Typify

Typify compiles JSON Schema documents into Rust types. It can be used in one of several ways:

• using the cargo typify command

• via the macro import_types!("types.json") to generate Rust types directly in your program

• via a builder interface to generate Rust types in build.rs or xtask

• via the builder functions to generate persistent files e.g. when building API bindings

If generation fails, doesn't compile or is generally lousy: Please file an issue and include the JSON Schema and Rust output (if there is any). Use cargo typify command to generate code from the command-line. It's even more helpful if you can articulate the output you'd ideally like to see.

JSON Schema → Rust types

JSON Schema is a constraint language designed for validation. As a result, it is not well-suited--and is often seemingly hostile--to translation into constructive type systems. It allows for expressions of arbitrary complexity with an infinity of ways to articulate a given set of constraints. As such, typify does its best to discern an appropriate interpretation, but it is far from perfect!

Typify translates JSON Schema types in a few different ways depending on some basic properties of the schema:

Built-in types

Integers, floating-point numbers, strings, etc. Those all have straightforward representations in Rust. The only significant nuance is how to select the appropriate built-in type based on type attributes. For example, a JSON Schema might specify a maximum and/or minimum that indicates the appropriate integral type to use.

String schemas that include a known format are represented with the appropriate Rust type. For example { "type": "string", "format": "uuid" } is represented as a uuid::Uuid (which requires the uuid crate be included as a dependency).

Arrays

JSON Schema arrays can turn into one of three Rust types Vec<T>, HashSet<T>, and tuples depending on the schema properties. An array may have a fixed length that matches a fixed list of item types; this is well represented by a Rust tuple. The distinction between Vec<T> and HashSet<T> is only if the schema's uniqueItems field is false or true respectively.

Objects

In general, objects turn into Rust structs. If, however, the schema defines no properties, Typify emits a HashMap<String, T> if the additionalProperties schema specifies T or a HashMap<String, serde_json::Value> otherwise.

Properties of generated struct that are not in the required set are typically represented as an Option<T> with the #[serde(default)] attribute applied. Non-required properties with types that already have a default value (such as a Vec<T>) simply get the #[serde(default)] attribute (so you won't see e.g. Option<Vec<T>>).

Alternate Map types

By default, Typify uses std::collections::HashMap as described above.

If you prefer to use std::collections::BTreeMap or a map type from a crate such as indexmap::IndexMap, you can specify this by calling with_map_type on the TypeSpaceSettings object, and providing the full path to the type you want to use. E.g. ::std::collections::BTreeMap or ::indexmap::IndexMap.

Note that for a custom map type to work you must have T defined to generate a struct as described in Objects. If T is not defined, typify will generate code using a serde_json::Map<String, serde_json::Value> instead.

See the documentation for TypeSpaceSettings::with_map_type for the requirements for a map type.

OneOf

The oneOf construct maps to a Rust enum. Typify maps this to the various serde enum types.

AllOf

The 'allOf' construct is handled by merging schemas. While most of the time, typify tries to preserve and share type names, it can't always do this when merging schemas. You may end up with fields replicated across type; optimizing this generation is an area of active work.

AnyOf

The anyOf construct is much trickier. If can be close to an enum (oneOf), but where no particular variant might be canonical or unique for particular data. While today we (imprecisely) model these as structs with optional, flattened members, this is one of the weaker areas of code generation.

Issues describing example schemas and desired output are welcome and helpful.

AdditionalProperties

The additionalProperties constraint lets a schema define what non-specified properties are permitted. A value of false means that no other properties are permitted (this is expressed in Rust with the #[serde(deny_unknown_fields)] annotation). The absence of additionalProperties or a value of true are equivalent constructions that mean that any other property is permitted.

Without other properties, an object that permits additional properties will be represented as a map type. In conjunction with other properties, typify employs a heuristic interpretation. Absent or with a value of true, additional properties are ignored. If, however, additionalProperties has another value (i.e. a schema), the generated type will have a map field annotated with #[serde(flatten)].

Note that this is true of any schema value for additionalProperties that is not a boolean. This includes values that would be equivalent with regard to validation such as the schema {} or { "not": false } or any of the other infinity of equivalent schemas. One can therefore construct a struct with named properties and a flattened map of additional properties by using a value for additionalProperties that is equivalent to true or absent with regard to validation, by using some e.g. {}.

Rust -> Schema -> Rust

Schemas derived from Rust types may include an extension that provides information about the original type:

{
  "type": "object",
  "properties": { .. },
  "x-rust-type": {
    "crate": "crate-o-types",
    "version": "1.0.0",
    "path": "crate_o_types::some_mod::SomeType"
  }
}

The extension includes the name of the crate, a Cargo-style version requirements spec, and the full path (that must start with ident-converted name of the crate).

Each of the modes of using typify allow for a list of crates and versions to be specified. In this case, if the user specifies "crate-o-types@1.0.1" for example, then typify would use its SomeType type rather than generating one according to the schema.

