cast

src/core.rs

@@ -5,11 +5,32 @@ use crate::func::*;
 pub trait Context {
     type T: Monad;
 
+    type LookupError: Error;
+
     fn hash(s: &[u8]) -> Hash;
 }
 
+pub enum ResolutionError<L, P> {
+    Lookup(L),
+    Parse(P),
+}
+
+pub type ResolutionResult<'a, Ctx, A> = Result<
+    Rc<A>,
+    ResolutionError<
+        <Ctx as Context>::LookupError,
+        <<A as Mentionable<'a, Ctx>>::Fctr as Factory<'a, Ctx>>::ParseError,
+    >,
+>;
+
+pub type Wrapped<'a, Ctx, A> = <<Ctx as Context>::T as WeakFunctor>::F<'a, A>;
+
+pub type Resolution<'a, Ctx, A> = Wrapped<'a, Ctx, ResolutionResult<'a, Ctx, A>>;
+
 pub const HASH_SIZE: usize = 32;
 
+pub const HASH_ZEROS: [u8; HASH_SIZE] = [0; HASH_SIZE];
+
 pub type Hash = [u8; HASH_SIZE];
 
 pub trait Serializer {
@@ -45,39 +66,45 @@ pub trait Mentionable<'a, Ctx: 'a + Context>: 'a + Serializable {
     fn points(&self) -> Vec<Point<'a, Ctx, TypelessMentionable<'a, Ctx>>>;
 }
 
+pub type HashResolution<'a, Ctx> =
+    Wrapped<'a, Ctx, Result<(Vec<u8>, Rc<dyn Resolver<'a, Ctx>>), <Ctx as Context>::LookupError>>;
+
+#[derive(Clone, Copy)]
+pub struct Address {
+    pub point: Hash,
+    pub index: usize,
+}
+
+pub trait Resolver<'a, Ctx: 'a + Context>: 'a {
+    fn resolve(self: Rc<Self>, address: Address) -> HashResolution<'a, Ctx>;
+}
+
 pub trait Deserializer {
     fn read_n(&mut self, n: usize) -> &[u8];
-    fn read_all(&mut self, n: usize) -> &[u8];
+    fn read_all(&mut self) -> &[u8];
+    fn tell(&self) -> usize;
 }
 
-pub trait ExtDeserialiser {
-    fn read_n_const<const N: usize>(&mut self) -> Result<[u8; N], &[u8]>;
-}
-
-impl<D: ?Sized + Deserializer> ExtDeserialiser for D {
-    fn read_n_const<const N: usize>(&mut self) -> Result<[u8; N], &[u8]> {
-        let slice = self.read_n(N);
-        match slice.try_into() {
-            Ok(array) => Ok(array),
-            Err(_) => Err(slice),
-        }
-    }
-}
+pub type ParseResult<'a, Ctx, F> =
+    Result<<F as Factory<'a, Ctx>>::Mtbl, <F as Factory<'a, Ctx>>::ParseError>;
 
 pub trait Factory<'a, Ctx: 'a + Context>: Clone + 'a {
     type Mtbl: Mentionable<'a, Ctx, Fctr = Self>;
-    type ParseError: Error;
+    type ParseError: 'a + Error;
 
     fn deserialize(
         &self,
         deserializer: &mut dyn Deserializer,
-    ) -> Result<Self::Mtbl, Self::ParseError>;
+        resolver: Rc<dyn Resolver<'a, Ctx>>,
+    ) -> ParseResult<'a, Ctx, Self>;
+
+    fn unexpected_tail(&self, tail: &[u8]) -> Self::ParseError;
 }
 
 pub trait Origin<'a, Ctx: 'a + Context>: 'a {
     type Mtbl: Mentionable<'a, Ctx>;
     fn factory(&self) -> <Self::Mtbl as Mentionable<'a, Ctx>>::Fctr;
-    fn resolve(&self) -> <<Ctx as Context>::T as WeakFunctor>::F<'a, Rc<Self::Mtbl>>;
+    fn resolve(self: Rc<Self>) -> Resolution<'a, Ctx, Self::Mtbl>;
 }
 
 struct LocalOrigin<A>(Rc<A>);
@@ -89,8 +116,8 @@ impl<'a, Ctx: 'a + Context, A: Mentionable<'a, Ctx>> Origin<'a, Ctx> for LocalOr
         self.0.factory()
     }
 
