Repository: komora-io/terrors
Branch: main
Commit: 8f3b237a32d5
Files: 10
Total size: 58.5 KB

Directory structure:
gitextract_i6pk_blr/

├── .gitignore
├── Cargo.toml
├── LICENSE-APACHE
├── LICENSE-MIT
├── README.md
├── src/
│   ├── lib.rs
│   ├── one_of.rs
│   ├── one_of_to_enum.rs
│   └── type_set.rs
└── tests/
    └── usability.rs

================================================
FILE CONTENTS
================================================

================================================
FILE: .gitignore
================================================
/target
Cargo.lock


================================================
FILE: Cargo.toml
================================================
[package]
name = "terrors"
version = "0.3.3"
edition = "2021"
authors = ["Tyler Neely <tylerneely@gmail.com>"]
documentation = "https://docs.rs/terrors/"
description = "ergonomic and precise error handling built atop type-level set arithmetic"
license = "MIT OR Apache-2.0"
repository = "https://github.com/komora-io/terrors"
categories = ["rust-patterns"]
keywords = ["error", "error-handling", "type-level", "anonymous", "sum"]
readme = "README.md"

[features]
error_provide = []
error_provide_feature = []


================================================
FILE: LICENSE-APACHE
================================================
                                 Apache License
                           Version 2.0, January 2004
                        http://www.apache.org/licenses/

   TERMS AND CONDITIONS FOR USE, REPRODUCTION, AND DISTRIBUTION

   1. Definitions.

      "License" shall mean the terms and conditions for use, reproduction,
      and distribution as defined by Sections 1 through 9 of this document.

      "Licensor" shall mean the copyright owner or entity authorized by
      the copyright owner that is granting the License.

      "Legal Entity" shall mean the union of the acting entity and all
      other entities that control, are controlled by, or are under common
      control with that entity. For the purposes of this definition,
      "control" means (i) the power, direct or indirect, to cause the
      direction or management of such entity, whether by contract or
      otherwise, or (ii) ownership of fifty percent (50%) or more of the
      outstanding shares, or (iii) beneficial ownership of such entity.

      "You" (or "Your") shall mean an individual or Legal Entity
      exercising permissions granted by this License.

      "Source" form shall mean the preferred form for making modifications,
      including but not limited to software source code, documentation
      source, and configuration files.

      "Object" form shall mean any form resulting from mechanical
      transformation or translation of a Source form, including but
      not limited to compiled object code, generated documentation,
      and conversions to other media types.

      "Work" shall mean the work of authorship, whether in Source or
      Object form, made available under the License, as indicated by a
      copyright notice that is included in or attached to the work
      (an example is provided in the Appendix below).

      "Derivative Works" shall mean any work, whether in Source or Object
      form, that is based on (or derived from) the Work and for which the
      editorial revisions, annotations, elaborations, or other modifications
      represent, as a whole, an original work of authorship. For the purposes
      of this License, Derivative Works shall not include works that remain
      separable from, or merely link (or bind by name) to the interfaces of,
      the Work and Derivative Works thereof.

      "Contribution" shall mean any work of authorship, including
      the original version of the Work and any modifications or additions
      to that Work or Derivative Works thereof, that is intentionally
      submitted to Licensor for inclusion in the Work by the copyright owner
      or by an individual or Legal Entity authorized to submit on behalf of
      the copyright owner. For the purposes of this definition, "submitted"
      means any form of electronic, verbal, or written communication sent
      to the Licensor or its representatives, including but not limited to
      communication on electronic mailing lists, source code control systems,
      and issue tracking systems that are managed by, or on behalf of, the
      Licensor for the purpose of discussing and improving the Work, but
      excluding communication that is conspicuously marked or otherwise
      designated in writing by the copyright owner as "Not a Contribution."

      "Contributor" shall mean Licensor and any individual or Legal Entity
      on behalf of whom a Contribution has been received by Licensor and
      subsequently incorporated within the Work.

   2. Grant of Copyright License. Subject to the terms and conditions of
      this License, each Contributor hereby grants to You a perpetual,
      worldwide, non-exclusive, no-charge, royalty-free, irrevocable
      copyright license to reproduce, prepare Derivative Works of,
      publicly display, publicly perform, sublicense, and distribute the
      Work and such Derivative Works in Source or Object form.

   3. Grant of Patent License. Subject to the terms and conditions of
      this License, each Contributor hereby grants to You a perpetual,
      worldwide, non-exclusive, no-charge, royalty-free, irrevocable
      (except as stated in this section) patent license to make, have made,
      use, offer to sell, sell, import, and otherwise transfer the Work,
      where such license applies only to those patent claims licensable
      by such Contributor that are necessarily infringed by their
      Contribution(s) alone or by combination of their Contribution(s)
      with the Work to which such Contribution(s) was submitted. If You
      institute patent litigation against any entity (including a
      cross-claim or counterclaim in a lawsuit) alleging that the Work
      or a Contribution incorporated within the Work constitutes direct
      or contributory patent infringement, then any patent licenses
      granted to You under this License for that Work shall terminate
      as of the date such litigation is filed.

   4. Redistribution. You may reproduce and distribute copies of the
      Work or Derivative Works thereof in any medium, with or without
      modifications, and in Source or Object form, provided that You
      meet the following conditions:

      (a) You must give any other recipients of the Work or
          Derivative Works a copy of this License; and

      (b) You must cause any modified files to carry prominent notices
          stating that You changed the files; and

      (c) You must retain, in the Source form of any Derivative Works
          that You distribute, all copyright, patent, trademark, and
          attribution notices from the Source form of the Work,
          excluding those notices that do not pertain to any part of
          the Derivative Works; and

      (d) If the Work includes a "NOTICE" text file as part of its
          distribution, then any Derivative Works that You distribute must
          include a readable copy of the attribution notices contained
          within such NOTICE file, excluding those notices that do not
          pertain to any part of the Derivative Works, in at least one
          of the following places: within a NOTICE text file distributed
          as part of the Derivative Works; within the Source form or
          documentation, if provided along with the Derivative Works; or,
          within a display generated by the Derivative Works, if and
          wherever such third-party notices normally appear. The contents
          of the NOTICE file are for informational purposes only and
          do not modify the License. You may add Your own attribution
          notices within Derivative Works that You distribute, alongside
          or as an addendum to the NOTICE text from the Work, provided
          that such additional attribution notices cannot be construed
          as modifying the License.

      You may add Your own copyright statement to Your modifications and
      may provide additional or different license terms and conditions
      for use, reproduction, or distribution of Your modifications, or
      for any such Derivative Works as a whole, provided Your use,
      reproduction, and distribution of the Work otherwise complies with
      the conditions stated in this License.

   5. Submission of Contributions. Unless You explicitly state otherwise,
      any Contribution intentionally submitted for inclusion in the Work
      by You to the Licensor shall be under the terms and conditions of
      this License, without any additional terms or conditions.
      Notwithstanding the above, nothing herein shall supersede or modify
      the terms of any separate license agreement you may have executed
      with Licensor regarding such Contributions.

   6. Trademarks. This License does not grant permission to use the trade
      names, trademarks, service marks, or product names of the Licensor,
      except as required for reasonable and customary use in describing the
      origin of the Work and reproducing the content of the NOTICE file.

   7. Disclaimer of Warranty. Unless required by applicable law or
      agreed to in writing, Licensor provides the Work (and each
      Contributor provides its Contributions) on an "AS IS" BASIS,
      WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or
      implied, including, without limitation, any warranties or conditions
      of TITLE, NON-INFRINGEMENT, MERCHANTABILITY, or FITNESS FOR A
      PARTICULAR PURPOSE. You are solely responsible for determining the
      appropriateness of using or redistributing the Work and assume any
      risks associated with Your exercise of permissions under this License.

