Filter a table view with where

view open_list = each t in tasks where !t.done { ... } has a row for each
item the condition keeps, in the list's order; the condition compiles to
the table's filter block, generated like a column so it reads the row's
item, and the view entry names the list in the byte after its type. A
count, sum, max or min inside the condition is a compile error, as Pure
refuses a fold in a filter. The random model check now compares a
filtered table too; two diagnostics join.

3 files changed, 38 insertions, 7 deletions

diff --git a/docs/design/002-dsl-direction.md b/docs/design/002-dsl-direction.md
index 9d695f9..7924083 100644
--- a/docs/design/002-dsl-direction.md
+++ b/docs/design/002-dsl-direction.md
@@ -147,6 +147,10 @@ view board = each task in tasks {        # 항목마다 한 행
 - **표 보기.** `view 이름 = each 행 in 리스트 { 열 = 값 ... }`는 리스트 항목마다 행 하나를 낸다. 열은 수나 text이고
   행 이름(인덱스)과 다른 상태를 읽는다. 호스트는 보여 줄 행의 칸만 Pure에 물어 계산한다. 목록을 화면 모델로 복사하거나
   보이는 범위를 앱이 계산하는 코드(가상 스크롤)가 필요 없다. 열 이름은 표 안에서만 유일하면 된다.
+- **걸러진 표.** `view open_list = each t in tasks where !t.done { label = t.title }`는 조건이 참인 항목마다 행
+  하나를 낸다(리스트 순서 그대로). 열의 행 이름은 여전히 항목이라 `t.position`은 리스트 안의 위치다. Pure가 상태가
+  바뀐 뒤 한 번 걸러 행과 항목의 대응을 두므로, 열린 일·끝난 일처럼 같은 리스트를 여러 목록으로 보여도 상태를
+  둘로 나눠 맞출 필요가 없다. 조건 안에는 집계를 둘 수 없다.
 - 목록·표·포커스·톤 같은 인터페이스 어휘는 여전히 aup에 없다. Vega가 이 투영 위에서 제공한다.
 
 검증: 표 열과 조건 보기를 더한 할 일 목록이 참조 모델과 4,800단계에서 같고, 고르지 않은 가지가 계산되지 않음을 확인한다.
diff --git a/src/compile.c b/src/compile.c
index 0c90e18..2eac523 100644
--- a/src/compile.c
+++ b/src/compile.c
@@ -134,6 +134,8 @@ typedef struct {
     /* Views in order; a table (no value) is followed by its columns. */
     char view_names[VIEWS][NAME], view_row[VIEWS][NAME], view_list[VIEWS][NAME];
     Expr *view_value[VIEWS];
+    Expr *view_filter[VIEWS];                         /* a table's where condition, or NULL */
+    unsigned char view_byte[VIEWS];                   /* a filtered table's list + 1 */
     int view_table[VIEWS];                            /* a column's table + 1, or 0 */
     int view_line[VIEWS][2], view_column[VIEWS][2];   /* a table's row name and list name */
     unsigned views;
@@ -144,7 +146,7 @@ typedef struct {
     int row;              /* the table whose column is being generated, or -1 */
     /* The fold whose block is being generated: its item's name and list, and
      * the fn scope it began in (whose parameters its block cannot read). */
-    bool folding;
+    bool folding, filtering;
     char fold_name[NAME];
     int fold_list;
     Scope *fold_scope;
@@ -1097,7 +1099,8 @@ static void state_declaration(Compiler *c) {
     ++c->states;
 }
 
