Anonymous sum type prototype

hsum.rs

use std::marker::PhantomData;

pub enum Void {}

impl Void {
    fn any(&self) -> ! {
        match *self {}
    }
}

trait UnwrapVoid<R> {
    fn unwrap_void(self) -> R;
}

impl<R> UnwrapVoid<R> for Result<R, Void> {
    fn unwrap_void(self) -> R {
        match self {
            Ok(r) => r,
            Err(void) => void.any()
        }
    }
}

#[allow(dead_code)]
enum Here {}
#[allow(dead_code)]
struct There<T>(PhantomData<T>);

enum Cans<Head, Tail> {
    Here(Head),
    There(Tail)
}

trait NewCans<Item, Where> {
    fn new(item: Item) -> Self;
}

impl<Item, Tail> NewCans<Item, Here> for Cans<Item, Tail> {
    fn new(item: Item) -> Self {
        Cans::Here(item)
    }
}

impl<Item, Head, Tail, TailIndex> NewCans<Item, There<TailIndex>> for Cans<Head, Tail>
    where Tail: NewCans<Item, TailIndex> {
    fn new(item: Item) -> Self {
        Cans::There(NewCans::new(item))
    }    
}

impl<Head, Tail> Cans<Head, Tail> {
    pub fn new<Item, Where>(item: Item) -> Self
    where Self: NewCans<Item, Where> {
        NewCans::new(item)
    }
}

trait Find<T, Where> {
    fn find(&self) -> Option<&T>;
    fn find_mut(&mut self) -> Option<&mut T>;
}

impl<T, Tail> Find<T, Here> for Cans<T, Tail> {
    fn find(&self) -> Option<&T> {
        match *self {
            Cans::Here(ref t) => Some(t),
            Cans::There(_)    => None,
        }
    }
    
    fn find_mut(&mut self) -> Option<&mut T> {
        match *self {
            Cans::Here(ref mut t) => Some(t),
            Cans::There(_)        => None
        }
    }
}

impl<Head, T, Tail, TailIndex> Find<T, There<TailIndex>> for Cans<Head, Tail>
    where Tail: Find<T, TailIndex> {
    fn find(&self) -> Option<&T> {
        match *self {
            Cans::Here(_)          => None,
            Cans::There(ref there) => there.find()
        }
    }
    
    fn find_mut(&mut self) -> Option<&mut T> {
        match *self {
            Cans::Here(_)              => None,
            Cans::There(ref mut there) => there.find_mut()
        }
    }
}

trait Select<T, R, Where> {
    type Remainder;
    fn select<F>(self, F) -> Result<R, Self::Remainder>
        where F: FnOnce(T) -> R;
}

impl<T, R, Tail> Select<T, R, Here> for Cans<T, Tail> {
    type Remainder = Tail;
    fn select<F>(self, f: F) -> Result<R, Self::Remainder>
    where F: FnOnce(T) -> R {
        match self {
            Cans::Here(t) => Ok(f(t)),
            Cans::There(there) => Err(there)
        }
    }
}

impl<T, R, Head, Tail, TailIndex> Select<T, R, There<TailIndex>> for Cans<Head, Tail>
    where Tail: Select<T, R, TailIndex> {
    type Remainder = Cans<Head, Tail::Remainder>;
    fn select<F>(self, f: F) -> Result<R, Self::Remainder>
    where F: FnOnce(T) -> R {
        match self {
            Cans::Here(t) => Err(Cans::Here(t)),
            Cans::There(there) => there.select(f).map_err(|tail| Cans::There(tail))
        }
    }
}

impl<T, R, Selectable, Index> Select<T, R, Index> for Result<R, Selectable>
    where Selectable: Select<T, R, Index> {
    type Remainder = Selectable::Remainder;
    fn select<F>(self, f: F) -> Result<R, Self::Remainder>
    where F: FnOnce(T) -> R{
        self.or_else(|sel| sel.select(f))
    }
}

trait Reshape<Dest, Indices> {
    fn reshape(self) -> Dest;
}

impl<Dest> Reshape<Dest, ()> for Void {
    fn reshape(self) -> Dest {
        self.any()
    }
}

impl<Dest, SHead, STail, HIndex, TIndices> Reshape<Dest, (HIndex, TIndices)> for Cans<SHead, STail>
    where STail: Reshape<Dest, TIndices>,
          Dest: NewCans<SHead, HIndex> {
    fn reshape(self) -> Dest {
        match self {
            Cans::Here(shead) => NewCans::new(shead),
            Cans::There(stail) => stail.reshape()
        }
    }          
}

fn main() {
    let cans: Cans<i32, Cans<String, Void>> = Cans::new("Hello".to_string());
    {
        let s: Option<&String> = cans.find();
        let t: Option<&i32> = cans.find();
        println!("{:?}", s);
        println!("{:?}", t);
    }
    let foo = cans.select(|s: String| "It's a string!").select(|i: i32| "It's an i32!").unwrap_void();
    println!("{:?}", foo);
    let cans2: Cans<i32, Cans<String, Void>> = Cans::new("Hello".to_string());
    let cans3: Cans<String, Cans<i32, Cans<i64, Void>>> = cans2.reshape();
}

I can work on uploading it to crates.io, but I can't promise to pay attention to maintenance long term.

May I ask why you want this specific code? I see some similar things on crates.io already, some of which are focused on error handling, some of which aren't.