-    fn resolve(&self) -> <<Ctx as Context>::T as WeakFunctor>::F<'a, Rc<Self::Mtbl>> {
-        <Ctx::T as Applicative>::pure(self.0.clone())
+    fn resolve(self: Rc<Self>) -> Resolution<'a, Ctx, Self::Mtbl> {
+        <Ctx::T as Applicative>::pure(Ok(self.0.clone()))
     }
 }
 
@@ -129,8 +156,8 @@ impl<'a, Ctx: 'a + Context, A: Mentionable<'a, Ctx>> Point<'a, Ctx, A> {
         Self::from_fields(point, Rc::new(origin))
     }
 
-    pub fn resolve(&self) -> <<Ctx as Context>::T as WeakFunctor>::F<'a, Rc<A>> {
-        self.origin.resolve()
+    pub fn resolve(&self) -> Resolution<'a, Ctx, A> {
+        self.origin.clone().resolve()
     }
 }
 
@@ -154,10 +181,14 @@ pub struct TypelessMentionable<'a, Ctx: 'a + Context> {
 
 type TypelessParsed<'a, Ctx> = Result<TypelessMentionable<'a, Ctx>, Box<dyn 'a + Error>>;
 
-type TypelessDeserialize<'a, Ctx> = dyn 'a + Fn(&mut dyn Deserializer) -> TypelessParsed<'a, Ctx>;
+type TypelessDeserialize<'a, Ctx> =
+    dyn 'a + Fn(&mut dyn Deserializer, Rc<dyn Resolver<'a, Ctx>>) -> TypelessParsed<'a, Ctx>;
+
+type TypelessUnexpectedTail<'a> = dyn 'a + Fn(&[u8]) -> TypelessError<'a>;
 
 pub struct TypelessFactory<'a, Ctx: 'a + Context> {
     t_deserialize: Rc<TypelessDeserialize<'a, Ctx>>,
+    t_unexpected_tail: Rc<TypelessUnexpectedTail<'a>>,
 }
 
 impl<'a, Ctx: 'a + Context> Serializable for TypelessMentionable<'a, Ctx> {
@@ -186,6 +217,7 @@ impl<'a, Ctx: 'a + Context> Clone for TypelessFactory<'a, Ctx> {
     fn clone(&self) -> Self {
         Self {
             t_deserialize: self.t_deserialize.clone(),
+            t_unexpected_tail: self.t_unexpected_tail.clone(),
         }
     }
 }
@@ -209,17 +241,22 @@ impl<'a, Ctx: 'a + Context> Factory<'a, Ctx> for TypelessFactory<'a, Ctx> {
     fn deserialize(
         &self,
         deserializer: &mut dyn Deserializer,
-    ) -> Result<Self::Mtbl, Self::ParseError> {
-        match (self.t_deserialize)(deserializer) {
+        resolver: Rc<dyn Resolver<'a, Ctx>>,
+    ) -> ParseResult<'a, Ctx, Self> {
+        match (self.t_deserialize)(deserializer, resolver) {
             Ok(mentionable) => Ok(mentionable),
             Err(error) => Err(TypelessError(error)),
         }
     }
+
+    fn unexpected_tail(&self, tail: &[u8]) -> Self::ParseError {
+        (self.t_unexpected_tail)(tail)
+    }
 }
 
 impl<'a, Ctx: 'a + Context> TypelessMentionable<'a, Ctx> {
-    pub fn from_mentionable<A: Mentionable<'a, Ctx>>(mentionable: Rc<A>) -> Self {
-        let factory = TypelessFactory::from_factory(mentionable.factory());
+    pub fn from_typed<A: Mentionable<'a, Ctx>>(mentionable: Rc<A>) -> Self {
+        let factory = TypelessFactory::from_typed(mentionable.factory());
         let topology = mentionable.topology();
         let points = mentionable.points();
         TypelessMentionable {
@@ -232,16 +269,24 @@ impl<'a, Ctx: 'a + Context> TypelessMentionable<'a, Ctx> {
 }
 
