Elm Assist LSP

A fast Elm development extension for VS Code, powered by a native Rust language server.

Real-time diagnostics, auto-fix code actions, hover documentation for 54 built-in lint rules, and document formatting matching elm-format output. No Node.js runtime, no Elm compiler dependency, no plugin system to configure.

Features

Real-time diagnostics

Lint errors appear as you type with 150ms debouncing. Parse errors are shown inline when syntax is invalid, while lint rules continue to run on the valid portions of your file.

Quick-fix code actions

25 rules offer auto-fixes via the lightbulb menu or Ctrl+. / Cmd+.. Fixes include removing unused imports, simplifying boolean expressions, collapsing redundant patterns, and more.

Hover documentation

Hover over any diagnostic to see the rule name, description, and whether an auto-fix is available.

Document formatting

Format Elm files with the same output as elm-format, using the native Rust implementation from elm-fmt. Works with Shift+Option+F (Format Document), Format on Save, and any other VS Code formatting entry point.

If you have another Elm extension installed that also provides formatting, set this as the default in your settings:

"[elm]": {
  "editor.defaultFormatter": "joshburgess.elm-assist-lsp"
}

File watching

External changes (git checkout, build tools) are detected automatically. Config changes to elm-assist.toml trigger a live reload without restarting the server.

Project-aware analysis

The server scans your entire project at startup, enabling cross-file rules like unused exports, unused modules, and unused custom type constructors.

Rules

54 rules covering simplification, unused code, code style, debugging, complexity, and port safety. 25 of these offer auto-fixes.

Full rule list

Simplification NoIfTrueFalse (fix), NoBooleanCase (fix), NoAlwaysIdentity (fix), NoRedundantCons (fix), NoUnnecessaryParens (fix), NoNegationOfBooleanOperator (fix), NoFullyAppliedPrefixOperator (fix), NoIdentityFunction (fix), NoListLiteralConcat (fix), NoEmptyListConcat (fix), NoStringConcat (fix), NoBoolOperatorSimplify (fix), NoMaybeMapWithNothing (fix), NoResultMapWithErr (fix), NoPipelineSimplify (fix), NoNestedNegation (fix)

Unused code NoUnusedImports (fix), NoUnusedVariables (fix), NoUnusedExports, NoUnusedCustomTypeConstructors, NoUnusedCustomTypeConstructorArgs, NoUnusedModules, NoUnusedParameters (fix), NoUnusedLetBinding (fix), NoUnusedPatterns

Code style NoMissingTypeAnnotation, NoSinglePatternCase (fix), NoExposingAll (fix), NoImportExposingAll (fix), NoDeprecated, NoMissingDocumentation, NoUnnecessaryTrailingUnderscore, NoPrematureLetComputation, NoSimpleLetBody (fix), NoUnnecessaryPortModule (fix), NoMissingTypeAnnotationInLetIn, NoMissingTypeExpose, NoRedundantlyQualifiedType (fix), NoRecordPatternInFunctionArgs

Debugging NoDebug (fix)

Complexity CognitiveComplexity, NoUnoptimizedRecursion, NoRecursiveUpdate, NoConfusingPrefixOperator, NoShadowing

Port safety NoDuplicatePorts, NoUnsafePorts, NoInconsistentAliases, NoUnusedDependencies

Other NoEmptyLet (fix), NoEmptyRecordUpdate (fix), NoWildcardPatternLast, NoMaxLineLength, NoTodoComment

Requirements

The elm-assist-lsp binary must be available on your system. Install it via:

npm install elm-assist

Or download a pre-built binary from GitHub Releases.

The extension searches for the binary in this order:

1. The elm-assist.serverPath setting (if set)
2. ./node_modules/.bin/elm-assist-lsp (project-local npm install)
3. elm-assist-lsp on PATH

Configuration

Extension settings

Project configuration

Create an elm-assist.toml in your project root to customize rules:

[rules]
disable = ["NoMissingTypeAnnotation"]

[rules.severity]
NoDebug = "error"
NoUnusedImports = "warning"

[rules.CognitiveComplexity]
threshold = 20

[rules.NoInconsistentAliases]
aliases = { "Json.Decode" = "Decode", "Json.Encode" = "Encode" }

Changes to elm-assist.toml are picked up automatically, no restart needed.

CLI tools

The elm-assist package also includes CLI tools that complement the editor experience:

• elm-lint: run lint checks from the command line with --fix, --watch, and --json output
• elm-unused: project-wide dead code detection
• elm-deps: module dependency graph analysis (DOT, Mermaid, cycle detection)
• elm-refactor: automated refactoring (rename, sort imports, qualify imports)
• elm-search: semantic AST-aware code search
• elm-fmt: fast formatter matching elm-format output, with --check, --write, and recursive directory support

See the elm-assist repository for full documentation.

License

MIT