coremem_types: enumerated: allow folding the enum into the first element

crates/types/src/compound/enumerated.rs

@@ -1,13 +1,25 @@
 use crate::compound::peano::{LessThan, P0, P1, P2, P3, Peano, PeanoNonZero, PNext};
-use crate::compound::list::{Indexable, IntoList, List};
+use crate::compound::list::{self, Indexable, IntoList, List};
 use core::marker::PhantomData;
 
+// anything which can encode a discriminant up to *and including* P
+pub trait DiscriminantCodable<P: Peano> {
+    fn discr(&self) -> Discr<P>;
+}
+
 // discriminant which encodes up to *and including* P.
-pub struct Discr<P: Peano>(u32, PhantomData<P>);
+#[derive(Copy, Clone, Default, PartialEq)]
+pub struct Discr<P: Peano>(u32, P::Unit);
+
+impl<P: Peano> DiscriminantCodable<P> for Discr<P> {
+    fn discr(&self) -> Self {
+        *self
+    }
+}
 
 impl<P: Peano> Discr<P> {
     fn new_unchecked(u: u32) -> Self {
-        Self(u, PhantomData)
+        Self(u, Default::default())
     }
     fn dispatch<H: DiscrHandler<P>>(&self, h: H) -> H::Output {
         match self.0 {
@@ -33,59 +45,88 @@ pub trait VariantHandler<N: Peano> {
 }
 
 /// helper used to call F with some (yet-to-be-determined) variant of E
-struct EnumDispatch<'a, E, F> {
-    enum_: &'a E,
+struct DispatchIndexable<'a, I, F> {
+    indexable: &'a I,
     f: F,
 }
 
-impl<'a, E, F> EnumDispatch<'a, E, F> {
-    fn new(enum_: &'a E, f: F) -> Self {
-        Self { enum_, f }
+impl<'a, I, F> DispatchIndexable<'a, I, F> {
+    fn new(indexable: &'a I, f: F) -> Self {
+        Self { indexable, f }
     }
 }
 
-impl<'a, L, F> DiscrHandler<P0> for EnumDispatch<'a, Enum<L>, F>
+impl<'a, I, F> DiscrHandler<P0> for DispatchIndexable<'a, I, F>
 where
-    L: Indexable<P0>,
-    L::Element: Copy,
-    F: VariantHandler<P0, Args=L::Element>,
+    I: Indexable<P0>,
+    I::Element: Copy,
+    F: VariantHandler<P0, Args=I::Element>,
 {
     type Output = F::Output;
     type PrevOrPanic = Self;
     fn call(self) -> Self::Output {
-        self.f.call(self.enum_.0.get())
+        self.f.call(self.indexable.get())
     }
     fn prev(self) -> Self::PrevOrPanic {
         unimplemented!()
     }
 }
 
-impl<'a, L, F, P: Peano> DiscrHandler<PNext<P>> for EnumDispatch<'a, Enum<L>, F>
+impl<'a, I, F, P: Peano> DiscrHandler<PNext<P>> for DispatchIndexable<'a, I, F>
 where
-    L: Indexable<PNext<P>>,
-    L::Element: Copy,
-    F: VariantHandler<PNext<P>, Args=L::Element, Output=<Self as DiscrHandler<P>>::Output>,
+    I: Indexable<PNext<P>>,
+    I::Element: Copy,
+    F: VariantHandler<PNext<P>, Args=I::Element, Output=<Self as DiscrHandler<P>>::Output>,
     Self: DiscrHandler<P>,
 {
     type Output = F::Output;
     type PrevOrPanic = Self;
     fn call(self) -> Self::Output {
-        self.f.call(self.enum_.0.get())
+        self.f.call(self.indexable.get())
     }
     fn prev(self) -> Self::PrevOrPanic {
         self
     }
 }
 
-pub struct Enum<L>(L);
+struct Enum<D, L>(D, L);
 