 impl<'a, Ctx: 'a + Context> TypelessFactory<'a, Ctx> {
-    pub fn from_factory<F: Factory<'a, Ctx>>(factory: F) -> Self {
+    pub fn from_typed<F: Factory<'a, Ctx>>(factory: F) -> Self {
+        let tail_factory = factory.clone();
         TypelessFactory {
-            t_deserialize: Rc::new(
-                move |deserializer| match factory.deserialize(deserializer) {
-                    Ok(mentionable) => {
-                        Ok(TypelessMentionable::from_mentionable(Rc::new(mentionable)))
-                    }
+            t_deserialize: Rc::new(move |deserializer, resolver| {
+                match factory.deserialize(deserializer, resolver) {
+                    Ok(mentionable) => Ok(TypelessMentionable::from_typed(Rc::new(mentionable))),
                     Err(error) => Err(Box::new(error)),
-                },
-            ),
+                }
+            }),
+            t_unexpected_tail: Rc::new(move |tail| {
+                TypelessError::from_typed(tail_factory.unexpected_tail(tail))
+            }),
         }
     }
 }
+
+impl<'a> TypelessError<'a> {
+    pub fn from_typed<E: 'a + Error>(error: E) -> Self {
+        TypelessError(Box::new(error))
+    }
+}

src/std.rs

@@ -1,6 +1,9 @@
 pub mod atomic;
+pub mod cast;
 pub mod inlining;
+pub mod nullable;
 
+use std::cmp::min;
 use std::{error::Error, fmt::Display, rc::Rc};
 
 use crate::core::*;
@@ -8,23 +11,38 @@ use crate::func::*;
 
 struct WrappedOrigin<'a, Ctx: 'a + Context, A: Mentionable<'a, Ctx>> {
     w_factory: A::Fctr,
-    w_resolve: Box<dyn 'a + Fn() -> <<Ctx as Context>::T as WeakFunctor>::F<'a, Rc<A>>>,
+    w_resolve: Box<dyn 'a + Fn() -> Resolution<'a, Ctx, A>>,
 }
 
 trait MappableOrigin<'a, Ctx: 'a + Context>: Origin<'a, Ctx> {
     fn map<B: Mentionable<'a, Ctx>>(
         self: Rc<Self>,
-        map_mentionable: impl 'a + Clone + Fn(Rc<Self::Mtbl>) -> B,
+        map_ok: impl 'a + Clone + Fn(Rc<Self::Mtbl>) -> B,
+        map_err: impl 'a
+            + Clone
+            + Fn(
+                <<Self::Mtbl as Mentionable<'a, Ctx>>::Fctr as Factory<'a, Ctx>>::ParseError,
+            ) -> <B::Fctr as Factory<'a, Ctx>>::ParseError,
         map_factory: impl 'a + FnOnce(<Self::Mtbl as Mentionable<'a, Ctx>>::Fctr) -> B::Fctr,
     ) -> WrappedOrigin<'a, Ctx, B>;
 }
 
