At the same time, linguistic checks would be very useful to be provided through the server.


The implementation list above lists the following generically-interesting implementations:

The implementation list also lists some languages of my ML list above:

See also, And using a search engine, I found the following implementation:


I can see some potential for language servers in the field of markup languages, but you barely find any implementations so far.