`Could not find module ‘Language.Javascript.JSaddle.WebKitGTK’` in default install of reflex-dom-0.6.3.4 under ghc 9.12

[emberdart]

Hello
While compiling a package requiring reflex-dom for ghc 9.12 I get the following:

Building library for reflex-dom-0.6.3.4...
[1 of 3] Compiling Reflex.Dom.Internal ( src/Reflex/Dom/Internal.hs, dist/build/Reflex/Dom/Internal.o, dist/build/Reflex/Dom/Internal.dyn_o )
src/Reflex/Dom/Internal.hs:99:1-50: error: [GHC-61948]
    Could not find module ‘Language.Javascript.JSaddle.WebKitGTK’.
    Perhaps you meant
      Language.Javascript.JSaddle.Debug (needs flag -package-id jsaddle-0.9.9.3)
      Language.Javascript.JSaddle.Helper (needs flag -package-id jsaddle-0.9.9.3)
      Language.Javascript.JSaddle.Native (needs flag -package-id jsaddle-0.9.9.3)
    Use -v to see a list of the files searched for.
   |
99 | import Language.Javascript.JSaddle.WebKitGTK (run)
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Am I doing something wrong? I have depended on reflex-dom ^>=0.6.3.4 in my cabal file.

My project didn't compile under js backend because of the lack of basement support but I thought it would at least build in ghc version for development...

Thanks

[alexfmpe]

My reading of the CPP conditionals around

reflex-dom/reflex-dom/src/Reflex/Dom/Internal.hs

Line 59 in f182a1b

import Language.Javascript.JSaddle.WKWebView (run)

is that this line gets included if you're depending on jsaddle-wkwebview but not building for iOS.
It also looks like jsaddle-wkwebview is only buildable on darwin https://github.com/ghcjs/jsaddle/blob/2513cd19184376ac8a2f0e3797a1ae7d2e522e87/jsaddle-wkwebview/jsaddle-wkwebview.cabal#L34-L35 so I would only expect this import to be present when compiling for MacOS

So maybe it's (transitively) on your dependencies while building for linux?
What system are you building from, and for?

[emberdart]

This is plain x86-64-linux, not cross-compiling, expecting a warp callable version, no flags.

[alexfmpe]

Inside the nix-shell, does jsaddle-wkwebview show up if you do ghc-pkg list ? If so, ghc-pkg dot would help figure out why

[emberdart]

No, the nix-shell isn't importing anything like that though, I'm doing it all through cabal, all I'm using in the nix shell file is haskell.compiler.ghc912 and cabal-install. What would be the equivalent of "what's cabal trying to build and why"?

[alexfmpe]

Try looking at cabal-plan output
e.g. nix-shell -p cabal-install haskell.compiler.ghc912 haskellPackages.cabal-plan --run "cabal-plan tred" or maybe cabal-plan dot

[alexfmpe]

Wow, brain fart. Why was I even looking as WKWebView?

The error is from

reflex-dom/reflex-dom/src/Reflex/Dom/Internal.hs

Line 99 in f182a1b

import Language.Javascript.JSaddle.WebKitGTK (run)

which indeed should be active on linux.

Now, unlike the wkwebview bit, since this is under a catch-all #else, it's not present only if jsaddle-webkit2gtk is found, which is questionable.
I wonder if we should gate that and have the catch-all #else just fail with a message about unrecognized platform.

I'm still not sure why it's failing for you since CI has been working just fine with...oh. There we're using

reflex-dom/cabal.project-nogtk

Lines 8 to 9 in f182a1b

	package reflex-dom
	flags: +use-warp -webkit2gtk +build-examples

So I guess something is borked with linux webkitgtk builds. Don't have access to my linux right now, will check in a couple days.

expecting a warp callable version, no flags.

In the meantime, maybe you want +use-warp -webkit2gtk on your cabal.project?

[alexfmpe]

Well, both warp and webkit2gtk builds of reflex-dom work on my x86_64 linux.

So I guess something is borked with linux webkitgtk builds.

Now that I think about it, in nixpkgs this is tested with haskellPackages.reflex-dom and there's been no issues.

What does your uname -a say?
Does this happen with 9.8, 9.10 ?
Can you isolate a minimal reproducible?

[noelemahcz]

Edit: Confirmed, using +use-warp -webkit2gtk is a working workaround. The default build still fails for me.

---

Same here, with flake.nix:

{
  inputs = {
    nixpkgs.url = "github:NixOS/nixpkgs/nixpkgs-unstable";
    flake-utils.url = "github:numtide/flake-utils";
  };

  outputs = {
    self,
    nixpkgs,
    flake-utils,
  } @ inputs:
    flake-utils.lib.eachDefaultSystem (
      system: let
        pkgs = nixpkgs.legacyPackages.${system};
      in {
        devShells = {
          default = pkgs.mkShell {
            nativeBuildInputs = with pkgs; [
              cabal-install
              haskell.compiler.ghc9122
              haskell.packages.ghc9122.haskell-language-server
            ];
          };
        };
      }
    );
}

The project is set up using a fresh cabal init. The only modification is adding reflex-dom to the build-depends section in the .cabal file:

...
build-depends:
  base ^>=4.21.0.0,
  reflex-dom
...

Running cabal build fails with the following error:

Starting     reflex-dom-0.6.3.4 (lib)
Building     reflex-dom-0.6.3.4 (lib)

Failed to build reflex-dom-0.6.3.4.
Build log (
/home/noelemahcz/.cache/cabal/logs/ghc-9.12.2/reflex-dom-0.6.3.4-00d347665f7e101ee4744b134a5ed36bffdb980ffa41bab4e16d81eb2be97430.log
):
Configuring library for reflex-dom-0.6.3.4...
Preprocessing library for reflex-dom-0.6.3.4...
Building library for reflex-dom-0.6.3.4...
[1 of 3] Compiling Reflex.Dom.Internal ( src/Reflex/Dom/Internal.hs, dist/build/Reflex/Dom/Internal.o, dist/build/Reflex/Dom/Internal.dyn_o )
src/Reflex/Dom/Internal.hs:99:1-50: error: [GHC-61948]
    Could not find module ‘Language.Javascript.JSaddle.WebKitGTK’.
    Perhaps you meant
      Language.Javascript.JSaddle.Debug (needs flag -package-id jsaddle-0.9.9.3)
      Language.Javascript.JSaddle.Helper (needs flag -package-id jsaddle-0.9.9.3)
      Language.Javascript.JSaddle.Native (needs flag -package-id jsaddle-0.9.9.3)
    Use -v to see a list of the files searched for.
   |
99 | import Language.Javascript.JSaddle.WebKitGTK (run)
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

[3 of 3] Compiling Reflex.Dom.Location.Platform ( src/Reflex/Dom/Location/Platform.hs, dist/build/Reflex/Dom/Location/Platform.o, dist/build/Reflex/Dom/Location/Platform.dyn_o )
Error: [Cabal-7125]
Failed to build reflex-dom-0.6.3.4

I'm not sure if this is related to WSL2, but here is the output of uname -a:

Linux nixos 6.6.87.2-microsoft-standard-WSL2 #1 SMP PREEMPT_DYNAMIC Thu Jun  5 18:30:46 UTC 2025 x86_64 GNU/Linux