Skip to content
Switch branches/tags



F# support for Vim/Neovim


Part of the Ionide plugin suite.

About Ionide-Vim

Development Status

Consider this to be beta since it's lacking features compared to Ionide-VSCode and not as battle-tested as that.

That being said, we maintainers use this plugin daily so it will someday become feature-rich and stable for sure.

Feel free to request features and/or file bug reports!


  • Neovim or Vim 8.0+

    • Python support is not required as of now. This may or may not change in the future.
  • .NET Core SDK

    • Required to run FsAutoComplete.
    • Very useful for command-line development.
  • If you are using Vim or Neovim (below version 0.5):

    • LanguageClient-neovim
      • Required to communicate with FsAutoComplete.
    • fzf (optional)
      • Optional dependency of LanguageClient-neovim.
      • Multi-entry selection UI.


  • Syntax highlighting
  • Auto completions
  • Error highlighting, error list, and quick fixes based on errors
  • Tooltips
  • Codelens
  • Go to Definition
  • Find all references
  • Highlighting usages
  • Rename
  • Show symbols in file
  • Find symbol in workspace
  • Show signature in status line
  • Integration with F# Interactive
  • Integration with FSharpLint (additional hints and quick fixes)
  • Integration with Fantomas (the best formatter available for F#)

Getting Started

Install a LSP Client

For Neovim 0.5+

No LSP client plugin is required.

If you are using neovim/nvim-lspconfig, do not enable fsautocomplete. Ionide-vim automatically integrates itself into nvim-lspconfig and will register itself as a server.

For Vim / Neovim (below 0.5)

Install LanguageClient-neovim. Refer to their

Here is the example for vim-plug package manager.

Plug 'autozimu/LanguageClient-neovim', {
    \ 'branch': 'next',
    \ 'do': 'bash',
    \ }

If you are running Windows, you will have to set the value of do to 'powershell -ExecutionPolicy Unrestricted .\install.ps1'.

Install an autocompletion plugin

We recommend using Shougo/deoplete.nvim.

if has('nvim')
  Plug 'Shougo/deoplete.nvim', { 'do': ':UpdateRemotePlugins' }
  Plug 'Shougo/deoplete.nvim'
  Plug 'roxma/nvim-yarp'
  Plug 'roxma/vim-hug-neovim-rpc'

If you are using Neovim 0.5+ and not using LanguageClient-neovim, you should also install deoplete-plugins/deoplete-lsp.

Plug 'deoplete-plugins/deoplete-lsp'

Install Ionide-vim

Installing with your plugin manager

Plug 'ionide/Ionide-vim', {
      \ 'do':  'make fsautocomplete',
call dein#add('ionide/Ionide-vim', {
    \ 'build': 'make fsautocomplete',
    \ })

Installing on Windows

Run install.ps1 instead of make fsautocomplete.

For example, if you are using vim-plug, you can use the following:

Plug 'ionide/Ionide-vim', {
     \ 'do':  'powershell -ExecutionPolicy Unrestricted .\install.ps1',

Installing manually

Clone Ionide-vim to some runtimepath and run make fsautocomplete.


Opening either *.fs, *.fsi or *.fsx files should trigger syntax highlighting and other depending runtime files as well.


Refer to LanguageClient-neovim for features provided via Language Server Protocol.

To be added as requested for F#-specific features.


  • Shows the projects currently loaded.

:FSharpParseProject <files>+

  • Loads specified projects (sln or fsproj).


  • Reloads all the projects currently loaded.
  • Automatically called when you save .fsproj files. Can be disabled in settings.



  • Downloads the latest build of FsAutoComplete to be used with Ionide-vim.
  • On Windows, it will run install.ps1 update.

Working with F# Interactive

Ionide-vim has an integration with F# Interactive.

FSI is displayed using the builtin :terminal feature introduced in Vim 8 / Neovim and can be used like in VSCode.


  • Shows a F# Interactive window.

:FsiEval <expr>

  • Evaluates given expression in FSI.


