Add new basic Emacs Lisp code formatting function

* lisp/emacs-lisp/pp.el (pp-emacs-lisp-code): New interface function.
(pp): Mention it.
(pp--insert-lisp, pp--format-vector, pp--format-list)
(pp--format-function, pp--format-definition, pp--insert-binding)
(pp--insert, pp--indent-buffer): New helper functions.

etc/NEWS

@@ -414,6 +414,11 @@ Use 'exif-parse-file' and 'exif-field' instead.
 
 * Lisp Changes in Emacs 29.1
 
+---
+*** New function 'pp-emacs-lisp-code'.
+'pp' formats general Lisp sexps.  This function does much the same,
+but applies formatting rules appropriate for Emacs Lisp code.
+
 +++
 *** New function 'file-has-changed-p'.
 This convenience function is useful when writing code that parses

lisp/emacs-lisp/pp.el

@@ -82,6 +82,10 @@ to make output that `read' can handle, whenever this is possible."
   "Output the pretty-printed representation of OBJECT, any Lisp object.
 Quoting characters are printed as needed to make output that `read'
 can handle, whenever this is possible.
+
+This function does not apply special formatting rules for Emacs
+Lisp code.  See `pp-emacs-lisp-code' instead.
+
 Output stream is STREAM, or value of `standard-output' (which see)."
   (princ (pp-to-string object) (or stream standard-output)))
 
@@ -180,6 +184,124 @@ Ignores leading comment characters."
       (insert (pp-to-string (macroexpand-1 (pp-last-sexp))))
     (pp-macroexpand-expression (pp-last-sexp))))
 
+;;;###autoload
+(defun pp-emacs-lisp-code (sexp)
+  "Insert SEXP into the current buffer, formatted as Emacs Lisp code."
+  (require 'edebug)
+  (let ((standard-output (current-buffer)))
+    (save-restriction
+      (narrow-to-region (point) (point))
+      (pp--insert-lisp sexp)
+      (insert "\n")
+      (goto-char (point-min))
+      (indent-sexp))))
+
+(defun pp--insert-lisp (sexp)
+  (cl-case (type-of sexp)
+    (vector (pp--format-vector sexp))
+    (cons (cond
+           ((consp (cdr sexp))
+            (if (and (length= sexp 2)
+                     (eq (car sexp) 'quote))
+                (let ((print-quoted t))
+                  (prin1 sexp))
+              (pp--format-list sexp)))
+           (t
+            (princ sexp))))
+    ;; Print some of the smaller integers as characters, perhaps?
+    (integer
+     (if (<= ?0 sexp ?z)
+         (let ((print-integers-as-characters t))
+           (princ sexp))
+       (princ sexp)))
+    (string
+     (let ((print-escape-newlines t))
+       (prin1 sexp)))
+    (otherwise (princ sexp))))
+
+(defun pp--format-vector (sexp)
+  (prin1 sexp))
+
+(defun pp--format-list (sexp)
+  (if (and (symbolp (car sexp))
+           (not (keywordp (car sexp))))
+      (pp--format-function sexp)
+    (prin1 sexp)))
+
+(defun pp--format-function (sexp)
+  (let* ((sym (car sexp))
+         (edebug (get sym 'edebug-form-spec))
+         (indent (get sym 'lisp-indent-function)))
+    (when (eq indent 'defun)
+      (setq indent 2))
+    (pp--insert "(" sym)
+    (pop sexp)
+    ;; Get the first entries on the first line.
+    (if indent
+        (pp--format-definition sexp indent edebug)
+      (while sexp
+        (pp--insert " " (pop sexp))))
+    (insert ")")))
+
+(defun pp--format-definition (sexp indent edebug)
+  (while (and (cl-plusp indent)
+              sexp)
+    (insert " ")
+    (if (and (consp (car edebug))
+             (eq (caar edebug) '&rest))
+        (pp--insert-binding (pop sexp))
+      (if (null (car sexp))
+          (insert "()")
+        (pp--insert-lisp (car sexp)))
+      (pop sexp))
+    (pop edebug)
+    (cl-decf indent))
+  (when (stringp (car sexp))
+    (insert "\n")
+    (prin1 (pop sexp)))
+  ;; Then insert the rest with line breaks before each form.
+  (while sexp
+    (insert "\n")
+    (if (keywordp (car sexp))
+        (progn
+          (pp--insert-lisp (pop sexp))
+          (when sexp
+            (pp--insert " " (pop sexp))))
+      (pp--insert-lisp (pop sexp)))))
+
+(defun pp--insert-binding (sexp)
+  (insert "(")
+  (while sexp
+    (if (consp (car sexp))
+        ;; Newlines after each (...) binding.
+        (progn
+          (pp--insert-lisp (car sexp))
+          (when (cdr sexp)
+            (insert "\n")))
+      ;; Keep plain symbols on the same line.
+      (pp--insert " " (car sexp)))
+    (pop sexp))
+  (insert ")"))
+
+(defun pp--insert (delim &rest things)
+  (let ((start (point)))
+    (when delim
+      (insert delim))
+    (dolist (thing things)
+      (pp--insert-lisp thing))
+    ;; We need to indent what we have so far to see if we have to fold.
+    (pp--indent-buffer)
+    (when (> (current-column) (window-width))
+      (save-excursion
+        (goto-char start)
+        (insert "\n")))))
+
+(defun pp--indent-buffer ()
+  (goto-char (point-min))
+  (while (not (eobp))
+    (lisp-indent-line)
+    (forward-line 1)))
+
 (provide 'pp)				; so (require 'pp) works
 