   8. Limitation of Liability. In no event and under no legal theory,
      whether in tort (including negligence), contract, or otherwise,
      unless required by applicable law (such as deliberate and grossly
      negligent acts) or agreed to in writing, shall any Contributor be
      liable to You for damages, including any direct, indirect, special,
      incidental, or consequential damages of any character arising as a
      result of this License or out of the use or inability to use the
      Work (including but not limited to damages for loss of goodwill,
      work stoppage, computer failure or malfunction, or any and all
      other commercial damages or losses), even if such Contributor
      has been advised of the possibility of such damages.

   9. Accepting Warranty or Additional Liability. While redistributing
      the Work or Derivative Works thereof, You may choose to offer,
      and charge a fee for, acceptance of support, warranty, indemnity,
      or other liability obligations and/or rights consistent with this
      License. However, in accepting such obligations, You may act only
      on Your own behalf and on Your sole responsibility, not on behalf
      of any other Contributor, and only if You agree to indemnify,
      defend, and hold each Contributor harmless for any liability
      incurred by, or claims asserted against, such Contributor by reason
      of your accepting any such warranty or additional liability.

   END OF TERMS AND CONDITIONS

   APPENDIX: How to apply the Apache License to your work.

      To apply the Apache License to your work, attach the following
      boilerplate notice, with the fields enclosed by brackets "{}"
      replaced with your own identifying information. (Don't include
      the brackets!)  The text should be enclosed in the appropriate
      comment syntax for the file format. We also recommend that a
      file or class name and description of purpose be included on the
      same "printed page" as the copyright notice for easier
      identification within third-party archives.

   Copyright 2024 Tyler Neely

   Licensed under the Apache License, Version 2.0 (the "License");
   you may not use this file except in compliance with the License.
   You may obtain a copy of the License at

       http://www.apache.org/licenses/LICENSE-2.0

   Unless required by applicable law or agreed to in writing, software
   distributed under the License is distributed on an "AS IS" BASIS,
   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
   See the License for the specific language governing permissions and
   limitations under the License.


================================================
FILE: LICENSE-MIT
================================================
Copyright (c) 2024 Tyler Neely

Permission is hereby granted, free of charge, to any
person obtaining a copy of this software and associated
documentation files (the "Software"), to deal in the
Software without restriction, including without
limitation the rights to use, copy, modify, merge,
publish, distribute, sublicense, and/or sell copies of
the Software, and to permit persons to whom the Software
is furnished to do so, subject to the following
conditions:

The above copyright notice and this permission notice
shall be included in all copies or substantial portions
of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF
ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED
TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A
PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT
SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY
CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR
IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
DEALINGS IN THE SOFTWARE.


================================================
FILE: README.md
================================================
# terrors - the Rust error **handling** library

Handling errors means taking a set of possible error
types, removing the ones that are locally addressible,
and then if the set of errors is not within those local
concerns, propagating the remainder to a caller. The
caller should not receive the local errors of the callee.

# Principles

* Error types should be precise.
  * `terrors::OneOf` solves this by making precise sets of possible errors:
    * low friction to specify
    * low friction to narrow by specific error handlers
    * low friction to broaden to pass up the stack
* Error handling should follow the single responsibility principle
    * if every error in a system is spread everywhere else, there
      is no clear responsibility for where it needs to be handled.
* No macros.
    * Users should not have to learn some new DSL for error handling that every macro entails.

# Examples

```rust
use terrors::OneOf;

let one_of_3: OneOf<(String, u32, Vec<u8>)> = OneOf::new(5);

let narrowed_res: Result<u32, OneOf<(String, Vec<u8>)>> =
    one_of_3.narrow();

assert_eq!(5, narrowed_res.unwrap());
```

OneOf can also be broadened to a superset, checked at compile-time.

```rust
use terrors::OneOf;

struct Timeout;
struct AllocationFailure;
struct RetriesExhausted;

fn allocate_box() -> Result<Box<u8>, OneOf<(AllocationFailure,)>> {
    Err(AllocationFailure.into())
}

fn send() -> Result<(), OneOf<(Timeout,)>> {
    Err(Timeout.into())
}

fn allocate_and_send() -> Result<(), OneOf<(AllocationFailure, Timeout)>> {
    let boxed_byte: Box<u8> = allocate_box().map_err(OneOf::broaden)?;
    send().map_err(OneOf::broaden)?;

    Ok(())
}

fn retry() -> Result<(), OneOf<(AllocationFailure, RetriesExhausted)>> {
    for _ in 0..3 {
        let Err(err) = allocate_and_send() else {
            return Ok(());
        };

        // keep retrying if we have a Timeout,
        // but punt allocation issues to caller.
        match err.narrow::<Timeout, _>() {
            Ok(_timeout) => {},
            Err(one_of_others) => return Err(one_of_others.broaden()),
        }
    }

    Err(OneOf::new(RetriesExhausted))
}
```

`OneOf` also implements `Clone`, `Debug`, `Display`, `Send`, `Sync` and/or `std::error::Error` if all types in the type set do as well:

```rust
use std::error::Error;
use std::io;
use terrors::OneOf;

let o_1: OneOf<(u32, String)> = OneOf::new(5_u32);

// Debug is implemented if all types in the type set implement Debug
dbg!(&o_1);

// Display is implemented if all types in the type set implement Display
println!("{}", o_1);

let cloned = o_1.clone();

type E = io::Error;
let e = io::Error::new(io::ErrorKind::Other, "wuaaaaahhhzzaaaaaaaa");

let o_2: OneOf<(E,)> = OneOf::new(e);

// std::error::Error is implemented if all types in the type set implement it
dbg!(o_2.description());
```

OneOf can also be turned into an owned or referenced enum form:

```rust
use terrors::{OneOf, E2};

let o_1: OneOf<(u32, String)> = OneOf::new(5_u32);

match o_1.as_enum() {
    E2::A(u) => {
        println!("handling reference {u}: u32")
    }
    E2::B(s) => {
        println!("handling reference {s}: String")
    }
}

match o_1.to_enum() {
    E2::A(u) => {
        println!("handling owned {u}: u32")
    }
    E2::B(s) => {
        println!("handling owned {s}: String")
    }
}
```

### Motivation

The paper [Simple Testing Can Prevent Most Critical Failures: An Analysis of Production Failures in Distributed Data-intensive Systems](https://www.eecg.toronto.edu/~yuan/papers/failure_analysis_osdi14.pdf)
is goldmine of fascinating statistics that illuminate the
software patterns that tend to correspond to system failures.
This is one of my favorites:

```no_compile
almost all (92%) of the catastrophic system failures
are the result of incorrect handling of non-fatal errors
explicitly signaled in software.
```

Our systems are falling over because we aren't handling
our errors. We're doing fine when it comes to signalling
their existence, but we need to actually handle them.

When we write Rust, we tend to encounter a variety of different
error types. Sometimes we need to put multiple possible errors
into a container that is then returned from a function, where
the caller or a transitive caller is expected to handle the
specific problem that arose.

As we grow a codebase, more of these situations pop up.
While it's not so much effort to write custom enums in
one or two places that hold the precise set of possible
errors, most people resort to one of two strategies for
minimizing the effort that goes into propagating their
error types:
* A large top-level enum that holds variants for errors
  originating across the codebase, tending to grow
  larger and larger over time, undermining the ability
  to use exhaustive pattern matching to confidently
  ensure that local concerns are not bubbling up the stack.
* A boxed trait that is easy to convert errors into, but
 then hides information about what may actually be inside.
 You don't know where it's been or where it's going.

As the number of different source error types that these
error containers hold increases, the amount of information
that the container communicates to people who encounter it
decreases. It becomes increasingly unclear what the error
container actually holds. As the precision of the type
goes down, so does a human's ability to reason about
where the appropriate place is to handle any particular
concern within it.

We have to increase the precision in our error types.

People don't write a precise enum for every function that
may only return some subset of errors because we would
end up with a ton of small enum types that only get used in
one or two places. This is the pain that drives people
to using overly-broad error enums or overly-smooth
boxed dynamic error traits, reducing their ability to
handle their errors.

### Cool stuff

This crate is built around `OneOf`, which functions as
a form of anonymous enum that can be narrowed in ways
that may be familiar for users of TypeScript etc...
Our error containers need to get smaller as individual
errors are peeled off and handled, leaving the reduced
remainder of possible error types if the local concerns
are not present.

