Support 3-argument form of GroupBy with reducer function

GroupBy[list, f, reducer] now applies the reducer function to each
group. E.g. GroupBy[{1,2,3,4,5,6}, EvenQ, Total] returns
<|False -> 9, True -> 12|>. Previously only the 2-argument form
was supported.

Author: ad-si

src/evaluator/dispatch/arg_count.rs

@@ -399,7 +399,7 @@ pub fn get_arg_count_range(name: &str) -> Option<(usize, usize)> {
     "GreaterEqual" => Some((2, usize::MAX)),
     "GreaterEqualThan" => Some((1, 1)),
     "GreaterThan" => Some((1, 1)),
-    "GroupBy" => Some((1, 2)),
+    "GroupBy" => Some((1, 3)),
     "GroupGenerators" => Some((1, 1)),
     "Groupings" => Some((2, 2)),
     "Gudermannian" => Some((1, 1)),

src/evaluator/dispatch/list_operations.rs

@@ -300,8 +300,28 @@ pub fn dispatch_list_operations(
     "CountBy" if args.len() == 2 => {
       return Some(list_helpers_ast::count_by_ast(&args[0], &args[1]));
     }
-    "GroupBy" if args.len() == 2 => {
-      return Some(list_helpers_ast::group_by_ast(&args[0], &args[1]));
+    "GroupBy" if args.len() == 2 || args.len() == 3 => {
+      let result = list_helpers_ast::group_by_ast(&args[0], &args[1]);
+      if args.len() == 3 {
+        // GroupBy[list, f, reducer] - apply reducer to each group
+        return Some(result.and_then(|grouped| match &grouped {
+          Expr::Association(pairs) => {
+            let new_pairs: Result<Vec<(Expr, Expr)>, InterpreterError> = pairs
+              .iter()
+              .map(|(k, v)| {
+                let reduced =
+                  crate::functions::list_helpers_ast::apply_func_ast(
+                    &args[2], v,
+                  )?;
+                Ok((k.clone(), reduced))
+              })
+              .collect();
+            Ok(Expr::Association(new_pairs?))
+          }
+          _ => Ok(grouped),
+        }));
+      }
+      return Some(result);
     }
     "SortBy" if args.len() == 2 => {
       return Some(list_helpers_ast::sort_by_ast(&args[0], &args[1]));

tests/interpreter_tests/list.rs

@@ -4225,6 +4225,22 @@ mod join_non_list {
     );
   }
 
+  #[test]
+  fn group_by_with_reducer() {
+    assert_eq!(
+      interpret("GroupBy[{1, 2, 3, 4, 5, 6}, EvenQ, Total]").unwrap(),
+      "<|False -> 9, True -> 12|>"
+    );
+  }
+
+  #[test]
+  fn group_by_with_length_reducer() {
+    assert_eq!(
+      interpret("GroupBy[{1, 2, 3, 4, 5, 6}, EvenQ, Length]").unwrap(),
+      "<|False -> 3, True -> 3|>"
+    );
+  }
+
   #[test]
   fn counts_by_operator_form() {
     assert_eq!(