Serde edn  

Serde is a framework for serializing and deserializing Rust data structures efficiently and generically.

---

[dependencies]
serde_edn = "1.0"

You may be looking for:

• edn API documentation
• Serde API documentation
• Detailed documentation about Serde
• Setting up #[derive(Serialize, Deserialize)]
• Release notes

edn is a ubiquitous open-standard format that uses human-readable text to transmit data objects consisting of key-value pairs.

{
  "name": "John Doe",
  "age": 43,
  "address": {
    "street": "10 Downing Street",
    "city": "London"
  },
  "phones": [
    "+44 1234567",
    "+44 2345678"
  ]
}

There are three common ways that you might find yourself needing to work with edn data in Rust.

• As text data. An unprocessed string of edn data that you receive on an HTTP endpoint, read from a file, or prepare to send to a remote server.
• As an untyped or loosely typed representation. Maybe you want to check that some edn data is valid before passing it on, but without knowing the structure of what it contains. Or you want to do very basic manipulations like insert a key in a particular spot.
• As a strongly typed Rust data structure. When you expect all or most of your data to conform to a particular structure and want to get real work done without edn's loosey-goosey nature tripping you up.

Serde edn provides efficient, flexible, safe ways of converting data between each of these representations.

Operating on untyped edn values

Any valid edn data can be manipulated in the following recursive enum representation. This data structure is serde_edn::Value.

enum Value {
    Nil,
    Bool(bool),
    Number(Number),
    String(String),
    Vector(Vec<Value>),
    Object(Map<String, Value>),
}

A string of edn data can be parsed into a serde_edn::Value by the serde_edn::from_str function. There is also from_slice for parsing from a byte slice &[u8] and from_reader for parsing from any io::Read like a File or a TCP stream.

extern crate serde_edn;

use serde_edn::{Value, Error};

fn untyped_example() -> Result<(), Error> {
    // Some edn input data as a &str. Maybe this comes from the user.
    let data = r#"{
                    "name": "John Doe",
                    "age": 43,
                    "phones": [
                      "+44 1234567",
                      "+44 2345678"
                    ]
                  }"#;

    // Parse the string of data into serde_edn::Value.
    let v: Value = serde_edn::from_str(data)?;

    // Access parts of the data by indexing with square brackets.
    println!("Please call {} at the number {}", v["name"], v["phones"][0]);

    Ok(())
}

The result of square bracket indexing like v["name"] is a borrow of the data at that index, so the type is &Value. A edn map can be indexed with string keys, while a edn array can be indexed with integer keys. If the type of the data is not right for the type with which it is being indexed, or if a map does not contain the key being indexed, or if the index into a vector is out of bounds, the returned element is Value::Nil.

When a Value is printed, it is printed as a edn string. So in the code above, the output looks like Please call "John Doe" at the number "+44 1234567". The quotation marks appear because v["name"] is a &Value containing a edn string and its edn representation is "John Doe". Printing as a plain string without quotation marks involves converting from a edn string to a Rust string with as_str() or avoiding the use of Value as described in the following section.

The Value representation is sufficient for very basic tasks but can be tedious to work with for anything more significant. Error handling is verbose to implement correctly, for example imagine trying to detect the presence of unrecognized fields in the input data. The compiler is powerless to help you when you make a mistake, for example imagine typoing v["name"] as v["nmae"] in one of the dozens of places it is used in your code.

Parsing edn as strongly typed data structures

Serde provides a powerful way of mapping edn data into Rust data structures largely automatically.

extern crate serde;
extern crate serde_edn;

#[macro_use]
extern crate serde_derive;

use serde_edn::Error;

#[derive(Serialize, Deserialize)]
struct Person {
    name: String,
    age: u8,
    phones: Vec<String>,
}

fn typed_example() -> Result<(), Error> {
    // Some edn input data as a &str. Maybe this comes from the user.
    let data = r#"{
                    "name": "John Doe",
                    "age": 43,
                    "phones": [
                      "+44 1234567",
                      "+44 2345678"
                    ]
                  }"#;

    // Parse the string of data into a Person object. This is exactly the
    // same function as the one that produced serde_edn::Value above, but
    // now we are asking it for a Person as output.
    let p: Person = serde_edn::from_str(data)?;

    // Do things just like with any other Rust data structure.
    println!("Please call {} at the number {}", p.name, p.phones[0]);

    Ok(())
}

This is the same serde_edn::from_str function as before, but this time we assign the return value to a variable of type Person so Serde will automatically interpret the input data as a Person and produce informative error messages if the layout does not conform to what a Person is expected to look like.