The cool thing about it is that it is built on top of a
type-level heterogenous set of possible error types,
where there's only one actual value among the different
possibilities.

Rather than having a giant ball of mud enum or
boxed trait object that is never clear what it actually
contains, causing you to never handle individual
concerns from, the idea of this is that you can
have a minimized set of actual error types that may
thread through the stack.

The nice thing about this type-level set of possibilities
is that any specific type can be peeled off while narrowing
the rest of the types if the narrowing fails. Both narrowing
and broadening are based on compile-time error type set checking.

### The Trade-Off

Type-level programming is something that I have tried hard to avoid
for most of my career due to confusing error messages resulting
from compilation errors. These complex type checking failures
produce errors that are challenging to reason about, and can often
take several minutes to understand.

I have tried hard to avoid exposing users of `terrors` to too many
of the sharp edges in the underlying type machinery, but it is likely
that if the source and destination type sets do not satisfy the `SupersetOf`
trait in the right direction depending on whether `narrow` or
`broaden` is being called, that the error will not be particularly
pleasant to read. Just know that errors pretty much always mean
that the superset relationship does not hold as required.

Going forward, I believe most of the required traits can be implemented
in ways that expose users to errors that look more like `(A, B) does not
implement SupersetOf<(C, D), _>` instead of `Cons<A, Cons<B, End>> does
not implement SupersetOf<Cons<C, Cons<D, End>>>` by leaning into the
bidirectional type mapping that exists between the heterogenous type
set `Cons` chains and more human-friendly type tuples.

### Special Thanks

Much of the fancy type-level logic for reasoning about sets of error types
was directly inspired by [frunk](https://docs.rs/frunk/latest/frunk/).
I had been wondering for years about the feasibility of a data structure
like `OneOf`, and had often assumed it was impossible, until I finally
had an extended weekend to give it a deep dive. After many false starts,
I finally came across [an article](https://archive.is/YwDMX) written by
[lloydmeta](https://github.com/lloydmeta) (the author of frunk) about how
frunk handles several related concerns in the context of a heterogenous
list structure. Despite having used Rust for over 10 years, that article
taught me a huge amount about how the language's type system can be
used in interesting ways that addressed very practical needs. In particular,
the general perspective in that blog post about how you can implement
traits in a recursive way that is familiar from other functional languages
was the missing primitive for working with Rust that I had not realized
was possible for my first decade with the language. Thank you very
much for creating frunk and telling the world about how you did it!


================================================
FILE: src/lib.rs
================================================
#![cfg_attr(
    feature = "error_provide_feature",
    feature(error_generic_member_access)
)]
#![doc = include_str!(concat!(env!("CARGO_MANIFEST_DIR"), "/README.md"))]

#[doc = include_str!(concat!(env!("CARGO_MANIFEST_DIR"), "/README.md"))]
#[cfg(doctest)]
pub struct ReadmeDoctests;

mod one_of;
mod one_of_to_enum;
mod type_set;

/// Similar to anonymous unions / enums in languages that support type narrowing.
pub use one_of::OneOf;

pub use type_set::{TypeSet, E1, E2, E3, E4, E5, E6, E7, E8, E9};

/* ------------------------- Helpers ----------------------- */

/// The final element of a type-level Cons list.
#[doc(hidden)]
#[derive(Debug)]
pub enum End {}

impl std::error::Error for End {}

/// A compile-time list of types, similar to other basic functional list structures.
#[doc(hidden)]
#[derive(Debug)]
pub struct Cons<Head, Tail>(core::marker::PhantomData<Head>, Tail);

#[doc(hidden)]
#[derive(Debug)]
pub struct Recurse<Tail>(Tail);


================================================
FILE: src/one_of.rs
================================================
use core::any::Any;
use core::fmt;
use core::marker::PhantomData;
use core::ops::Deref;
use std::error::Error;

use crate::type_set::{
    CloneFold, Contains, DebugFold, DisplayFold, ErrorFold, IsFold, Narrow, SupersetOf, TupleForm,
    TypeSet,
};

use crate::{Cons, End};

/* ------------------------- OneOf ----------------------- */

/// `OneOf` is an open sum type. It differs from an enum
/// in that you do not need to define any actual new type
/// in order to hold some specific combination of variants,
/// but rather you simply describe the OneOf as holding
/// one value out of several specific possibilities,
/// defined by using a tuple of those possible variants
/// as the generic parameter for the `OneOf`.
///
/// For example, a `OneOf<(String, u32)>` contains either
/// a `String` or a `u32`. The value over a simple `Result`
/// or other traditional enum starts to become apparent in larger
/// codebases where error handling needs to occur in
/// different places for different errors. `OneOf` allows
/// you to quickly specify a function's return value as
/// involving a precise subset of errors that the caller
/// can clearly reason about.
pub struct OneOf<E: TypeSet> {
    pub(crate) value: Box<dyn Any>,
    _pd: PhantomData<E>,
}

fn _send_sync_error_assert() {
    use std::io;

    fn is_send<T: Send>(_: &T) {}
    fn is_sync<T: Sync>(_: &T) {}
    fn is_error<T: Error>(_: &T) {}

    let o: OneOf<(io::Error,)> = OneOf::new(io::Error::new(io::ErrorKind::Other, "yooo"));
    is_send(&o);
    is_sync(&o);
    is_error(&o);
}

unsafe impl<T> Send for OneOf<T> where T: TypeSet + Send {}
unsafe impl<T> Sync for OneOf<T> where T: TypeSet + Sync {}

impl<T> Deref for OneOf<(T,)>
where
    T: 'static,
{
    type Target = T;

    fn deref(&self) -> &T {
        self.value.downcast_ref::<T>().unwrap()
    }
}

impl<T> From<T> for OneOf<(T,)>
where
    T: 'static,
{
    fn from(t: T) -> OneOf<(T,)> {
        OneOf::new(t)
    }
}

impl<E> Clone for OneOf<E>
where
    E: TypeSet,
    E::Variants: Clone + CloneFold,
{
    fn clone(&self) -> Self {
        let value = E::Variants::clone_fold(&self.value);

        OneOf {
            value,
            _pd: PhantomData,
        }
    }
}
impl<E> fmt::Debug for OneOf<E>
where
    E: TypeSet,
    E::Variants: fmt::Debug + DebugFold,
{
    fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
        E::Variants::debug_fold(&self.value, formatter)
    }
}

impl<E> fmt::Display for OneOf<E>
where
    E: TypeSet,
    E::Variants: fmt::Display + DisplayFold,
{
    fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
        E::Variants::display_fold(&self.value, formatter)
    }
}

impl<E> Error for OneOf<E>
where
    E: TypeSet,
    E::Variants: Error + DebugFold + DisplayFold + ErrorFold,
{
    fn source(&self) -> Option<&(dyn Error + 'static)> {
        E::Variants::source_fold(&self.value)
    }
}

impl<E> OneOf<E>
where
    E: TypeSet,
{
    /// Create a new `OneOf`.
    pub fn new<T, Index>(t: T) -> OneOf<E>
    where
        T: Any,
        E::Variants: Contains<T, Index>,
    {
        OneOf {
            value: Box::new(t),
            _pd: PhantomData,
        }
    }

    /// Attempt to downcast the `OneOf` into a specific type, and
    /// if that fails, return a `OneOf` which does not contain that
    /// type as one of its possible variants.
    pub fn narrow<Target, Index>(
        self,
    ) -> Result<
        Target,
        OneOf<<<E::Variants as Narrow<Target, Index>>::Remainder as TupleForm>::Tuple>,
    >
    where
        Target: 'static,
        E::Variants: Narrow<Target, Index>,
    {
        if self.value.is::<Target>() {
            Ok(*self.value.downcast::<Target>().unwrap())
        } else {
            Err(OneOf {
                value: self.value,
                _pd: PhantomData,
            })
        }
    }