 fn map_resolve<'a, Ctx: 'a + Context, A: Mentionable<'a, Ctx>, B: Mentionable<'a, Ctx>>(
-    resolve: impl 'a + Fn() -> <<Ctx as Context>::T as WeakFunctor>::F<'a, Rc<A>>,
-    map_mentionable: impl 'a + Fn(Rc<A>) -> B,
-) -> <<Ctx as Context>::T as WeakFunctor>::F<'a, Rc<B>> {
+    resolve: impl 'a + Fn() -> Resolution<'a, Ctx, A>,
+    map_ok: impl 'a + Fn(Rc<A>) -> B,
+    map_err: impl 'a
+        + Fn(<A::Fctr as Factory<'a, Ctx>>::ParseError) -> <B::Fctr as Factory<'a, Ctx>>::ParseError,
+) -> Resolution<'a, Ctx, B> {
     Ctx::T::fmap(
-        move |resolved| Rc::new(map_mentionable(resolved)),
+        move |resolved| match resolved {
+            Ok(mentionable) => Ok(Rc::new(map_ok(mentionable))),
+            Err(ResolutionError::Parse(parse_error)) => {
+                Err(ResolutionError::Parse(map_err(parse_error)))
+            }
+            Err(ResolutionError::Lookup(lookup_error)) => {
+                Err(ResolutionError::Lookup(lookup_error))
+            }
+        },
         resolve(),
     )
 }
@@ -32,7 +50,12 @@ fn map_resolve<'a, Ctx: 'a + Context, A: Mentionable<'a, Ctx>, B: Mentionable<'a
 impl<'a, Ctx: 'a + Context, O: ?Sized + Origin<'a, Ctx>> MappableOrigin<'a, Ctx> for O {
     fn map<B: Mentionable<'a, Ctx>>(
         self: Rc<Self>,
-        map_mentionable: impl 'a + Clone + Fn(Rc<Self::Mtbl>) -> B,
+        map_ok: impl 'a + Clone + Fn(Rc<Self::Mtbl>) -> B,
+        map_err: impl 'a
+            + Clone
+            + Fn(
+                <<Self::Mtbl as Mentionable<'a, Ctx>>::Fctr as Factory<'a, Ctx>>::ParseError,
+            ) -> <B::Fctr as Factory<'a, Ctx>>::ParseError,
         map_factory: impl 'a + FnOnce(<Self::Mtbl as Mentionable<'a, Ctx>>::Fctr) -> B::Fctr,
     ) -> WrappedOrigin<'a, Ctx, B> {
         let origin = self.clone();
@@ -40,11 +63,9 @@ impl<'a, Ctx: 'a + Context, O: ?Sized + Origin<'a, Ctx>> MappableOrigin<'a, Ctx>
             w_factory: map_factory(self.factory()),
             w_resolve: Box::new(move || {
                 let origin = origin.clone();
-                let map_mentionable = map_mentionable.clone();
-                map_resolve(
-                    move || origin.resolve(),
-                    map_mentionable,
-                )
+                let map_ok = map_ok.clone();
+                let map_err = map_err.clone();
+                map_resolve(move || origin.clone().resolve(), map_ok, map_err)
             }),
         }
     }
@@ -57,7 +78,7 @@ impl<'a, Ctx: 'a + Context, A: Mentionable<'a, Ctx>> Origin<'a, Ctx> for Wrapped
         self.w_factory.clone()
     }
 
-    fn resolve(&self) -> <<Ctx as Context>::T as WeakFunctor>::F<'a, Rc<Self::Mtbl>> {
+    fn resolve(self: Rc<Self>) -> Resolution<'a, Ctx, A> {
         (self.w_resolve)()
     }
 }
@@ -67,8 +88,9 @@ impl<'a, Ctx: 'a + Context, A: Mentionable<'a, Ctx>> Point<'a, Ctx, A> {
         Point {
             point: self.point,
             origin: Rc::new(self.origin.clone().map(
-                TypelessMentionable::from_mentionable,
-                TypelessFactory::from_factory,
+                TypelessMentionable::from_typed,
+                TypelessError::from_typed,
+                TypelessFactory::from_typed,
             )),
         }
     }
@@ -121,6 +143,64 @@ impl Display for PointParseError {
 
 impl Error for PointParseError {}
 
+impl From<&[u8]> for PointParseError {
+    fn from(value: &[u8]) -> Self {
+        PointParseError::WrongLength(value.len())
+    }
+}
+
+impl<'a, Ctx: 'a + Context, A: Mentionable<'a, Ctx>> Point<'a, Ctx, A> {
+    fn from_point(address: Address, factory: A::Fctr, resolver: Rc<dyn Resolver<'a, Ctx>>) -> Self {
+        Point {
+            point: address.point,
+            origin: Rc::new(ResolverOrigin {
+                r_factory: factory,
+                r_address: address,
+                r_resolver: resolver,
+            }),
+        }
+    }
+}
+
+impl<'a, Ctx: 'a + Context, A: Mentionable<'a, Ctx>> AsRef<[u8]> for Point<'a, Ctx, A> {
+    fn as_ref(&self) -> &[u8] {
+        &self.point
+    }
+}
+
+struct Addresses {
+    current: usize,
+}
+
+impl Addresses {
+    fn next<'a>(&mut self, deserializer: &'a mut dyn Deserializer) -> Result<Address, &'a [u8]> {
+        let point = deserializer.read_n_const::<HASH_SIZE>()?;
+        let address = Address {
+            point,
+            index: self.current,
+        };
+        self.current += 1;
+        Ok(address)
+    }
+
+    fn next_point<'a, 'b, Ctx: 'a + Context, A: Mentionable<'a, Ctx>>(
+        &mut self,
+        deserializer: &'b mut dyn Deserializer,
+        resolver: Rc<dyn Resolver<'a, Ctx>>,
+        factory: A::Fctr,
+    ) -> Result<Point<'a, Ctx, A>, &'b [u8]> {
+        Ok(Point::from_point(
+            self.next(deserializer)?,
+            factory,
+            resolver,
+        ))
+    }
+
+    fn start() -> Self {
+        Addresses { current: 0 }
+    }
+}
+
 impl<'a, Ctx: 'a + Context, F: Factory<'a, Ctx>> Factory<'a, Ctx> for PointFactory<F> {
     type Mtbl = Point<'a, Ctx, F::Mtbl>;
 
