Support non-file buffers (#52) (#59)

* Support non-file buffers (#52)

Closes #52

* Apply suggestions from code review

* Oops, my bad

Co-authored-by: Radon Rosborough <radon.neon@gmail.com>

3 files changed, 45 insertions, 14 deletions

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 07a971d..27dea21 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -10,6 +10,7 @@ The format is based on [Keep a Changelog].
   be run in sequence.
 * Support evaluating items in `apheleia-formatters` to make formatter
   commands more dynamic ([#50], [#55]).
+* Allow apheleia to format buffers without an underlying file ([#52]).
 
 ### Formatters
 * [ClangFormat](https://clang.llvm.org/docs/ClangFormat.html) for
@@ -46,6 +47,7 @@ The format is based on [Keep a Changelog].
 [#48]: https://github.com/raxod502/apheleia/pull/48
 [#49]: https://github.com/raxod502/apheleia/pull/49
 [#50]: https://github.com/raxod502/apheleia/pull/50
+[#52]: https://github.com/raxod502/apheleia/issues/52
 [#54]: https://github.com/raxod502/apheleia/pull/54
 [#55]: https://github.com/raxod502/apheleia/issues/55
 
diff --git a/README.md b/README.md
index c4f7241..00fa7f1 100644
--- a/README.md
+++ b/README.md
@@ -107,6 +107,18 @@ variables:
       only when it is bound. Observe that one of these evaluations
       returns a list of flags whereas the other returns a single
       string. These are substituted into the command as you'd expect.
+    * You can also use Apheleia to format buffers that have no underlying
+      files. In this case the value of `file` and `filepath` will be
+      the name of the current buffer with any special characters for
+      the file-system (such as `*` on windows) being stripped out.
+
+      This is also how the extension for any temporary files apheleia
+      might create will be determined. If you're using a formatter
+      that determines the file-type from the extension you should name
+      such buffers such that their suffixed with the extension. For
+      example a buffer called `*foo-bar.c*` that has no associated
+      file will have an implicit file-name of `foo-bar.c` and any
+      temporary files will be suffixed with a `.c` extension.
 * `apheleia-mode-alist`: Alist mapping major modes and filename
   regexps to names of formatters to use in those modes and files. See
   the docstring for more information.
diff --git a/apheleia.el b/apheleia.el
index 6fe4b8f..a815a2e 100644
--- a/apheleia.el
+++ b/apheleia.el
@@ -27,6 +27,9 @@
 (require 'map)
 (require 'subr-x)
 
+(eval-when-compile
+  (require 'rx))
+
 (defgroup apheleia nil
   "Reformat buffer without moving point."
   :group 'external
@@ -394,6 +397,15 @@ as its sole argument."
                       ;; changes, and 2 if error.
                       (memq status '(0 1)))))))
 
+(defun apheleia--safe-buffer-name ()
+  "Return `buffer-name' without special file-system characters."
+  ;; See https://stackoverflow.com/q/1976007 for a list of supported
+  ;; characters on all systems.
+  (replace-regexp-in-string
+   (rx (or "/" "<" ">" ":" "\"" "\\" "|" "?" "*"))
+   ""
+   (buffer-name)))
+
 (defun apheleia--format-command (command &optional stdin-buffer)
   "Format COMMAND into a shell command and list of file paths.
 Returns a list with the car being the optional input file-name, the
@@ -435,9 +447,10 @@ cmd is to be run."
       (when (memq 'input command)
         (let ((input-fname (make-temp-file
                             "apheleia" nil
-                            (and buffer-file-name
-                                 (file-name-extension
-                                  buffer-file-name 'period)))))
+                            (when-let ((file-name
+                                        (or buffer-file-name
+                                            (apheleia--safe-buffer-name))))
+                              (file-name-extension file-name 'period)))))
           (with-current-buffer stdin
             (apheleia--write-region-silently nil nil input-fname))
           (setq command (mapcar (lambda (arg)
@@ -458,15 +471,18 @@ cmd is to be run."
         (when stdin-buffer
           (error "Cannot run formatter using `file' or `filepath' in a \
 sequence unless it's first in the sequence"))
-        (setq command (mapcar (lambda (arg)
-                                (when (eq arg 'file)
-                                  (setq stdin nil))
-                                (if (memq arg '(file filepath))
-                                    (prog1 buffer-file-name
-                                      (when (buffer-modified-p)
-                                        (cl-return)))
-                                  arg))
-                              command)))
+        (let ((file-name (or buffer-file-name
+                             (concat default-directory
+                                     (apheleia--safe-buffer-name)))))
+          (setq command (mapcar (lambda (arg)
+                                  (when (eq arg 'file)
+                                    (setq stdin nil))
+                                  (if (memq arg '(file filepath))
+                                      (prog1 file-name
+                                        (when (buffer-modified-p)
+                                          (cl-return)))
+                                    arg))
+                                command))))
       ;; Evaluate each element of arg that isn't a string and replace
       ;; it with the evaluated value. The result of an evaluation should
       ;; be a string or a list of strings. If the former its replaced as
@@ -772,8 +788,9 @@ operating, to prevent an infinite loop.")
          commands
          (lambda ()
            (with-demoted-errors "Apheleia: %s"
-             (let ((apheleia--format-after-save-in-progress t))
-               (apheleia--write-file-silently buffer-file-name))
+             (when buffer-file-name
+               (let ((apheleia--format-after-save-in-progress t))
+                 (apheleia--write-file-silently buffer-file-name)))
              (run-hooks 'apheleia-post-format-hook))))))))
 
 ;; Use `progn' to force the entire minor mode definition to be copied