Support Alex ".x" files

[brandonchinn178]

Is your enhancement request related to a problem? Please describe.

Currently, HLS fails when run on .x files:

2026-02-01T17:13:19-08:00 ERROR [project::lsp_store] Error handling response for request GetCodeLens: Code Lens via hls failed: No plugins are available to handle this SMethod_TextDocumentCodeLens request.
 Plugins installed for this method, but not available to handle this request are:
eval does not support .x filetypes).
ghcide-type-lenses does not support .x filetypes).
importLens does not support .x filetypes).
class does not support .x filetypes).
moduleName does not support .x filetypes).

Unsure if this is a duplicate of #689, #4409

Describe the solution you'd like

Right now, I just want HLS support on the Haskell part of the file.

Describe alternatives you've considered

Additional context

[fendor]

I dont understand .x files enough, is code always delimited by { and }? And the Haskell code can only be referenced from
the parser but parsers cannot be referenced from haskell code?

We'd just need to parse the AST without losing all of the other context into an GHC AST...

[wz1000]

alex (and happy) already turn .x files into normal .hs files with LINE pragmas pointing to the original .x file. All we would need to do in HLS is to run alex on the file, and then use the generated .hs files to provide diagnostics/info etc.

In the context of use with GHC, hadrian already generates a .hs file for us and passses it in place of a .x file. In general, we are not passed the original .x file from the build tools hie-bios integrates with, rather the tool generates the .hs file and gives us that instead.

So the real problem is that we have no knowledge of the original .x file, and need to convince the build tools to give us this info somehow.

An alternative is to rejigger things so that ghc uses alex/happy as a GHC preprocessor. It is currently possible to have a semi-functional workaround by applying the following patch to GHC:

diff --git a/compiler/GHC/Parser.y b/compiler/GHC/Parser.hs
similarity index 99%
rename from compiler/GHC/Parser.y
rename to compiler/GHC/Parser.hs
index e85af521fd0..2cf14efb0b5 100644
--- a/compiler/GHC/Parser.y
+++ b/compiler/GHC/Parser.hs
@@ -1,3 +1,4 @@
+{-# OPTIONS_GHC -F -pgmF my-happy #-}
 --                                                              -*-haskell-*-
 -- ---------------------------------------------------------------------------
 -- (c) The University of Glasgow 1997-2003

where my-happy is:

#!/bin/bash
input_file="$2"
y_file="${input_file%.*}.y"
cp "$input_file" "$y_file"
happy "$y_file" -o "$3"
rm "$y_file"

HLS seems to "work" after applying this patch and hadrian continues to function. Hover information is all messed up in the file though, that will need further investigation of the file that happy generates and the LINE pragmas within.

[wz1000]

It looks like neither alex nor happy produce the LINE pragmas for this to work well.