Strum

Strum is a set of macros and traits for working with enums and strings easier in Rust.

Including Strum in Your Project

Import strum and strum_macros into your project by adding the following lines to your Cargo.toml. Strum_macros contains the macros needed to derive all the traits in Strum.

[dependencies]
strum = "*"
strum_macros = "*"

And add these lines to the root of your project, either lib.rs or main.rs.

// Strum contains all the trait definitions
extern crate strum;
#[macro_use]
extern crate strum_macros;

Strum Macros

Strum has implemented the following macros:

1. EnumString: auto-derives std::str::FromStr on the enum. Each variant of the enum will match on it's own name. This can be overridden using serialize="DifferentName" on the attribute as shown below. Multiple deserializations can be added to the same variant. If the variant contains additional data, they will be set to their default values upon deserialization.

   The default attribute can be applied to a tuple variant with a single data parameter. When a match isn't found, the given variant will be returned and the input string will be captured in the parameter.

   Here is an example of the code generated by deriving EnumString.

   extern crate strum;
   #[macro_use] extern crate strum_macros;
   #[derive(EnumString)]
   enum Color {
       Red,
   
       // The Default value will be inserted into range if we match "Green".
       Green { range:usize },
   
       // We can match on multiple different patterns.
       #[strum(serialize="blue",serialize="b")]
       Blue(usize),
   
       // Notice that we can disable certain variants from being found
       #[strum(disabled="true")]
       Yellow,
   }
   
   /*
   //The generated code will look like:
   impl std::str::FromStr for Color {
       type Err = strum::ParseError;
   
       fn from_str(s: &str) -> Result<Color, strum::ParseError> {
           match s {
               "Red" => Ok(Color::Red),
               "Green" => Ok(Color::Green { range:Default::default() }),
               "blue" | "b" => Ok(Color::Blue(Default::default())),
               _ => Err(strum::ParseError::VariantNotFound),
           }
       }
   }
   */

   Note that the implementation of FromStr only matches on the name of the variant. Strum, where possible, avoids operations that have an unknown runtime cost, and parsing strings is potentially an expensive operation. If you do need that behavior, consider the more powerful Serde library for your serialization.

2. EnumIter: iterate over the variants of an Enum. Any additional data on your variants will be set to Default::default(). The macro implements strum::IntoEnumIter on your enum and creates a new type called YourEnumIter that is the iterator object. You cannot derive EnumIter on any type with a lifetime bound (<'a>) because the iterator would surely create [unbounded lifetimes] (https://doc.rust-lang.org/nightly/nomicon/unbounded-lifetimes.html).

   // You need to bring the type into scope to use it!!!
   use strum::IntoEnumIterator;
   
   #[derive(EnumIter,Debug)]
   enum Color {
       Red,
       Green { range:usize },
       Blue(usize),
       Yellow,
   }
   
   // It's simple to iterate over the variants of an enum.
   fn debug_colors() {
       for color in Color::iter() {
           println!("My favorite color is {:?}", color);
       }
   }
   
   fn main() {
       debug_colors();
   }

3. EnumMessage: encode strings into the enum itself. This macro implements the strum::EnumMessage trait. EnumMessage looks for #[strum(message="...")] attributes on your variants. You can also provided a detailed_message="..." attribute to create a seperate more detailed message than the first.

