fsharp-ts-mode¶

A tree-sitter-based Emacs major mode for F#, a functional-first programming language running on .NET.

Requires Emacs 29.1+ with tree-sitter support.

Features¶

Syntax highlighting via tree-sitter, organized into 4 configurable levels
Indentation via tree-sitter with support for all major F# constructs
Imenu (symbol index) with fully-qualified names (e.g., Module.func)
Navigation (beginning-of-defun, end-of-defun, forward-sexp)
F# Interactive (REPL) with tree-sitter input highlighting and persistent history
Eglot/LSP integration with automatic FsAutoComplete installation, custom commands, and feature toggles
dotnet CLI integration -- build, test, run, clean, format, restore, watch mode
Type signature overlays (LineLens) -- inline inferred types after definitions
Documentation info panel -- persistent side window with rich type documentation
Pipeline type hints and inlay hints via LSP
.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 (F#[ProjectName])
Bug report helpers with environment info collection

Why tree-sitter?¶

Traditional Emacs major modes rely on regular expressions for syntax highlighting and heuristic-based indentation. This works reasonably well for many languages, but F# presents unique challenges:

Indentation-sensitive syntax -- whitespace is semantically meaningful
Complex syntax -- computation expressions, type providers, active patterns
Rich type system -- generic types, type annotations, discriminated unions

Tree-sitter provides a full incremental parser that understands F#'s actual grammar. This gives fsharp-ts-mode structural awareness that regex-based modes can't achieve -- more accurate highlighting, better indentation, and reliable navigation.

See the Migration guide for a detailed comparison with the older regex-based fsharp-mode.

Background¶

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.

Funding¶

fsharp-ts-mode is free software and will always be free. If you find it useful, consider supporting its development via GitHub Sponsors.

License¶

Distributed under the GNU General Public License, version 3.