feat: add hls-document-link-plugin to support LSP document links

[jetjinser]

This PR introduces a new plugin hls-document-link-plugin that adds LSP documentLink support to Haskell Language Server.

This plugin automatically extracts documentation links for symbols in source code. These links are the same ones that power the [Documentation] link in the hover information.

Changes

• New plugin: hls-document-link-plugin – extracts clickable links and provides them as document links. Editors can then open these links directly from the source code.
• Configuration: Added documentLinkOn option to enable/disable the feature (default: True).

Usage

In Neovim 0.12 or later, you can place the cursor on the link and press gx to open it directly in your default browser.

The feature can be turned off by setting documentLinkOn: false in the HLS configuration.

---

Note. The getDocumentLinks helper for lsp-test is implemented manually here, but a same change is also pending in haskell/lsp#638. After that PR is merged and the dependency is updated, this part can be simplified.

[fendor]

Welcome back and happy to see you tackle this issue!

This looks pretty good to my cursory review. I have mostly nitpicks and a couple of clarification questions.
Other than that, LGTM :)

[fendor]

Suggested change

	-- custom Eq to ignore some details, such as specific URI
	-- not symmetry
	-- Custom Eq to ignore some details, such as specific URI.
	-- This is not a lawful instance as it doesn't obey the `symmetry` law. Use at your own risk and only in tests.

[fendor]

Also, I am wondering whether this has any overlap with #4746 and the Documentation and Source links in the hover message?

[fendor]

Screencast_20260418_150352.webm

This is the behaviour on vscode... I suppose that's not the expected behaviour?

[jetjinser]

Also, I am wondering whether this has any overlap with #4746 and the Documentation and Source links in the hover message?

Yes. The links provided in this PR share the same source as the 'Documentation' links in the hover messages. The goal here is to expose that information through a more native and convenient LSP feature (DocumentLink).

Therefore, documentLink provides a local hackage link, not the online link provided in #4746.

[jetjinser]

Screencast_20260418_150352.webm

This is the behaviour on vscode... I suppose that's not the expected behaviour?

I’ve done some local testing and noticed the same behavior in VSCodium.

1. If I manually run xdg-open on the generated URI (e.g., file:///nix/store/.../docs/index.html#v:nodeChildren), it opens correctly in the external browser. This is also why gx works as expected in Neovim. It seems VSCodium's default behavior is to capture file:// URIs and render them in an internal webview/tab instead of passing them to the system handler?

2. Additionally, I found a minor UX conflict: when a DocumentLink is present, VSCodium's Ctrl+Click defaults to following the link, which "shadows" the Go to Definition action. This might cause some confusion?

---

EDIT:
For the first question,

    "workbench.externalUriOpeners": {
        "file:///*": "simpleBrowser.open"
    },

Ideally, this might work? However, I still couldn't get it to open in either an internal or external browser.

[crtschin]

However, I still couldn't get it to open in either an internal or external browser.

I think this might be a lsp consumer problem. I'm using Helix and by default all file:// links get opened by the editor as well, and only other links get the xdg-open treatment. I guess from an editor's pov, there is a legitimate usecase for opening html files in the editor.

I wonder, what's the hueristic nvim uses to determine whether to open files with xdg-open or locally in the editor?

[jetjinser]

I wonder, what's the hueristic nvim uses to determine whether to open files with xdg-open or locally in the editor?

Nvim doesn't attempt any heuristics.

It simply uses vim.ui.open, which always uses xdg-open (the equivalent command on other platforms).

https://github.com/neovim/neovim/blob/2ca31eddae0748de5d54207c2d34f7405fed4f29/runtime/lua/vim/_core/defaults.lua#L147-L181

It's possible that Neovim doesn't have a particular reason to open HTML, while a GUI editor does, and the former distinguishes between gx (opens ... using the system default handler) and gf (Edit the file ...). XX

[vidit-od]

Additionally, I found a minor UX conflict:

We also should take into consideration how to disable the underlines.
The by default view makes everything look too cluttered due to lots of underlines.

example :

[fendor]

We also should take into consideration how to disable the underlines.

Disabling should work, otherwise this is a VSCode / theme setting, so not in the scope of this PR