    /// Turns the `OneOf` into a `OneOf` with a set of variants
    /// which is a superset of the current one. This may also be
    /// the same set of variants, but in a different order.
    pub fn broaden<Other, Index>(self) -> OneOf<Other>
    where
        Other: TypeSet,
        Other::Variants: SupersetOf<E::Variants, Index>,
    {
        OneOf {
            value: self.value,
            _pd: PhantomData,
        }
    }

    /// Attempt to split a subset of variants out of the `OneOf`,
    /// returning the remainder of possible variants if the value
    /// does not have one of the `TargetList` types.
    pub fn subset<TargetList, Index>(
        self,
    ) -> Result<
        OneOf<TargetList>,
        OneOf<<<E::Variants as SupersetOf<TargetList::Variants, Index>>::Remainder as TupleForm>::Tuple>,
    >
    where
        TargetList: TypeSet,
        E::Variants: IsFold + SupersetOf<TargetList::Variants, Index>,
    {
        if E::Variants::is_fold(&self.value) {
            Ok(OneOf {
                value: self.value,
                _pd: PhantomData,
            })
        } else {
            Err(OneOf {
                value: self.value,
                _pd: PhantomData,
            })
        }
    }

    /// For a `OneOf` with a single variant, return
    /// the contained value.
    pub fn take<Target>(self) -> Target
    where
        Target: 'static,
        E: TypeSet<Variants = Cons<Target, End>>,
    {
        *self.value.downcast::<Target>().unwrap()
    }

    /// Convert the `OneOf` to an owned enum for
    /// use in pattern matching etc...
    pub fn to_enum(self) -> E::Enum
    where
        E::Enum: From<Self>,
    {
        E::Enum::from(self)
    }

    /// Borrow the enum as an enum for use in
    /// pattern matching etc...
    pub fn as_enum<'a>(&'a self) -> E::EnumRef<'a>
    where
        E::EnumRef<'a>: From<&'a Self>,
    {
        E::EnumRef::from(&self)
    }
}


================================================
FILE: src/one_of_to_enum.rs
================================================
use super::{OneOf, E1, E2, E3, E4, E5, E6, E7, E8, E9};

/* ------------------------- Enum conversions ----------------------- */

impl<A> From<OneOf<(A,)>> for E1<A>
where
    A: 'static,
{
    fn from(one_of: OneOf<(A,)>) -> Self {
        E1::A(*one_of.value.downcast().unwrap())
    }
}

impl<'a, A> From<&'a OneOf<(A,)>> for E1<&'a A>
where
    A: 'static,
{
    fn from(one_of: &'a OneOf<(A,)>) -> Self {
        E1::A(one_of.value.downcast_ref().unwrap())
    }
}

impl<A, B> From<OneOf<(A, B)>> for E2<A, B>
where
    A: 'static,
    B: 'static,
{
    fn from(one_of: OneOf<(A, B)>) -> Self {
        if one_of.value.is::<A>() {
            E2::A(*one_of.value.downcast().unwrap())
        } else {
            E2::B(*one_of.value.downcast().unwrap())
        }
    }
}

impl<'a, A, B> From<&'a OneOf<(A, B)>> for E2<&'a A, &'a B>
where
    A: 'static,
    B: 'static,
{
    fn from(one_of: &'a OneOf<(A, B)>) -> Self {
        if one_of.value.is::<A>() {
            E2::A(one_of.value.downcast_ref().unwrap())
        } else {
            E2::B(one_of.value.downcast_ref().unwrap())
        }
    }
}

impl<A, B, C> From<OneOf<(A, B, C)>> for E3<A, B, C>
where
    A: 'static,
    B: 'static,
    C: 'static,
{
    fn from(one_of: OneOf<(A, B, C)>) -> Self {
        if one_of.value.is::<A>() {
            E3::A(*one_of.value.downcast().unwrap())
        } else if one_of.value.is::<B>() {
            E3::B(*one_of.value.downcast().unwrap())
        } else {
            E3::C(*one_of.value.downcast().unwrap())
        }
    }
}

impl<'a, A, B, C> From<&'a OneOf<(A, B, C)>> for E3<&'a A, &'a B, &'a C>
where
    A: 'static,
    B: 'static,
    C: 'static,
{
    fn from(one_of: &'a OneOf<(A, B, C)>) -> Self {
        if one_of.value.is::<A>() {
            E3::A(one_of.value.downcast_ref().unwrap())
        } else if one_of.value.is::<B>() {
            E3::B(one_of.value.downcast_ref().unwrap())
        } else {
            E3::C(one_of.value.downcast_ref().unwrap())
        }
    }
}

impl<A, B, C, D> From<OneOf<(A, B, C, D)>> for E4<A, B, C, D>
where
    A: 'static,
    B: 'static,
    C: 'static,
    D: 'static,
{
    fn from(one_of: OneOf<(A, B, C, D)>) -> Self {
        if one_of.value.is::<A>() {
            E4::A(*one_of.value.downcast().unwrap())
        } else if one_of.value.is::<B>() {
            E4::B(*one_of.value.downcast().unwrap())
        } else if one_of.value.is::<C>() {
            E4::C(*one_of.value.downcast().unwrap())
        } else {
            E4::D(*one_of.value.downcast().unwrap())
        }
    }
}

impl<'a, A, B, C, D> From<&'a OneOf<(A, B, C, D)>> for E4<&'a A, &'a B, &'a C, &'a D>
where
    A: 'static,
    B: 'static,
    C: 'static,
    D: 'static,
{
    fn from(one_of: &'a OneOf<(A, B, C, D)>) -> Self {
        if one_of.value.is::<A>() {
            E4::A(one_of.value.downcast_ref().unwrap())
        } else if one_of.value.is::<B>() {
            E4::B(one_of.value.downcast_ref().unwrap())
        } else if one_of.value.is::<C>() {
            E4::C(one_of.value.downcast_ref().unwrap())
        } else {
            E4::D(one_of.value.downcast_ref().unwrap())
        }
    }
}

impl<A, B, C, D, E> From<OneOf<(A, B, C, D, E)>> for E5<A, B, C, D, E>
where
    A: 'static,
    B: 'static,
    C: 'static,
    D: 'static,
    E: 'static,
{
    fn from(one_of: OneOf<(A, B, C, D, E)>) -> Self {
        if one_of.value.is::<A>() {
            E5::A(*one_of.value.downcast().unwrap())
        } else if one_of.value.is::<B>() {
            E5::B(*one_of.value.downcast().unwrap())
        } else if one_of.value.is::<C>() {
            E5::C(*one_of.value.downcast().unwrap())
        } else if one_of.value.is::<D>() {
            E5::D(*one_of.value.downcast().unwrap())
        } else {
            E5::E(*one_of.value.downcast().unwrap())
        }
    }
}

impl<'a, A, B, C, D, E> From<&'a OneOf<(A, B, C, D, E)>> for E5<&'a A, &'a B, &'a C, &'a D, &'a E>
where
    A: 'static,
    B: 'static,
    C: 'static,
    D: 'static,
    E: 'static,
{
    fn from(one_of: &'a OneOf<(A, B, C, D, E)>) -> Self {
        if one_of.value.is::<A>() {
            E5::A(one_of.value.downcast_ref().unwrap())
        } else if one_of.value.is::<B>() {
            E5::B(one_of.value.downcast_ref().unwrap())
        } else if one_of.value.is::<C>() {
            E5::C(one_of.value.downcast_ref().unwrap())
        } else if one_of.value.is::<D>() {
            E5::D(one_of.value.downcast_ref().unwrap())
        } else {
            E5::E(one_of.value.downcast_ref().unwrap())
        }
    }
}

impl<A, B, C, D, E, F> From<OneOf<(A, B, C, D, E, F)>> for E6<A, B, C, D, E, F>
where
    A: 'static,
    B: 'static,
    C: 'static,
    D: 'static,
    E: 'static,
    F: 'static,
{
    fn from(one_of: OneOf<(A, B, C, D, E, F)>) -> Self {
        if one_of.value.is::<A>() {
            E6::A(*one_of.value.downcast().unwrap())
        } else if one_of.value.is::<B>() {
            E6::B(*one_of.value.downcast().unwrap())
        } else if one_of.value.is::<C>() {
            E6::C(*one_of.value.downcast().unwrap())
        } else if one_of.value.is::<D>() {
            E6::D(*one_of.value.downcast().unwrap())
        } else if one_of.value.is::<E>() {
            E6::E(*one_of.value.downcast().unwrap())
        } else {
            E6::F(*one_of.value.downcast().unwrap())
        }
    }
}

