RFC: d: add support for %printer

[Akim Demaille]

Now the traces look more decent:

> $ echo '1+2' | YYDEBUG=1 ./_build/g9d/examples/d/calc/calc
> Starting parse
> Entering state 0
> [...]
> Reducing stack by rule 7 (line 63):
>    $1 = nterm exp (1.1: 1)
>    $2 = token + (1.2: )
>    $3 = nterm exp (1.3: 2)
> -> $$ = nterm exp (1.1-3: 3)

I'd like to get some approval before installing it.  The test suite is happy.  
But we need a real test for %printer in D.


commit ec584adae379f63f7083166ea797abe0407e8658
Author: Akim Demaille <akim.demaille@gmail.com>
Date:   Wed Jan 6 19:22:18 2021 +0100

    d: add support for %printer
    
    Currently we display the addresses of the semantic values.  Instead,
    print the values.
    
    Add support for YY_USE across languages.
    
    * data/skeletons/c.m4, data/skeletons/d.m4 (b4_use): New.
    * data/skeletons/bison.m4 (b4_symbol_actions): Use b4_use to be
    portable to D.
    
    Add support for %printer, and use it.
    
    * data/skeletons/d.m4: Instead of duplicating what's already in
    c-like.m4, include it.
    (b4_symbol_action): New.
    Differs from the one in bison.m4 in that it uses yyval/yyloc instead
    of *yyvaluep and *yylocationp.
    
    * data/skeletons/lalr1.d (yy_symbol_print): Avoid calls to formatting,
    just call write directly.
    Use the %printer.
    * examples/d/calc/calc.y: Specify a printer.
    Enable traces when $YYDEBUG is set.
    * tests/calc.at: Fix the use of %printer with D.

diff --git a/data/skeletons/bison.m4 b/data/skeletons/bison.m4
index c405b62d..ace9d524 100644
--- a/data/skeletons/bison.m4
+++ b/data/skeletons/bison.m4
@@ -555,7 +555,7 @@ m4_defn([b4_actions_])[]dnl
         break;
     }dnl
 ],
-[YY_USE (m4_default([$2], [yykind]));])dnl
+[b4_use(m4_default([$2], [yykind]));])dnl
 m4_popdef([b4_actions_])dnl
 ])
 
diff --git a/data/skeletons/c.m4 b/data/skeletons/c.m4
index 1a2eb917..ed2c762c 100644
--- a/data/skeletons/c.m4
+++ b/data/skeletons/c.m4
@@ -163,13 +163,20 @@ m4_popdef([$2])dnl
 m4_popdef([$1])dnl
 ])])
 
+
+# b4_use(EXPR)
+# ------------
+# Pacify the compiler about some maybe unused value.
+m4_define([b4_use],
+[YY_USE ($1)])
+
 # b4_parse_param_use([VAL], [LOC])
 # --------------------------------
 # 'YY_USE' VAL, LOC if locations are enabled, and all the parse-params.
 m4_define([b4_parse_param_use],
-[m4_ifvaln([$1], [  YY_USE ([$1]);])dnl
-b4_locations_if([m4_ifvaln([$2], [  YY_USE ([$2]);])])dnl
-b4_parse_param_for([Decl], [Formal], [  YY_USE (Formal);
+[m4_ifvaln([$1], [  b4_use([$1]);])dnl
+b4_locations_if([m4_ifvaln([$2], [  b4_use([$2]);])])dnl
+b4_parse_param_for([Decl], [Formal], [  b4_use(Formal);
 ])dnl
 ])
 
diff --git a/data/skeletons/d.m4 b/data/skeletons/d.m4
index cfb87944..e0a554b9 100644
--- a/data/skeletons/d.m4
+++ b/data/skeletons/d.m4
@@ -18,24 +18,33 @@
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
 
