Configuration¶

This page covers core fsharp-ts-mode settings. For LSP-related configuration, see Eglot (LSP). For REPL settings, see REPL Integration. For dotnet CLI settings, see dotnet CLI.

Indentation¶

The default indentation offset is 4 spaces:

;; Change indentation offset (default: 4)
(setq fsharp-ts-indent-offset 2)

To auto-detect the offset from file contents on open:

(setq fsharp-ts-guess-indent-offset t)

You can also run M-x fsharp-ts-mode-guess-indent-offset manually at any time.

Font-Lock (Syntax Highlighting)¶

Syntax highlighting is organized into 4 levels, controlled by treesit-font-lock-level (default: 3):

Level	Features
1	Comments, definitions (function/value/type/member names)
2	Keywords, strings, type annotations, DU constructors
3	Attributes, builtins, constants (`true`/`false`), numbers, escape sequences
4	Operators, brackets, delimiters, all variables, properties, function calls

;; Maximum highlighting (includes operators, all variables, function calls)
(setq treesit-font-lock-level 4)

To switch the level for the current buffer without changing the global default, use M-x fsharp-ts-mode-set-font-lock-level (also on the F# > Font-Lock Level menu).

Toggling Individual Features¶

You can enable or disable individual font-lock features without changing the level:

;; Enable function call highlighting (level 4) while keeping level 3 default
(add-hook 'fsharp-ts-mode-hook
          (lambda () (treesit-font-lock-recompute-features '(function) nil)))

;; Disable operator highlighting
(add-hook 'fsharp-ts-mode-hook
          (lambda () (treesit-font-lock-recompute-features nil '(operator))))

Available feature names for .fs/.fsx files: comment, definition, keyword, string, type, attribute, builtin, constant, escape-sequence, number, operator, bracket, delimiter, variable, property, function.

Note

Signature files (.fsi) use a separate tree-sitter grammar with a reduced set of font-lock rules. Only comment, definition, keyword, string, type, bracket, delimiter, and variable are available for .fsi buffers.

Applying to Both Modes¶

Face customizations via hooks need to target both modes if you want them to apply to implementation and signature files:

(dolist (hook '(fsharp-ts-mode-hook fsharp-ts-signature-mode-hook))
  (add-hook hook #'my-fsharp-faces))

Face Customization¶

Tree-sitter modes use the standard font-lock-*-face faces (the named styles Emacs applies to highlighted code). You can customize them globally or locally:

;; Globally change how function names look
(set-face-attribute 'font-lock-function-name-face nil :weight 'bold)

;; Override faces only in fsharp-ts-mode buffers
(defun my-fsharp-faces ()
  (face-remap-add-relative 'font-lock-keyword-face :foreground "#ff6600")
  (face-remap-add-relative 'font-lock-type-face :foreground "#2aa198"))

(add-hook 'fsharp-ts-mode-hook #'my-fsharp-faces)

Prettify Symbols¶

Enable prettify-symbols-mode to display common operators as Unicode equivalents (the underlying text is unchanged -- only the display is affected):

(add-hook 'fsharp-ts-mode-hook #'prettify-symbols-mode)

Default replacements include:

Symbol	Displayed as
`->`	`→`
`<-`	`←`
`>=`	`≥`
`<=`	`≤`
`<>`	`≠`
`fun`	`λ`

Project Name in Mode-Line¶

When the buffer belongs to a .fsproj project, the mode-line (the status bar at the bottom of each window) shows F#[ProjectName]. Disable with:

(setq fsharp-ts-show-project-name nil)

Comment Handling¶

fsharp-ts-mode auto-continues /// doc comments and // line comments when you press RET at the end of a comment line. This makes writing multi-line doc comments more convenient.

URLs and bug references in comments are clickable (via goto-address-prog-mode and bug-reference-prog-mode). To resolve bare references like #123, set bug-reference-url-format, e.g. in a .dir-locals.el:

((fsharp-ts-mode
  . ((bug-reference-url-format
      . "https://github.com/your-org/your-repo/issues/%s"))))

Formatting¶

C-c C-f (fsharp-ts-format-buffer) formats the buffer with Fantomas. Point fsharp-ts-fantomas-program at your install if it isn't on PATH, and set fsharp-ts-format-on-save to format automatically before saving:

(setq fsharp-ts-fantomas-program "fantomas")
(setq fsharp-ts-format-on-save t)

Build Directory Awareness¶

When you open a file under a bin/ or obj/ directory, fsharp-ts-mode prompts you to switch to the corresponding source file instead. This helps avoid accidentally editing build artifacts.