For a small custom language, start by teaching your editor to recognize its files, then add syntax highlighting and basic editing conveniences. Add snippets for reusable text patterns. If completion must understand variables, symbols, or project files, build or connect a language server; highlighting alone cannot provide that analysis.
This guide uses Visual Studio Code (VS Code) for a worked path, then explains the corresponding choices in Neovim. The exact extension and configuration formats depend on the editor and its version.
Choose the features your language actually needs
Syntax highlighting and code completion are related but separate features. In VS Code, a TextMate grammar identifies tokens and assigns scopes that themes can style. Extension contributions can also provide snippets and basic editing behavior. Completion based on symbols, project files, or language rules requires programmatic analysis, commonly provided through a language server.
Use the simplest approach that meets the need. Fixed keywords or reusable templates may be enough for a small language. If suggestions must reflect declarations, types, or project context, plan for a language server. A parser such as Tree-sitter is useful when structural syntax-tree queries are needed; it does not replace a language server for semantic completion.
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| Approach | What it provides | Best fit |
|---|---|---|
| TextMate grammar | Lexical token scopes that themes can style | Comments, strings, keywords, operators, and other recognizable text patterns |
| Snippets | Template-like text insertion | Common constructs that do not require analysis of the program |
| Tree-sitter parser and queries | Syntax-tree-based structural highlighting | Syntax where structural matching is useful, including cases that are difficult to handle with regular-expression rules alone |
| Language server | Analysis features such as context-aware completion; it may also provide diagnostics and navigation | Suggestions that depend on symbols, project context, or language semantics |
These approaches are not mutually exclusive. A language can use a TextMate grammar for highlighting and a language server for completion, for example. A parser and an LSP server also solve different problems: one supplies syntax structure; the other communicates analysis features to compatible editors.
Set up a custom language in VS Code
In VS Code, package language support as an extension. Keep the language identifier consistent: file recognition, grammar contribution, and any language client should refer to the same ID. The official syntax highlighting guide documents grammar contributions and TextMate scopes, while the language extensions overview distinguishes declarative features from programmatic ones.
1. Choose a language ID and recognize its files
Pick a unique language ID, such as mylang, and list the extensions or filenames your language uses. The extension’s file association and its grammar contribution must point to that same ID. If the file is not recognized, the grammar may never be applied.
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For example, a language contribution might associate .myl files with mylang, and the grammar contribution would name mylang as its language. Treat this as a structural example, not a complete installable extension manifest: the exact manifest fields and packaging should follow the VS Code documentation for the version you target.
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A TextMate grammar is a JSON file with a root scope and rules. Start with the constructs readers need to distinguish—typically comments, strings, numbers, keywords, operators, and punctuation. Group reusable rules in a repository and include them where appropriate. Prefer established scope names so existing themes can style tokens without requiring users to install a custom theme.
Grammar rules are a practical fit for lexical patterns, not a substitute for a parser or semantic analyzer. Pay particular attention to escaped quotes, nested delimiters, embedded syntax, and unfinished code: people often edit incomplete files, and rules that work on finished examples can behave poorly while typing.
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3. Add language configuration for editor behavior
Language configuration can describe line and block comments, bracket pairs, auto-closing and surrounding pairs, indentation, and folding. These conveniences do not analyze program meaning. Configure them to match the language, and avoid bracket matching or auto-closing inside strings or other scopes where those behaviors would be disruptive.
4. Add snippets only for repeatable templates
Snippets insert known text patterns, such as a standard declaration or block. They can appear alongside completion suggestions, but they do not know whether a variable exists or what symbols a project defines. Use them for templates; use a language server when suggestions need that context.
5. Inspect scopes and test edge cases
Open a sample file and run VS Code’s Developer: Inspect Editor Tokens and Scopes command on representative text. Check that VS Code selected the intended language, the grammar loaded, and tokens received sensible scopes.
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- Test comments, strings, escaped quotes, numbers, operators, and punctuation.
- Try nested delimiters and incomplete or malformed code, not just valid finished programs.
- Open a file with the wrong extension to confirm whether it is intentionally unrecognized or needs another association.
- Check whether the theme styles the scopes as expected; a recognized token can still look unstyled if its scope is unconventional.
6. Add a language server for semantic completion
A language server separates editor communication from language analysis. The VS Code extension has a client that starts or connects to a server; the server advertises supported capabilities and handles requests such as completion. Microsoft’s language server extension guide walks through a client/server implementation and also demonstrates diagnostics.
Begin with the smallest useful completion behavior, then add symbol resolution, documentation, diagnostics, navigation, or project-wide analysis only if the language’s semantics support it. The Language Server Protocol (LSP) is useful when the same analysis server should work with multiple compatible editors. Microsoft describes LSP as standardizing communication between a language server and a language client so an analysis program can be reused across editors in its language extensions overview.
7. Validate suggestions in a development host
Launch the extension in a VS Code Extension Development Host using the workflow in the official guide. Check that the language ID is active, the server starts without errors, and suggestions appear in the intended contexts. Test a file containing declared and undeclared names, if those distinctions are part of the language, and verify that selecting a suggestion inserts or resolves the expected text. A client starting successfully is not evidence that completion behavior is correct; inspect server output or logs when requests fail.
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Set up the equivalent features in Neovim
Traditional syntax highlighting
For lexical highlighting in Neovim, install a syntax file in a user runtime directory and ensure filetype detection associates the language’s files with the intended filetype. Defining highlighting rules without recognizing the files will not make Neovim select them automatically. Neovim’s syntax documentation describes syntax files and runtime behavior.
Tree-sitter highlighting
For structural highlighting, provide or install a Tree-sitter parser for the language and write query patterns in queries/<language>/highlights.scm. Queries match parser nodes and assign captures such as @keyword, @function, @type, and @string. Neovim searches query files through runtimepath; if the editor’s filetype name differs from the parser language name, register the mapping. See the Tree-sitter documentation and nvim-treesitter documentation for parser and integration details.
Completion through LSP
Tree-sitter queries provide highlighting from syntax structure, not project-aware completion. For semantic suggestions in Neovim, connect a compatible language client to a language server that implements the needed features. LSP is the shared client/server interface; the editor still needs an integration that can launch or connect to the server.
Decide when to move beyond a grammar
- Editor coverage: TextMate grammars are useful in editors that support the format. Tree-sitter and LSP require compatible editor integrations and suitable packaging.
- Language complexity: Regular-expression tokenization may be sufficient for straightforward lexical constructs. Nested or context-sensitive syntax is a reason to consider a real parser.
- Feature depth: Snippets and fixed keywords are lightweight. Symbol-aware completion, diagnostics, and navigation require analysis, often exposed through LSP.
- Maintenance: A grammar is a smaller first deliverable. A parser and server bring additional implementation and compatibility work.
- Editing conditions: Validate embedded syntax and incomplete files with the approach you choose; users need editor feedback while code is still being written.
A practical progression is file recognition, grammar, and basic language configuration; then snippets if they remove repetitive typing; then a parser or language server when specific structural or semantic features justify the extra work. That sequence is a design recommendation, not a requirement imposed by VS Code or LSP.
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