impl<'a, A, B, C, D, E, F> From<&'a OneOf<(A, B, C, D, E, F)>>
    for E6<&'a A, &'a B, &'a C, &'a D, &'a E, &'a F>
where
    A: 'static,
    B: 'static,
    C: 'static,
    D: 'static,
    E: 'static,
    F: 'static,
{
    fn from(one_of: &'a OneOf<(A, B, C, D, E, F)>) -> Self {
        if one_of.value.is::<A>() {
            E6::A(one_of.value.downcast_ref().unwrap())
        } else if one_of.value.is::<B>() {
            E6::B(one_of.value.downcast_ref().unwrap())
        } else if one_of.value.is::<C>() {
            E6::C(one_of.value.downcast_ref().unwrap())
        } else if one_of.value.is::<D>() {
            E6::D(one_of.value.downcast_ref().unwrap())
        } else if one_of.value.is::<E>() {
            E6::E(one_of.value.downcast_ref().unwrap())
        } else {
            E6::F(one_of.value.downcast_ref().unwrap())
        }
    }
}

impl<A, B, C, D, E, F, G> From<OneOf<(A, B, C, D, E, F, G)>> for E7<A, B, C, D, E, F, G>
where
    A: 'static,
    B: 'static,
    C: 'static,
    D: 'static,
    E: 'static,
    F: 'static,
    G: 'static,
{
    fn from(one_of: OneOf<(A, B, C, D, E, F, G)>) -> Self {
        if one_of.value.is::<A>() {
            E7::A(*one_of.value.downcast().unwrap())
        } else if one_of.value.is::<B>() {
            E7::B(*one_of.value.downcast().unwrap())
        } else if one_of.value.is::<C>() {
            E7::C(*one_of.value.downcast().unwrap())
        } else if one_of.value.is::<D>() {
            E7::D(*one_of.value.downcast().unwrap())
        } else if one_of.value.is::<E>() {
            E7::E(*one_of.value.downcast().unwrap())
        } else if one_of.value.is::<F>() {
            E7::F(*one_of.value.downcast().unwrap())
        } else {
            E7::G(*one_of.value.downcast().unwrap())
        }
    }
}

impl<'a, A, B, C, D, E, F, G> From<&'a OneOf<(A, B, C, D, E, F, G)>>
    for E7<&'a A, &'a B, &'a C, &'a D, &'a E, &'a F, &'a G>
where
    A: 'static,
    B: 'static,
    C: 'static,
    D: 'static,
    E: 'static,
    F: 'static,
    G: 'static,
{
    fn from(one_of: &'a OneOf<(A, B, C, D, E, F, G)>) -> Self {
        if one_of.value.is::<A>() {
            E7::A(one_of.value.downcast_ref().unwrap())
        } else if one_of.value.is::<B>() {
            E7::B(one_of.value.downcast_ref().unwrap())
        } else if one_of.value.is::<C>() {
            E7::C(one_of.value.downcast_ref().unwrap())
        } else if one_of.value.is::<D>() {
            E7::D(one_of.value.downcast_ref().unwrap())
        } else if one_of.value.is::<E>() {
            E7::E(one_of.value.downcast_ref().unwrap())
        } else if one_of.value.is::<F>() {
            E7::F(one_of.value.downcast_ref().unwrap())
        } else {
            E7::G(one_of.value.downcast_ref().unwrap())
        }
    }
}

impl<A, B, C, D, E, F, G, H> From<OneOf<(A, B, C, D, E, F, G, H)>> for E8<A, B, C, D, E, F, G, H>
where
    A: 'static,
    B: 'static,
    C: 'static,
    D: 'static,
    E: 'static,
    F: 'static,
    G: 'static,
    H: 'static,
{
    fn from(one_of: OneOf<(A, B, C, D, E, F, G, H)>) -> Self {
        if one_of.value.is::<A>() {
            E8::A(*one_of.value.downcast().unwrap())
        } else if one_of.value.is::<B>() {
            E8::B(*one_of.value.downcast().unwrap())
        } else if one_of.value.is::<C>() {
            E8::C(*one_of.value.downcast().unwrap())
        } else if one_of.value.is::<D>() {
            E8::D(*one_of.value.downcast().unwrap())
        } else if one_of.value.is::<E>() {
            E8::E(*one_of.value.downcast().unwrap())
        } else if one_of.value.is::<F>() {
            E8::F(*one_of.value.downcast().unwrap())
        } else if one_of.value.is::<G>() {
            E8::G(*one_of.value.downcast().unwrap())
        } else {
            E8::H(*one_of.value.downcast().unwrap())
        }
    }
}

impl<'a, A, B, C, D, E, F, G, H> From<&'a OneOf<(A, B, C, D, E, F, G, H)>>
    for E8<&'a A, &'a B, &'a C, &'a D, &'a E, &'a F, &'a G, &'a H>
where
    A: 'static,
    B: 'static,
    C: 'static,
    D: 'static,
    E: 'static,
    F: 'static,
    G: 'static,
    H: 'static,
{
    fn from(one_of: &'a OneOf<(A, B, C, D, E, F, G, H)>) -> Self {
        if one_of.value.is::<A>() {
            E8::A(one_of.value.downcast_ref().unwrap())
        } else if one_of.value.is::<B>() {
            E8::B(one_of.value.downcast_ref().unwrap())
        } else if one_of.value.is::<C>() {
            E8::C(one_of.value.downcast_ref().unwrap())
        } else if one_of.value.is::<D>() {
            E8::D(one_of.value.downcast_ref().unwrap())
        } else if one_of.value.is::<E>() {
            E8::E(one_of.value.downcast_ref().unwrap())
        } else if one_of.value.is::<F>() {
            E8::F(one_of.value.downcast_ref().unwrap())
        } else if one_of.value.is::<G>() {
            E8::G(one_of.value.downcast_ref().unwrap())
        } else {
            E8::H(one_of.value.downcast_ref().unwrap())
        }
    }
}

impl<A, B, C, D, E, F, G, H, I> From<OneOf<(A, B, C, D, E, F, G, H, I)>>
    for E9<A, B, C, D, E, F, G, H, I>
where
    A: 'static,
    B: 'static,
    C: 'static,
    D: 'static,
    E: 'static,
    F: 'static,
    G: 'static,
    H: 'static,
    I: 'static,
{
    fn from(one_of: OneOf<(A, B, C, D, E, F, G, H, I)>) -> Self {
        if one_of.value.is::<A>() {
            E9::A(*one_of.value.downcast().unwrap())
        } else if one_of.value.is::<B>() {
            E9::B(*one_of.value.downcast().unwrap())
        } else if one_of.value.is::<C>() {
            E9::C(*one_of.value.downcast().unwrap())
        } else if one_of.value.is::<D>() {
            E9::D(*one_of.value.downcast().unwrap())
        } else if one_of.value.is::<E>() {
            E9::E(*one_of.value.downcast().unwrap())
        } else if one_of.value.is::<F>() {
            E9::F(*one_of.value.downcast().unwrap())
        } else if one_of.value.is::<G>() {
            E9::G(*one_of.value.downcast().unwrap())
        } else if one_of.value.is::<H>() {
            E9::H(*one_of.value.downcast().unwrap())
        } else {
            E9::I(*one_of.value.downcast().unwrap())
        }
    }
}

impl<'a, A, B, C, D, E, F, G, H, I> From<&'a OneOf<(A, B, C, D, E, F, G, H, I)>>
    for E9<&'a A, &'a B, &'a C, &'a D, &'a E, &'a F, &'a G, &'a H, &'a I>
where
    A: 'static,
    B: 'static,
    C: 'static,
    D: 'static,
    E: 'static,
    F: 'static,
    G: 'static,
    H: 'static,
    I: 'static,
{
    fn from(one_of: &'a OneOf<(A, B, C, D, E, F, G, H, I)>) -> Self {
        if one_of.value.is::<A>() {
            E9::A(one_of.value.downcast_ref().unwrap())
        } else if one_of.value.is::<B>() {
            E9::B(one_of.value.downcast_ref().unwrap())
        } else if one_of.value.is::<C>() {
            E9::C(one_of.value.downcast_ref().unwrap())
        } else if one_of.value.is::<D>() {
            E9::D(one_of.value.downcast_ref().unwrap())
        } else if one_of.value.is::<E>() {
            E9::E(one_of.value.downcast_ref().unwrap())
        } else if one_of.value.is::<F>() {
            E9::F(one_of.value.downcast_ref().unwrap())
        } else if one_of.value.is::<G>() {
            E9::G(one_of.value.downcast_ref().unwrap())
        } else if one_of.value.is::<H>() {
            E9::H(one_of.value.downcast_ref().unwrap())
        } else {
            E9::I(one_of.value.downcast_ref().unwrap())
        }
    }
}