Using types from other crates

Each mode of using typify has a method for controlling the use of types with x-rust-type annotations. The default is to ignore them. The recommended method is to specify each crate and version you intend to use. You can additionally supply the * version for crates (which may result in incompatibilities) or you can define a policy to allow the use of all "unknown" crates (which may require that addition of dependencies for those crates).

For the CLI:

$ cargo typify --unknown-crates allow --crate oxnet@1.0.0 ...

For the builder:

let mut settings = typify::TypeSpaceSettings::default();
settings.with_unknown_crates(typify::UnknownPolicy::Allow)
    .with_crate("oxnet", typify::CrateVers::Version("1.0.0".parse().unwrap()));

For the macro:

typify::import_types!(
  schema = "schema.json",
  unknown_crates = Allow,
  crates = {
    "oxnet" = "1.0.0"
  }
)

Version requirements

The version field within the x-rust-type extension follows the Cargo version requirements specification. If the extension specifies 0.1.0 of a crate and the user states that they're using 0.1.1, then the type is used; conversely, if the extension specifies 0.2.2 and the user is only using 0.2.0 the type is not used.

Crate authors may choose to adhere to greater stability than otherwise provided by semver. If the extension version is >=0.1.0, <1.0.0 then the crate author is committing to the schema compatibility of the given type on all releases until 1.0.0. It is important that crate authors populate the version field in a way that upholds type availability. For example, while * is a valid value, it is only conceivably valid if the type in question were available in the first ever version of a crate published and never changed incompatibly in any subsequent version.

Type parameters

The x-rust-type extension may also specify type parameters:

{
  "$defs": {
    "Sprocket": {
      "type": "object",
      "properties": { .. },
      "x-rust-type": {
        "crate": "util",
        "version": "0.1.0",
        "path": "util::Sprocket",
        "parameters": [
          {
            "$ref": "#/$defs/Gizmo"
          }
        ]
      }
    },
    "Gizmo": {
      "type": "object",
      "properties": { .. },
      "x-rust-type": {
        "crate": "util",
        "version": "0.1.0",
        "path": "util::Gizmo"
      }
    }
  }
}

With the util@0.1.0 crate specified during type generation, schemas referencing #/$defs/Sprocket would use the (non-generated) type util::Sprocket<util::Gizmo>.

The parameters field is an array of schemas. They may be inline schemas or referenced schemas.

Including x-rust-type in your library

The schema for the expected value is as follows:

{
  "description": "schema for the x-rust-type extension",
  "type": "object",
  "properties": {
    "crate": {
      "type": "string",
      "pattern": "^[a-zA-Z0-9_-]+$"
    },
    "version": {
      "description": "semver requirements per a Cargo.toml dependencies entry",
      "type": "string"
    },
    "path": {
      "type": "string",
      "pattern": "^[a-zA-Z0-9_]+(::[a-zA-Z0-9+]+)*$"
    },
    "parameters": {
      "type": "array",
      "items": {
        "$ref": "#/definitions/Schema"
      }
    }
  },
  "required": [
    "crate",
    "path",
    "version"
  ]
}

The version field expresses the stability of your type. For example, if 0.1.0 indicates that 0.1.1 users would be fine whereas 0.2.0 users would not use the type (instead generating it). You can communicate a future commitment beyond what semver implies by using the Cargo version requirement syntax. For example >=0.1.0, <1.0.0 says that the type will remain structurally compatible from version 0.1.0 until 1.0.0.

Formatting

You can format generated code using crates such as rustfmt-wrapper and prettyplease. This can be particularly useful when checking in code or emitting code from a build.rs.

The examples below show different ways to convert a TypeSpace to a string (typespace is a typify::TypeSpace).

rustfmt

Best for generation of code that might be checked in alongside hand-written code such as in the case of an xtask or stand-alone code generator (such as cargo-typify).

rustfmt_wrapper::rustfmt(typespace.to_stream().to_string())?

prettyplease

Best for build.rs scripts where transitive dependencies might not have rustfmt installed so should be self-contained.

prettyplease::unparse(&syn::parse2::<syn::File>(typespace.to_stream())?)

No formatting

If no human will ever see the code (and this is almost never the case).

typespace.to_stream().to_string()

WIP

Typify is a work in progress. Changes that affect output will be indicated with a breaking change to the crate version number.

In general, if you have a JSON Schema that causes Typify to fail or if the generated type isn't what you expect, please file an issue.

There are some known areas where we'd like to improve:

Complex JSON Schema types

JSON schema can express a wide variety of types. Some of them are easy to model in Rust; others aren't. There's a lot of work to be done to handle esoteric types. Examples from users are very helpful in this regard.

Bounded numbers

Bounded numbers aren't very well handled. Consider, for example, the schema:

{
  "type": "integer",
  "minimum": 1,
  "maximum": 6
}

The resulting types won't enforce those value constraints.

Configurable dependencies

A string schema with format set to uuid will result in the uuid::Uuid type; similarly, a format of date translates to chrono::naive::NaiveDate. For users that don't want dependencies on uuid or chrono it would be useful for Typify to optionally represent those as String (or as some other, consumer-specified type).