Crate enum_derive [] [src]

This crate provides several macros for deriving some useful methods for unitary enums (i.e. enums where variants do not have payloads).

All of these macros are designed to be used with the custom_derive crate, though they can be used independent of it.

Note: see also the TryFrom! macro provided by the conv crate to derive a function for creating enum values from integer values.


Derive iterators that yield all variants of an enum.

#[macro_use] extern crate custom_derive;
#[macro_use] extern crate enum_derive;

custom_derive! {
    #[derive(Debug, PartialEq, Eq,
        IterVariants(CandyVariants), IterVariantNames(CandyVariantNames))]
    pub enum Candy { Musk, FruitRock, BoPeeps, LemonSherbert }

let vars: CandyVariants = Candy::iter_variants();
let names: CandyVariantNames = Candy::iter_variant_names();
assert_eq!(&*<Vec<_>>(), &[
    (Candy::Musk, "Musk"),
    (Candy::FruitRock, "FruitRock"),
    (Candy::BoPeeps, "BoPeeps"),
    (Candy::LemonSherbert, "LemonSherbert"),

Alternately, derive next_variant and prev_variant methods.

#[macro_use] extern crate custom_derive;
#[macro_use] extern crate enum_derive;

use Hanagami::*;

custom_derive! {
    #[derive(Debug, PartialEq, Eq, NextVariant, PrevVariant)]
    pub enum Hanagami { Sakigami, Hasugami, Tsutagami }

assert_eq!(Sakigami.next_variant(), Some(Hasugami));
assert_eq!(Hasugami.next_variant(), Some(Tsutagami));
assert_eq!(Tsutagami.next_variant(), None);

assert_eq!(Sakigami.prev_variant(), None);
assert_eq!(Hasugami.prev_variant(), Some(Sakigami));
assert_eq!(Tsutagami.prev_variant(), Some(Hasugami));


This crate provides macros to derive the following methods for unitary variant enums:

  • EnumDisplay derives Display, which outputs the name of the variant. Note that for unitary variants, this is identical to the behaviour of a derived Debug implementation.
  • EnumFromStr derives FromStr, allowing str::parse to be used. It requires an exact match of the variant name.
  • IterVariants derives iter_variants(), which returns an iterator over the variants of the enum in lexical order.
  • IterVariantNames derives iter_variant_names(), which returns an iterator over the string names of the variants of the enum in lexical order.
  • NextVariant derives next_variant(&self), which returns the next variant, or None when called for the last.
  • PrevVariant derives prev_variant(&self), which returns the previous variant, or None when called for the first.
  • EnumFromInner derives From<T> for each variant's payload, assuming all variants are unary.
  • EnumInnerAsTrait derives a method to return a borrowed pointer to the inner value, cast to a trait object.

Both of the IterVariant* macros accept a single deriving form. Taking IterVariants as an example, it must be invoked like so:

custom_derive! {
    pub enum Get { Up, Down, AllAround }

The argument is the name of the iterator type that will be generated. Neither macro imposes any naming requirements, save the obvious: the name must not conflict with any other types.

EnumInnerAsTrait accepts a single deriving form that specifies the name of the method to be derived, whether the borrow should be mutable, and the trait of interest. For example:

custom_derive! {
    #[derive(EnumInnerAsTrait(pub as_display -> &std::fmt::Display))]
    enum Value {

let s = format!("{}", Value::U64(42).as_display());
assert_eq!(&s[..], "42");

The other macros take no arguments.

The methods and iterator types generated will be public if the enum itself is public; otherwise, they will be private.

Using Without custom_derive!

Although designed to be used with custom_derive!, all of the macros in this crate can be used without it. The following:

custom_derive! {
    #[derive(Copy, Clone, Debug, IterVariants(Vars))]
    enum ItAintRight { BabeNo, NoNo, BoyBoy }

Can also be written as:

#[derive(Copy, Clone, Debug)]
enum ItAintRight { BabeNo, NoNo, BoyBoy }

IterVariants! { (Vars) enum ItAintRight { BabeNo, NoNo, BoyBoy } }

Other Examples

This shows how to use Display and FromStr to perform string round-tripping of enums.

#[macro_use] extern crate custom_derive;
#[macro_use] extern crate enum_derive;

custom_derive! {
    #[derive(Debug, PartialEq, EnumDisplay, EnumFromStr)]
    pub enum TrollDigit { One, Two, Three, Many, Lots }

fn to_troll(mut n: u32) -> String {
    use std::fmt::Write;
    let mut s = String::new();

    if n == 0 {
        panic!("I dun' see nuffin'; how's I s'posed to count it?!");

    while n > 0 {
        let (del, dig) = match n {
            n if n >= 16 => (16, TrollDigit::Lots),
            n if n >= 4 => (4, TrollDigit::Many),
            n if n >= 3 => (3, TrollDigit::Three),
            n if n >= 2 => (2, TrollDigit::Two),
            _ => (1, TrollDigit::One),
        n -= del;
        if s.len() > 0 { s.push_str(" "); }
        write!(&mut s, "{}", dig).unwrap();


fn from_troll(s: &str) -> Result<u32, enum_derive::ParseEnumError> {
    let mut n = 0;
    for word in s.split_whitespace() {
        n += match try!(word.parse()) {
            TrollDigit::One => 1,
            TrollDigit::Two => 2,
            TrollDigit::Three => 3,
            TrollDigit::Many => 4,
            TrollDigit::Lots => 16,
    if n == 0 {
    } else {

let number = 42;
let troll_number = to_troll(number);
assert_eq!(troll_number, "Lots Lots Many Many Two");
assert_eq!(from_troll(&troll_number), Ok(number));





This is the error type used for derived implementations of FromStr for unitary enums.