ADDED: portray_var/2 and flag auto_name_variables

boot/init.pl

@@ -999,9 +999,9 @@
     !.
 
 :- dynamic
-    user:portray/1.
+    user:portray/1, system:portray_var/2.
 :- multifile
-    user:portray/1.
+    user:portray/1, system:portray_var/2.
 
 
                  /*******************************

boot/toplevel.pl

@@ -606,6 +606,7 @@
                        Keep),
     create_prolog_flag(toplevel_residue_vars, false, Keep),
     create_prolog_flag(toplevel_list_wfs_residual_program, true, Keep),
+    create_prolog_flag(auto_name_variables, false, Keep),
     '$set_debugger_write_options'(print).
 
 %!  setup_backtrace
@@ -840,6 +841,10 @@
     ),
     trim_stacks,
     trim_heap,
+    (   BreakLev == 0
+    ->  reset_variable_names
+    ;   true
+    ),
     repeat,
       read_query(Prompt, Query, Bindings),
       prompt(_, Old),
@@ -1283,6 +1288,68 @@
     diff21(T2, H1, T1, Diff).
 
 
+:- multifile system:portray_var/2, user:portray_var/2.
+:- dynamic system:portray_var/2, user:portray_var/2.
+
+system:portray_var(Var, Index) :-
+    predicate_property(user:portray_var(_,_), ssu),
+    catch(
+        \+ \+ user:portray_var(Var, Index),
+        error(existence_error(matching_rule, _), _),
+        fail
+    ),
+    !.
+
+system:portray_var(Var, _Index) :-
+    current_prolog_flag(auto_name_variables, true),
+    $,
+    (   nb_current('$named_variables', NVars)
+    ->  true
+    ;   NVars = [end]
+    ),
+
+    var_nth0(Index, NVars, Var),
+    phrase(portray_var_name(Index), Codes),
+    format('~s', [Codes]),
+    nb_linkval('$named_variables', NVars).
+
+var_nth0(N, List, Var) :-
+    var_nth(List, Var, 0, N).
+var_nth(End, Var, N0, N) :-
+    End = [end],
+    !,
+    nb_linkarg(1, End, Var),
+    nb_setarg(2, End, [end]),
+    N = N0.
+var_nth([H|_], Var, N0, N) :-
+    H == Var,
+    !,
+    N = N0.
+var_nth([_|T], Var, N0, N) :-
+    succ(N0, N1),
+    var_nth(T, Var, N1, N).
+
+portray_var_name(Index) -->
+    `__`,
+    {   divmod(Index, 26, Rank, Letter),
+        LCode is Letter + 0'A
+    },
+    [LCode],
+    (   { Rank > 0, number_codes(Rank, RCodes) }
+    ->  RCodes
+    ;   []
+    ).
+
+%!  reset_variable_names is det.
+%
+%   Reset the current list of "variables seen" by the Prolog flag
+%   `auto_name_variables`, causing the next variable it prints to be
+%   named `__A`. This is called automatically by the toplevel whenever
+%   a query prompt is issued at break level 0.
+
+reset_variable_names :-
+    nb_delete('$named_variables').
+
 %!  project_constraints(+Bindings, +ResidueVars) is det.
 %
 %   Call   <module>:project_attributes/2   if   the    Prolog   flag

customize/init.pl

@@ -80,6 +80,11 @@
 
 % :- set_prolog_flag(toplevel_print_anon, false).
 
+%	If you want unnamed variables to be printed using names (__A, __B)
+%	instead of ids (_1246, _1248).
+
+% :- set_prolog_flag(auto_name_variables, true).
+
 %	If you do not want the tracer to stop at at the exit port.
 
 % :- leash(-exit).

man/builtin.doc

@@ -6314,7 +6314,8 @@ current priority.
     \termitem{portrayed}{Bool}
 If \const{true}, the hook portray/1 is called before printing a term
 that is not a variable.  If portray/1 succeeds, the term is considered
-printed.  See also print/1.  The default is \const{false}.  This option
+printed.  Similarly, the hook portray_var/3 is called before printing
+a variable.  See also print/1.  The default is \const{false}.  This option
 is an extension to the ISO write_term options.
 
     \termitem{priority}{Integer}
@@ -6472,6 +6473,21 @@ is not a variable print/1 first calls portray/1 using the term as
 argument.  For lists, only the list as a whole is given to portray/1.  If
 portray/1 succeeds print/1 assumes the term has been written.
 
