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();
}