Any type that implements Serde's Deserialize trait can be deserialized this way. This includes built-in Rust standard library types like Vec<T> and HashMap<K, V>, as well as any structs or enums annotated with #[derive(Deserialize)].

Once we have p of type Person, our IDE and the Rust compiler can help us use it correctly like they do for any other Rust code. The IDE can autocomplete field names to prevent typos, which was impossible in the serde_edn::Value representation. And the Rust compiler can check that when we write p.phones[0], then p.phones is guaranteed to be a Vec<String> so indexing into it makes sense and produces a String.

Constructing edn values

Serde edn provides a edn! macro to build serde_edn::Value objects with very natural edn syntax. In order to use this macro, serde_edn needs to be imported with the #[macro_use] attribute.

#[macro_use]
extern crate serde_edn;

fn main() {
    // The type of `john` is `serde_edn::Value`
    let john = edn!({
      "name": "John Doe",
      "age": 43,
      "phones": [
        "+44 1234567",
        "+44 2345678"
      ]
    });

    println!("first phone number: {}", john["phones"][0]);

    // Convert to a string of edn and print it out
    println!("{}", john.to_string());
}

The Value::to_string() function converts a serde_edn::Value into a String of edn text.

One neat thing about the edn! macro is that variables and expressions can be interpolated directly into the edn value as you are building it. Serde will check at compile time that the value you are interpolating is able to be represented as edn.

let full_name = "John Doe";
let age_last_year = 42;

// The type of `john` is `serde_edn::Value`
let john = edn!({
  "name": full_name,
  "age": age_last_year + 1,
  "phones": [
    format!("+44 {}", random_phone())
  ]
});

This is amazingly convenient but we have the problem we had before with Value which is that the IDE and Rust compiler cannot help us if we get it wrong. Serde edn provides a better way of serializing strongly-typed data structures into edn text.

Creating edn by serializing data structures

A data structure can be converted to a edn string by serde_edn::to_string. There is also serde_edn::to_vec which serializes to a Vec<u8> and serde_edn::to_writer which serializes to any io::Write such as a File or a TCP stream.

extern crate serde;
extern crate serde_edn;

#[macro_use]
extern crate serde_derive;

use serde_edn::Error;

#[derive(Serialize, Deserialize)]
struct Address {
    street: String,
    city: String,
}

fn print_an_address() -> Result<(), Error> {
    // Some data structure.
    let address = Address {
        street: "10 Downing Street".to_owned(),
        city: "London".to_owned(),
    };

    // Serialize it to a edn string.
    let j = serde_edn::to_string(&address)?;

    // Print, write to a file, or send to an HTTP server.
    println!("{}", j);

    Ok(())
}

Any type that implements Serde's Serialize trait can be serialized this way. This includes built-in Rust standard library types like Vec<T> and HashMap<K, V>, as well as any structs or enums annotated with #[derive(Serialize)].

Performance

It is fast. You should expect in the ballpark of 500 to 1000 megabytes per second deserialization and 600 to 900 megabytes per second serialization, depending on the characteristics of your data. This is competitive with the fastest C and C++ edn libraries or even 30% faster for many use cases. Benchmarks live in the serde-rs/edn-benchmark repo.

Getting help

Serde developers live in the #serde channel on irc.mozilla.org. The #rust channel is also a good resource with generally faster response time but less specific knowledge about Serde. If IRC is not your thing, we are happy to respond to GitHub issues as well.

No-std support

This crate currently requires the Rust standard library. For edn support in Serde without a standard library, please see the serde-edn-core crate.

License

Serde edn is licensed under either of

• Apache License, Version 2.0, (LICENSE-APACHE or http://www.apache.org/licenses/LICENSE-2.0)
• MIT license (LICENSE-MIT or http://opensource.org/licenses/MIT)

at your option.

Contribution

Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in Serde edn by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.