Re: POC: converting Lists into arrays

Andres Freund <andres@anarazel.de> writes:
> On 2019-07-31 19:40:09 -0400, Tom Lane wrote:
>> That makes the other idea (of a foreach-ish macro declaring the
>> listcell value variable) problematic, too :-(.

> Hm. One way partially around that would be using an anonymous struct
> inside the for(). Something like
> #define foreach_node(membertype, name, lst)    \
> for (struct {membertype *node; ListCell *lc; const List *l; int i;} name = {...}; \
>      ...)
> which then would allow code like

> foreach_node(OpExpr, cur, list)
> {
>     do_something_with_node(cur.node);
>     foreach_delete_current(cur);
> }

I'm hesitant to change the look of our loops quite that much, mainly
because it'll be a pain for back-patching.  If you write some code
for HEAD like this, and then have to back-patch it, you'll need to
insert/change significantly more code than if it's just a matter
of whether there's a ListCell variable or not.

I experimented with the "aforeach" idea I suggested upthread,
to the extent of writing the macros and then converting
parse_clause.c (a file chosen more or less at random) to use
aforeach instead of foreach.  I was somewhat surprised to find
that every single foreach() did convert pleasantly.  (There are
several forboth's that I didn't try to do anything with, though.)

If we do go in this direction, I wouldn't suggest trying to
actually do wholesale conversion of existing code like this;
that seems more likely to create back-patching land mines than
do anything helpful.  I am slightly tempted to try to convert
everyplace using foreach_delete_current, though, since those
loops are different from v12 already.

Thoughts?

            regards, tom lane

diff --git a/src/backend/parser/parse_clause.c b/src/backend/parser/parse_clause.c
index 2a6b2ff..39d8d8e 100644
--- a/src/backend/parser/parse_clause.c
+++ b/src/backend/parser/parse_clause.c
@@ -117,8 +117,6 @@ static Node *transformFrameOffset(ParseState *pstate, int frameOptions,
 void
 transformFromClause(ParseState *pstate, List *frmList)
 {
-    ListCell   *fl;
-
     /*
      * The grammar will have produced a list of RangeVars, RangeSubselects,
      * RangeFunctions, and/or JoinExprs. Transform each one (possibly adding
@@ -128,9 +126,9 @@ transformFromClause(ParseState *pstate, List *frmList)
      * Note we must process the items left-to-right for proper handling of
      * LATERAL references.
      */
-    foreach(fl, frmList)
+    aforeach(frmList)
     {
-        Node       *n = lfirst(fl);
+        Node       *n = (Node *) aforeach_current();
         RangeTblEntry *rte;
         int            rtindex;
         List       *namespace;
@@ -267,11 +265,10 @@ extractRemainingColumns(List *common_colnames,
     {
         char       *colname = strVal(lfirst(lnames));
         bool        match = false;
-        ListCell   *cnames;

-        foreach(cnames, common_colnames)
+        aforeach(common_colnames)
         {
-            char       *ccolname = strVal(lfirst(cnames));
+            char       *ccolname = strVal(aforeach_current());

             if (strcmp(colname, ccolname) == 0)
             {
@@ -475,7 +472,6 @@ transformRangeFunction(ParseState *pstate, RangeFunction *r)
     List       *coldeflists = NIL;
     bool        is_lateral;
     RangeTblEntry *rte;
-    ListCell   *lc;

     /*
      * We make lateral_only names of this level visible, whether or not the
@@ -505,9 +501,9 @@ transformRangeFunction(ParseState *pstate, RangeFunction *r)
      * Likewise, collect column definition lists if there were any.  But
      * complain if we find one here and the RangeFunction has one too.
      */
-    foreach(lc, r->functions)
+    aforeach(r->functions)
     {
-        List       *pair = (List *) lfirst(lc);
+        List       *pair = aforeach_current_node(List);
         Node       *fexpr;
         List       *coldeflist;
         Node       *newfexpr;
@@ -551,11 +547,9 @@ transformRangeFunction(ParseState *pstate, RangeFunction *r)
                 fc->over == NULL &&
                 coldeflist == NIL)
             {
-                ListCell   *lc;
-
-                foreach(lc, fc->args)
+                aforeach(fc->args)
                 {
-                    Node       *arg = (Node *) lfirst(lc);
+                    Node       *arg = (Node *) aforeach_current();
                     FuncCall   *newfc;

                     last_srf = pstate->p_last_srf;
@@ -700,7 +694,6 @@ transformRangeTableFunc(ParseState *pstate, RangeTableFunc *rtf)
     Oid            docType;
     RangeTblEntry *rte;
     bool        is_lateral;
-    ListCell   *col;
     char      **names;
     int            colno;

