coremem_types: List: add an apply_at method

crates/types/src/compound.rs

@@ -31,6 +31,12 @@ impl<E, N> LLNode<E, N> {
 trait ListOp<F, L> {
     /// apply `f` to all elements in the list.
     fn apply(&self, f: F);
+    /// apply `f` *only* to the element at the provided index
+    fn apply_at(&self, i: u32, f: F) {
+        self.apply_at_rel(i, 0, f);
+    }
+    // helper: tracks the current index into the list to know when to call `f`
+    fn apply_at_rel(&self, i: u32, cur: u32, f: F);
 }
 
 /// some operation which can be applied to the provided element type `T`.
@@ -46,12 +52,24 @@ impl<F: ElementOp<E>, E, N: ListOp<F, N>> ListOp<F, LLNode<E, N>> for LLNode<E,
         // apply the operation to all tail elements
         self.next.apply(f)
     }
+    fn apply_at_rel(&self, i: u32, cur: u32, f: F) {
+        if i == cur {
+            f.call(&self.e)
+        }
+        self.next.apply_at_rel(i, cur + 1, f);
+    }
 }
 
 impl<F> ListOp<F, Null> for Null {
     fn apply(&self, _f: F) {
         // the tail element simply terminates recursion
     }
+    fn apply_at_rel(&self, i: u32, cur: u32, _f: F) {
+        if i >= cur {
+            // if the requested index was past the length of the list, undef
+            unreachable!();
+        }
+    }
 }
 
 #[cfg(test)]
@@ -144,6 +162,27 @@ mod test {
         assert_eq!(acc.delayed2.into_inner(), V0(2u32).to_flat());
         assert_eq!(acc.delayed1.into_inner(), V1(4f32).to_flat());
         assert_eq!(acc.delayed0.into_inner(), V0(5u32).to_flat());
+    }
 
+    #[test]
+    fn test_apply_at() {
+        let list = LLNode::new(V0(5u32))
+            .prepend(V1(4f32))
+            .prepend(V0(2u32));
+
+        let acc = ApplyAccumulator::default();
+        list.apply_at(0, &acc);
+        assert_eq!(acc.delayed0.into_inner(), V0(2u32).to_flat());
+        assert_eq!(acc.delayed1.into_inner(), Flat::default());
+
+        let acc = ApplyAccumulator::default();
+        list.apply_at(1, &acc);
+        assert_eq!(acc.delayed0.into_inner(), V1(4f32).to_flat());
+        assert_eq!(acc.delayed1.into_inner(), Flat::default());
+
+        let acc = ApplyAccumulator::default();
+        list.apply_at(2, &acc);
+        assert_eq!(acc.delayed0.into_inner(), V0(5u32).to_flat());
+        assert_eq!(acc.delayed1.into_inner(), Flat::default());
     }
 }