This section presents an annotated example of a complete working Flymake backend. The example illustrates the process of writing a backend as outlined above.
The backend in question is used for checking Ruby source files. It uses asynchronous sub-processes (see Asynchronous Processes in The Emacs Lisp Reference Manual), a common technique for performing parallel processing in Emacs.
The following code needs lexical binding (see Using Lexical Binding in The Emacs Lisp Reference Manual) to be active.
;;; ruby-flymake.el --- A ruby Flymake backend -*- lexical-binding: t; -*- (require 'cl-lib) (defvar-local ruby--flymake-proc nil)
(defun ruby-flymake (report-fn &rest _args) ;; Not having a ruby interpreter is a serious problem which should cause ;; the backend to disable itself, so an
erroris signaled. ;; (unless (executable-find "ruby") (error "Cannot find a suitable ruby")) ;; If a live process launched in an earlier check was found, that ;; process is killed. When that process's sentinel eventually runs, ;; it will notice its obsoletion, since it have since reset ;; `ruby-flymake-proc' to a different value ;; (when (process-live-p ruby--flymake-proc) (kill-process ruby--flymake-proc))
;; Save the current buffer, the narrowing restriction, remove any ;; narrowing restriction. ;; (let ((source (current-buffer))) (save-restriction (widen) ;; Reset the `ruby--flymake-proc' process to a new process ;; calling the ruby tool. ;; (setq ruby--flymake-proc (make-process :name "ruby-flymake" :noquery t :connection-type 'pipe ;; Make output go to a temporary buffer. ;; :buffer (generate-new-buffer " *ruby-flymake*") :command '("ruby" "-w" "-c") :sentinel (lambda (proc _event) ;; Check that the process has indeed exited, as it might ;; be simply suspended. ;; (when (memq (process-status proc) '(exit signal)) (unwind-protect ;; Only proceed if `proc' is the same as ;; `ruby--flymake-proc', which indicates that ;; `proc' is not an obsolete process. ;; (if (with-current-buffer source (eq proc ruby--flymake-proc)) (with-current-buffer (process-buffer proc) (goto-char (point-min)) ;; Parse the output buffer for diagnostic's ;; messages and locations, collect them in a list ;; of objects, and call `report-fn'. ;; (cl-loop while (search-forward-regexp "^\\(?:.*.rb\\|-\\):\\([0-9]+\\): \\(.*\\)$" nil t) for msg = (match-string 2) for (beg . end) = (flymake-diag-region source (string-to-number (match-string 1))) for type = (if (string-match "^warning" msg) :warning :error) when (and beg end) collect (flymake-make-diagnostic source beg end type msg) into diags finally (funcall report-fn diags))) (flymake-log :warning "Canceling obsolete check %s" proc)) ;; Cleanup the temporary buffer used to hold the ;; check's output. ;; (kill-buffer (process-buffer proc))))))) ;; Send the buffer contents to the process's stdin, followed by ;; an EOF. ;; (process-send-region ruby--flymake-proc (point-min) (point-max)) (process-send-eof ruby--flymake-proc))))
(defun ruby-setup-flymake-backend () (add-hook 'flymake-diagnostic-functions 'ruby-flymake nil t)) (add-hook 'ruby-mode-hook 'ruby-setup-flymake-backend)