@@ -129,10 +209,128 @@ impl<'a, Ctx: 'a + Context, F: Factory<'a, Ctx>> Factory<'a, Ctx> for PointFacto
     fn deserialize(
         &self,
         deserializer: &mut dyn Deserializer,
-    ) -> Result<Self::Mtbl, Self::ParseError> {
-        match deserializer.read_n_const::<HASH_SIZE>() {
-            Ok(point) => Ok(Point {point, origin: todo!()}),
-            Err(slice) => Err(PointParseError::WrongLength(slice.len())),
+        resolver: Rc<dyn Resolver<'a, Ctx>>,
+    ) -> ParseResult<'a, Ctx, Self> {
+        let mut addresses = Addresses::start();
+        Ok(addresses.next_point(deserializer, resolver, self.factory.clone())?)
+    }
+
+    fn unexpected_tail(&self, tail: &[u8]) -> Self::ParseError {
+        PointParseError::WrongLength(HASH_SIZE + tail.len())
+    }
+}
+
+struct ResolverOrigin<'a, Ctx: 'a + Context, F: Factory<'a, Ctx>> {
+    r_factory: F,
+    r_address: Address,
+    r_resolver: Rc<dyn Resolver<'a, Ctx>>,
+}
+
+fn _resolve_origin<'a, Ctx: 'a + Context, A: Mentionable<'a, Ctx>>(
+    origin: Rc<ResolverOrigin<'a, Ctx, A::Fctr>>,
+) -> Resolution<'a, Ctx, A> {
+    let resolution = origin.r_resolver.clone().resolve(origin.r_address);
+    Ctx::T::fmap(
+        move |resolved| match resolved {
+            Ok((src, resolver)) => match origin.r_factory.parse_slice(&src, resolver) {
+                Ok(mentionable) => Ok(Rc::new(mentionable)),
+                Err(parse_error) => Err(ResolutionError::Parse(parse_error)),
+            },
+            Err(lookup_error) => Err(ResolutionError::Lookup(lookup_error)),
+        },
+        resolution,
+    )
+}
+
+impl<'a, Ctx: 'a + Context, F: Factory<'a, Ctx>> Origin<'a, Ctx> for ResolverOrigin<'a, Ctx, F> {
+    type Mtbl = F::Mtbl;
+
+    fn factory(&self) -> <Self::Mtbl as Mentionable<'a, Ctx>>::Fctr {
+        self.r_factory.clone()
+    }
+
+    fn resolve(self: Rc<Self>) -> Resolution<'a, Ctx, Self::Mtbl> {
+        _resolve_origin(self)
+    }
+}
+
+struct SliceDeserializer<'a> {
+    slice: &'a [u8],
+    pos: usize,
+}
+
+impl<'a> From<&'a [u8]> for SliceDeserializer<'a> {
+    fn from(value: &'a [u8]) -> Self {
+        SliceDeserializer {
+            slice: value,
+            pos: 0,
+        }
+    }
+}
+
+impl<'a> Deserializer for SliceDeserializer<'a> {
+    fn read_n(&mut self, n: usize) -> &[u8] {
+        let (left, right) = self.slice.split_at(min(n, self.slice.len()));
+        self.slice = right;
+        self.pos += left.len();
+        left
+    }
+
+    fn read_all(&mut self) -> &[u8] {
+        let left = self.slice;
+        self.slice = &[];
+        self.pos += left.len();
+        left
+    }
+
+    fn tell(&self) -> usize {
+        self.pos
+    }
+}
+
+fn _parse_slice<'a, Ctx: 'a + Context, A: Mentionable<'a, Ctx>>(
+    factory: &A::Fctr,
+    slice: &[u8],
+    resolver: Rc<dyn Resolver<'a, Ctx>>,
+) -> ParseResult<'a, Ctx, A::Fctr> {
+    let mut deserializer = SliceDeserializer::from(slice);
+    let mentionable = factory.deserialize(&mut deserializer, resolver)?;
+    let tail = deserializer.read_all();
+    if tail.is_empty() {
+        Ok(mentionable)
+    } else {
+        Err(factory.unexpected_tail(tail))
+    }
+}
+
+pub trait ExtFactory<'a, Ctx: 'a + Context>: Factory<'a, Ctx> {
+    fn parse_slice(
+        &self,
+        slice: &[u8],
+        resolver: Rc<dyn Resolver<'a, Ctx>>,
+    ) -> ParseResult<'a, Ctx, Self>;
+}
+
+impl<'a, Ctx: 'a + Context, F: Factory<'a, Ctx>> ExtFactory<'a, Ctx> for F {
+    fn parse_slice(
+        &self,
+        slice: &[u8],
+        resolver: Rc<dyn Resolver<'a, Ctx>>,
+    ) -> ParseResult<'a, Ctx, Self> {
+        _parse_slice::<Ctx, F::Mtbl>(self, slice, resolver)
+    }
+}
+
+pub trait ExtDeserializer {
+    fn read_n_const<const N: usize>(&mut self) -> Result<[u8; N], &[u8]>;
+}
+
+impl<D: ?Sized + Deserializer> ExtDeserializer for D {
+    fn read_n_const<const N: usize>(&mut self) -> Result<[u8; N], &[u8]> {
+        let slice = self.read_n(N);
+        match slice.try_into() {
+            Ok(array) => Ok(array),
+            Err(_) => Err(slice),
         }
     }
 }