================================================
FILE: src/type_set.rs
================================================
//! Type-level set inclusion and difference, inspired by frunk's approach: <https://archive.is/YwDMX>
use core::any::Any;
use core::fmt;
use std::error::Error;

use crate::{Cons, End, Recurse};

/* ------------------------- std::error::Error support ----------------------- */

pub trait ErrorFold {
    fn source_fold(any: &Box<dyn Any>) -> Option<&(dyn Error + 'static)>;

    #[cfg(feature = "error_provide")]
    fn provide_fold<'a>(any: &'a Box<dyn Any>, request: &mut std::error::Request<'a>);
}

impl ErrorFold for End {
    fn source_fold(_: &Box<dyn Any>) -> Option<&(dyn Error + 'static)> {
        unreachable!("source_fold called on End");
    }

    #[cfg(feature = "error_provide")]
    fn provide_fold<'a>(_: &Box<dyn Any>, _: &mut std::error::Request<'a>) {
        unreachable!("provide_fold called on End");
    }
}

impl<Head, Tail> Error for Cons<Head, Tail>
where
    Head: Error,
    Tail: Error,
{
}

impl<Head, Tail> ErrorFold for Cons<Head, Tail>
where
    Cons<Head, Tail>: Error,
    Head: 'static + Error,
    Tail: ErrorFold,
{
    fn source_fold(any: &Box<dyn Any>) -> Option<&(dyn Error + 'static)> {
        if let Some(head_ref) = any.downcast_ref::<Head>() {
            head_ref.source()
        } else {
            Tail::source_fold(any)
        }
    }

    #[cfg(feature = "error_provide")]
    fn provide_fold<'a>(any: &'a Box<dyn Any>, request: &mut std::error::Request<'a>) {
        if let Some(head_ref) = any.downcast_ref::<Head>() {
            head_ref.provide(request)
        } else {
            Tail::provide_fold(any, request)
        }
    }
}

/* ------------------------- Display support ----------------------- */

impl<Head, Tail> fmt::Display for Cons<Head, Tail>
where
    Head: fmt::Display,
    Tail: fmt::Display,
{
    fn fmt(&self, _: &mut fmt::Formatter<'_>) -> fmt::Result {
        unreachable!("Display called for Cons which is not constructable")
    }
}

impl fmt::Display for End {
    fn fmt(&self, _: &mut fmt::Formatter<'_>) -> fmt::Result {
        unreachable!("Display::fmt called for an End, which is not constructible.")
    }
}

pub trait DisplayFold {
    fn display_fold(any: &Box<dyn Any>, formatter: &mut fmt::Formatter<'_>) -> fmt::Result;
}

impl DisplayFold for End {
    fn display_fold(_: &Box<dyn Any>, _: &mut fmt::Formatter<'_>) -> fmt::Result {
        unreachable!("display_fold called on End");
    }
}

impl<Head, Tail> DisplayFold for Cons<Head, Tail>
where
    Cons<Head, Tail>: fmt::Display,
    Head: 'static + fmt::Display,
    Tail: DisplayFold,
{
    fn display_fold(any: &Box<dyn Any>, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
        if let Some(head_ref) = any.downcast_ref::<Head>() {
            head_ref.fmt(formatter)
        } else {
            Tail::display_fold(any, formatter)
        }
    }
}

/* ------------------------- Debug support ----------------------- */

pub trait DebugFold {
    fn debug_fold(any: &Box<dyn Any>, formatter: &mut fmt::Formatter<'_>) -> fmt::Result;
}

impl DebugFold for End {
    fn debug_fold(_: &Box<dyn Any>, _: &mut fmt::Formatter<'_>) -> fmt::Result {
        unreachable!("debug_fold called on End");
    }
}

impl<Head, Tail> DebugFold for Cons<Head, Tail>
where
    Cons<Head, Tail>: fmt::Debug,
    Head: 'static + fmt::Debug,
    Tail: DebugFold,
{
    fn debug_fold(any: &Box<dyn Any>, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
        if let Some(head_ref) = any.downcast_ref::<Head>() {
            head_ref.fmt(formatter)
        } else {
            Tail::debug_fold(any, formatter)
        }
    }
}

/* ------------------------- Clone support ----------------------- */

pub trait CloneFold {
    fn clone_fold(any: &Box<dyn Any>) -> Box<dyn Any>;
}

impl Clone for End {
    fn clone(&self) -> End {
        unreachable!("clone called for End");
    }
}

impl<Head, Tail> Clone for Cons<Head, Tail>
where
    Head: 'static + Clone,
    Tail: CloneFold,
{
    fn clone(&self) -> Self {
        unreachable!("clone called for Cons which is not constructable");
    }
}

impl CloneFold for End {
    fn clone_fold(_: &Box<dyn Any>) -> Box<dyn Any> {
        unreachable!("clone_fold called on End");
    }
}

impl<Head, Tail> CloneFold for Cons<Head, Tail>
where
    Head: 'static + Clone,
    Tail: CloneFold,
{
    fn clone_fold(any: &Box<dyn Any>) -> Box<dyn Any> {
        if let Some(head_ref) = any.downcast_ref::<Head>() {
            Box::new(head_ref.clone())
        } else {
            Tail::clone_fold(any)
        }
    }
}

fn _clone_test() {
    fn is_clone<T: Clone>() {}

    type T0 = <(String, u32) as TypeSet>::Variants;

    is_clone::<T0>();
}

/* ------------------------- Any::is support ----------------------- */

pub trait IsFold {
    fn is_fold(any: &Box<dyn Any>) -> bool;
}

impl IsFold for End {
    fn is_fold(_: &Box<dyn Any>) -> bool {
        false
    }
}

impl<Head, Tail> IsFold for Cons<Head, Tail>
where
    Head: 'static,
    Tail: IsFold,
{
    fn is_fold(any: &Box<dyn Any>) -> bool {
        if any.is::<Head>() {
            true
        } else {
            Tail::is_fold(any)
        }
    }
}

