improvise-lnkb: the folded category reads as absent to IS and WHERE, as on None

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VdfEGXA7aJ6ftMsCc9PJze

crates/improvise-core/src/model/types.rs

@@ -222,7 +222,10 @@ fn eval_expr(
             agg_filter.as_ref(),
             &|e, at| eval_expr(e, at, model, target_category, depth, Reach::Record),
         ),
-        Expr::Is(filter) => Ok(eval_is(context, filter)),
+        Expr::Is(filter) => Ok(eval_is(
+            model.read_coordinate(context, &filter.category, reach),
+            filter,
+        )),
         Expr::If(cond, then, else_) => {
             match eval_bool(cond, context, model, target_category, depth, reach)? {
                 true => eval(then),
@@ -270,10 +273,8 @@ fn eval_bool(
 
 /// `Cat IS Item` as a number. A cell with no `Cat` coordinate is **not** that
 /// item, matching `Formula::filter`'s own `unwrap_or(false)` — §13.
-fn eval_is(context: &CellKey, filter: &Filter) -> f64 {
-    context
-        .get(&filter.category)
-        .is_some_and(|v| v == filter.item.as_str()) as i32 as f64
+fn eval_is(coordinate: Option<&str>, filter: &Filter) -> f64 {
+    coordinate.is_some_and(|v| v == filter.item.as_str()) as i32 as f64
 }
 
 /// Whether a grid hides dimensions a cell key does not name. The emptiness of
@@ -678,6 +679,23 @@ impl Model {
         }
     }
 
+    /// A coordinate as a formula reads it. At a fold a group's name is no item, so
+    /// the folded category reads as absent, as on `Axis::None` (improvise-lnkb).
+    fn read_coordinate<'k>(&self, context: &'k CellKey, cat: &str, reach: Reach) -> Option<&'k str> {
+        let value = context.get(cat)?;
+        match reach {
+            Reach::Fold { .. } if self.names_a_group(cat, value) => None,
+            Reach::Fold { .. } | Reach::Record => Some(value),
+        }
+    }
+
+    /// Whether `name` is a group of `cat` rather than one of its items.
+    fn names_a_group(&self, cat: &str, name: &str) -> bool {
+        self.category(cat).is_some_and(|c| {
+            !c.items.contains_key(name) && !c.items_in_group(name).is_empty()
+        })
+    }
+
     /// The contexts an aggregate inside a formula pools over at `context`.
     fn contexts_of(&self, context: &CellKey, reach: Reach) -> Vec<CellKey> {
         let members = match reach {
@@ -717,10 +735,9 @@ impl Model {
         agg: Aggregation,
         reach: Reach,
     ) -> Option<CellValue> {
-        let applies = formula
-            .filter
-            .as_ref()
-            .is_none_or(|f| key.get(&f.category) == Some(f.item.as_str()));
+        let applies = formula.filter.as_ref().is_none_or(|f| {
+            self.read_coordinate(key, &f.category, reach) == Some(f.item.as_str())
+        });
         if !applies {
             return self.fold_stored(key, members, agg);
         }
@@ -905,7 +922,7 @@ impl Model {
                         eval_expr(e, at, model, target_category, depth, Reach::Record)
                     },
                 ),
-                Expr::Is(filter) => Ok(eval_is(context, filter)),
+                Expr::Is(filter) => Ok(eval_is(context.get(&filter.category), filter)),
                 Expr::If(cond, then, else_) => {
                     let cv = eval_bool_cached(cond, context, model, target_category, none_cats)?;
                     if cv {

crates/improvise-core/src/view/layout.rs

@@ -1819,6 +1819,28 @@ mod tests {
         }
     }
 
+    /// improvise-lnkb: the folded category reads as absent, as on `Axis::None` — see
+    /// context/test-evidence/core.md.
+    #[test]
+    fn a_fold_answers_is_and_where_on_its_category_as_the_none_axis_does() {
+        let formulas = [
+            "Flag = Month IS \"Q1\"",
+            "Marked = Revenue * 2 WHERE Month = \"Q1\"",
+        ];
+        for measure in ["Flag", "Marked"] {
+            let mut folded = margin_book(&formulas);
+            folded.active_view_mut().toggle_group_collapse("Month", "Q1");
+            let fold = grid_value(&mut folded, "Q1", measure);
+            let mut none = margin_book(&formulas);
+            none.active_view_mut().set_axis("Month", Axis::None);
+            let over_none = grid_value(&mut none, "", measure);
+            assert_eq!(fold, over_none, "{measure}: the fold and the None axis differ");
+        }
+        let mut folded = margin_book(&formulas);
+        folded.active_view_mut().toggle_group_collapse("Month", "Q1");
+        assert_eq!(grid_value(&mut folded, "Q1", "Flag"), Some(0.0), "false, not absent");
+    }
+
     /// improvise-eccl's guard on the other side: an aggregate INSIDE the formula
     /// still pools every record the fold covers.
     #[test]