src/std/atomic.rs

@@ -1,10 +1,11 @@
-use std::{error::Error, marker::PhantomData};
+use std::{error::Error, marker::PhantomData, rc::Rc};
 
 use crate::core::*;
 
 pub trait Atomic: 'static + Sized + Clone + Serializable {
     type ParseError: Error;
     fn deserialize(deserializer: &mut dyn Deserializer) -> Result<Self, Self::ParseError>;
+    fn unexpected_tail(tail: &[u8]) -> Self::ParseError;
 }
 
 pub struct AtomicFactory<A: Atomic>(PhantomData<A>);
@@ -45,7 +46,12 @@ impl<'a, Ctx: 'a + Context, A: Atomic> Factory<'a, Ctx> for AtomicFactory<A> {
     fn deserialize(
         &self,
         deserializer: &mut dyn Deserializer,
-    ) -> Result<Self::Mtbl, Self::ParseError> {
+        _resolver: Rc<dyn Resolver<'a, Ctx>>,
+    ) -> ParseResult<'a, Ctx, Self> {
         A::deserialize(deserializer)
     }
+
+    fn unexpected_tail(&self, tail: &[u8]) -> Self::ParseError {
+        A::unexpected_tail(tail)
+    }
 }

src/std/cast.rs

@@ -0,0 +1,61 @@
+use crate::core::*;
+use crate::std::*;
+
+struct CastResolver<'a, Ctx: 'a + Context> {
+    points: Vec<Point<'a, Ctx, TypelessMentionable<'a, Ctx>>>,
+}
+
+impl<'a, Ctx: 'a + Context> Resolver<'a, Ctx> for CastResolver<'a, Ctx>
+where
+    Ctx::LookupError: From<TypelessError<'a>>,
+{
+    fn resolve(self: Rc<Self>, address: Address) -> HashResolution<'a, Ctx> {
+        let point = &self
+            .points
+            .get(address.index)
+            .expect("CastResolved received an invalid address.");
+        if point.point != address.point {
+            panic!("CastResolved received an invalid address.")
+        }
+        Ctx::T::fmap(
+            |resolved| match resolved {
+                Ok(mentionable) => {
+                    let mut vec = Vec::new();
+                    mentionable.serialize(&mut vec);
+                    let resolver: Rc<dyn Resolver<'a, Ctx>> = Rc::new(CastResolver {
+                        points: mentionable.points(),
+                    });
+                    Ok((vec, resolver))
+                }
+                Err(error) => Err(match error {
+                    ResolutionError::Lookup(lookup_error) => lookup_error,
+                    ResolutionError::Parse(parse_error) => parse_error.into(),
+                }),
+            },
+            point.resolve(),
+        )
+    }
+}
+
+pub type CastResult<'a, Ctx, A> =
+    Result<A, <<A as Mentionable<'a, Ctx>>::Fctr as Factory<'a, Ctx>>::ParseError>;
+impl<'a, Ctx: Context> TypelessMentionable<'a, Ctx>
+where
+    Ctx::LookupError: From<TypelessError<'a>>,
+{
+    /// .
+    ///
+    /// # Errors
+    ///
+    /// This function will return an error if .
