Setting Eglot's server executable based on web-mode-engine

[jasalt]

LSP (Language Server Protocol, not Lisp Server Pages) can provide some handy project specific auto completion and navigation tools but it's bit unclear how to integrate well with web-mode.

Generally Eglot server executables can be configured on mode basis, but as web-mode uses multiple "engines" each relying on their own LSP server executables, resolving the correct one requires some extra tricks.

With some trial and error I figured out how to conditionally set the LSP server program after the web-mode-engine has been resolved, before initializing Eglot:

;; Add hook after web-mode's hack-local-variables-hook logic that has
;; access to the web-mode-engine variable for setting eglot's lsp server
;; program accordingly, and finally start eglot
(add-hook 'web-mode-hook
          (lambda ()
            (add-hook 'hack-local-variables-hook
                      (lambda ()
                        (when (derived-mode-p 'web-mode)
                          (cond
                           ((string= web-mode-engine "django")
                            ;; Django-specific code here
                            (message "Django engine detected")
                            (add-to-list 'eglot-server-programs
                                         `(web-mode . ("/home/user/.local/bin/django-template-lsp"))))
                          ((string= web-mode-engine "twig")
                            ;; Twig-specific code here (TODO)
                            (message "Twig engine detected"))
                          ((string= web-mode-engine "astro")
                            ;; Astro-specific code here (TODO)
                            (message "Astro engine detected"))))
                        (eglot-ensure))
                      90 t)))  ;; <- inner hook here is delayed after web-mode does it's magic

Quick demo with django-template-lsp using Eglot xrefs navigation and completion for static files (Emacs 31).

Kooha-2025-11-15-09-47-42.mp4

It's a bit hacky but improvement ideas are welcome. Maybe it would require some changes to web-mode for having cleanest way to do it, something like following might be more ergonomic:

;; This is pseudocode / example
(setq web-mode-engine-eglot-executables-alist '(("django" . "/home/user/.local/bin/django-template-lsp")
                                                ("twig" . "/path/to/twiggy-lsp")
                                                ("astro" . "/path/to/astro-lsp"))