cross: list: more tests for fold operation

crates/cross/src/compound/list/mod.rs

@@ -142,16 +142,48 @@ where
 #[cfg(test)]
 mod test {
     use super::*;
+
+    impl FoldOp<i32> for i32 {
+        type Output = i32;
+        fn feed(self, a: i32) -> Self::Output {
+            self + a
+        }
+    }
+
     #[test]
     fn fold_prim() {
-        impl FoldOp<i32> for i32 {
-            type Output = i32;
-            fn feed(self, a: i32) -> Self::Output {
-                self + a
-            }
-        }
-
         let list = (3, 4, 5i32).into_list();
         assert_eq!(fold(list, 2i32), 2+3+4+5);
     }
+
+    impl FoldOp<f32> for i32 {
+        type Output = f32;
+        fn feed(self, a: f32) -> Self::Output {
+            self as f32 + a
+        }
+    }
+    impl FoldOp<f32> for f32 {
+        type Output = f32;
+        fn feed(self, a: f32) -> Self::Output {
+            self + a
+        }
+    }
+    impl FoldOp<i32> for f32 {
+        type Output = f32;
+        fn feed(self, a: i32) -> Self::Output {
+            self + a as f32
+        }
+    }
+
+    #[test]
+    fn fold_mixed() {
+        // we fold:
+        // 2i32 + 3i32
+        // 5i32 + 4i32
+        // 9i32 + 5.5f32
+        // 14.5f32 + 6.5f32
+        // 21f32 + 7i32
+        let list = (3i32, 4i32, 5.5f32, 6.5f32, 7i32).into_list();
+        assert_eq!(fold(list, 2i32), 28f32);
+    }
 }