add weak sets

* libguile/weak-set.c:
* libguile/weak-set.h: New files, implementing weak sets, for use in the
  symbol table and port set.  Eventually we will be able to remove weak
  pairs.

* libguile.h:
* libguile/Makefile.am: Add new files.

* libguile/evalext.c:
* libguile/gc.c:
* libguile/init.c:
* libguile/print.c:
* libguile/tags.h: Add support for the new types.

libguile/weak-set.h

@@ -0,0 +1,69 @@
+/* classes: h_files */
+
+#ifndef SCM_WEAK_SET_H
+#define SCM_WEAK_SET_H
+
+/* Copyright (C) 2011 Free Software Foundation, Inc.
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public License
+ * as published by the Free Software Foundation; either version 3 of
+ * the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
+ * 02110-1301 USA
+ */
+
+
+
+#include "libguile/__scm.h"
+
+
+
+/* The weak set API is currently only used internally.  We could make it
+   public later, after some API review.  */
+
+/* Function that returns nonzero if the given object is the one we are
+   looking for.  */
+typedef int (*scm_t_set_predicate_fn) (SCM obj, void *closure);
+
+/* Function to fold over the elements of a set.  */
+typedef SCM (*scm_t_set_fold_fn) (void *closure, SCM key, SCM result);
+
+SCM_INTERNAL SCM scm_c_make_weak_set (unsigned long k);
+SCM_INTERNAL SCM scm_weak_set_p (SCM h);
+SCM_INTERNAL SCM scm_c_weak_set_lookup (SCM set, unsigned long raw_hash,
+                                        scm_t_set_predicate_fn pred,
+                                        void *closure, SCM dflt);
+SCM_INTERNAL SCM scm_c_weak_set_add_x (SCM set, unsigned long raw_hash,
+                                       scm_t_set_predicate_fn pred,
+                                       void *closure, SCM obj);
+SCM_INTERNAL void scm_c_weak_set_remove_x (SCM set, unsigned long raw_hash,
+                                           scm_t_set_predicate_fn pred,
+                                           void *closure);
+SCM_INTERNAL SCM scm_weak_set_add_x (SCM set, SCM obj);
+SCM_INTERNAL SCM scm_weak_set_remove_x (SCM set, SCM obj);
+SCM_INTERNAL SCM scm_weak_set_clear_x (SCM set);
+SCM_INTERNAL SCM scm_c_weak_set_fold (scm_t_set_fold_fn proc, void *closure,
+                                      SCM init, SCM set);
+SCM_INTERNAL SCM scm_weak_set_fold (SCM proc, SCM init, SCM set);
+SCM_INTERNAL SCM scm_weak_set_for_each (SCM proc, SCM set);
+SCM_INTERNAL SCM scm_weak_set_map_to_list (SCM proc, SCM set);
+
+SCM_INTERNAL void scm_i_weak_set_print (SCM exp, SCM port, scm_print_state *pstate);
+SCM_INTERNAL void scm_init_weak_set (void);
+
+#endif  /* SCM_WEAK_SET_H */
+
+/*
+  Local Variables:
+  c-file-style: "gnu"
+  End:
+*/