 ;;; pp.el ends here

test/lisp/emacs-lisp/pp-resources/code-formats.erts

@@ -0,0 +1,97 @@
+Code:
+  (lambda ()
+    (emacs-lisp-mode)
+    (let ((code (read (current-buffer))))
+      (erase-buffer)
+      (pp-emacs-lisp-code code)
+      (untabify (point-min) (point-max))))
+
+Name: code-formats1
+
+=-=
+(defun foo (bar)
+  "Yes."
+  (let ((a 1)
+        (b 2))
+    (zot 1 2 (funcall bar 2))))
+=-=-=
+
+
+Name: code-formats2
+
+=-=
+(defun pp-emacs-lisp-code (sexp)
+  "Insert SEXP into the current buffer, formatted as Emacs Lisp code."
+  (require 'edebug)
+  (let ((start (point))
+        (standard-output (current-buffer)))
+    (pp--insert-lisp sexp)
+    (insert "\n")
+    (goto-char start)
+    (indent-sexp)))
+=-=-=
+
+
+Name: code-formats3
+
+=-=
+(defun foo (bar)
+  "Yes."
+  (let ((a 1)
+        (b 2))
+    (zot-zot-zot-zot-zot-zot 1 2 (funcall
+                                  bar-bar-bar-bar-bar-bar-bar-bar-bar-bar 2))))
+=-=-=
+
+
+Name: code-formats4
+
+=-=
+(defun foo (bar)
+  "Yes."
+  (let ((a 1)
+        (b 2)
+        foo bar zotfoo bar zotfoo bar zotfoo bar zotfoo bar zotfoo bar zotfoo
+        bar zot)
+    (zot 1 2 (funcall bar 2))))
+=-=-=
+
+
+Name: code-formats5
+
+=-=
+(defgroup pp ()
+  "Pretty printer for Emacs Lisp."
+  :prefix "pp-"
+  :group 'lisp)
+=-=-=
+
+Name: code-formats6
+
+=-=
+(defcustom pp-escape-newlines t
+  "Value of `print-escape-newlines' used by pp-* functions."
+  :type 'boolean
+  :group 'pp)
+=-=-=
+
+Name: code-formats7
+
+=-=
+(defun pp (object &optional stream)
+  (princ (pp-to-string object) (or stream standard-output)))
+=-=-=
+
+
+Name: code-formats8
+
+=-=
+(defun pp-eval-expression (expression)
+  "Evaluate EXPRESSION and pretty-print its value.
+Also add the value to the front of the list in the variable `values'."
+  (interactive (list (read--expression "Eval: ")))
+  (message "Evaluating...")
+  (let ((result (eval expression lexical-binding)))
+    (values--store-value result)
+    (pp-display-expression result "*Pp Eval Output*")))
+=-=-=

test/lisp/emacs-lisp/pp-tests.el

@@ -20,6 +20,7 @@
 ;;; Code:
 
 (require 'pp)
+(require 'ert-x)
 
 (ert-deftest pp-print-quote ()
   (should (string= (pp-to-string 'quote) "quote"))
@@ -32,4 +33,7 @@
   (should (string= (pp-to-string '(quotefoo)) "(quotefoo)\n"))
   (should (string= (pp-to-string '(a b)) "(a b)\n")))
 
+(ert-deftest test-indentation ()
+  (ert-test-erts-file (ert-resource-file "code-formats.erts")))
+
 ;;; pp-tests.el ends here.