+    \predicate{portray_var}{3}{+Var, +Id, +WriteOptions}
+A dynamic predicate, which can be defined by the user to change the
+behavior of print/1 on unbound variables. It is used by the Prolog flag
+\prologflag{auto_name_variables} to print variables with names. If defined
+by the user, it will be called before the auto_name_variables functionality
+regardless of the state of that flag, and passed the variable (which the
+hook should be careful not to instantiate), the variable's ID (the number
+that would be printed following an underscore) and the options to write_term/2
+that caused it to be printed. If portray_var/3 fails, Prolog will fall back
+to the \prologflag{auto_name_variables} functionality if enabled, or otherwise
+just print the default numeric variable ID following an underscore.
+
+This predicate must be defined using SSU (=>)/2 rules, to avoid accidental
+instantiation of the variable being portrayed.
+
     \predicate[ISO]{read}{1}{-Term}
 Read the next \textbf{Prolog term} from the current input stream and
 unify it with \arg{Term}. On reaching end-of-file \arg{Term} is unified

man/overview.doc

@@ -1165,6 +1165,13 @@ definition is more practical.}
 Set if Prolog was started with a prolog file as argument.  Used by
 e.g., edit/0 to edit the initial file.
 
+    \prologflagitem{auto_name_variables}{bool}{rw}
+If \const{true}, variables printed with the \const{portray(true)} option
+of write_term/2 will be tracked and named by identity in the order they
+appear, in the sequence (\const{__A} .. \const{__Z}, \const{__A1} ..
+\const{__Z1}, etc). The sequence will be reset every time a query prompt
+is issued at break level 0.
+
     \prologflagitem{autoload}{atom}{rw}
 This flag controls autoloading predicates based on autoload/1 and
 autoload/2 as well as predicates from \jargon{autoload libraries}.
@@ -2299,6 +2306,9 @@ portability.
 \begin{itemlist}
     \item [portray/1]
 Hook into write_term/3 to alter the way terms are printed (ISO).
+    \item [portray_var/3]
+Extension to the portray/1 functionality to alter the way variables
+are printed.
     \item [message_hook/3]
 Hook into print_message/2 to alter the way system messages are printed
 (Quintus/SICStus).

src/pl-global.h

@@ -321,6 +321,7 @@ struct PL_global_data
     Procedure	prolog_trace_interception4;
     Procedure	prolog_break_hook6;	/* prolog:break_hook/6 */
     Procedure	portray;		/* portray/1 */
+    Procedure	portray_var2;		/* system:portray_var/2 */
     Procedure   dcall1;			/* $call/1 */
     Procedure   call3;			/* call/3*/
     Procedure	setup_call_catcher_cleanup4; /* setup_call_catcher_cleanup/4 */

src/pl-write.c

@@ -76,6 +76,7 @@ typedef struct
 } write_options;
 
 #if USE_LD_MACROS
+#define	var_index_ptr(p)	LDFUNC(var_index_ptr, p)
 #define	enterPortray(_)		LDFUNC(enterPortray, _)
 #define	leavePortray(_)		LDFUNC(leavePortray, _)
 #endif /*USE_LD_MACROS*/
@@ -96,8 +97,8 @@ static void	leavePortray(void);
 
 #undef LDFUNC_DECLARATIONS
 
-char *
-var_name_ptr(DECL_LD Word p, char *name)
+static int64_t
+var_index_ptr(DECL_LD Word p)
 { size_t iref;
 
   deRef(p);
@@ -106,8 +107,12 @@ var_name_ptr(DECL_LD Word p, char *name)
     iref = ((Word)p - (Word)lBase)*2+1;
   else
     iref = ((Word)p - (Word)gBase)*2;
+  return (int64_t) iref;
+}
 
-  Ssprintf(name, "_%lld", (int64_t)iref);
+char *
+var_name_ptr(DECL_LD Word p, char *name)
+{ Ssprintf(name, "_%lld", var_index_ptr(p));
 
   return name;
 }
@@ -1315,7 +1320,9 @@ callPortray(term_t arg, int prec, write_options *options)
   if ( GD->cleaning > CLN_PROLOG )
     fail;				/* avoid dangerous callbacks */
 
-  if ( options->portray_goal )
+  if ( PL_is_variable(arg) )
+  { pred = _PL_predicate("portray_var", 2, "system", &GD->procedures.portray_var2);
+  } else if ( options->portray_goal )
   { pred = _PL_predicate("call", 3, "user", &GD->procedures.call3);
   } else
   { pred = _PL_predicate("portray", 1, "user", &GD->procedures.portray);
@@ -1334,7 +1341,11 @@ callPortray(term_t arg, int prec, write_options *options)
     if ( !saveWakeup(&wstate, TRUE) )
       return -1;
     Scurout = options->out;
-    if ( options->portray_goal )
+    if ( PL_is_variable(arg) )
+    { av = PL_new_term_refs(2);
+      PL_put_term(av+0, arg);
+      PL_put_int64(av+1, var_index_ptr(valTermRef(arg)));
+    } else if ( options->portray_goal )
     { av = PL_new_term_refs(3);
 
       PL_put_term(av+0, options->portray_goal);
@@ -1510,8 +1521,7 @@ writeTerm2(term_t t, int prec, write_options *options, bool arg)
   atom_t a;
   IOSTREAM *out = options->out;
 
-  if ( !PL_is_variable(t) &&
-       true(options, PL_WRT_PORTRAY) )
+  if ( true(options, PL_WRT_PORTRAY) )
   { switch( callPortray(t, prec, options) )
     { case TRUE:
 	return TRUE;