Sorry for 2 weeks of silence

This is the cleanest implementation I've seen so far, and also the smallest one. It could be refactored to be a little more better named, like Polymorph or Poly instead of Cans

#![feature(decl_macro)]
use std::marker::PhantomData;

#[derive(Clone, Debug)]
pub enum Void {}

impl Void {
  fn any(&self) -> ! {
    match *self {}
  }
}

#[allow(dead_code)]
pub trait Unwrap<R> {
  fn unwrap(self) -> R;
}

impl<R> Unwrap<R> for Result<R, Void> {
  fn unwrap(self) -> R {
    match self {
      Ok(r) => r,
      Err(void) => void.any(),
    }
  }
}

pub enum HeadMarker {}
pub struct TailMarker<T>(PhantomData<T>);

#[derive(Clone, Debug)]
pub enum Poly_<Head, Tail> {
  Head(Head),
  Tail(Tail),
}

pub trait Poly<Item, Where> {
  fn new(item: Item) -> Self;
}

impl<Item, Tail> Poly<Item, HeadMarker> for Poly_<Item, Tail> {
  fn new(item: Item) -> Self {
    Poly_::Head(item)
  }
}

impl<Item, Head, Tail, TailIndex> Poly<Item, TailMarker<TailIndex>> for Poly_<Head, Tail>
where
  Tail: Poly<Item, TailIndex>,
{
  fn new(item: Item) -> Self {
    Poly_::Tail(Poly::new(item))
  }
}

#[allow(dead_code)]
impl<A, B> Poly_<A, B> {
  pub fn new<Item, Where>(item: Item) -> Self
  where
    Self: Poly<Item, Where>,
  {
    Poly::new(item)
  }
}

pub trait Find<T, Where> {
  /// Given that `P` is isomorphic to `poly![A, B]`, one of the following method calls 
  /// will successfully return `Some(&T)`: `P.find() as A`, `P.find() as B`
  /// 
  /// Example:
  /// ```rs
  /// use poly::{poly, Poly, Find, Void};
  /// 
  /// fn test() {
  ///   let p: poly![u32, i32] = Poly::new(123u32);
  ///   let a: Option<&u32> = p.find();
  ///   if let Some(a) = a {
  ///     println!("{a}");
  ///   }
  /// }
  /// ```
  fn find(&self) -> Option<&T>;
  fn find_mut(&mut self) -> Option<&mut T>;
}

impl<T, Tail> Find<T, HeadMarker> for Poly_<T, Tail> {
  fn find(&self) -> Option<&T> {
    match *self {
      Poly_::Head(ref t) => Some(t),
      Poly_::Tail(_) => None,
    }
  }

  fn find_mut(&mut self) -> Option<&mut T> {
    match *self {
      Poly_::Head(ref mut t) => Some(t),
      Poly_::Tail(_) => None,
    }
  }
}

impl<Head, T, Tail, TailIndex> Find<T, TailMarker<TailIndex>> for Poly_<Head, Tail>
where
  Tail: Find<T, TailIndex>,
{
  fn find(&self) -> Option<&T> {
    match *self {
      Poly_::Head(_) => None,
      Poly_::Tail(ref there) => there.find(),
    }
  }

  fn find_mut(&mut self) -> Option<&mut T> {
    match *self {
      Poly_::Head(_) => None,
      Poly_::Tail(ref mut there) => there.find_mut(),
    }
  }
}

pub trait Select<T, R, Where> {
  type Remainder;

  fn select<F>(self, _: F) -> Result<R, Self::Remainder>
  where
    F: FnOnce(T) -> R;
}

impl<T, R, Tail> Select<T, R, HeadMarker> for Poly_<T, Tail> {
  type Remainder = Tail;

  fn select<F>(self, f: F) -> Result<R, Self::Remainder>
  where
    F: FnOnce(T) -> R,
  {
    match self {
      Poly_::Head(t) => Ok(f(t)),
      Poly_::Tail(there) => Err(there),
    }
  }
}

impl<T, R, Head, Tail, TailIndex> Select<T, R, TailMarker<TailIndex>> for Poly_<Head, Tail>
where
  Tail: Select<T, R, TailIndex>,
{
  type Remainder = Poly_<Head, Tail::Remainder>;

  fn select<F>(self, f: F) -> Result<R, Self::Remainder>
  where
    F: FnOnce(T) -> R,
  {
    match self {
      Poly_::Head(t) => Err(Poly_::Head(t)),
      Poly_::Tail(there) => there.select(f).map_err(|tail| Poly_::Tail(tail)),
    }
  }
}

impl<T, R, Selectable, Index> Select<T, R, Index> for Result<R, Selectable>
where
  Selectable: Select<T, R, Index>,
{
  type Remainder = Selectable::Remainder;

  fn select<F>(self, f: F) -> Result<R, Self::Remainder>
  where
    F: FnOnce(T) -> R,
  {
    self.or_else(|sel| sel.select(f))
  }
}

pub trait Mash<Dest, Indices> {
  fn mash(self) -> Dest;
}

impl<Dest> Mash<Dest, ()> for Void {
  fn mash(self) -> Dest {
    self.any()
  }
}

impl<Dest, SHead, STail, HIndex, TIndices> Mash<Dest, (HIndex, TIndices)> for Poly_<SHead, STail>
where
  STail: Mash<Dest, TIndices>,
  Dest: Poly<SHead, HIndex>,
{
  fn mash(self) -> Dest {
    match self {
      Poly_::Head(shead) => Poly::new(shead),
      Poly_::Tail(stail) => stail.mash(),
    }
  }
}

pub macro poly {
  () => {
    $crate::poly::Void
  },

  ($head:ty $(, $tail:ty)*) => {
    $crate::poly::Poly_<$head, poly!($($tail),*) >
  }
}

For example

Reshape could be left as is instead of Mash