Problems setting up haskell-tools.nvim on NixOS

Question

I'm trying to setup the Neovim plugin https://github.com/mrcjkb/haskell-tools.nvim for a while now and I don't see any progress. I mostly want this plugin for Haskell CodeLenses, so that I can write a comment

module Test (fact) where

fact :: Integer -> Integer
fact 0 = 1
fact n = n * fact (n-1)

fact5 :: Integer
fact5 = fact 5
-- >>>

and the plugin would add the result of fact5 (120) after -- >>>.

I'm on NixOS and tried both, setting it up with Lazy.nvim and with Nix as the plugin installer. I will attach a minimal configuration using Lazy.nvim plugin manager where only those two plugins are installed.

In all three versions (My full config with Lazy.nvim, My full config where Nix home-manager installs packages, Minimal config with Lazy.nvim) I have the same behaviour:

I can open the interactive repl as a Neovim Window above my buffer with :Haskell repl toggle and the keymaps shown on the GitHub page (so the plugin and the keymaps get loaded)
When I type :Hls start for starting haskell-language-server nothing happens. The command is still in the prompt and nothing changes. Same with :Hls evalAll and others.
The plugin displays grey text right after -- >>> namely "Evaluate..." but never finishes with the evaluation
The keybinding for code actions tells me that no actions are available
When I remove the line fact5 :: Integer and use the keymap for code lenses it adds this line right above.

I don't know what else I could try so I'm very happy about recommendations.

Versions

NixOS: 25.05 – Info from flake.lock, for nixpkgs:

nixpkgs:
   "narHash": "sha256-zyEsoxHTMIbyYWpc4n+jiKwZ9TcIE4DPotdxAe2Jrso=",
   "rev": "0ee3848fea3e9c7dadf47cf1e89f8c13878e9f6f",
   ...

lazy-lock.json

{
  "haskell-tools.nvim": { "branch": "master", "commit": "5fb8eda3c4430cb1d473d3e2138b070bdca1c224" },
  "lazy.nvim": { "branch": "main", "commit": "6c3bda4aca61a13a9c63f1c1d1b16b9d3be90d7a" }
}

Checkhealth

I exported the result of :checkhealth with :TOhtml, I don't notice interesting errors either https://heureka-code.github.io/haskell-tools-question/checkhealth.html

Files

I uploaded them to GitHub: https://github.com/heureka-code/haskell-tools-question

Thanks in advance! hc

MrcJkb · Accepted Answer · 2025-08-22 12:48:18Z

4

fact5 :: Integer
fact5 = fact 5
-- >>> 
and the plugin would add the result of fact5 (120) after -- >>>.

That's not how haskell-language-server's doctest evaluation works. It's evaluating nothing, because there's nothing after the -- >>> to evaluate.

You want something like this:


-- | >>> fact 5
fact :: Integer -> Integerfact 0 = 1
fact n = n * fact (n-1)

answered Aug 22 at 12:48

MrcJkb

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1 Comment

h-c Aug 22 at 18:36

Thank you very much! -- >>> fact 5 worked for me.

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