  • Sends the content of current file to FSI.


  • Resets the current FSI session.


  • When in normal mode, sends the current line to FSI.
  • When in visual mode, sends the selection to FSI.
  • Sending code to FSI opens FSI window but the cursor does not focus to it. Unlike Neovim, Vim doesn't support asynchronous buffer updating so you have to input something (e.g. moving cursor) to see the result. You can change this behavior in settings.


  • Toggles FSI window. FSI windows shown in different tabpages share the same FSI session.
  • When opened, the cursor automatically focuses to the FSI window (unlike in Alt-Enter by default).

You can customize the location of FSI, key mappings, etc. See the documentation below.


Refer to LanguageClient-neovim's recommended settings for features provided via Language Server Protocol.

To be added as requested for F#-specific features.

See some examples in our wiki if you're not sure what you would want to set.

LSP Client Settings

Set the LSP client used by Ionide-vim (default: see below)

Set g:fsharp#backend to

  • nvim if you want to use neovim's built-in LSP client.
  • languageclient-neovim if you want to use autozimu/LanguageClient-neovim.
  • disable if you only want the syntax highlighting and the FSI integration.

Default: nvim if you are using Neovim 0.5+, languageclient-neovim otherwise.

let g:fsharp#backend = "languageclient-neovim"
Set the path to FSAC (default: bundled with Ionide-vim)

By default, Ionide-vim automatically downloads FSAC to its plugin directory. If you have a different one e.g. installed as a .NET global tool, you can use it in Ionide-vim by:

let g:fsharp#fsautocomplete_command =
    \ [ 'dotnet',
    \   'fsautocomplete',
    \   '--background-service-enabled'
    \ ]

Note: You have to use an array here. Setting a string value to this option will result in an error.

Set the keybindings for LSP features (default: not set)

Ionide-vim does not provide default keybindings for various LSP features, so you will have to set them yourself.

  • If you are using neovim's built-in LSP client, see here.
  • If you are using LanguageClient-neovim, refer to their docs.

Examples are available at our wiki.

Settings specific to neovim's built-in LSP client

Enable/disable the default colorscheme for diagnostics (default: enable)

Neovim's LSP client comes with no default colorscheme, so Ionide-vim sets a VSCode-like one for LSP diagnostics by default. You can disable this by the following:

let g:fsharp#lsp_recommended_colorscheme = 0
Enable/disable automatic setup (default: enabled)

With nvim-lspconfig, you would manually call the setup function for each LSP servers. Ionide-vim does this automatically by default, but you can disable it.

let g:fsharp#lsp_auto_setup = 0

lua << EOF
Enable/disable automatic refreshing CodeLens (default: enabled)

By default, Ionide-vim sets the following so that CodeLens gets refreshed automatically.

augroup FSharp_AutoRefreshCodeLens
    autocmd CursorHold,InsertLeave <buffer> lua vim.lsp.codelens.refresh()
augroup END

You can disable this by setting the below option:

let g:fsharp#lsp_codelens = 0

Note: this setting does not affect LanguageClient-neovim's CodeLens feature. Please see their docs for how to configure it.

FsAutoComplete Settings

  • Ionide-vim uses snake_case for the setting names.
    • For FSAC settings only, CamelCase can also be used (as it gets serialized to a F# record).
    • If both snake_case and CamelCase are specified, the snake_case one will be preferred.
  • You can change the values at runtime and then notify the changes to FSAC by :FSharpUpdateServerConfig.
  • Some of the settings may not work in Ionide-vim as it is lacking the corresponding feature of Ionide-VSCode.
  • If not specified, the recommended default values described in the FSAC's documentation will be used.
    • You can disable this by let g:fsharp#use_recommended_server_config = 0.