/* ------------------------- TypeSet implemented for tuples ----------------------- */

pub trait TypeSet {
    type Variants: TupleForm;
    type Enum;
    type EnumRef<'a>
    where
        Self: 'a;
}

impl TypeSet for () {
    type Variants = End;
    type Enum = E0;
    type EnumRef<'a> = E0 where Self: 'a;
}

impl<A> TypeSet for (A,) {
    type Variants = Cons<A, End>;
    type Enum = E1<A>;
    type EnumRef<'a> = E1<&'a A> where Self: 'a;
}

impl<A, B> TypeSet for (A, B) {
    type Variants = Cons<A, Cons<B, End>>;
    type Enum = E2<A, B>;
    type EnumRef<'a> = E2<&'a A, &'a B> where Self: 'a;
}

impl<A, B, C> TypeSet for (A, B, C) {
    type Variants = Cons<A, Cons<B, Cons<C, End>>>;
    type Enum = E3<A, B, C>;
    type EnumRef<'a> = E3<&'a A, &'a B, &'a C> where Self: 'a;
}

impl<A, B, C, D> TypeSet for (A, B, C, D) {
    type Variants = Cons<A, Cons<B, Cons<C, Cons<D, End>>>>;
    type Enum = E4<A, B, C, D>;
    type EnumRef<'a> = E4<&'a A, &'a B, &'a C, &'a D> where Self: 'a;
}

impl<A, B, C, D, E> TypeSet for (A, B, C, D, E) {
    type Variants = Cons<A, Cons<B, Cons<C, Cons<D, Cons<E, End>>>>>;
    type Enum = E5<A, B, C, D, E>;
    type EnumRef<'a> = E5<&'a A, &'a B, &'a C, &'a D, &'a E> where Self: 'a;
}

impl<A, B, C, D, E, F> TypeSet for (A, B, C, D, E, F) {
    type Variants = Cons<A, Cons<B, Cons<C, Cons<D, Cons<E, Cons<F, End>>>>>>;
    type Enum = E6<A, B, C, D, E, F>;
    type EnumRef<'a> = E6<&'a A, &'a B, &'a C, &'a D, &'a E, &'a F> where Self: 'a;
}

impl<A, B, C, D, E, F, G> TypeSet for (A, B, C, D, E, F, G) {
    type Variants = Cons<A, Cons<B, Cons<C, Cons<D, Cons<E, Cons<F, Cons<G, End>>>>>>>;
    type Enum = E7<A, B, C, D, E, F, G>;
    type EnumRef<'a> = E7<&'a A, &'a B, &'a C, &'a D, &'a E, &'a F, &'a G> where Self: 'a;
}

impl<A, B, C, D, E, F, G, H> TypeSet for (A, B, C, D, E, F, G, H) {
    type Variants = Cons<A, Cons<B, Cons<C, Cons<D, Cons<E, Cons<F, Cons<G, Cons<H, End>>>>>>>>;
    type Enum = E8<A, B, C, D, E, F, G, H>;
    type EnumRef<'a> = E8<&'a A, &'a B, &'a C, &'a D, &'a E, &'a F, &'a G, &'a H> where Self: 'a;
}

impl<A, B, C, D, E, F, G, H, I> TypeSet for (A, B, C, D, E, F, G, H, I) {
    type Variants =
        Cons<A, Cons<B, Cons<C, Cons<D, Cons<E, Cons<F, Cons<G, Cons<H, Cons<I, End>>>>>>>>>;
    type Enum = E9<A, B, C, D, E, F, G, H, I>;
    type EnumRef<'a> = E9<&'a A, &'a B, &'a C, &'a D, &'a E, &'a F, &'a G, &'a H, &'a I> where Self: 'a;
}

/* ------------------------- TupleForm implemented for TypeSet ----------------------- */

pub trait TupleForm {
    type Tuple: TypeSet;
}

impl TupleForm for End {
    type Tuple = ();
}

impl<A> TupleForm for Cons<A, End> {
    type Tuple = (A,);
}

impl<A, B> TupleForm for Cons<A, Cons<B, End>> {
    type Tuple = (A, B);
}

impl<A, B, C> TupleForm for Cons<A, Cons<B, Cons<C, End>>> {
    type Tuple = (A, B, C);
}

impl<A, B, C, D> TupleForm for Cons<A, Cons<B, Cons<C, Cons<D, End>>>> {
    type Tuple = (A, B, C, D);
}

impl<A, B, C, D, E> TupleForm for Cons<A, Cons<B, Cons<C, Cons<D, Cons<E, End>>>>> {
    type Tuple = (A, B, C, D, E);
}

impl<A, B, C, D, E, F> TupleForm for Cons<A, Cons<B, Cons<C, Cons<D, Cons<E, Cons<F, End>>>>>> {
    type Tuple = (A, B, C, D, E, F);
}

impl<A, B, C, D, E, F, G> TupleForm
    for Cons<A, Cons<B, Cons<C, Cons<D, Cons<E, Cons<F, Cons<G, End>>>>>>>
{
    type Tuple = (A, B, C, D, E, F, G);
}

impl<A, B, C, D, E, F, G, H> TupleForm
    for Cons<A, Cons<B, Cons<C, Cons<D, Cons<E, Cons<F, Cons<G, Cons<H, End>>>>>>>>
{
    type Tuple = (A, B, C, D, E, F, G, H);
}

impl<A, B, C, D, E, F, G, H, I> TupleForm
    for Cons<A, Cons<B, Cons<C, Cons<D, Cons<E, Cons<F, Cons<G, Cons<H, Cons<I, End>>>>>>>>>
{
    type Tuple = (A, B, C, D, E, F, G, H, I);
}

/* ------------------------- Lifted ----------------------- */

pub enum E0 {}
pub enum E1<A> {
    A(A),
}
impl<A> From<A> for E1<A> {
    fn from(a: A) -> E1<A> {
        E1::A(a)
    }
}
pub enum E2<A, B> {
    A(A),
    B(B),
}
pub enum E3<A, B, C> {
    A(A),
    B(B),
    C(C),
}
pub enum E4<A, B, C, D> {
    A(A),
    B(B),
    C(C),
    D(D),
}
pub enum E5<A, B, C, D, E> {
    A(A),
    B(B),
    C(C),
    D(D),
    E(E),
}
pub enum E6<A, B, C, D, E, F> {
    A(A),
    B(B),
    C(C),
    D(D),
    E(E),
    F(F),
}
pub enum E7<A, B, C, D, E, F, G> {
    A(A),
    B(B),
    C(C),
    D(D),
    E(E),
    F(F),
    G(G),
}
pub enum E8<A, B, C, D, E, F, G, H> {
    A(A),
    B(B),
    C(C),
    D(D),
    E(E),
    F(F),
    G(G),
    H(H),
}
pub enum E9<A, B, C, D, E, F, G, H, I> {
    A(A),
    B(B),
    C(C),
    D(D),
    E(E),
    F(F),
    G(G),
    H(H),
    I(I),
}

/* ------------------------- Contains ----------------------- */

/// A trait that assists with compile-time type set inclusion testing.
/// The `Index` parameter is either `End` or `Cons<...>` depending on
/// whether the trait implementation is a base case or the recursive
/// case.
pub trait Contains<T, Index> {}

/// Base case implementation for when the Cons Head is T.
impl<T, Tail> Contains<T, End> for Cons<T, Tail> {}

/// Recursive case for when the Cons Tail contains T.
impl<T, Index, Head, Tail> Contains<T, Cons<Index, ()>> for Cons<Head, Tail> where
    Tail: Contains<T, Index>
{
}

/* ------------------------- Narrow ----------------------- */

/// A trait for pulling a specific type out of a Variants at compile-time
/// and having access to the other types as the Remainder.
pub trait Narrow<Target, Index>: TupleForm {
    type Remainder: TupleForm;
}

/// Base case where the search Target is in the Head of the Variants.
impl<Target, Tail> Narrow<Target, End> for Cons<Target, Tail>
where
    Tail: TupleForm,
    Cons<Target, Tail>: TupleForm,
{
    type Remainder = Tail;
}

/// Recursive case where the search Target is in the Tail of the Variants.
impl<Head, Tail, Target, Index> Narrow<Target, Recurse<Index>> for Cons<Head, Tail>
where
    Tail: Narrow<Target, Index>,
    Tail: TupleForm,
    Cons<Head, Tail>: TupleForm,
    Cons<Head, <Tail as Narrow<Target, Index>>::Remainder>: TupleForm,
{
    type Remainder = Cons<Head, <Tail as Narrow<Target, Index>>::Remainder>;
}

fn _narrow_test() {
    fn can_narrow<Types, Target, Remainder, Index>()
    where
        Types: Narrow<Target, Index, Remainder = Remainder>,
    {
    }

    type T0 = <(u32, String) as TypeSet>::Variants;

    can_narrow::<T0, u32, _, _>();
    can_narrow::<T0, String, Cons<u32, End>, _>();
}

