Features

This table gives a summary of the features that HLS supports. Many of these are standard LSP features, but a lot of special features are provided as code actions and code lenses.

Feature	LSP method
Diagnostics	`textDocument/publishDiagnostics`
Hovers	`textDocument/hover`
Jump to definition	`textDocument/definition`
Jump to type definition	`textDocument/typeDefinition`
Find references	`textDocument/references`
Completions	`textDocument/completion`
Formatting	`textDocument/formatting`, `textDocument/rangeFormatting`
Document symbols	`textDocument/documentSymbol`
Workspace symbols	`workspace/symbol`
Call hierarchy	`textDocument/prepareCallHierarchy`, `callHierarchy/incomingCalls`, `callHierarchy/outgoingCalls`
Highlight references	`textDocument/documentHighlight`
Code actions	`textDocument/codeAction`
Code lenses	`textDocument/codeLens`
Selection range	`textDocument/selectionRange`
Rename	`textDocument/rename`
Semantic tokens	`textDocument/semanticTokens/full`

The individual sections below also identify which HLS plugin is responsible for providing the given functionality, which is useful if you want to raise an issue report or contribute! Additionally, not all plugins are supported on all versions of GHC, see the plugin support page for details.

Diagnostics

GHC compiler errors and warnings

Provided by: ghcide

Provides errors and warnings from GHC as diagnostics.

Hlint hints

Provided by: hls-hlint-plugin

Provides hlint hints as diagnostics.

Stan hints

Provided by: hls-stan-plugin

Provides Stan hints as diagnostics.

Cabal parse errors and warnings

Provided by: hls-cabal-plugin

Provides errors and warnings from Cabal as diagnostics

Hovers

Provided by: ghcide

Type information and documentation on hover, including from local definitions.

Show fixity

Provided by: hls-explicit-fixity-plugin

Provides fixity information.

Jump to definition

Provided by: ghcide

Jump to the definition of a name.

Known limitations:

Only works for local definitions.

Jump to type definition

Provided by: ghcide

Known limitations:

Only works for local definitions.

Jump to note definition

Provided by: hls-notes-plugin

Jump to the definition of a GHC-style note.

Find references

Provided by: ghcide

Find references to a name within the project.

Completions

Code completions

Provided by: ghcide

Completion of names from qualified imports.
Completion of names from non-imported modules.

Pragma completions

Provided by: hls-pragmas-plugin

Completions for language pragmas.

Formatting

Format your code with various Haskell code formatters.

Formatter	Provided by
Floskell	`hls-floskell-plugin`
Fourmolu	`hls-fourmolu-plugin`
Ormolu	`hls-ormolu-plugin`
Stylish Haskell	`hls-stylish-haskell-plugin`

Format your cabal files with a cabal code formatter.

Formatter	Provided by
cabal-fmt	`hls-cabal-fmt-plugin`

Document symbols

Provided by: ghcide

Provides listing of the symbols defined in a module, used to power outline displays.

Workspace symbols

Provided by: ghcide

Provides listing of the symbols defined in the project, used to power searches.

Call hierarchy

Provided by: hls-call-hierarchy-plugin

Shows ingoing and outgoing calls for a function.

Call Hierarchy in VSCode

Highlight references

Provided by: ghcide

Highlights references to a name in a document.

Code actions

Insert missing pragmas

Provided by: hls-pragma-plugin

Code action kind: quickfix

Inserts missing pragmas needed by GHC.

Apply Hlint fixes

Provided by: hls-hlint-plugin

Code action kind: quickfix

Applies hints, either individually or for the whole file. Uses apply-refact.

Hlint Demo

Known limitations:

May have strange behaviour in files with CPP, since apply-refact does not support CPP.
The hlint executable by default turns on many extensions when parsing a file because it is not certain about the exact extensions that apply to the file (they may come from project files). This differs from HLS which uses only the extensions the file needs to parse the file. Hence it is possible for the hlint executable to report a parse error on a file, but the hlint plugin to work just fine on the same file. This does mean that the turning on/off of extensions in the hlint config may be ignored by the hlint plugin.
Hlint restrictions do not work (yet). This PR should enable that functionality, but this requires a newer version of hlint to be used in HLS.

Make import lists fully explicit

Provided by: hls-explicit-imports-plugin

Code action kind: quickfix.literals.style

Make import lists fully explicit (same as the code lens).

Refine import

Provided by: hls-explicit-imports-plugin

Code action kind: quickfix.import.refine

Refines imports to more specific modules when names are re-exported (same as the code lens).

