Toml Example

A lib help generate toml example

Introduction

This crate provides the TomlExample trait and an accompanying derive macro.

Deriving TomlExample on a struct will provide to_example function help generate toml example file base documentation

• support #[serde(default)], #[serde(default = "function_name")] attributes on both structs and struct fields (serde feature, opt-in)
• support #[serde(rename)], #[serde(rename_all = "renaming rules")], the renaming rules can be lowercase, UPPERCASE, PascalCase, camelCase, snake_case, SCREAMING_SNAKE_CASE, kebab-case, SCREAMING-KEBAB-CASE
• provide #[toml_example(default)], #[toml_example(default = 0)], #[toml_example(default = "default_string")] attributes on struct fields
• #[toml_example(default)] is also supported as an outer attribute for structs
• The order matter of attribute macro, if #[serde(default = ..] and #[toml_example(default = ..)] existing at the same time with different value

Quick Example

use toml_example::TomlExample;
use serde::Deserialize;

/// Config is to arrange something or change the controls on a computer or other device
/// so that it can be used in a particular way
#[derive(TomlExample, Deserialize)]
struct Config {
    /// Config.a should be a number
    a: usize,
    /// Config.b should be a string
    b: String,
    /// Optional Config.c is a number
    c: Option<usize>,
    /// Config.d is a list of number
    d: Vec<usize>,
    /// Config.e should be a number
    #[serde(default = "default_int")]
    e: usize,
    /// Config.f should be a string
    #[serde(default = "default_str")]
    f: String,
    /// Config.g should be a number
    #[toml_example(default =7)]
    g: usize,
    /// Config.f should be a string
    #[toml_example(default = "seven")]
    h: String,
}
fn default_int() -> usize {
    7
}
fn default_str() -> String {
    "seven".into()
}

Config::to_toml_example("example.toml");  // write example to a file
let example = Config::toml_example();

Toml example base on the docstring of each field

# Config is to arrange something or change the controls on a computer or other device
# so that it can be used in a particular way

# Config.a should be a number
a = 0

# Config.b should be a string
b = ""

# Optional Config.c is a number
# c = 0

# Config.d is a list of number
# d = [ 0, ]

# Config.e should be a number
e = 7

# Config.f should be a string
f = "seven"

# Config.g should be a number
g = 7

# Config.h should be a string
h = "seven"

The fields of a struct can inherit their defaults from the parent struct when the #[toml_example(default)], #[serde(default)] or #[serde(default = "default_fn")] attribute is set as an outer attribute of the parent struct:

use serde::Serialize;
use toml_example::TomlExample;

#[derive(TomlExample, Serialize)]
#[serde(default)]
struct Config {
    /// Name of the theme to use
    theme: String,
}

impl Default for Config {
    fn default() -> Self {
        Self {
            theme: String::from("Dark"),
        }
    }
}

assert_eq!(Config::toml_example(),
r#"# Name of the theme to use
theme = "Dark"

"#);

Nesting Struct

A nesting struct wrap with Option<T>, Vec<T>, HashMap<String, T>, BTreeMap<String, T> are handled. Please add #[toml_example(nesting)], or #[toml_example(nesting = prefix)] on the field. #[toml_example(nesting)]

  /// Service with specific port
  #[derive(TomlExample)]
  struct Service {
      /// port should be a number
      #[toml_example(default = 80)]
      port: usize,
  }
  #[derive(TomlExample)]
  #[allow(dead_code)]
  struct Node {
      /// Services are running in the node
      #[toml_example(default = http, nesting)]
      services: HashMap<String, Service>,
  }

Node::toml_example() will be following string.

# Services are running in the node
# Service with specific port
[services.http]
# port should be a number
port = 80

Flattening

Flattening means treating the fields of a nested struct as if they were defined directly in the wrapping struct.

#[derive(TomlExample)]
struct ItemWrapper {
    #[toml_example(flatten, nesting)]
    item: Item,
}
#[derive(TomlExample)]
struct Item {
    value: String,
}

assert_eq!(ItemWrapper::toml_example(), Item::toml_example());

This works with maps too!

#[derive(TomlExample, Deserialize)]
struct MainConfig {
    #[serde(flatten)]
    #[toml_example(nesting)]
    nested: HashMap<String, ConfigItem>,
}
#[derive(TomlExample, Deserialize)]
struct ConfigItem {
    #[toml_example(default = false)]
    enabled: bool,
}

let example = MainConfig::toml_example();
assert!(toml::from_str::<MainConfig>(&example).is_ok());
println!("{example}");

[example]
enabled = false

Enum Field

You can also use fieldless enums, but you have to annotate them with #[toml_example(enum)] or #[toml_example(is_enum)] if you mind the keyword highlight you likely get when writing "enum". When annotating a field with #[toml_example(default)] it will use the Debug implementation. However for non-TOML data types like enums, this does not work as the value needs to be treated as a string in TOML. The #[toml_example(enum)] attribute just adds the needed quotes around the Debug implementation and can be omitted if a custom Debug already includes those.

use toml_example::TomlExample;
#[derive(TomlExample)]
struct Config {
    /// Config.priority is an enum
    #[toml_example(enum, default)]
    priority: Priority,
}
#[derive(Debug, Default)]
enum Priority {
    #[default]
    Important,
    Trivial,
}
assert_eq!(Config::toml_example(),
r#"# Config.priority is an enum
priority = "Important"

"#)

More

If you want an optional field become a required field in example, place the #[toml_example(require)] on the field. If you want to skip some field you can use #[toml_example(skip)], the #[serde(skip)], #[serde(skip_deserializing)] also works.

use toml_example::TomlExample;
#[derive(TomlExample)]
struct Config {
    /// Config.a is an optional number
    #[toml_example(require)]
    a: Option<usize>,
    /// Config.b is an optional string
    #[toml_example(require)]
    b: Option<String>,
    #[toml_example(require, default = "third")]
    c: Option<String>,
    #[toml_example(skip)]
    d: usize,
}

# Config.a is an optional number
a = 0

# Config.b is an optional string
b = ""

c = "third"

Why TOML is good for program configuration file

There are several common config file solutions: INI, JSON, YAML, TOML.

• INI is legacy and no nesting support, and the comment char is not designed at first, so some parser will use ; or #.
• JSON does not support documentation. It is efficient for data exchange, but configuration files should include documentation, so it is not good for this use case.
• YAML is too greedy on covering derserialize issue, so it is too complex for a configuration and vulnerable, for example:
  • y ambiguously stand for "y" or true
  • CVE-2019-11253
• TOML is good for now, and easy to write and read for human, that is a real need for configuration file.