Let a fold in a column read the column's row

A count, sum, max or min inside a table's column may now name the row the
column is evaluated for: per branch, the sum of its accounts
(sum(a.balance for a in accounts where a.bid == b.bid)), or per account,
how many are richer. The column's row inside the fold compiles to
OUTER_ROW, which Pure's verifier accepts only in a fold that runs in a
column; the fold's own item stays ROW. This is a correlated subquery, the
form SQL writes GROUP BY and self-joins with.

2 files changed, 6 insertions, 6 deletions

diff --git a/compiler/compile.c b/compiler/compile.c
index 3c4f0cc..6cfb903 100644
--- a/compiler/compile.c
+++ b/compiler/compile.c
@@ -17,7 +17,7 @@ enum { CONST, LOAD, DT, ADD, SUB, MUL, DIV, NEG, LT, LE, GT, GE, EQ, NE, AND, OR
        CUR_LOAD, CUR_START, CUR_END, CUR_MOVE, CUR_STORE, TEXT_INSERT, TEXT_REMOVE, TEXT_LINE,
        TEXT_SET, TEXT_FROM, BYTES_FROM, CUR_COUNT, LIST_COUNT, LIST_APPEND, LIST_INSERT, LIST_REMOVE,
        ITEM_GET, ITEM_SET, INDEX_LOAD, INDEX_STORE, INDEX_MOVE, INDEX_FIRST, INDEX_LAST, INDEX_NUM, ROW,
-       PICK, SLIDE, FOLD, TEXT_COMPARE, LIST_FIND, LIST_HAS, NOW, FLOOR, MOD, SEND };
+       PICK, SLIDE, FOLD, TEXT_COMPARE, LIST_FIND, LIST_HAS, NOW, FLOOR, MOD, SEND, OUTER_ROW };
 enum { FOLD_COUNT, FOLD_SUM, FOLD_MAX, FOLD_MIN, FOLD_FIRST };
 enum { KIND_KEY = 1, KIND_MESSAGE = 2, KIND_TEXT = 3, KIND_TIMER = 4, KIND_OUTPUT = 5, FIELD_U32 = 1, FIELD_BYTES = 2, FIELD_NUMBER = 3, FIELD_TEXT = 4 };
 enum { STATE_NUM = 0, STATE_TEXT = 1, STATE_CURSOR = 2, STATE_LIST = 3, STATE_INDEX = 4, UNIT_CHAR = 0, UNIT_LINE = 1 };
@@ -1650,10 +1650,10 @@ static Kind value(Compiler *c, const Expr *e, bool handler, int height) {
         if (fn_index(c, c->module, e->name) >= 0) return inline_fn(c, e, c->module, e->name, handler, height);
         if (c->module) fail_at(c, e->line, e->column, "'%s' is not a parameter; a library fn reads only its parameters", e->name);
         if (!c->scope && c->row >= 0 && !strcmp(e->name, c->view_row[c->row])) {
-            if (c->folding)
-                fail_at(c, e->line, e->column, "'%s' is the column's row; a count, sum, max or min reads only its own item", e->name);
+            /* In a count, sum, max or min the column's row is still there (a
+             * correlated fold: per branch, the sum of its accounts). */
             int list = state_index(c, c->view_list[c->row]);
-            emit(c, e->line, e->column, ROW, (uint32_t)list, 0);
+            emit(c, e->line, e->column, c->folding ? OUTER_ROW : ROW, (uint32_t)list, 0);
             return (Kind){INDEX, list};
         }
         int index = state_index(c, e->name);
diff --git a/tests/check.py b/tests/check.py
index 98d6e37..84fc9bd 100644
--- a/tests/check.py
+++ b/tests/check.py
@@ -196,8 +196,6 @@ DIAGNOSTICS = [
     (LIST + 'view v = count(x in at)\n', '6:10', "'at' is not a list"),
     ('record T {\n  t: text\n}\nstate xs: list of T\nview v = sum(x.t for x in xs)\n', '5:14',
      'sum, max and min take numbers, not text'),
-    (LIST + 'view w = each r in xs {\n  n = count(x in xs where x.a == r.a)\n}\n', '7:34',
-     "'r' is the column's row; a count, sum, max or min reads only its own item"),
     (LIST + 'fn f(n) = count(x in xs where x.a == n)\nview v = f(1)\n', '6:38',
      "'n' is a parameter outside this count, sum, max or min"),
     (LIST + 'view v = sum(dt for x in xs)\n', '6:14', 'dt is not available in count, sum, max or min'),
@@ -234,6 +232,8 @@ DIAGNOSTICS = [
 ]
 
 ACCEPTED = [
+    # A fold in a column reads the column's row too (a correlated subquery).
+    LIST + 'view w = each r in xs {\n  n = count(x in xs where x.a == r.a)\n}\n',
     SENDS + 'on buy {\n  n = n + 1\n  send ordered { to: { city: "Seoul" }, count: buy.count, item: buy.item }\n}\nview sent = n\n',
     # fns named like folds stay calls, and max of two values stays max
     'fn sum(a, b) = a + b\nfn count(x) = x\nview v = sum(1, count(2)) + max(3, 4) + min(\n  5,\n  6\n)\n',