Qualify imported names

Provided by: hls-qualify-imported-names-plugin

Code action kind: quickfix

Rewrites imported names to be qualified.

Qualify Imported Names Demo

For usage see the readme.

Add missing class methods

Provided by: hls-class-plugin

Code action kind: quickfix

Adds placeholders for missing class methods in a class instance definition.

Unfold definition

Provided by: hls-retrie-plugin

Code action kind: refactor.extract

Extracts a definition from the code.

Fold definition

Provided by: hls-retrie-plugin

Code action kind: refactor.inline

Inlines a definition from the code.

Retrie Demo

Insert contents of Template Haskell splice

Provided by: hls-splice-plugin

Code action kind: refactor.rewrite

Evaluates a Template Haskell splice and inserts the resulting code in its place.

Convert numbers to alternative formats

Provided by: hls-alternate-number-format-plugin

Code action kind: quickfix.literals.style

Converts numeric literals to different formats.

Alternate Number Format Demo

Change Type Signature

Provided by: hls-change-type-signature-plugin

Code action kind: quickfix

Change/Update a type signature to match implementation.

Status: Until GHC 9.4, the implementation is ad-hoc and relies on GHC error messages to create a new signature. Not all GHC error messages are supported.

Known Limitations:

Not all GHC error messages are supported
Top-level and Function-local bindings with the same names can cause issues, such as incorrect signature changes or no code actions available.

Change Type Signature Demo

Link to Docs

Add argument to function

Provided by: hls-refactor-plugin

Code action kind: quickfix

Add an undefined variable as an argument to the top-level binding.

Convert to GADT syntax

Provided by: hls-gadt-plugin

Code action kind: refactor.rewrite

Convert a datatype to GADT syntax.

GADT Demo

Link to Docs

Expand record wildcard

Provided by: hls-explicit-record-fields-plugin

Code action kind: refactor.rewrite

Expand record wildcards, explicitly listing all record fields as field puns.

Unknown SPDX License suggestion

Provided by: hls-cabal-plugin

Code action kind: quickfix

Correct common misspelling of SPDX Licenses such as BSD-3-Clause.

Code lenses

Add type signature

Provided by: ghcide

Shows the type signature for bindings without type signatures, and adds it with a click.

Evaluation code snippets in comments

Provided by: hls-eval-plugin

Evaluates code blocks in comments with a click. Tutorial.

Eval Demo

Known limitations:

Standard input is shared with HLS, so e.g. getLine breaks the connection to server.
Standard (error) output is not captured.
While similar to doctest, some of its features are unsupported, see Differences with doctest.

Make import lists fully explicit code lens

Provided by: hls-explicit-imports-plugin

Shows fully explicit import lists and rewrites them with a click (same as the code action).

Imports code lens Demo

Refine import code lens

Provided by: hls-explicit-imports-plugin

Shows refined imports and applies them with a click (same as the code action).

Fix module names

Provided by: hls-module-name-plugin

Shows module name matching file path, and applies it with a click.

Module Name Demo

Selection range

Provided by: hls-code-range-plugin

Provides haskell specific shrink/expand selection support.

Selection range demo

Folding range

Provided by: hls-code-range-plugin

Provides haskell specific Folding support.

Folding range demo

Rename

Provided by: hls-rename-plugin

Provides renaming of symbols within a module. Experimental cross-module renaming can be enabled via the configuration.

Rename Demo

Known limitations:

Cross-module renaming requires all components to be indexed, which sometimes causes partial renames in multi-component projects.

Semantic tokens

Provided by: hls-semantic-tokens-plugin

Provides semantic tokens for each token in the source code to support semantic highlighting.

Rewrite to overloaded record syntax

Provided by: hls-overloaded-record-dot-plugin

Code action kind: refactor.rewrite

Rewrites record selectors to use overloaded dot syntax

Explicit Wildcard Demo

Missing features

The following features are supported by the LSP specification but not implemented in HLS. Contributions welcome!

Feature	Status	LSP method
Signature help	Unimplemented	`textDocument/signatureHelp`
Jump to declaration	Unclear if useful	`textDocument/declaration`
Jump to implementation	Unclear if useful	`textDocument/implementation`
Linked editing	Unimplemented	`textDocument/linkedEditingRange`
Document links	Unimplemented	`textDocument/documentLink`
Document color	Unclear if useful	`textDocument/documentColor`
Color presentation	Unclear if useful	`textDocument/colorPresentation`
Monikers	Unclear if useful	`textDocument/moniker`