xref-fallback.el

(use-package lsp-haskell
  :config
  (setq lsp-haskell-process-path-hie "haskell-language-server-wrapper")
  ;; xref backends don't compose.  This creates one that first consults the
  ;; xref-lsp backend for the latest information from the language server,
  ;; and then falls through to etags for library definitions.
  (defun my/xref-lsp-etags-backend () 'my/lsp-etags)
  (cl-defmethod xref-backend-identifier-at-point ((_backend (eql my/lsp-etags)))
    (xref-backend-identifier-at-point 'xref-lsp))
  (cl-defmethod xref-backend-identifier-completion-table ((_backend (eql my/lsp-etags)))
    (xref-backend-identifier-completion-table 'xref-lsp))
  (cl-defmethod xref-backend-definitions ((_backend (eql my/lsp-etags)) identifier)
    (or
     (xref-backend-definitions 'xref-lsp identifier)
     (xref-backend-definitions 'etags identifier)))
  (cl-defmethod xref-backend-references ((_backend (eql my/lsp-etags)) identifier)
    (or
     ;; HLS doesn't yet support textDocument/references
     ;; (xref-backend-references 'xref-lsp identifier))
     (xref-backend-references 'etags identifier)))
  (cl-defmethod xref-backend-apropos ((_backend (eql my/lsp-etags)) pattern)
    (append
     ;; HLS doesn't yet support workspace/symbol
     ;; (xref-backend-apropos 'xref-lsp pattern))
     (xref-backend-apropos 'etags pattern)))
  (defun my/xref-add-lsp-etags-backend ()
    (add-hook 'xref-backend-functions #'my/xref-lsp-etags-backend nil t))
  :hook
  (haskell-mode . lsp)
  ;; lsp-managed-mode installs the lsp backend.  We need to
  ;; come after that, or it won't fall through to us, either.
  (lsp-managed-mode . my/xref-add-lsp-etags-backend)
  :general
  (:keymaps 'interactive-haskell-mode-map
            "M-." 'xref-find-definitions))