osury

Generate ReScript types with Sury schemas from OpenAPI specifications.

Acknowledgements

Huge thanks to the ReScript team for an amazing language, and special thanks to @DZakh for the incredible Sury library that made this project possible.

Project Status

Now at v1.0.0. The codegen is in production use against real-world OpenAPI specs (both Pydantic/FastAPI and Django/DRF generated). The pipeline is feature-complete for the OpenAPI 3.x patterns most code-first generators emit; edge cases beyond that are added on demand. Issues and PRs welcome.

Features

• OpenAPI 3.x → ReScript types with full discriminated-union support
• @schema annotations for Sury PPX runtime validation
• @genType for TypeScript interop with type-safe literal unions
• Discriminated unions via discriminator.mapping (OpenAPI-standard, primary) with fallback to _tag.const (Effect-style convention)
• Custom discriminator property names via discriminator.propertyName (@tag("type"), @tag("kind"), etc., not just _tag)
• Inline enum auto-promotion to named top-level types with field-based naming and structural deduplication
• Path operation types — both Response types from responses[200] and Params types from query/path parameters[]
• JSON null support via Nullable.t<T> (maps to T | null in TypeScript, distinct from option<T> for undefined)
• Untyped/Unknown handling via JSON.t with @s.matches(S.json) so untyped fields don't poison @schema propagation through enclosing types
• Automatic deduplication of identical union/enum structures
• TypeScript shims generated alongside (Dict.gen.ts, JSON.gen.ts, Nullable.res, Nullable.shim.ts)

Installation

npm install -D osury

Usage

CLI

# Generate to default ./Generated.res + shims
npx osury openapi.json

# Generate to specific directory
npx osury openapi.json src/API.res
# Creates: src/API.res, src/Dict.gen.ts, src/JSON.gen.ts,
#         src/Nullable.res, src/Nullable.shim.ts

# With explicit output flag
npx osury generate openapi.json -o src/Schema.res

# Show help
npx osury --help

Full Example: OpenAPI → ReScript → TypeScript

OpenAPI Input:

{
  "components": {
    "schemas": {
      "User": {
        "type": "object",
        "properties": {
          "id": { "type": "integer" },
          "email": { "type": "string" },
          "status": { "$ref": "#/components/schemas/Status" },
          "role": {
            "anyOf": [
              { "$ref": "#/components/schemas/Admin" },
              { "$ref": "#/components/schemas/Guest" }
            ]
          }
        },
        "required": ["id", "email"]
      },
      "Status": {
        "type": "string",
        "enum": ["pending", "active", "blocked"]
      },
      "Admin": {
        "type": "object",
        "properties": {
          "_tag": { "type": "string", "const": "Admin" },
          "permissions": { "type": "array", "items": { "type": "string" } }
        }
      },
      "Guest": {
        "type": "object",
        "properties": {
          "_tag": { "type": "string", "const": "Guest" },
          "expiresAt": {
            "anyOf": [{ "type": "string" }, { "type": "null" }]
          }
        }
      }
    }
  }
}

Generated ReScript (Schema.res):

module S = Sury

@genType
@schema
type status = [#pending | #active | #blocked]

@genType
@schema
type admin = {
  permissions: option<array<string>>
}

@genType
@schema
type guest = {
  expiresAt: @s.null Nullable.t<string>
}

@genType
@tag("_tag")
@schema
type adminOrGuest = Admin({
  permissions: option<array<string>>
}) | Guest({
  expiresAt: @s.null Nullable.t<string>
})

@genType
@schema
type user = {
  id: int,
  email: string,
  status: option<status>,
  role: option<adminOrGuest>
}

Generated TypeScript (Schema.gen.ts via genType):

import type {t as Nullable_t} from './Nullable.gen';

export type status = "pending" | "active" | "blocked";

export type admin = { readonly permissions: (undefined | string[]) };

export type guest = { readonly expiresAt: Nullable_t<string> };

export type adminOrGuest =
    { _tag: "Admin"; readonly permissions: (undefined | string[]) }
  | { _tag: "Guest"; readonly expiresAt: Nullable_t<string> };

export type user = {
  readonly id: number;
  readonly email: string;
  readonly status: (undefined | status);
  readonly role: (undefined | adminOrGuest)
};

The library uses sury-ppx for code-first approach — @schema annotation automatically generates runtime validators from type definitions.

Demo Playground

Try it online: osury-production.up.railway.app

This repository also includes a local demo for development:

Run locally

npm run demo

Open http://localhost:4173/demo/.

