bug#44155: Print integers as characters

[Juri Linkov <juri@linkov.net>]

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>> > 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.
>
> I don't think it's right, FWIW.  Displaying something like \100 would
> be better, IMO.

Sorry, I don't understand why 4194176 could be printed as \100.

>> +@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.
>
> This should mention the functions affected by the variable.
>
>> +** New variable 'integer-output-format' defines the format of integers.
>                                            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> "determines how to print integer values"
>
>> +When this variable is bound to the value 't', integers are printed by
>> +printing functions as characters when an integer represents a character.
>
> Please give at least one example of a function affected by this.

Ok, fixed:


[-- Attachment #2: integer-output-format-4.patch --]
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diff --git a/doc/lispref/streams.texi b/doc/lispref/streams.texi
index 2cd61ad04f..08d8032e6f 100644
--- a/doc/lispref/streams.texi
+++ b/doc/lispref/streams.texi
@@ -902,3 +902,12 @@ 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.
+This variable affects all print functions.
+@end defvar
diff --git a/etc/NEWS b/etc/NEWS
index 5e159480e0..202e449b16 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -1641,6 +1641,12 @@ ledit.el, lmenu.el, lucid.el and old-whitespace.el.
 \f
 * Lisp Changes in Emacs 28.1
 
+** New variable 'integer-output-format' determines how to print integer values.
+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..7b3dc61065 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 +2270,15 @@ 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.
+This variable affects all print functions, for example, such function
+as `print'.  */);
+  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