-/* view name = each row in list { column = value ... }: one row per item. */
+/* view name = each row in list [where condition] { column = value ... }: one
+ * row per item, or per item the condition keeps. */
 static void table_declaration(Compiler *c, unsigned table) {
     take(c); /* each */
     Token row = expect_name(c, "a name for each item");
@@ -1110,6 +1113,10 @@ static void table_declaration(Compiler *c, unsigned table) {
     c->view_column[table][0] = row.column;
     c->view_line[table][1] = list.line;
     c->view_column[table][1] = list.column;
+    if (word(c, "where")) { /* a row only for the items it keeps */
+        take(c);
+        c->view_filter[table] = expression(c, 0, false);
+    }
     expect(c, "{", "'{' on the line of the table");
     separators(c);
     while (!is(c, "}")) {
@@ -1650,6 +1657,7 @@ static Kind value(Compiler *c, const Expr *e, bool handler, int height) {
         int list = state_index(c, e->member);
         if (list < 0 || c->state_types[list] != STATE_LIST) fail_at(c, e->line, e->column, "'%s' is not a list", e->member);
         if (c->folding) fail_at(c, e->line, e->column, "a count, sum, max or min cannot hold another");
+        if (c->filtering) fail_at(c, e->line, e->column, "a table's where cannot hold a count, sum, max or min");
         unsigned at = c->code_count, joins = c->joins;
         emit(c, e->line, e->column, FOLD, 0, 0);
         c->folding = true;
@@ -1920,6 +1928,16 @@ static void generate(Compiler *c, unsigned char **module, size_t *size) {
             c->view_types[i] = 2;
             c->view_code[i][0] = (uint32_t)list;
             c->view_code[i][1] = 0;
+            if (c->view_filter[i]) {
+                /* The filter runs as a column of the list: it reads the table's row. */
+                c->row = (int)i;
+                c->filtering = true;
+                c->view_code[i][0] = c->code_count;
+                number(c, c->view_filter[i], false, 0);
+                c->view_code[i][1] = c->code_count - c->view_code[i][0];
+                c->view_byte[i] = (unsigned char)(list + 1);
+                c->filtering = false;
+            }
             continue;
         }
         c->row = c->view_table[i] - 1;
@@ -2021,7 +2039,7 @@ static void generate(Compiler *c, unsigned char **module, size_t *size) {
     for (unsigned i = 0; i < c->views; ++i) {
         p = put32(p, view_name[i]);
         *p++ = (unsigned char)c->view_types[i];
-        *p++ = 0;
+        *p++ = c->view_byte[i];
         p = put16(p, (uint32_t)c->view_table[i]);
         p = put32(p, c->view_code[i][0]);
         p = put32(p, c->view_code[i][1]);
diff --git a/tests/check.py b/tests/check.py
index ef8549f..3716ff6 100644
--- a/tests/check.py
+++ b/tests/check.py
@@ -176,6 +176,9 @@ DIAGNOSTICS = [
      "'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'),
     (LIST + 'view v = sum(x.a for x xs)\n', '6:24', "expected 'in' and a list, as in sum(x in tasks)"),
+    (LIST + 'view w = each r in xs where r {\n  n = r.a\n}\n', '6:29', 'expected a number, not an index'),
+    (LIST + 'view w = each r in xs where count(y in xs) > 0 {\n  n = r.a\n}\n', '6:29',
+     "a table's where cannot hold a count, sum, max or min"),
 ]
 
 ACCEPTED = [
@@ -989,13 +992,18 @@ view done_points = sum(
 view longest = max(t.title.chars for t in tasks)
 view fewest_open = min(t.points for t in tasks where !t.done)
 view summary = "{count(t in tasks where !t.done)} open of {tasks.count}"
+view open_list = each t in tasks where !t.done {
+  label = t.title
+  item = t.position
+}
 """
 
 
 def folds(aup, pure_run, work, seeds, steps=300):
-    """count, sum, max and min over a list against a Python model after
-    random adds, toggles, moves and removals (an empty max or min has no
-    value)."""
+    """count, sum, max and min over a list, and a table filtered by a where,
+    against a Python model after random adds, toggles, moves and removals (an
+    empty max or min has no value; a filtered table's rows keep the list's
+    order and its columns read the item)."""
     source = work / 'folds.aup'
     source.write_text(FOLDS_SOURCE)
     compile_module(aup, source, work / 'folds.pmod')
@@ -1024,7 +1032,8 @@ def folds(aup, pure_run, work, seeds, steps=300):
                              'done_points': sum(t['points'] for t in tasks if t['done']),
                              'longest': max((len(t['title']) for t in tasks), default=None),
                              'fewest_open': min((t['points'] for t in open_tasks), default=None),
-                             'summary': f'{len(open_tasks)} open of {len(tasks)}'})
+                             'summary': f'{len(open_tasks)} open of {len(tasks)}',
+                             'open_list': [{'label': t['title'], 'item': i} for i, t in enumerate(tasks) if not t['done']]})
         # Toggling or removing in an empty list fails; pure-run then exits 1 but shows every event.
         lines = run(pure_run, work / 'folds.pmod', stdin='\n'.join(commands) + '\n', check=False).stdout.splitlines()
         shown = [json.loads(line)['views'] for line in lines]