#type #union #polymorphic #variant #anonymous


A sketch implementation of anonymous, tagged unions in stable Rust

9 unstable releases (3 breaking)

Uses old Rust 2015

0.4.4 Sep 13, 2017
0.4.2 Sep 13, 2017
0.4.0 Aug 31, 2017
0.3.0 Aug 10, 2017
0.1.0 Dec 2, 2016

#964 in Data structures





In your Cargo.toml file:

fed = "0.4"

In order to be able to declare type federations, the various Fed* types need to be owned by your crate. To enable this, the code declaring the generic FedX<*_> is contained in a macro that the user needs to call in their crate root:

extern crate fed as fed_;

This will create a fed module in your crate root.

Since much of the functionality (especially the .into() trick) can currently only be defined on concrete types, you'll need to explicitly declare a type federation with the fed! macro:


This will implement the traits in ::fed::* for the concrete type Fed4<usize, bool, Option<String>, char> as well as the concrete Fed2 and Fed3 types for all of the 2- and 3-type subsets of usize, bool, Option<String>, and char (e.g., Fed2<usize, bool>, Fed2<usize, char>, Fed3<usize, bool, Option<String>>).

Fed1, since it's a federation of exactly one type, is able to have totally generic implementations of the ::fed::* traits.

Three immediately relevant limitations:

  1. When using any of the generated concrete type federations and specifying the types in the federation, the types must be listed in the same order in which they were declared in the fed! call. In the above example, Fed2<usize, bool> is now usable, but Fed2<bool, usize> is not.

  2. Because Rust forbids duplicate, identical trait implementations, the types listed in a second fed! declaration cannot have a common subset of more than one type with the set of types in the first type federation (unless you order the second type federation in such a way that no ordered subset is identical to an ordered subset of the first type federation (as mentioned in the first limitation)).

  3. Type federations of more than 8 types are not supported.

Now, you can create collections of any subset of the types in the federation you declared without having to come up with names for a dedicated enum:

use ::fed::*;

let vec: Vec<Fed3<usize, bool, char>> = vec![

assert_eq!(vec.iter().filter(Fed::is::<usize>).count(), 2);
assert_eq!(vec[3].extract::<char>(), Ok('A'));

Deriving non-std traits

When declared via the init_fed! macro call, the type federation structs (Fed0, Fed1, ... Fed8) derive all the built-in #![derive(...)] traits (e.g., Copy, Debug). If you need additional derivable traits (e.g., serde's Serialize and Deserialize) , include them as a special argument to init_fed! using the following syntax:

extern crate serde_derive;

init_fed!(@deriving: [Serialize, Deserialize]);

No runtime deps