/* ------------------------- SupersetOf ----------------------- */

/// When all types in a Variants are present in a second Variants
pub trait SupersetOf<Other, Index> {
    type Remainder: TupleForm;
}

/// Base case
impl<T: TupleForm> SupersetOf<End, End> for T {
    type Remainder = T;
}

/// Recursive case - more complex because we have to reason about the Index itself as a
/// heterogenous list.
impl<SubHead, SubTail, SuperHead, SuperTail, HeadIndex, TailIndex>
    SupersetOf<Cons<SubHead, SubTail>, Cons<HeadIndex, TailIndex>> for Cons<SuperHead, SuperTail>
where
    Cons<SuperHead, SuperTail>: Narrow<SubHead, HeadIndex>,
    <Cons<SuperHead, SuperTail> as Narrow<SubHead, HeadIndex>>::Remainder:
        SupersetOf<SubTail, TailIndex>,
{
    type Remainder =
        <<Cons<SuperHead, SuperTail> as Narrow<SubHead, HeadIndex>>::Remainder as SupersetOf<
            SubTail,
            TailIndex,
        >>::Remainder;
}

fn _superset_test() {
    fn is_superset<S1, S2, Remainder, Index>()
    where
        S1: SupersetOf<S2, Index, Remainder = Remainder>,
    {
    }

    type T0 = <(u32,) as TypeSet>::Variants;
    type T1A = <(u32, String) as TypeSet>::Variants;
    type T1B = <(String, u32) as TypeSet>::Variants;
    type T2 = <(String, i32, u32) as TypeSet>::Variants;
    type T3 = <(Vec<u8>, Vec<i8>, u32, f32, String, f64, i32) as TypeSet>::Variants;

    is_superset::<T0, T0, _, _>();
    is_superset::<T1A, T1A, _, _>();
    is_superset::<T1A, T1B, _, _>();
    is_superset::<T1B, T1A, _, _>();
    is_superset::<T2, T2, _, _>();
    is_superset::<T1A, T0, _, _>();
    is_superset::<T1B, T0, _, _>();
    is_superset::<T2, T0, <(String, i32) as TypeSet>::Variants, _>();
    is_superset::<T2, T1A, <(i32,) as TypeSet>::Variants, _>();
    is_superset::<T2, T1B, <(i32,) as TypeSet>::Variants, _>();
    is_superset::<T3, T1A, <(Vec<u8>, Vec<i8>, f32, f64, i32) as TypeSet>::Variants, _>();
    is_superset::<T3, T1B, _, _>();
    is_superset::<T3, T0, _, _>();
    is_superset::<T3, T2, _, _>();

    type T5sup = <(u8, u16, u32, u64, u128) as TypeSet>::Variants;
    type T5sub = <(u8, u128) as TypeSet>::Variants;
    type T5rem = <(u16, u32, u64) as TypeSet>::Variants;

    is_superset::<T5sup, T5sub, T5rem, _>();
}


================================================
FILE: tests/usability.rs
================================================
use terrors::OneOf;

#[derive(Debug)]
struct NotEnoughMemory;

#[derive(Debug)]
struct Timeout;

#[derive(Debug)]
struct RetriesExhausted;

#[test]
fn retry() {
    fn inner() -> Result<(), OneOf<(NotEnoughMemory, RetriesExhausted)>> {
        for _ in 0..3 {
            let Err(err) = does_stuff() else {
                return Ok(());
            };

            match err.narrow::<Timeout, _>() {
                Ok(_timeout) => continue,
                Err(allocation_oneof) => {
                    println!("didn't get Timeout, now trying to get NotEnoughMemory");
                    let allocation_oneof: OneOf<(NotEnoughMemory,)> = allocation_oneof;
                    let allocation = allocation_oneof.narrow::<NotEnoughMemory, _>().unwrap();

                    return Err(OneOf::new(allocation));
                }
            }
        }

        Err(OneOf::new(RetriesExhausted))
    }

    let _ = dbg!(inner());
}

fn does_stuff() -> Result<(), OneOf<(NotEnoughMemory, Timeout)>> {
    // TODO Try impl after superset type work
    let _allocation = match allocates() {
        Ok(a) => a,
        Err(e) => return Err(e.broaden()),
    };

    // TODO Try impl after superset type work
    let _chat = match chats() {
        Ok(c) => c,
        Err(e) => return Err(OneOf::new(e)),
    };

    Ok(())
}

fn allocates() -> Result<(), OneOf<(NotEnoughMemory,)>> {
    let result: Result<(), NotEnoughMemory> = Err(NotEnoughMemory);

    result?;

    Ok(())
}

fn chats() -> Result<(), Timeout> {
    Err(Timeout)
}

#[test]
fn smoke() {
    let o_1: OneOf<(u32, String)> = OneOf::new(5_u32);
    let _narrowed_1: u32 = o_1.narrow::<u32, _>().unwrap();

    let o_2: OneOf<(String, u32)> = OneOf::new(5_u32);
    let _narrowed_2: u32 = o_2.narrow::<u32, _>().unwrap();

    let o_3: OneOf<(String, u32)> = OneOf::new("5".to_string());
    let _narrowed_3: OneOf<(String,)> = o_3.narrow::<u32, _>().unwrap_err();

    let o_4: OneOf<(String, u32)> = OneOf::new("5".to_string());

    let _: String = o_4.narrow().unwrap();

    let o_5: OneOf<(String, u32)> = OneOf::new("5".to_string());
    o_5.narrow::<String, _>().unwrap();

    let o_6: OneOf<(String, u32)> = OneOf::new("5".to_string());
    let o_7: OneOf<(u32, String)> = o_6.broaden();
    let o_8: OneOf<(String, u32)> = o_7.subset().unwrap();
    let _: OneOf<(u32, String)> = o_8.subset().unwrap();

    let o_9: OneOf<(u8, u16, u32)> = OneOf::new(3_u32);
    let _: Result<OneOf<(u16,)>, OneOf<(u8, u32)>> = o_9.subset();
    let o_10: OneOf<(u8, u16, u32)> = OneOf::new(3_u32);
    let _: Result<u16, OneOf<(u8, u32)>> = o_10.narrow();
}

#[test]
fn debug() {
    use std::error::Error;
    use std::io;

    let o_1: OneOf<(u32, String)> = OneOf::new(5_u32);

    // Debug is implemented if all types in the type set implement Debug
    dbg!(&o_1);

    // Display is implemented if all types in the type set implement Display
    println!("{}", o_1);

    type E = io::Error;
    let e = io::Error::new(io::ErrorKind::Other, "wuaaaaahhhzzaaaaaaaa");

    let o_2: OneOf<(E,)> = OneOf::new(e);

    // std::error::Error is implemented if all types in the type set implement it
    dbg!(o_2.source());

    let o_3: OneOf<(u32, String)> = OneOf::new("hey".to_string());
    dbg!(o_3);
}

#[test]
fn multi_match() {
    use terrors::E2;

    let o_1: OneOf<(u32, String)> = OneOf::new(5_u32);

    match o_1.as_enum() {
        E2::A(u) => {
            println!("handling {u}: u32")
        }
        E2::B(s) => {
            println!("handling {s}: String")
        }
    }

    match o_1.to_enum() {
        E2::A(u) => {
            println!("handling {u}: u32")
        }
        E2::B(s) => {
            println!("handling {s}: String")
        }
    }
}

#[test]
fn multi_narrow() {
    use terrors::E2;

    struct Timeout {}
    struct Backoff {}

    let o_1: OneOf<(u8, u16, u32, u64, u128)> = OneOf::new(5_u32);

    let _narrow_res: Result<OneOf<(u8, u128)>, OneOf<(u16, u32, u64)>> = o_1.subset();

    let o_2: OneOf<(u8, u16, Backoff, Timeout, u32, u64, u128)> = OneOf::new(Timeout {});

    match o_2.subset::<(Timeout, Backoff), _>().unwrap().to_enum() {
        E2::A(Timeout {}) => {
            println!(":)");
        }
        E2::B(Backoff {}) => {
            unreachable!()
        }
    }
}