+    pub fn cast<A: Mentionable<'a, Ctx>>(&self, factory: A::Fctr) -> CastResult<'a, Ctx, A> {
+        let mut vec = Vec::new();
+        self.serialize(&mut vec);
+        factory.parse_slice(
+            &vec,
+            Rc::new(CastResolver {
+                points: self.points(),
+            }),
+        )
+    }
+}

src/std/nullable.rs

@@ -0,0 +1,77 @@
+use crate::core::*;
+use crate::std::*;
+
+enum Nullable<'a, Ctx: 'a + Context, A: Mentionable<'a, Ctx>> {
+    Null(A::Fctr),
+    NotNull(Point<'a, Ctx, A>),
+}
+
+struct NullableFactory<'a, Ctx: 'a + Context, A: Mentionable<'a, Ctx>> {
+    factory: A::Fctr,
+}
+
+impl<'a, Ctx: 'a + Context, A: Mentionable<'a, Ctx>> Serializable for Nullable<'a, Ctx, A> {
+    fn serialize(&self, serializer: &mut dyn Serializer) {
+        serializer.write(match self {
+            Nullable::Null(_) => &HASH_ZEROS,
+            Nullable::NotNull(point) => &point.point,
+        })
+    }
+}
+
+impl<'a, Ctx: 'a + Context, A: Mentionable<'a, Ctx>> Mentionable<'a, Ctx> for Nullable<'a, Ctx, A> {
+    type Fctr = NullableFactory<'a, Ctx, A>;
+
+    fn factory(&self) -> Self::Fctr {
+        match self {
+            Nullable::Null(factory) => NullableFactory {
+                factory: factory.clone(),
+            },
+            Nullable::NotNull(point) => NullableFactory {
+                factory: point.origin.factory(),
+            },
+        }
+    }
+
+    fn points(&self) -> Vec<Point<'a, Ctx, TypelessMentionable<'a, Ctx>>> {
+        match self {
+            Nullable::Null(_) => vec![],
+            Nullable::NotNull(point) => vec![point.typeless()],
+        }
+    }
+}
+
+impl<'a, Ctx: 'a + Context, A: Mentionable<'a, Ctx>> Clone for NullableFactory<'a, Ctx, A> {
+    fn clone(&self) -> Self {
+        Self {
+            factory: self.factory.clone(),
+        }
+    }
+}
+
+impl<'a, Ctx: 'a + Context, A: Mentionable<'a, Ctx>> Factory<'a, Ctx>
+    for NullableFactory<'a, Ctx, A>
+{
+    type Mtbl = Nullable<'a, Ctx, A>;
+
+    type ParseError = PointParseError;
+
+    fn deserialize(
+        &self,
+        deserializer: &mut dyn Deserializer,
+        resolver: std::rc::Rc<dyn Resolver<'a, Ctx>>,
+    ) -> ParseResult<'a, Ctx, Self> {
+        let point = deserializer.read_n_const::<HASH_SIZE>()?;
+        let factory = self.factory.clone();
+        Ok(if point == HASH_ZEROS {
+            Nullable::Null(factory)
+        } else {
+            let mut addresses = Addresses::start();
+            Nullable::NotNull(addresses.next_point(deserializer, resolver, factory)?)
+        })
+    }
+
+    fn unexpected_tail(&self, tail: &[u8]) -> Self::ParseError {
+        PointParseError::WrongLength(HASH_SIZE + tail.len())
+    }
+}