Implement print-circle, allowing recursive and circular structures to be read. src/ChangeLog addition: 2011-09-04 Aidan Kehoe <kehoea@parhasard.net> * alloc.c: * alloc.c (ALLOC_FROB_BLOCK_LISP_OBJECT_1): * alloc.c (ALLOC_FROB_BLOCK_LISP_OBJECT): * alloc.c (cons_print_preprocess): * alloc.c (vector_print_preprocess): * alloc.c (vector_nsubst_structures_descend): * alloc.c (Fmake_symbol): * alloc.c (UNMARK_symbol): * alloc.c (sweep_symbols): * alloc.c (reinit_alloc_objects_early): * alloc.c (reinit_alloc_early): * bytecode.c: * bytecode.c (compiled_function_print_preprocess): * bytecode.c (compiled_function_nsubst_structures_descend): * bytecode.c (set_compiled_function_arglist): * bytecode.c (set_compiled_function_interactive): * bytecode.c (bytecode_objects_create): * chartab.c: * chartab.c (print_preprocess_mapper): * chartab.c (nsubst_structures_mapper): * chartab.c (char_table_nsubst_structures_descend): * chartab.c (chartab_objects_create): * elhash.c: * elhash.c (nsubst_structures_map_hash_table): * elhash.c (hash_table_nsubst_structures_descend): * elhash.c (print_preprocess_mapper): * elhash.c (hash_table_print_preprocess): * elhash.c (inchash_eq): * elhash.c (hash_table_objects_create): * elhash.c (syms_of_elhash): * elhash.h: * emacs.c (main_1): * fns.c: * fns.c (check_eq_nokey): * fns.c (Fnsubst): * fns.c (syms_of_fns): * lisp.h: * lisp.h (struct Lisp_Symbol): * lisp.h (IN_OBARRAY): * lisp.h (struct): * lisp.h (PRINT_PREPROCESS): * lread.c (read1): * lrecord.h: * lrecord.h (struct lrecord_implementation): * lrecord.h (DEFINE_DUMPABLE_MODULE_LISP_OBJECT): * print.c: * print.c (PRINT_CIRCLE_LIMIT): * print.c (print_continuous_numbering_changed): * print.c (print_prepare): * print.c (print_finish): * print.c (Fprin1_to_string): * print.c (print_cons): * print.c (print_preprocess_inchash_eq): * print.c (print_preprocess): * print.c (print_sort_get_numbers): * print.c (print_sort_compare_ordinals): * print.c (print_gensym_or_circle): * print.c (nsubst_structures_descend): * print.c (nsubst_structures): * print.c (print_internal): * print.c (print_symbol): * print.c (vars_of_print): * rangetab.c: * rangetab.c (range_table_print_preprocess): * rangetab.c (range_table_nsubst_structures_descend): * rangetab.c (rangetab_objects_create): * rangetab.c (syms_of_rangetab): * symbols.c: * symbols.c (symbol_print_preprocess): * symbols.c (Fintern): * symbols.c (Funintern): * symbols.c (reinit_symbol_objects_early): * symbols.c (init_symbols_once_early): * symsinit.h: Implement print-circle, printing circular structures in a readable fashion, and treating them appropriately on read. This is by means of two new object methods, print_preprocess (detecting circularities), and nsubst_structures_descend (replacing placeholders with the read objects). Expose the substitution to Lisp via #'nsubst and its new :descend-structures keyword. Store information as to whether symbols are interned in obarray or not in their header, making checking for keywords and uninterned symbols (and thus printing) cheaper. Default print_gensym to t, as Common Lisp does, and as a more-than-decade old comment suggests. lisp/ChangeLog addition: 2011-09-04 Aidan Kehoe <kehoea@parhasard.net> * bytecomp.el (byte-compile-output-file-form): * bytecomp.el (byte-compile-output-docform): Bind print-circle, print-continuous-numbering in these functions, now those variables are available. * lisp.el (forward-sexp): * lisp.el (backward-sexp): Recognise leading #N= as being part of an expression. tests/ChangeLog addition: 2011-09-04 Aidan Kehoe <kehoea@parhasard.net> * automated/lisp-reader-tests.el: * automated/lisp-tests.el (literal-with-uninterned): * automated/symbol-tests.el (foo): Test print-circle, for printing (mutually-)recursive and circular structures. Bind print-continuous-numbering where appropriate.