@@ -743,9 +736,9 @@ transformRangeTableFunc(ParseState *pstate, RangeTableFunc *rtf)
     names = palloc(sizeof(char *) * list_length(rtf->columns));

     colno = 0;
-    foreach(col, rtf->columns)
+    aforeach(rtf->columns)
     {
-        RangeTableFuncCol *rawc = (RangeTableFuncCol *) lfirst(col);
+        RangeTableFuncCol *rawc = aforeach_current_node(RangeTableFuncCol);
         Oid            typid;
         int32        typmod;
         Node       *colexpr;
@@ -837,15 +830,13 @@ transformRangeTableFunc(ParseState *pstate, RangeTableFunc *rtf)
     /* Namespaces, if any, also need to be transformed */
     if (rtf->namespaces != NIL)
     {
-        ListCell   *ns;
-        ListCell   *lc2;
         List       *ns_uris = NIL;
         List       *ns_names = NIL;
         bool        default_ns_seen = false;

-        foreach(ns, rtf->namespaces)
+        aforeach(rtf->namespaces)
         {
-            ResTarget  *r = (ResTarget *) lfirst(ns);
+            ResTarget  *r = aforeach_current_node(ResTarget);
             Node       *ns_uri;

             Assert(IsA(r, ResTarget));
@@ -858,9 +849,9 @@ transformRangeTableFunc(ParseState *pstate, RangeTableFunc *rtf)
             /* Verify consistency of name list: no dupes, only one DEFAULT */
             if (r->name != NULL)
             {
-                foreach(lc2, ns_names)
+                aforeach(ns_names)
                 {
-                    Value       *ns_node = (Value *) lfirst(lc2);
+                    Value       *ns_node = (Value *) aforeach_current();

                     if (ns_node == NULL)
                         continue;
@@ -1268,19 +1259,17 @@ transformFromClauseItem(ParseState *pstate, Node *n,
         if (j->isNatural)
         {
             List       *rlist = NIL;
-            ListCell   *lx,
-                       *rx;

             Assert(j->usingClause == NIL);    /* shouldn't have USING() too */

-            foreach(lx, l_colnames)
+            aforeach(l_colnames)
             {
-                char       *l_colname = strVal(lfirst(lx));
+                char       *l_colname = strVal(aforeach_current());
                 Value       *m_name = NULL;

-                foreach(rx, r_colnames)
+                aforeach(r_colnames)
                 {
-                    char       *r_colname = strVal(lfirst(rx));
+                    char       *r_colname = strVal(aforeach_current());

                     if (strcmp(l_colname, r_colname) == 0)
                     {
@@ -1313,24 +1302,21 @@ transformFromClauseItem(ParseState *pstate, Node *n,
             List       *ucols = j->usingClause;
             List       *l_usingvars = NIL;
             List       *r_usingvars = NIL;
-            ListCell   *ucol;

             Assert(j->quals == NULL);    /* shouldn't have ON() too */

-            foreach(ucol, ucols)
+            aforeach(ucols)
             {
-                char       *u_colname = strVal(lfirst(ucol));
-                ListCell   *col;
-                int            ndx;
+                char       *u_colname = strVal(aforeach_current());
                 int            l_index = -1;
                 int            r_index = -1;
                 Var           *l_colvar,
                            *r_colvar;

                 /* Check for USING(foo,foo) */
-                foreach(col, res_colnames)
+                aforeach(res_colnames)
                 {
-                    char       *res_colname = strVal(lfirst(col));
+                    char       *res_colname = strVal(aforeach_current());

                     if (strcmp(res_colname, u_colname) == 0)
                         ereport(ERROR,
@@ -1340,10 +1326,9 @@ transformFromClauseItem(ParseState *pstate, Node *n,
                 }

                 /* Find it in left input */
-                ndx = 0;
-                foreach(col, l_colnames)
+                aforeach(l_colnames)
                 {
-                    char       *l_colname = strVal(lfirst(col));
+                    char       *l_colname = strVal(aforeach_current());

                     if (strcmp(l_colname, u_colname) == 0)
                     {
@@ -1352,9 +1337,8 @@ transformFromClauseItem(ParseState *pstate, Node *n,
                                     (errcode(ERRCODE_AMBIGUOUS_COLUMN),
                                      errmsg("common column name \"%s\" appears more than once in left table",
                                             u_colname)));
-                        l_index = ndx;
+                        l_index = aforeach_current_index();
                     }
-                    ndx++;
                 }
                 if (l_index < 0)
                     ereport(ERROR,
@@ -1363,10 +1347,9 @@ transformFromClauseItem(ParseState *pstate, Node *n,
                                     u_colname)));

