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+;;; Haunt --- Static site generator for GNU Guile
+;;; Copyright © 2022 David Thompson <davet@gnu.org>
+;;;
+;;; This file is part of Haunt.
+;;;
+;;; Haunt is free software; you can redistribute it and/or modify it
+;;; under the terms of the GNU General Public License as published by
+;;; the Free Software Foundation; either version 3 of the License, or
+;;; (at your option) any later version.
+;;;
+;;; Haunt is distributed in the hope that it will be useful, but
+;;; WITHOUT ANY WARRANTY; without even the implied warranty of
+;;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+;;; General Public License for more details.
+;;;
+;;; You should have received a copy of the GNU General Public License
+;;; along with Haunt.  If not, see <http://www.gnu.org/licenses/>.
+
+;;; Commentary:
+;;
+;; Inotify-based file watching for Linux.
+;;
+;;; Code:
+
+(define-module (haunt watch linux)
+  #:use-module (haunt inotify)
+  #:use-module (haunt site)
+  #:use-module (ice-9 ftw)
+  #:export (watch))
+
+;; TODO: Detect new directories and watch them, too.
+(define (watch thunk check-dir? check-file?)
+  (let ((inotify (make-inotify)))
+    (define (no-op name stat result) result)
+    (define (watch-directory name stat result)
+      (and (check-dir? name)
+           (inotify-add-watch! inotify name
+                               '(create delete close-write moved-to moved-from))
+           #t))
+    ;; Drop .scm extension, remove working directory,
+    ;; and transform into a symbolic module name.
+    (define (file-name->module-name file-name)
+      (map string->symbol
+           (string-split (string-drop (string-take file-name
+                                                   (- (string-length file-name)
+                                                      4))
+                                      (+ (string-length (getcwd)) 1))
+                         #\/)))
+    (file-system-fold watch-directory no-op no-op no-op no-op no-op #t (getcwd))
+    (let loop ((processed-event? #f))
+      (cond
+       ((inotify-pending-events? inotify)
+        (let* ((event (inotify-read-event inotify))
+               (type (inotify-event-type event))
+               (file-name (string-append (inotify-watch-file-name
+                                          (inotify-event-watch event))
+                                         "/"
+                                         (inotify-event-file-name event))))
+          (if (and (check-dir? file-name) (check-file? file-name))
+              (let ((action (case type
+                              ((create) "create")
+                              ((delete) "delete")
+                              ((close-write) "write")
+                              ((moved-to moved-from) "move"))))
+                (format #t "watch: observed ~a '~a'~%" action file-name)
+                ;; Reload Scheme modules when they are changed.
+                (when (%search-load-path file-name)
+                  (let ((module (resolve-module
+                                 (file-name->module-name file-name))))
+                    (when (module-filename module)
+                      (format #t "watch: reload module ~s~%"
+                              (module-name module))
+                      (reload-module module))))
+                (loop #t))
+              (loop processed-event?))))
+       (processed-event?
+        (thunk)
+        (loop #f))
+       (else
+        (sleep 1)
+        (loop #f))))))