How CUE works with JSON Schema | CUE

Jonathan Matthews

Paul Jolly

CUE has first class support for JSON Schema: both the cue command and the Go API understand the format.

Constraints stored as JSON Schema are available for cue commands to use as if they were expressed in CUE. This allows you to work with JSON Schema constraints directly, using them to validate data, and to represent them natively in CUE’s more succinct and expressive form.

In this guide we’ll see:

cue import converting a JSON Schema to CUE,
cue vet using JSON Schema constraints directly,
and the encoding/jsonschema Go API validating data against a JSON Schema.

The ability to export CUE constraints as JSON Schema is tracked in issue #929.

Using JSON Schema with the `cue` command

The cue import command can produce CUE from JSON Schema.

Let’s start with this JSON Schema:

{
    "$schema": "https://json-schema.org/draft/2020-12/schema",
    "type": "object",
    "title": "Main Person schema.",
    "description": "This schema defines a person.",
    "required": [
        "name"
    ],
    "properties": {
        "name": {
            "description": "What is this person called?",
            "type": "string",
            "minLength": 1
        },
        "address": {
            "description": "Where does this person live?",
            "type": "string",
            "minLength": 1,
            "maxLength": 200
        },
        "children": {
            "description": "This is a very long comment for some reason, which will keep going and going past the point where it should probably have stopped.",
            "type": "array",
            "items": {
                "type": "string"
            },
            "default": null
        },
        "home phone": {
            "type": "string",
            "deprecated": true
        }
    }
}

We use cue import to convert the JSON Schema to CUE:

TERMINAL

$ cue import -l '#Person:' schema.json

cue import recognises JSON Schema from its signature fields, and uses the schema’s constraints to create a shorter, more readable CUE representation. Our -l parameter tells cue to place the constraints inside the #Person definition:

// Main Person schema.
//
// This schema defines a person.

import "strings"

#Person: {
	@jsonschema(schema="https://json-schema.org/draft/2020-12/schema")

	// What is this person called?
	name!: strings.MinRunes(1)

	// Where does this person live?
	address?: strings.MinRunes(1) & strings.MaxRunes(200)

	// This is a very long comment for some reason, which will keep
	// going and going past the point where it should probably have
	// stopped.
	children?: [...string] | *null
	"home phone"?: string @deprecated()
	...
}

We use the imported schema to validate some known-good data (good.json) and known-bad data (bad.json):

{
    "name": "Dorothy Cartwright",
    "address": "Ripon, North Yorkshire"
}

{
    "name": [
        "Charlie",
        "Cartwright"
    ],
    "address": "Ripon, North Yorkshire"
}

The cue vet command validates our data against the #Person constraint:

TERMINAL

$ cue vet -d '#Person' schema.cue good.json bad.json
name: conflicting values strings.MinRunes(1) and ["Charlie","Cartwright"] (mismatched types string and list):
    ./bad.json:2:13
    ./schema.cue:11:9

The cue vet command can also validate the data using the JSON Schema directly:

TERMINAL

$ cue vet schema.json good.json bad.json
name: conflicting values strings.MinRunes(1) and ["Charlie","Cartwright"] (mismatched types string and list):
    ./bad.json:2:13
    ./schema.json:13:13

The cue command normally recognises JSON Schema’s signature fields and treats the contents of JSON Schema as data constraints - not just additional data. A qualifier can be used to change this behaviour, as outlined in cue help filetypes:

TERMINAL

$ cue def json: schema.json
$schema:     "https://json-schema.org/draft/2020-12/schema"
type:        "object"
title:       "Main Person schema."
description: "This schema defines a person."
...

Using JSON Schema with the Go API

The encoding/jsonschema API allows you to work with JSON Schema in Go code.

As with the cue command examples shown above, the API can be used to convert JSON Schema to CUE. However, in this next example, we’ll use the API in a more fully-formed context: controlling data validation at a lower level.

This Go program validates a JSON data file against a JSON Schema:

package main

import (
	"flag"
	"fmt"
	"log"
	"os"

	"cuelang.org/go/cue"
	"cuelang.org/go/cue/cuecontext"
	"cuelang.org/go/cue/errors"
	"cuelang.org/go/encoding/json"
	"cuelang.org/go/encoding/jsonschema"
)

func main() {
	log.SetFlags(0)
	flag.Parse()
	args := flag.Args()

	// A cue.Context is used for building/compiling CUE at a low-level.
	// It replaces cue.Runtime.
	ctx := cuecontext.New()

	if len(args) != 2 {
		log.Fatalf("usage:\n\t%s SCHEMA.json DATA.json\n", os.Args[0])
	}

	// Load the schema file JSON
	schemaFile, err := os.ReadFile(args[0])
	if err != nil {
		log.Fatal(err)
	}
	schemaJsonAst, err := json.Extract(args[0], schemaFile)
	if err != nil {
		log.Fatal(err)
	}
	schemaJson := ctx.BuildExpr(schemaJsonAst)

	// Extract JSON Schema from the JSON
	schemaAst, err := jsonschema.Extract(schemaJson, &jsonschema.Config{
		Strict: true,
	})
	if err != nil {
		log.Fatal(err)
	}

	// Build a cue.Value of the schema
	schema := ctx.BuildFile(schemaAst)

	// Load the data file JSON
	dataFile, err := os.ReadFile(args[1])
	if err != nil {
		log.Fatal(err)
	}
	dataAst, err := json.Extract(args[1], dataFile)
	if err != nil {
		log.Fatal(err)
	}

	// Build a cue.Value of the data
	data := ctx.BuildExpr(dataAst)

	// Unify the schema and data
	res := schema.Unify(data)

	// Validate whether the combined (unified) result has errors or not.
	if err := res.Validate(cue.Concrete(true)); err != nil {
		// If errors, report them and fail.
		log.Fatal(errors.Details(err, nil))
	}
	// If no errors, print the data value
	fmt.Printf("%v\n", res)
}

Running the command validates the data file in the second argument against the JSON schema in the first argument - printing the data if it’s valid and displaying a validation error otherwise. Here we use it to validate the same good.json and bad.json files from above:

TERMINAL

$ go run . schema.json good.json
{
	name:    "Dorothy Cartwright"
	address: "Ripon, North Yorkshire"
}
$ go run . schema.json bad.json
name: conflicting values strings.MinRunes(1) and ["Charlie","Cartwright"] (mismatched types string and list):
    bad.json:2:13
    schema.json:13:13
exit status 1

Future plans

One of CUE’s goals is to act as an interlingua: a bidirectional bridge between all the formats that CUE speaks, linking constraints and data sources of truth, no matter where they exist.

To meet this goal, CUE will gain the ability to export native CUE constraints as JSON Schema, enabling their use by tools that aren’t aware of CUE. This is tracked in issue #929.

Reference: cue help import
The encoding/jsonschema Go API
Reference: cue help vet
Reference: cue help filetypes
Issue #929 tracks the conversion of CUE to JSON Schema

Using JSON Schema with the cue command

Using JSON Schema with the Go API

Future plans

Related content

Using JSON Schema with the `cue` command