                 /* Find it in right input */
-                ndx = 0;
-                foreach(col, r_colnames)
+                aforeach(r_colnames)
                 {
-                    char       *r_colname = strVal(lfirst(col));
+                    char       *r_colname = strVal(aforeach_current());

                     if (strcmp(r_colname, u_colname) == 0)
                     {
@@ -1375,9 +1358,8 @@ transformFromClauseItem(ParseState *pstate, Node *n,
                                     (errcode(ERRCODE_AMBIGUOUS_COLUMN),
                                      errmsg("common column name \"%s\" appears more than once in right table",
                                             u_colname)));
-                        r_index = ndx;
+                        r_index = aforeach_current_index();
                     }
-                    ndx++;
                 }
                 if (r_index < 0)
                     ereport(ERROR,
@@ -1390,7 +1372,7 @@ transformFromClauseItem(ParseState *pstate, Node *n,
                 r_colvar = list_nth(r_colvars, r_index);
                 r_usingvars = lappend(r_usingvars, r_colvar);

-                res_colnames = lappend(res_colnames, lfirst(ucol));
+                res_colnames = lappend(res_colnames, aforeach_current());
                 res_colvars = lappend(res_colvars,
                                       buildMergedJoinVar(pstate,
                                                          j->jointype,
@@ -1643,11 +1625,9 @@ makeNamespaceItem(RangeTblEntry *rte, bool rel_visible, bool cols_visible,
 static void
 setNamespaceColumnVisibility(List *namespace, bool cols_visible)
 {
-    ListCell   *lc;
-
-    foreach(lc, namespace)
+    aforeach(namespace)
     {
-        ParseNamespaceItem *nsitem = (ParseNamespaceItem *) lfirst(lc);
+        ParseNamespaceItem *nsitem = (ParseNamespaceItem *) aforeach_current();

         nsitem->p_cols_visible = cols_visible;
     }
@@ -1660,11 +1640,9 @@ setNamespaceColumnVisibility(List *namespace, bool cols_visible)
 static void
 setNamespaceLateralState(List *namespace, bool lateral_only, bool lateral_ok)
 {
-    ListCell   *lc;
-
-    foreach(lc, namespace)
+    aforeach(namespace)
     {
-        ParseNamespaceItem *nsitem = (ParseNamespaceItem *) lfirst(lc);
+        ParseNamespaceItem *nsitem = (ParseNamespaceItem *) aforeach_current();

         nsitem->p_lateral_only = lateral_only;
         nsitem->p_lateral_ok = lateral_ok;
@@ -1822,8 +1800,6 @@ static TargetEntry *
 findTargetlistEntrySQL92(ParseState *pstate, Node *node, List **tlist,
                          ParseExprKind exprKind)
 {
-    ListCell   *tl;
-
     /*----------
      * Handle two special cases as mandated by the SQL92 spec:
      *
@@ -1895,9 +1871,9 @@ findTargetlistEntrySQL92(ParseState *pstate, Node *node, List **tlist,
         {
             TargetEntry *target_result = NULL;

-            foreach(tl, *tlist)
+            aforeach(*tlist)
             {
-                TargetEntry *tle = (TargetEntry *) lfirst(tl);
+                TargetEntry *tle = aforeach_current_node(TargetEntry);

                 if (!tle->resjunk &&
                     strcmp(tle->resname, name) == 0)
@@ -1944,9 +1920,9 @@ findTargetlistEntrySQL92(ParseState *pstate, Node *node, List **tlist,
                      parser_errposition(pstate, location)));

         target_pos = intVal(val);
-        foreach(tl, *tlist)
+        aforeach(*tlist)
         {
-            TargetEntry *tle = (TargetEntry *) lfirst(tl);
+            TargetEntry *tle = aforeach_current_node(TargetEntry);

             if (!tle->resjunk)
             {
@@ -1990,7 +1966,6 @@ findTargetlistEntrySQL99(ParseState *pstate, Node *node, List **tlist,
                          ParseExprKind exprKind)
 {
     TargetEntry *target_result;
-    ListCell   *tl;
     Node       *expr;