   The generated code will look something like:

   // You need to bring the type into scope to use it!!!
   use strum::EnumMessage;
   
   #[derive(EnumMessage,Debug)]
   enum Color {
       #[strum(message="Red",detailed_message="This is very red")]
       Red,
       #[strum(message="Simply Green")]
       Green { range:usize },
       #[strum(serialize="b",serialize="blue")]
       Blue(usize),
   }
   
   /*
   // Generated code
   impl EnumMessage for Color {
       fn get_message(&self) -> Option<&str> {
           match self {
               &Color::Red => Some("Red"),
               &Color::Green {..} => Some("Simply Green"),
               _ => None
           }
       }
   
       fn get_detailed_message(&self) -> Option<&str> {
           match self {
               &Color::Red => Some("This is very red"),
               &Color::Green {..}=> Some("Simply Green"),
               _ => None
           }
       }
   
       fn get_serializations(&self) -> &[&str] {
           match self {
               &Color::Red => {
                   static ARR: [&'static str; 1] = ["Red"];
                   &ARR
               },
               &Color::Green {..}=> {
                   static ARR: [&'static str; 1] = ["Green"];
                   &ARR
               },
               &Color::Blue (..) => {
                   static ARR: [&'static str; 2] = ["b", "blue"];
                   &ARR
               },
           }
       }
   }
   */

Additional Attributes

Strum supports several custom attributes to modify the generated code. Custom attributes are applied to a variant by adding #[strum(parameter="value")] to the variant.

• serialize="...": Changes the text that FromStr() looks for when parsing a string. This attribute can be applied multiple times to an element and the enum variant will be parsed if any of them match.

• default="true": Applied to a single variant of an enum. The variant must be a Tuple-like variant with a single piece of data that can be create from a &str i.e. T: From<&str>. The generated code will now return the variant with the input string captured as shown below instead of failing.

  // Replaces this:
  _ => Err(strum::ParseError::VariantNotFound)
  // With this in generated code:
  default => Ok(Variant(default.into()))

  The plugin will fail if the data doesn't implement From<&str>. You can only have one default on your enum.

• disabled="true": removes variant from generated code.

• message="..": Adds a message to enum variant. This is used in conjunction with the EnumMessage trait to associate a message with a variant. If detailed_message is not provided, then message will also be returned when get_detailed_message() is called.

• detailed_message="..": Adds a more detailed message to a variant. If this value is omitted, then message will be used in it's place.

Examples

Using EnumMessage for quickly implementing Error

extern crate strum;
#[macro_use]
extern crate strum_macros;
use std::error::Error;
use std::fmt::*;
use strum::EnumMessage;

#[derive(Debug, EnumMessage)]
enum ServerError {
    #[strum(message="A network error occured")]
    #[strum(detailed_message="Try checking your connection.")]
    NetworkError,
    #[strum(message="User input error.")]
    #[strum(detailed_message="There was an error parsing user input. Please try again.")]
    InvalidUserInputError,
}

impl Display for ServerError {
    fn fmt(&self, f: &mut Formatter) -> Result {
        write!(f, "{}", self.get_message().unwrap())
    }
}

impl Error for ServerError {
    fn description(&self) -> &str {
        self.get_detailed_message().unwrap()
    }
}

Using EnumString to tokenize a series of inputs:

extern crate strum;
#[macro_use]
extern crate strum_macros;
use std::str::FromStr;

#[derive(Eq, PartialEq, Debug, EnumString)]
enum Tokens {
    #[strum(serialize="fn")]
    Function,
    #[strum(serialize="(")]
    OpenParen,
    #[strum(serialize=")")]
    CloseParen,
    #[strum(default="true")]
    Ident(String)
}

fn main() {
    let toks = ["fn", "hello_world", "(", ")"].iter()
                   .map(|tok| Tokens::from_str(tok).unwrap())
                   .collect::<Vec<_>>();

    assert_eq!(toks, vec![Tokens::Function,
                          Tokens::Ident(String::from("hello_world")),
                          Tokens::OpenParen,
                          Tokens::CloseParen]);
}

Debugging

To see the generated code, set the DEBUG_STRUM environment variable before compiling your code. DEBUG_STRUM=1 will dump all of the generated code for every type. DEBUG_STRUM=YourType will only dump the code generated on a type named YourType.

Name

Strum is short for STRing enUM because it's a library for augmenting enums with additional information through strings.

Strumming is also a very whimsical motion, much like writing Rust code.