-trait DiscriminantedEnum<P: Peano>: Sized {
-    fn discr(&self) -> Discr<P>;
-    fn dispatch<'a, F>(&'a self, f: F) -> <EnumDispatch<'a, Self, F> as DiscrHandler<P>>::Output
-    where
-        EnumDispatch<'a, Self, F>: DiscrHandler<P>
-    {
-        self.discr().dispatch(EnumDispatch::new(self, f))
+trait EnumRequirements {
+    type MaxDiscr: Peano;
+    fn discr(&self) -> Discr<Self::MaxDiscr>;
+}
+
+impl<D, L> EnumRequirements for Enum<(D,), L>
+where
+    D: DiscriminantCodable<<<L as list::Meta>::Length as PeanoNonZero>::Prev>,
+    L: list::Meta,
+{
+    type MaxDiscr = <<L as list::Meta>::Length as PeanoNonZero>::Prev;
+    fn discr(&self) -> Discr<Self::MaxDiscr> {
+        self.0.0.discr()
+    }
+}
+
+impl<L> EnumRequirements for Enum<(), L>
+where
+    L: list::Meta + Indexable<P0>,
+    list::ElementAt<P0, L>: Copy + DiscriminantCodable<<<L as list::Meta>::Length as PeanoNonZero>::Prev>,
+{
+    type MaxDiscr = <<L as list::Meta>::Length as PeanoNonZero>::Prev;
+    fn discr(&self) -> Discr<Self::MaxDiscr> {
+        self.1.get().discr()
+    }
+}
+
+
+impl<D, L> Enum<D, L>
+where Self: EnumRequirements
+{
+    fn dispatch<'a, F>(&'a self, f: F) -> <DispatchIndexable<'a, L, F> as DiscrHandler<<Self as EnumRequirements>::MaxDiscr>>::Output
+    where
+        DispatchIndexable<'a, L, F>: DiscrHandler<<Self as EnumRequirements>::MaxDiscr>
+    {
+        self.discr().dispatch(DispatchIndexable::new(&self.1, f))
     }
 }
 

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

@@ -1,4 +1,4 @@
-use crate::compound::peano::{LessThan, P0, P1, P2, P3, Peano};
+use crate::compound::peano::{LessThan, P0, P1, P2, P3, Peano, PeanoNonZero, PNext};
 
 #[cfg(feature = "serde")]
 use serde::{Serialize, Deserialize};
@@ -96,3 +96,16 @@ impl<E0, E1, E2, E3> IntoList for (E0, E1, E2, E3) {
         Node::new(self.0, (self.1, self.2, self.3).into_list())
     }
 }
+
+pub trait Meta {
+    type Length: PeanoNonZero;
+}
+
+impl<H> Meta for Node<H, Null> {
+    type Length = P1;
+}
+impl<H, T: Meta> Meta for Node<H, T> {
+    type Length = PNext<T::Length>;
+}
+
+pub type ElementAt<P, L> = <L as Indexable<P>>::Element;

crates/types/src/compound/peano.rs

@@ -1,6 +1,6 @@
-#[derive(Default)]
+#[derive(Copy, Clone, Default, PartialEq)]
 pub struct PNext<P>(P);
-#[derive(Default)]
+#[derive(Copy, Clone, Default, PartialEq)]
 pub struct P0;
 
 pub type P1 = PNext<P0>;
@@ -11,9 +11,12 @@ pub type P5 = PNext<P4>;
 pub type P6 = PNext<P5>;
 pub type P7 = PNext<P6>;
 
-pub trait Peano: Default {
+pub trait Peano: Copy + Clone + Default + PartialEq {
     type Next: PeanoNonZero;
     type PrevOrZero: Peano;
+    /// always set to ()
+    /// this exists to allow Peano numbers to be used as struct parameters without PhantomData
+    type Unit: Copy + Clone + Default + PartialEq;
     const VALUE: u32;
 }
 
@@ -24,11 +27,13 @@ pub trait PeanoNonZero: Peano {
 impl Peano for P0 {
     type Next = P1;
     type PrevOrZero = P0;
+    type Unit = ();
     const VALUE: u32 = 0;
 }
 impl<P: Peano> Peano for PNext<P> {
     type Next = PNext<PNext<P>>;
     type PrevOrZero = P;
+    type Unit = ();
     const VALUE: u32 = 1 + P::VALUE;
 }
 impl<P: Peano> PeanoNonZero for PNext<P> {