     /*
@@ -2002,9 +1977,9 @@ findTargetlistEntrySQL99(ParseState *pstate, Node *node, List **tlist,
      */
     expr = transformExpr(pstate, node, exprKind);

-    foreach(tl, *tlist)
+    aforeach(*tlist)
     {
-        TargetEntry *tle = (TargetEntry *) lfirst(tl);
+        TargetEntry *tle = aforeach_current_node(TargetEntry);
         Node       *texpr;

         /*
@@ -2094,7 +2069,6 @@ flatten_grouping_sets(Node *expr, bool toplevel, bool *hasGroupingSets)
         case T_GroupingSet:
             {
                 GroupingSet *gset = (GroupingSet *) expr;
-                ListCell   *l2;
                 List       *result_set = NIL;

                 if (hasGroupingSets)
@@ -2109,9 +2083,9 @@ flatten_grouping_sets(Node *expr, bool toplevel, bool *hasGroupingSets)
                 if (toplevel && gset->kind == GROUPING_SET_EMPTY)
                     return (Node *) NIL;

-                foreach(l2, gset->content)
+                aforeach(gset->content)
                 {
-                    Node       *n1 = lfirst(l2);
+                    Node       *n1 = (Node *) aforeach_current();
                     Node       *n2 = flatten_grouping_sets(n1, false, NULL);

                     if (IsA(n1, GroupingSet) &&
@@ -2139,13 +2113,13 @@ flatten_grouping_sets(Node *expr, bool toplevel, bool *hasGroupingSets)
         case T_List:
             {
                 List       *result = NIL;
-                ListCell   *l;

-                foreach(l, (List *) expr)
+                aforeach((List *) expr)
                 {
-                    Node       *n = flatten_grouping_sets(lfirst(l), toplevel, hasGroupingSets);
+                    Node       *n = (Node *) aforeach_current();

-                    if (n != (Node *) NIL)
+                    n = flatten_grouping_sets(n, toplevel, hasGroupingSets);
+                    if (n)
                     {
                         if (IsA(n, List))
                             result = list_concat(result, (List *) n);
@@ -2200,8 +2174,6 @@ transformGroupClauseExpr(List **flatresult, Bitmapset *seen_local,

     if (tle->ressortgroupref > 0)
     {
-        ListCell   *sl;
-
         /*
          * Eliminate duplicates (GROUP BY x, x) but only at local level.
          * (Duplicates in grouping sets can affect the number of returned
@@ -2239,10 +2211,9 @@ transformGroupClauseExpr(List **flatresult, Bitmapset *seen_local,
          * another sort step is going to be inevitable, but that's the
          * planner's problem.
          */
-
-        foreach(sl, sortClause)
+        aforeach(sortClause)
         {
-            SortGroupClause *sc = (SortGroupClause *) lfirst(sl);
+            SortGroupClause *sc = aforeach_current_node(SortGroupClause);

             if (sc->tleSortGroupRef == tle->ressortgroupref)
             {
@@ -2298,11 +2269,10 @@ transformGroupClauseList(List **flatresult,
 {
     Bitmapset  *seen_local = NULL;
     List       *result = NIL;
-    ListCell   *gl;

-    foreach(gl, list)
+    aforeach(list)
     {
-        Node       *gexpr = (Node *) lfirst(gl);
+        Node       *gexpr = (Node *) aforeach_current();

         Index        ref = transformGroupClauseExpr(flatresult,
                                                    seen_local,
@@ -2349,14 +2319,13 @@ transformGroupingSet(List **flatresult,
                      List **targetlist, List *sortClause,
                      ParseExprKind exprKind, bool useSQL99, bool toplevel)
 {
-    ListCell   *gl;
     List       *content = NIL;

     Assert(toplevel || gset->kind != GROUPING_SET_SETS);

-    foreach(gl, gset->content)
+    aforeach(gset->content)
     {
-        Node       *n = lfirst(gl);
+        Node       *n = (Node *) aforeach_current();

         if (IsA(n, List))
         {
@@ -2371,7 +2340,7 @@ transformGroupingSet(List **flatresult,
         }
         else if (IsA(n, GroupingSet))
         {
-            GroupingSet *gset2 = (GroupingSet *) lfirst(gl);
+            GroupingSet *gset2 = (GroupingSet *) n;

             content = lappend(content, transformGroupingSet(flatresult,
                                                             pstate, gset2,
@@ -2455,7 +2424,6 @@ transformGroupClause(ParseState *pstate, List *grouplist, List **groupingSets,
     List       *result = NIL;
     List       *flat_grouplist;
     List       *gsets = NIL;
-    ListCell   *gl;
     bool        hasGroupingSets = false;
     Bitmapset  *seen_local = NULL;

