cross: list: fold MaybeMeta and Meta into one trait

crates/coremem/src/stim.rs

@@ -45,7 +45,7 @@ impl<S: AbstractStimulus> list::AgnosticVisitor<S> for StimulusListVisitor {
 
 impl<L> AbstractStimulus for L
 where
-    L: list::MaybeMeta + Sync,
+    L: list::Meta + Sync,
     StimulusListVisitor: ListVisitor<L>,
 {
     fn at(&self, t_sec: f32, pos: Meters) -> Fields {

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

@@ -1,5 +1,5 @@
-use crate::compound::list::{Indexable, MaybeMeta, Meta};
-use crate::compound::peano::{P0, P1, Peano, PNext};
+use crate::compound::list::{Indexable, Meta};
+use crate::compound::peano::{P0, Peano, PNext};
 
 #[cfg(feature = "serde")]
 use serde::{Serialize, Deserialize};
@@ -291,13 +291,10 @@ impl<H, T, E> Appendable<E> for Node<H, T>
     }
 }
 
-impl MaybeMeta for Null {
+impl Meta for Null {
     type Length = P0;
 }
 
-impl<H> Meta for Node<H, Null> {
-    type Length = P1;
-}
 impl<H, T: Meta> Meta for Node<H, T> {
     type Length = PNext<T::Length>;
 }

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

@@ -1,4 +1,4 @@
-use crate::compound::peano::{Peano, PeanoNonZero, P0, PNext};
+use crate::compound::peano::{Peano, P0, PNext};
 
 mod flat;
 // mod linked;
@@ -23,20 +23,11 @@ pub trait Indexable<P: Peano> {
 /// convenience to lookup the type of the element at index `P` of list `L`.
 pub type ElementAt<P, L> = <L as Indexable<P>>::Element;
 
-/// implemented by any List.
+/// implemented by any List (including the Null, empty list)
 pub trait Meta {
-    type Length: PeanoNonZero;
-}
-
-/// implemented by any List, or Null (empty list)
-pub trait MaybeMeta {
     type Length: Peano;
 }
 
-impl<M: Meta> MaybeMeta for M {
-    type Length = M::Length;
-}
-
 /// implement on your own type for all `N` of a given list if you want to be able to walk the list.
 pub trait Visitor<N: Peano, Arg> {
     fn visit(&mut self, a: &Arg);
@@ -79,17 +70,17 @@ where
 }
 
 /// marker trait for a type which can visit every element in the list `L`.
-pub trait ListVisitor<L: MaybeMeta>: PartialListVisitor<L, <L as MaybeMeta>::Length> {}
+pub trait ListVisitor<L: Meta>: PartialListVisitor<L, <L as Meta>::Length> {}
 
-impl<L: MaybeMeta, V> ListVisitor<L> for V
+impl<L: Meta, V> ListVisitor<L> for V
 where
-    V: PartialListVisitor<L, <L as MaybeMeta>::Length>
+    V: PartialListVisitor<L, <L as Meta>::Length>
 {}
 
 /// user-facing API to visit a list
 pub fn visit<L, V>(l: &L, v: &mut V)
 where
-    L: MaybeMeta,
+    L: Meta,
     V: ListVisitor<L>,
 {
     v.visit_partial(l)