A tree-sitter-based Emacs major mode for F# development.

Requires Emacs 29.1+ with tree-sitter support.

Online documentation

The package is available on MELPA and MELPA Stable.

(use-package fsharp-ts-mode
  :ensure t)

Then install the required tree-sitter grammars:

M-x fsharp-ts-mode-install-grammars

This installs both the fsharp grammar (for .fs and .fsx files) and the fsharp-signature grammar (for .fsi files) from ionide/tree-sitter-fsharp.

See the installation guide for alternative installation methods and prerequisites.

• Syntax highlighting (font-lock) via tree-sitter, organized into 4 levels
• Indentation via tree-sitter
• Imenu support with fully-qualified names
• Navigation (beginning-of-defun, end-of-defun, forward-sexp)
• F# Interactive (REPL) with tree-sitter highlighting for input
• Eglot integration for FsAutoComplete with auto-install, feature toggles, and custom commands
• Type signature overlays (LineLens)
• Documentation info panel
• Pipeline type hints and inlay hints
• dotnet CLI integration (build, test, run, clean, format, restore, watch mode)
• .NET API documentation lookup at point
• Compilation error parsing for dotnet build output
• Prettify symbols (-> to →, fun to λ, etc.)
• Switch between .fs and .fsi files with C-c C-a
• Shift region left/right for quick re-indentation
• Auto-detect indentation offset from file contents
• Build directory awareness (prompts to switch from bin//obj/ to source)
• Outline mode integration (Emacs 30+)
• Project name in mode-line
• Bug report helpers

(use-package fsharp-ts-mode
  :ensure t
  :hook ((fsharp-ts-mode . fsharp-ts-repl-minor-mode)
         (fsharp-ts-mode . fsharp-ts-dotnet-mode)
         (fsharp-ts-mode . eglot-ensure)
         (fsharp-ts-mode . prettify-symbols-mode))
  :config
  (require 'fsharp-ts-eglot)
  (require 'fsharp-ts-lens)
  (require 'fsharp-ts-info)
  (add-hook 'fsharp-ts-mode-hook #'fsharp-ts-lens-mode)
  (add-hook 'fsharp-ts-mode-hook #'fsharp-ts-info-mode)
  (setq fsharp-ts-guess-indent-offset t))

See the online documentation for detailed configuration, usage guides, and more.

This package was inspired by neocaml, my tree-sitter-based OCaml mode. After spending time in the OCaml community I got curious about its .NET cousin and wanted a modern Emacs editing experience for F# as well. I strongly considered naming this package "Fa Dièse" (French for F sharp -- because naming things after spending time with OCaml does that to you), but ultimately chickened out and went with the boring-but-obvious fsharp-ts-mode. Naming is hard!

Copyright (C) 2026 Bozhidar Batsov

Distributed under the GNU General Public License, version 3.