@@ -2481,9 +2449,9 @@ transformGroupClause(ParseState *pstate, List *grouplist, List **groupingSets,
                                                     exprLocation((Node *) grouplist)));
     }

-    foreach(gl, flat_grouplist)
+    aforeach(flat_grouplist)
     {
-        Node       *gexpr = (Node *) lfirst(gl);
+        Node       *gexpr = (Node *) aforeach_current();

         if (IsA(gexpr, GroupingSet))
         {
@@ -2555,11 +2523,10 @@ transformSortClause(ParseState *pstate,
                     bool useSQL99)
 {
     List       *sortlist = NIL;
-    ListCell   *olitem;

-    foreach(olitem, orderlist)
+    aforeach(orderlist)
     {
-        SortBy       *sortby = (SortBy *) lfirst(olitem);
+        SortBy       *sortby = aforeach_current_node(SortBy);
         TargetEntry *tle;

         if (useSQL99)
@@ -2587,11 +2554,10 @@ transformWindowDefinitions(ParseState *pstate,
 {
     List       *result = NIL;
     Index        winref = 0;
-    ListCell   *lc;

-    foreach(lc, windowdefs)
+    aforeach(windowdefs)
     {
-        WindowDef  *windef = (WindowDef *) lfirst(lc);
+        WindowDef  *windef = aforeach_current_node(WindowDef);
         WindowClause *refwc = NULL;
         List       *partitionClause;
         List       *orderClause;
@@ -2805,8 +2771,6 @@ transformDistinctClause(ParseState *pstate,
                         List **targetlist, List *sortClause, bool is_agg)
 {
     List       *result = NIL;
-    ListCell   *slitem;
-    ListCell   *tlitem;

     /*
      * The distinctClause should consist of all ORDER BY items followed by all
@@ -2823,9 +2787,9 @@ transformDistinctClause(ParseState *pstate,
      * effect will be that the TLE value will be made unique according to both
      * sortops.
      */
-    foreach(slitem, sortClause)
+    aforeach(sortClause)
     {
-        SortGroupClause *scl = (SortGroupClause *) lfirst(slitem);
+        SortGroupClause *scl = aforeach_current_node(SortGroupClause);
         TargetEntry *tle = get_sortgroupclause_tle(scl, *targetlist);

         if (tle->resjunk)
@@ -2843,9 +2807,9 @@ transformDistinctClause(ParseState *pstate,
      * Now add any remaining non-resjunk tlist items, using default sort/group
      * semantics for their data types.
      */
-    foreach(tlitem, *targetlist)
+    aforeach(*targetlist)
     {
-        TargetEntry *tle = (TargetEntry *) lfirst(tlitem);
+        TargetEntry *tle = aforeach_current_node(TargetEntry);

         if (tle->resjunk)
             continue;            /* ignore junk */
@@ -2902,9 +2866,9 @@ transformDistinctOnClause(ParseState *pstate, List *distinctlist,
      * Also notice that we could have duplicate DISTINCT ON expressions and
      * hence duplicate entries in sortgrouprefs.)
      */
-    foreach(lc, distinctlist)
+    aforeach(distinctlist)
     {
-        Node       *dexpr = (Node *) lfirst(lc);
+        Node       *dexpr = (Node *) aforeach_current();
         int            sortgroupref;
         TargetEntry *tle;

@@ -2923,9 +2887,9 @@ transformDistinctOnClause(ParseState *pstate, List *distinctlist,
      * in DISTINCT ON.
      */
     skipped_sortitem = false;
-    foreach(lc, sortClause)
+    aforeach(sortClause)
     {
-        SortGroupClause *scl = (SortGroupClause *) lfirst(lc);
+        SortGroupClause *scl = aforeach_current_node(SortGroupClause);

         if (list_member_int(sortgrouprefs, scl->tleSortGroupRef))
         {
@@ -3021,11 +2985,10 @@ resolve_unique_index_expr(ParseState *pstate, InferClause *infer,
                           Relation heapRel)
 {
     List       *result = NIL;
-    ListCell   *l;

-    foreach(l, infer->indexElems)
+    aforeach(infer->indexElems)
     {
-        IndexElem  *ielem = (IndexElem *) lfirst(l);
+        IndexElem  *ielem = aforeach_current_node(IndexElem);
         InferenceElem *pInfer = makeNode(InferenceElem);
         Node       *parse;

