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

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 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 tuples. 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 in to 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 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>>).

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.

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 (list 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).