See the documentation of FSAC for the complete list of available settings. Frequently used ones are:

Enable/disable automatic loading of the workspace on opening F# files (default: enabled)
let g:fsharp#automatic_workspace_init = 1 " 0 to disable.
Set the deep level of directory hierarchy when searching for sln/fsprojs (default: 2)
let g:fsharp#workspace_mode_peek_deep_level = 2
Ignore specific directories when loading a workspace (default: empty)
let g:fsharp#exclude_project_directories = ['paket-files']
Enable/disable linter and unused opens/declarations analyzer (default: all enabled)

You may want to bind LanguageClient#textDocument_codeAction() to some shortcut key. Refer to their docs.

" 0 to disable.
let g:fsharp#linter = 1
let g:fsharp#unused_opens_analyzer = 1
let g:fsharp#unused_declarations_analyzer = 1

Editor Settings

Enable/disable automatic calling of :FSharpReloadWorkspace on saving fsproj (default: enabled)
let g:fsharp#automatic_reload_workspace = 1 " 0 to disable.
Show type signature at cursor position (default: enabled)
let g:fsharp#show_signature_on_cursor_move = 1 " 0 to disable.

F# Interactive Settings

Change the F# Interactive command to be used within Ionide-vim (default: dotnet fsi)

If you want to use a .NET Framework FSI instead of .NET Core one, set g:fsharp#use_sdk_scripts to 0. See:

let g:fsharp#fsi_command = "fsharpi"
let g:fsharp#use_sdk_scripts = 0 " for net462 FSI
Set additional runtime arguments passed to FSI (default: [] (empty))

Sets additional arguments of the FSI instance Ionide-vim spawns and changes the behavior of FSAC accordingly when editing fsx files.

let g:fsharp#fsi_extra_parameters = ['--langversion:preview']
Customize how FSI window is opened (default: botright 10new)

It must create a new empty window and then focus to it.

See :help opening-window for details.

let g:fsharp#fsi_window_command = "botright vnew"
Set if sending line/selection to FSI shoule make the cursor focus to FSI window (default: disabled)

If you are using Vim, you might want to enable this to see the result without inputting something.

let g:fsharp#fsi_focus_on_send = 1 " 0 to not to focus.
Change the key mappings (default: vscode)
  • vscode: Default. Same as in Ionide-VSCode (Alt-Enter to send, Alt-@ to toggle terminal).
    • <M-CR> in Neovim / <ESC><CR> in Vim: Sends line/selection to FSI.
    • <M-@> in Neovim / <ESC>@ in Vim: Toggles FSI window.
  • vim-fsharp: Same as in fsharp/vim-fsharp. Note that <leader> is mapped to backslash by default. See :help mapleader.
    • <leader>i : Sends line/selecion to FSI.
    • <leader>e : Toggles FSI window.
  • custom: You must set both g:fsharp#fsi_keymap_send and g:fsharp#fsi_keymap_toggle by yourself.
    • g:fsharp#fsi_keymap_send : Sends line/selection to FSI.
    • g:fsharp#fsi_keymap_toggle : Toggles FSI window.
  • none: Disables mapping.
" custom mapping example
let g:fsharp#fsi_keymap = "custom"
let g:fsharp#fsi_keymap_send   = "<C-e>"
let g:fsharp#fsi_keymap_toggle = "<C-@>"

Linter & Formatter Settings

Linting (other than the basic ones described above) and formatting is powered by independent tools, FSharpLint and Fantomas respectively.

Both uses their own JSON file for configuration and Ionide-vim does not control them. See their docs about configuration: FSharpLint and Fantomas.

Advanced Tips

Show tooltips on CursorHold

If you are using neovim 0.4.0 or later, floating windows will be used for tooltips and you might find it convenient to make them appear if the cursor does not move for several seconds.

if has('nvim') && exists('*nvim_open_win')
  augroup FSharpShowTooltip
    autocmd CursorHold *.fs,*.fsi,*.fsx call fsharp#showTooltip()
  augroup END

Note that you can set the delay time to show the tooltip by set updatetime=<ms>. The default delay is 4 seconds, which you may find too slow.


  • The primary maintainer for this repository is @cannorin.