--- a/src/symbols.c	Sun Aug 28 10:34:54 2011 +0100
+++ b/src/symbols.c	Sun Sep 04 19:51:35 2011 +0100
@@ -139,6 +139,16 @@
   return external_remprop (&XSYMBOL (symbol)->plist, property, 0, ERROR_ME);
 }
 
+static void
+symbol_print_preprocess (Lisp_Object UNUSED (symbol),
+                         Lisp_Object UNUSED (print_number_table),
+                         Elemcount * UNUSED (seen_object_count))
+{
+  /* This method is empty; symbols are handled specially in
+     print_preprocess, because print_preprocess_inchash_eq() is conditional
+     for them, rather than a given. */
+}
+
 DEFINE_DUMPABLE_FROB_BLOCK_LISP_OBJECT ("symbol", symbol,
 					mark_symbol, print_symbol,
 					0, 0, 0, symbol_description,
@@ -250,6 +260,9 @@
     XSYMBOL_NEXT (symbol) = 0;
   *ptr = object;
 
+  XSYMBOL (object)->u.v.package_count = 1;
+  XSYMBOL (object)->u.v.first_package_id = (EQ (obarray, Vobarray)) ? 1 : 2;
+
   if (string_byte (XSYMBOL_NAME (symbol), 0) == ':' && EQ (obarray, Vobarray))
     {
       /* The LISP way is to put keywords in their own package, but we
@@ -329,10 +342,19 @@
 
   if (EQ (XVECTOR_DATA (obarray)[hash], tem))
     {
+      unsigned int package_count = XSYMBOL (tem)->u.v.package_count;
       if (XSYMBOL (tem)->next)
 	XVECTOR_DATA (obarray)[hash] = wrap_symbol (XSYMBOL (tem)->next);
       else
 	XVECTOR_DATA (obarray)[hash] = Qzero;
+      if (package_count > 0)
+        {
+          if (1 == package_count)
+            {
+              XSYMBOL (tem)->u.v.first_package_id = 0;
+            }
+          XSYMBOL (tem)->u.v.package_count = package_count - 1;
+        }
     }
   else
     {
@@ -345,7 +367,17 @@
 	  following = wrap_symbol (XSYMBOL (tail)->next);
 	  if (EQ (following, tem))
 	    {
+              unsigned int package_count = XSYMBOL (tem)->u.v.package_count;
 	      XSYMBOL (tail)->next = XSYMBOL (following)->next;
+
+              if (package_count > 0)
+                {
+                  if (1 == package_count)
+                    {
+                      XSYMBOL (tem)->u.v.first_package_id = 0;
+                    }
+                  XSYMBOL (tem)->u.v.package_count = package_count - 1;
+                }
 	      break;
 	    }
 	}
@@ -3536,6 +3568,7 @@
   OBJECT_HAS_METHOD (symbol, getprop);
   OBJECT_HAS_METHOD (symbol, putprop);
   OBJECT_HAS_METHOD (symbol, remprop);
+  OBJECT_HAS_METHOD (symbol, print_preprocess);
   OBJECT_HAS_NAMED_METHOD (symbol, plist, Fsymbol_plist);
   OBJECT_HAS_NAMED_METHOD (symbol, setplist, Fsetplist);
 }
@@ -3557,6 +3590,8 @@
   XSTRING_PLIST (XSYMBOL (Qnil)->name) = Qnil;
   XSYMBOL (Qnil)->value = Qnil; /* Nihil ex nihilo */
   XSYMBOL (Qnil)->plist = Qnil;
+  XSYMBOL (Qnil)->u.v.package_count = 1;
+  XSYMBOL (Qnil)->u.v.first_package_id = 1;
 
   Vobarray = make_vector (OBARRAY_SIZE, Qzero);
   initial_obarray = Vobarray;