bug#44155: Print integers as characters

[Juri Linkov]

>> > And, btw, why not allow raw bytes here as well? is there some problem?
>>
>> Because of ambiguity, both these return the same value:
>>
>> (read (concat "?" (string 128)))     => 128
>> (read (concat "?" (string 4194176))) => 128
>
> And why is that a problem?

I don't know, Andreas remarked that it creates ambiguous output,
and I fixed the reported problem.

> Alternatively, we could print raw bytes in some special way.  But not
> treating them as characters sounds some subtlety that will be hard to
> explain.

The existing 'prin1-char' used as a reference implementation
doesn't print integers like 4194176 as characters, so the patch
does the same.

Anyway, here is a complete patch with tests and documentation:

diff --git a/doc/lispref/streams.texi b/doc/lispref/streams.texi
index 2cd61ad04f..f171f13779 100644
--- a/doc/lispref/streams.texi
+++ b/doc/lispref/streams.texi
@@ -902,3 +902,11 @@ Output Variables
 in the C function @code{sprintf}.  For further restrictions on what
 you can use, see the variable's documentation string.
 @end defvar
+
+@defvar integer-output-format
+This variable specifies how to print integer numbers.  The default is
+@code{nil}, meaning use the decimal format.  When bound to @code{t},
+print integers as characters when an integer represents a character
+(@pxref{Basic Char Syntax}).  When bound to the number @code{16},
+print non-negative integers in the hexadecimal format.
+@end defvar
diff --git a/etc/NEWS b/etc/NEWS
index a77c1c883e..2f7d08ad08 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -1631,6 +1631,12 @@ ledit.el, lmenu.el, lucid.el and old-whitespace.el.
 
 * Lisp Changes in Emacs 28.1
 
+** New variable 'integer-output-format' defines the format of integers.
+When this variable is bound to the value 't', integers are printed by
+printing functions as characters when an integer represents a character.
+When bound to the number 16, non-negative integers are printed in the
+hexadecimal format.
+
 +++
 ** 'define-globalized-minor-mode' now takes a :predicate parameter.
 This can be used to control which major modes the minor mode should be
diff --git a/src/print.c b/src/print.c
index 53aa353769..a5c56c6b48 100644
--- a/src/print.c
+++ b/src/print.c
@@ -1908,8 +1908,31 @@ print_object (Lisp_Object obj, Lisp_Object printcharfun, 
bool escapeflag)
     {
     case_Lisp_Int:
       {
-       int len = sprintf (buf, "%"pI"d", XFIXNUM (obj));
-       strout (buf, len, len, printcharfun);
+        int c;
+        intmax_t i;
+
+        if (EQ (Vinteger_output_format, Qt) && CHARACTERP (obj)
+            && (c = XFIXNUM (obj)) && ! CHAR_BYTE8_P (c))
+          {
+            printchar ('?', printcharfun);
+            if (escapeflag
+                && (c == ';' || c == '(' || c == ')' || c == '{' || c == '}'
+                    || c == '[' || c == ']' || c == '\"' || c == '\'' || c == 
'\\'))
+              printchar ('\\', printcharfun);
+            printchar (c, printcharfun);
+          }
+        else if (INTEGERP (Vinteger_output_format)
+                 && integer_to_intmax (Vinteger_output_format, &i)
+                 && i == 16 && Fnatnump (obj))
+          {
+            int len = sprintf (buf, "#x%"pI"x", (EMACS_UINT) XFIXNUM (obj));
+            strout (buf, len, len, printcharfun);
+          }
+        else
+          {
+            int len = sprintf (buf, "%"pI"d", XFIXNUM (obj));
+            strout (buf, len, len, printcharfun);
+          }
       }
       break;
 
@@ -2247,6 +2270,13 @@ syms_of_print (void)
 that represents the number without losing information.  */);
   Vfloat_output_format = Qnil;
 
+  DEFVAR_LISP ("integer-output-format", Vinteger_output_format,
+              doc: /* The format used to print integers.
+When t, print characters from integers that represent a character.
+When a number 16, print non-negative integers in the hexadecimal format.
+Otherwise, by default print integers in the decimal format.  */);
+  Vinteger_output_format = Qnil;
+
   DEFVAR_LISP ("print-length", Vprint_length,
               doc: /* Maximum length of list to print before abbreviating.
 A value of nil means no limit.  See also `eval-expression-print-length'.  */);
diff --git a/test/src/print-tests.el b/test/src/print-tests.el
index eb9572dbdf..7b026b6b21 100644
--- a/test/src/print-tests.el
+++ b/test/src/print-tests.el
@@ -383,5 +383,25 @@ print-hash-table-test
       (let ((print-length 1))
         (format "%S" h))))))
 
+(print-tests--deftest print-integer-output-format ()
+  ;; Bug#44155.
+  (let ((integer-output-format t)
+        (syms (list ?? ?\; ?\( ?\) ?\{ ?\} ?\[ ?\] ?\" ?\' ?\\ ?Á)))
+    (should (equal (read (print-tests--prin1-to-string syms)) syms))
+    (should (equal (print-tests--prin1-to-string syms)
+                   (concat "(" (mapconcat #'prin1-char syms " ") ")"))))
+  (let ((integer-output-format t)
+        (syms (list -1 0 1 ?\120 4194175 4194176 (max-char) (1+ (max-char)))))
+    (should (equal (read (print-tests--prin1-to-string syms)) syms)))
+  (let ((integer-output-format 16)
+        (syms (list -1 0 1 most-positive-fixnum (1+ most-positive-fixnum))))
+    (should (equal (read (print-tests--prin1-to-string syms)) syms))
+    (should (equal (print-tests--prin1-to-string syms)
+                   (concat "(" (mapconcat
+                                (lambda (i)
+                                  (if (and (>= i 0) (<= i 
most-positive-fixnum))
+                                      (format "#x%x" i) (format "%d" i)))
+                                syms " ") ")")))))
+
 (provide 'print-tests)
 ;;; print-tests.el ends here