cross: list: allow enumerating by u32 instead of Tagged

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

@@ -1,4 +1,4 @@
-use crate::compound::peano::{Peano, P0, PNext};
+use crate::compound::peano::Peano;
 
 mod flat;
 // mod linked;
@@ -286,10 +286,34 @@ where
     }
 }
 
+pub struct MapTagToValueOp<T>(T /* unused */);
+impl<P: Peano, V, T: From<u32>> MapVisitor<Tagged<P, V>> for MapTagToValueOp<T> {
+    type Output = (T, V);
+    fn map(&self, elem: Tagged<P, V>) -> Self::Output {
+        (P::VALUE.into(), elem.into_inner())
+    }
+}
+
+pub trait EnumerateU32 {
+    type Output;
+    fn enumerate_u32(self) -> Self::Output;
+}
+impl<L> EnumerateU32 for L
+where
+    L: Enumerate,
+    L::Output: Map<MapTagToValueOp<u32>>,
+{
+    type Output = <L::Output as Map<MapTagToValueOp<u32>>>::Output;
+    fn enumerate_u32(self) -> Self::Output {
+        self.enumerate().map(MapTagToValueOp(0u32 /* unused */))
+    }
+}
+
 
 #[cfg(test)]
 mod test {
     use super::*;
+    use crate::compound::peano::{P0, P1, P2};
 
     struct SumVal;
 
@@ -518,7 +542,6 @@ mod test {
 
     #[test]
     fn enumerate_multiple() {
-        use crate::compound::peano::{P1, P2};
         let list = (2i32, (), 4f32).into_list();
         let expected = (
             Tagged::<P0, _>::new(2i32),
@@ -527,4 +550,15 @@ mod test {
         ).into_list();
         assert!(list.enumerate() == expected);
     }
+
+    #[test]
+    fn enumerate_u32_multiple() {
+        let list = (2i32, (), 4f32).into_list();
+        let expected = (
+            (0u32, 2i32),
+            (1u32, ()),
+            (2u32, 4f32)
+        ).into_list();
+        assert!(list.enumerate_u32() == expected);
+    }
 }