-# _b4_comment(TEXT, OPEN, CONTINUE, END)
-# --------------------------------------
-# Put TEXT in comment.  Avoid trailing spaces: don't indent empty lines.
-# Avoid adding indentation to the first line, as the indentation comes
-# from OPEN.  That's why we don't patsubst([$1], [^\(.\)], [   \1]).
-#
-# Prefix all the output lines with PREFIX.
-m4_define([_b4_comment],
-[$2[]m4_bpatsubst(m4_expand([[$1]]), [
-\(.\)], [
-$3\1])$4])
-
-
-# b4_comment(TEXT, [PREFIX])
-# --------------------------
-# Put TEXT in comment.  Prefix all the output lines with PREFIX.
-m4_define([b4_comment],
-[_b4_comment([$1], [$2/* ], [$2   ], [  */])])
+m4_include(b4_skeletonsdir/[c-like.m4])
+
+
+# b4_symbol_action(SYMBOL-NUM, ACTION)
+# ------------------------------------
+# Run the action ACTION ("destructor" or "printer") for SYMBOL-NUM.
+m4_define([b4_symbol_action],
+[b4_symbol_if([$1], [has_$2],
+[b4_dollar_pushdef([yyval],
+                   [$1],
+                   [],
+                   [yyloc])dnl
+    _b4_symbol_case([$1])[]dnl
+b4_syncline([b4_symbol([$1], [$2_line])], [b4_symbol([$1], [$2_file])])dnl
+b4_symbol([$1], [$2])
+b4_syncline([@oline@], [@ofile@])dnl
+        break;
+
+b4_dollar_popdef[]dnl
+])])
+
+
+# b4_use(EXPR)
+# ------------
+# Pacify the compiler about some maybe unused value.
+m4_define([b4_use],
+[])
 
 
 # b4_sync_start(LINE, FILE)
diff --git a/data/skeletons/lalr1.d b/data/skeletons/lalr1.d
index e97613b6..538d6d91 100644
--- a/data/skeletons/lalr1.d
+++ b/data/skeletons/lalr1.d
@@ -399,20 +399,17 @@ b4_user_union_members
   `--------------------------------*/
 
   private final void yy_symbol_print (string s, SymbolKind yykind,
-    ref Value yyvaluep]dnl
-b4_locations_if([, ref Location yylocationp])[)
+    ref Value yyval]b4_locations_if([, ref Location yyloc])[)
   {
     if (0 < yydebug)
     {
-      string message = s ~ (yykind < yyntokens_ ? " token " : " nterm ")
-              ~ format("%s", yykind) ~ " ("]b4_locations_if([
-              ~ yylocationp.toString() ~ ": "])[;
-      static if (__traits(compiles, message ~= yyvaluep.toString()))
-              message ~= yyvaluep.toString();
-      else
-              message ~= format("%s", &yyvaluep);
-      message ~= ")";
-      yycdebugln (message);
+      File yyo = yyDebugStream;
+      yyo.write(s);
+      yyo.write(yykind < yyntokens_ ? " token " : " nterm ");
+      yyo.write(format("%s", yykind));
+      yyo.write(" ("]b4_locations_if([ ~ yyloc.toString() ~ ": "])[);
+      ]b4_symbol_actions([printer])[
+      yyo.write(")\n");
     }
   }
 ]])[
diff --git a/examples/d/calc/calc.y b/examples/d/calc/calc.y
index 572b10b6..765ba382 100644
--- a/examples/d/calc/calc.y
+++ b/examples/d/calc/calc.y
@@ -21,6 +21,7 @@
 
 %define api.parser.class {Calc}
 %define parse.error verbose
+%define parse.trace
 
 %locations
 
@@ -38,6 +39,7 @@
        EOL    "end of line"
 %token <ival> NUM "number"
 %type  <ival> exp
+%printer { yyo.write($$); } <ival>
 
 %left "-" "+"
 %left "*" "/"
@@ -168,6 +170,9 @@ int main()
 {
   auto l = calcLexer(stdin);
   auto p = new Calc(l);
+  import core.stdc.stdlib : getenv;
+  if (getenv("YYDEBUG"))
+    p.setDebugLevel(1);
   p.parse();
   return l.exit_status;
 }
diff --git a/tests/calc.at b/tests/calc.at
index 3d319645..77707b89 100644
--- a/tests/calc.at
+++ b/tests/calc.at
@@ -652,7 +652,7 @@ m4_define([_AT_DATA_CALC_Y(d)],
 {
   semantic_value ival;
 };
-%printer { fprintf (yyo, "%d", $$); } <ival>;
+%printer { yyo.write($$); } <ival>;
 
 %code {
 ]AT_TOKEN_TRANSLATE_IF([[