Eglot (LSP)

fsharp-ts-mode works with Eglot (Emacs's built-in LSP client) and FsAutoComplete (the F# language server) for a rich IDE-like experience: completions, diagnostics, type information, code actions, and more.

Basic Setup

For minimal setup, install FsAutoComplete manually and enable Eglot:

dotnet tool install -g fsautocomplete

(add-hook 'fsharp-ts-mode-hook #'eglot-ensure)

Tip

On macOS, GUI Emacs may not see ~/.dotnet/tools in its PATH. See Troubleshooting for the fix.

Enhanced Setup with fsharp-ts-eglot

For automatic server installation, custom LSP commands, and feature toggles, load fsharp-ts-eglot:

(require 'fsharp-ts-eglot)
(add-hook 'fsharp-ts-mode-hook #'eglot-ensure)

FsAutoComplete will be downloaded automatically on first use -- no manual installation needed.

Pinning a Server Version

By default, the latest version is fetched. To pin a specific version:

(setq fsharp-ts-eglot-server-version "0.76.0")

Custom Install Directory

(setq fsharp-ts-eglot-server-install-dir "/path/to/fsautocomplete")

Feature Toggles

Individual FsAutoComplete features can be toggled. These settings take effect when Eglot connects (restart the server with M-x eglot-reconnect to apply changes).

;; Linter (FSharpLint) -- default: t
(setq fsharp-ts-eglot-linter nil)

;; Unused opens analyzer -- default: t
(setq fsharp-ts-eglot-unused-opens-analyzer nil)

;; Unused declarations analyzer -- default: t
(setq fsharp-ts-eglot-unused-declarations-analyzer nil)

;; Simplify name analyzer -- default: nil
(setq fsharp-ts-eglot-simplify-name-analyzer t)

;; Unnecessary parentheses analyzer -- default: nil
(setq fsharp-ts-eglot-unnecessary-parens-analyzer t)

;; Enable third-party analyzers -- default: nil
(setq fsharp-ts-eglot-enable-analyzers t)

;; Code lenses (reference counts, signatures) -- default: t
(setq fsharp-ts-eglot-code-lenses nil)

;; Inlay hints (types, parameter names) -- default: t
(setq fsharp-ts-eglot-inlay-hints nil)

;; Pipeline type hints -- default: nil
(setq fsharp-ts-eglot-pipeline-hints t)

Custom LSP Commands

These commands use FsAutoComplete-specific LSP endpoints:

Command	Description
fsharp-ts-eglot-signature-at-point	Display type signature of symbol at point
fsharp-ts-eglot-f1-help	Open MSDN docs for symbol (falls back to .NET search)
fsharp-ts-eglot-generate-doc-comment	Generate XML doc comment stub for the definition at point

.fsproj Manipulation

File ordering matters in F# projects -- the compiler processes files in the order they appear in the .fsproj. These commands manipulate the current file's position:

Command	Description
fsharp-ts-eglot-fsproj-move-file-up	Move file up in compilation order
fsharp-ts-eglot-fsproj-move-file-down	Move file down in compilation order
fsharp-ts-eglot-fsproj-add-file	Add current file to the project
fsharp-ts-eglot-fsproj-remove-file	Remove current file from the project

Eldoc Integration

When fsharp-ts-eglot is loaded, the echo area (the line at the bottom of the Emacs frame) shows F#-specific type signatures for the symbol at point via the fsharp/signature endpoint, providing richer information than the standard LSP hover.

Pipeline Type Hints and Inlay Hints

FsAutoComplete can show intermediate types at each step of |> pipeline chains, as well as parameter names and type annotations as inlay hints. These use the standard LSP inlay hints protocol and are rendered by Eglot's built-in eglot-inlay-hints-mode:

;; Enable pipeline type hints (off by default)
(setq fsharp-ts-eglot-pipeline-hints t)

;; Enable inlay hints display
(add-hook 'fsharp-ts-mode-hook #'eglot-inlay-hints-mode)

This shows types inline as you write pipeline chains:

[1; 2; 3]                        // int list
|> List.map string               // string list
|> String.concat ", "            // string

Type Signature Overlays (LineLens)

fsharp-ts-lens.el is a separate library that shows inferred type signatures as overlays after function definitions, similar to Ionide's LineLens feature. It requires an active Eglot connection.

(require 'fsharp-ts-lens)
(add-hook 'fsharp-ts-mode-hook #'fsharp-ts-lens-mode)

Overlays are refreshed on save and can be updated manually with M-x fsharp-ts-lens-refresh.

Configuration

;; Change the overlay prefix (default: " // ")
(setq fsharp-ts-lens-prefix " // ")

;; Change the overlay face (default: font-lock-comment-face)
(setq fsharp-ts-lens-face 'shadow)

Documentation Info Panel

fsharp-ts-info.el is a separate library that provides a persistent side window showing rich type documentation for the symbol at point -- signature, documentation comment, constructors, interfaces, fields, functions, and attributes. It requires an active Eglot connection.

(require 'fsharp-ts-info)

Command	Description
fsharp-ts-info-show	Show documentation for symbol at point (opens side window)
fsharp-ts-info-mode	Toggle auto-update as you navigate code

Configuration

;; Change the auto-update delay (default: 0.5 seconds)
(setq fsharp-ts-info-idle-delay 0.5)

The panel updates automatically after fsharp-ts-info-idle-delay seconds of idle time when fsharp-ts-info-mode is active and the panel window is visible.