Editor integration

Harn provides first-class editor support through an LSP server, a DAP debugger, and a tree-sitter grammar. These cover most modern editors and IDE workflows.

VS Code

The editors/vscode/ directory contains a VS Code extension that bundles syntax highlighting (via tree-sitter) and automatic LSP/DAP client configuration.

Install from the extension directory:

cd editors/vscode && npm install && npm run build

Then use Extensions: Install from VSIX or symlink into ~/.vscode/extensions/.

The extension contributes:

• A Harn: Run Pipeline command (terminal harn run on the active file)
• A Harn: Format File command (terminal harn fmt)
• A Harn: Apply All Autofixes command — invokes the LSP's bulk source.fixAll.harn code action against the current document
• Language-scoped defaults that turn on editor.formatOnSave and editor.codeActionsOnSave: { "source.fixAll.harn": "always" } for [harn] files. Override either in user settings if you don't want autoformat-on-save or autofix-on-save.

Language server (LSP)

Start the LSP server with:

cargo run --bin harn-lsp

Or use the compiled binary directly (harn-lsp). The server communicates over stdin/stdout using the Language Server Protocol.

Supported capabilities

Feature	Description
Diagnostics	Real-time parse errors, type errors (including cross-module undefined-call errors), lint warnings, and @invariant(...) violations. Shares the same module graph used by harn check and harn run, so harn check --invariants and editor squiggles stay aligned.
Completions	Scope-aware: pipelines, functions, variables, parameters, enums, structs, interfaces. Dot-completions for methods plus inferred shape fields, struct members, and enum payload fields. Builtins and keywords.
Go-to-definition	Jump to the declaration of pipelines, functions, variables, enums, structs, and interfaces. Cross-file navigation walks the recursive module graph (relative paths and .harn/packages/), so symbols reachable through any number of transitive imports resolve.
Find references	Locate all usages of a symbol across the document
Call hierarchy	Navigate incoming and outgoing calls for pipelines, functions, tools, and methods in open documents
Hover	Shows type information and documentation for builtins
Signature help	Parameter hints while typing function arguments
Document symbols	Outline view of pipelines, functions, structs, enums
Workspace symbols	Cross-file search for pipelines and functions
Semantic tokens	Fine-grained syntax highlighting for keywords, types, functions, parameters, enums, and more
Code actions	Per-diagnostic quick fixes for lint warnings (var→let, boolean simplification, unused-import removal, string-interpolation conversion, unnecessary-cast removal, unnecessary-parentheses removal) and type errors. A bulk source.fixAll.harn action applies every available autofix in the document at once — wire it into editor.codeActionsOnSave to autofix on save.
Rename	Rename symbols across the document
Document formatting	Delegates to harn-fmt for format-on-save support
On-type formatting	Reuses harn-fmt after semicolons and closing braces for format-as-you-type
Folding ranges	Folds declaration bodies, multiline strings/comments, block expressions, and multiline match arms

Configuration

Most editors auto-detect the LSP binary. For manual configuration, point your editor's LSP client at the harn-lsp binary with no arguments. The server uses TextDocumentSyncKind::FULL and debounces full-document reparses so diagnostics stay responsive while you are typing.

The VS Code extension starts harn-lsp by default. Set harn.lspPath if the language server is installed somewhere else.

Debug adapter (DAP)

Start the debugger with:

cargo run --bin harn-dap

The DAP server communicates over stdin/stdout using the Debug Adapter Protocol. It supports:

• Breakpoints (line-based)
• Step in / step over / step out
• Variable inspection in scopes
• Stack frame navigation
• Continue / pause execution

VS Code launch configuration

The VS Code extension now contributes a harn debugger type and an initial Debug Current Harn File launch configuration. You can also add it manually:

{
  "type": "harn",
  "request": "launch",
  "name": "Debug Harn",
  "program": "${file}",
  "cwd": "${workspaceFolder}"
}

Set harn.dapPath if harn-dap is not on your PATH.

Tree-sitter grammar

The tree-sitter-harn/ directory contains a tree-sitter grammar for Harn. This powers syntax highlighting in editors that support tree-sitter (Neovim, Helix, Zed, etc.).

Build the grammar:

cd tree-sitter-harn && npx tree-sitter generate

Highlight queries are in tree-sitter-harn/queries/highlights.scm. Language injections, including PostgreSQL highlighting for sql(...) and named_sql(...) template strings from std/postgres/query, are in tree-sitter-harn/queries/injections.scm.

Formatter

Format Harn files from the command line or integrate with editor format-on-save:

harn fmt file.harn          # format in place
harn fmt --check file.harn  # check without modifying

Linter

Run the linter for static analysis:

harn lint file.harn
harn lint --fix file.harn   # automatically apply safe fixes

The linter checks for: shadow variables, unused variables, unused pattern bindings, unused types, undefined functions, dead code after terminating statements, pointless comparisons, redundant clones, missing harndoc comments, naming convention drift, branch-heavy functions, prompt-injection risks such as interpolated llm_call system prompts, and unnecessary conversion calls (to_string("hi"), to_int(42), etc.). With --fix, the linter automatically rewrites fixable issues (e.g., var → let, boolean comparison simplification, let-then-return simplification, redundant clone removal, unused import removal, unnecessary-cast removal, unnecessary parentheses removal). The same fixes are surfaced through the LSP as both per-diagnostic quick-fixes and a bulk source.fixAll.harn code action suitable for editor.codeActionsOnSave.