cross: list: remove special access to Node::{head,tail}

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

@@ -14,8 +14,8 @@ use serde::{Serialize, Deserialize};
 #[cfg_attr(feature = "fmt", derive(Debug))]
 #[derive(Copy, Clone, Default, PartialEq)]
 pub struct Node<H, T: ?Sized> {
-    pub(crate) head: H,
+    head: H,
-    pub(crate) tail: T,
+    tail: T,
 }
 #[cfg_attr(feature = "serde", derive(Deserialize, Serialize))]
 #[cfg_attr(feature = "fmt", derive(Debug))]
@@ -305,6 +305,19 @@ impl<H, T: Meta> Meta for Node<H, T> {
     type Length = PNext<T::Length>;
 }
 
+pub trait SplitHead {
+    type Head;
+    type Tail;
+    fn split(self) -> (Self::Head, Self::Tail);
+}
+impl<H, T> SplitHead for Node<H, T> {
+    type Head = H;
+    type Tail = T;
+    fn split(self) -> (Self::Head, Self::Tail) {
+        (self.head, self.tail)
+    }
+}
+
 /// these are exported for the convenience of potential consumers: not needed internally
 pub(crate) mod exports {
     #![allow(dead_code)]

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

@@ -7,7 +7,7 @@ mod flat;
 // pub use linked::*;
 pub use flat::{IntoList, Appendable, Appended, Prependable, Prepended};
 pub use flat::exports::*;
-use flat::Node;
+use flat::{Node, SplitHead};
 
 pub type Empty = flat::Null;
 
@@ -128,7 +128,7 @@ where
     type Output = <FoldImpl<Op, Op::Output> as ListConsumer<T>>::Output;
     fn consume(self, l: Node<H, T>) -> Self::Output {
         let FoldImpl(op, state) = self;
-        let (head, tail) = (l.head, l.tail); // TODO: hide behind a SplitHead trait
+        let (head, tail) = l.split();
         let next_state = op.feed(state, head);
         FoldImpl(op, next_state).consume(tail)
     }