Flycheck and HLS

I've been using LSP for most programming languages for a while now. HLS is really very good now, but I've found that it doesn't warn on quite all things I'd like it to so I find myself having to swap between the 'lsp and 'haskell-ghc checkers. However, since flycheck supports chaining checkers I thought there must be a way to have both checkers active at the same time.

The naive approach didn't work due to load order of things in Spacemacs so I had to experiment a bit to find something that works.

The first issue was to make sure that HLS is available at all. I use shell.nix together with direnv extensively and I had noticed that lsp-mode tried to load HLS before direnv had put it in the $PATH. I think the 'lsp-beforeinitialize-hook is the hook to use for this:

(add-hook 'lsp-before-initialize-hook #'direnv-update-environment))

I made a several attempt to chain the checkers but kept on getting errors due to the 'lsp checker not being defined yet. Another problem I ran into was that the checkers were chained too late, resulting in having to manually run flycheck-buffer on the first file I opened. (Deferred loading is a brilliant thing, but make some things really difficult to debug.) After quite a bit of experimenting and reading the description of various hooks I did find something that works:

(with-eval-after-load 'lsp-mode
  (defun magthe:lsp-next-checker ()
    (flycheck-add-next-checker 'lsp '(warning . haskell-ghc)))
  (add-hook 'lsp-lsp-haskell-after-open-hook
            #'magthe:lsp-next-checker))

Of course I have no idea if this is the easiest or most elegant solution but it does work for my testcases:

1. Open a file in a project, SPC p l - choose project - choose a Haskell file.
2. Open a project, SPC p l followed by C-d, and then open a Haskell file.

Suggestions for improvements are more than welcome, of course.