What it supports

• Upload OpenAPI JSON as a file
• Paste OpenAPI JSON into a text area
• Formatted ReScript output
• Formatted TypeScript output (derived from osury AST and matching generated ReScript structures)

Helper Files

Generated alongside the main Schema.res:

Dict.gen.ts — TypeScript shim for dictionaries:

export type t<T> = { [key: string]: T };

JSON.gen.ts — TypeScript shim for untyped/Unknown fields:

export type t = unknown;

Nullable.res — ReScript nullable type (option<T> with T | null TS mapping):

@genType.import(("./Nullable.shim.ts", "t"))
type t<'a> = option<'a>

Nullable.shim.ts — TypeScript shim for nullable:

export type t<T> = T | null;

Generated Annotations

Annotation	Purpose
@genType	TypeScript type generation
@schema	Sury PPX validation schema
@tag("_tag")	Discriminated union tag — default Effect TS convention; overridable via discriminator.propertyName (e.g. @tag("type"))
@s.null	Field-level JSON null support (for Nullable.t<T> fields)
@s.matches(S.json)	Per-field synthesizer for JSON.t so untyped fields don't poison enclosing @schema
@unboxed	Primitive-only union optimization
@as("name")	Reserved keyword field mapping

Requirements

For the generated code to compile, your project needs:

• rescript >= 12.0
• sury >= 11.0 (for @schema)
• sury-ppx >= 11.0 (for @schema PPX)
• gentype (for @genType)

Type Mapping

OpenAPI	ReScript
string	string
number	float
integer	int
boolean	bool
null	unit
array	array<T>
object	record { field: T }
$ref	type reference
enum (inline)	extracted to named type sortDirection = [#asc | #desc]
enum (top-level)	poly variant [#A | #B]
const (single string)	one-element enum (used for discriminator tags)
schema with no type	JSON.t (TS: unknown) with @s.matches(S.json)
anyOf: [T, null]	Nullable.t<T> (TS: T | null)
anyOf/oneOf of string literals (enum/const, incl. behind $ref)	merged into one named poly variant [#a | #b | #all]
anyOf: [A, B, ...] (no discriminator)	extracted variant type with structural name
oneOf + discriminator	poly variant with tags from discriminator.mapping
allOf	merged object type
additionalProperties	Dict.t<T>
default value	ignored for requiredness — optionality follows required[] only
parameters[] (query + path)	synthetic <method><Path>Params record
responses[200].schema	<method><Path>Response type

Discriminated Unions

osury resolves variant case tags through a three-level priority chain, so the ReScript-side tag always matches the wire-format truth, never the class name on the backend:

1. discriminator.mapping — OpenAPI 3.x standard, primary source. Works with any property name (_tag, tag, type, kind, …).
2. _tag.const — fallback when no explicit mapping is declared (Effect-style implicit convention).
3. Ref name — last-resort default for $ref items with no const information.

The practical effect: class names on the backend can diverge from wire-format discriminator values without breaking osury. Pydantic's natural style (class MetricGridBlock ↔ _tag: "MetricGrid") works out of the box as long as the discriminator mapping is declared in the schema.

Block:
  oneOf:
    - { $ref: "#/components/schemas/MetricGridBlock" }
    - { $ref: "#/components/schemas/ProseBlock" }
  discriminator:
    propertyName: _tag
    mapping:
      MetricGrid: "#/components/schemas/MetricGridBlock"
      Prose:      "#/components/schemas/ProseBlock"

Generates:

@genType @tag("_tag") @schema
type block = MetricGrid({
  metrics: array<string>
}) | Prose({
  text: string
})

Note the case names (MetricGrid, Prose) come from the mapping keys, not from the schema class names (MetricGridBlock, ProseBlock). Case payloads are inlined records — fields are copied from the referenced schema and the discriminator property is filtered out to avoid duplication with @tag.

Path Types

Types are generated from both responses and parameters of each path operation.

Response types

{
  "paths": {
    "/users": {
      "get": {
        "responses": {
          "200": {
            "content": {
              "application/json": {
                "schema": { "$ref": "#/components/schemas/UserList" }
              }
            }
          }
        }
      }
    }
  }
}

Generates: type getUsersResponse = userList

Params types

Query and path parameters are folded into a synthetic object schema and pushed through the same parsing pipeline (so all rules — default → required, anyOf [T, null] → Nullable.t<T>, inline enum → named type — apply uniformly).

{
  "paths": {
    "/products": {
      "get": {
        "parameters": [
          { "in": "query", "name": "sort_field",
            "schema": { "type": "string", "enum": ["sales", "clicks", "impressions"] } },
          { "in": "query", "name": "limit",
            "schema": { "type": "integer", "default": 50 } }
        ]
      }
    }
  }
}

Generates:

@genType @schema
type sortField = [#sales | #clicks | #impressions]  // promoted from inline enum

@genType @schema
type getProductsParams = {
  sort_field: option<sortField>,
  limit: int,  // has default → required
}

Headers and serialization details (style/explode) are intentionally excluded — they belong to the HTTP client layer, not the schema contract.

License

MIT