@@ -3422,16 +3385,15 @@ Index
 assignSortGroupRef(TargetEntry *tle, List *tlist)
 {
     Index        maxRef;
-    ListCell   *l;

     if (tle->ressortgroupref)    /* already has one? */
         return tle->ressortgroupref;

     /* easiest way to pick an unused refnumber: max used + 1 */
     maxRef = 0;
-    foreach(l, tlist)
+    aforeach(tlist)
     {
-        Index        ref = ((TargetEntry *) lfirst(l))->ressortgroupref;
+        Index        ref = aforeach_current_node(TargetEntry)->ressortgroupref;

         if (ref > maxRef)
             maxRef = ref;
@@ -3463,15 +3425,14 @@ bool
 targetIsInSortList(TargetEntry *tle, Oid sortop, List *sortList)
 {
     Index        ref = tle->ressortgroupref;
-    ListCell   *l;

     /* no need to scan list if tle has no marker */
     if (ref == 0)
         return false;

-    foreach(l, sortList)
+    aforeach(sortList)
     {
-        SortGroupClause *scl = (SortGroupClause *) lfirst(l);
+        SortGroupClause *scl = aforeach_current_node(SortGroupClause);

         if (scl->tleSortGroupRef == ref &&
             (sortop == InvalidOid ||
@@ -3489,11 +3450,9 @@ targetIsInSortList(TargetEntry *tle, Oid sortop, List *sortList)
 static WindowClause *
 findWindowClause(List *wclist, const char *name)
 {
-    ListCell   *l;
-
-    foreach(l, wclist)
+    aforeach(wclist)
     {
-        WindowClause *wc = (WindowClause *) lfirst(l);
+        WindowClause *wc = aforeach_current_node(WindowClause);

         if (wc->name && strcmp(wc->name, name) == 0)
             return wc;
diff --git a/src/include/nodes/pg_list.h b/src/include/nodes/pg_list.h
index 1463408..21d0a67 100644
--- a/src/include/nodes/pg_list.h
+++ b/src/include/nodes/pg_list.h
@@ -381,6 +381,59 @@ lnext(const List *l, const ListCell *c)
 #define foreach_current_index(cell)  (cell##__state.i)

 /*
+ * aforeach (anonymous foreach) -
+ *      another convenience macro for looping through a list
+ *
+ * This is the same as foreach() except that no ListCell variable is needed.
+ * Instead, reference the current list element with the appropriate variant
+ * of aforeach_current().
+ */
+#define aforeach(lst)    \
+    for (ForEachState aforeach__state = {(lst), 0}; \
+         aforeach__state.l != NIL && \
+         aforeach__state.i < aforeach__state.l->length; \
+         aforeach__state.i++)
+
+/*
+ * aforeach_current... -
+ *
+ * Access the current element of the most closely nested aforeach() loop.
+ * These exist in the same variants that lfirst...() has.
+ */
+#define aforeach_current()    \
+    (aforeach__state.l->elements[aforeach__state.i].ptr_value)
+#define aforeach_current_int()    \
+    (aforeach__state.l->elements[aforeach__state.i].int_value)
+#define aforeach_current_oid()    \
+    (aforeach__state.l->elements[aforeach__state.i].oid_value)
+#define aforeach_current_node(type)    \
+    castNode(type, aforeach_current())
+
+/*
+ * aforeach_delete_current -
+ *      delete the current list element from the List associated with the most
+ *      closely nested aforeach() loop, returning the new List pointer.
+ *
+ * This is equivalent to list_delete_nth_cell(), but it also adjusts the
+ * aforeach loop's state so that no list elements will be missed.  Do not
+ * delete elements from an active aforeach loop's list in any other way!
+ */
+#define aforeach_delete_current()    \
+    ((List *) (aforeach__state.l = list_delete_nth_cell(aforeach__state.l, \
+                                                        aforeach__state.i--)))
+
+/*
+ * aforeach_current_index -
+ *      get the zero-based list index of the most closely nested aforeach()
+ *      loop's current element.
+ *
+ * Beware of using this after aforeach_delete_current(); the value will be
+ * out of sync for the rest of the current loop iteration.  Anyway, since
+ * you just deleted the current element, the value is pretty meaningless.
+ */
+#define aforeach_current_index()  (aforeach__state.i)
+
+/*
  * for_each_cell -
  *      a convenience macro which loops through a list starting from a
  *      specified cell