This guide demonstrates how to use the cuego API to validate Go values against CUE constraints expressed in Go struct tags.

Create a Go program using cuego

1

Initialize a Go module, or use an existing one if that’s more suitable for your situation:

TERMINAL
$ go mod init go.example
...
2

Create a main program that uses the cuego API to perform the validation you require.

You can use this example code as a starting point:

main.go
package main

import (
	"fmt"

	"cuelang.org/go/cue/errors"
	"cuelang.org/go/cuego"
)

// Policy contains fields with struct tags in the "cue" namespace.
// These tags are read by the cuego API, and define field-level CUE constraints.
type Policy struct {
	Level  int    `cue:">10"`
	Action string `cue:" \"Allow\" | \"Deny\" "`
}

func main() {
	// check uses the cuego API to validate an instance of a Policy against the
	// CUE constraints embedded in the type definition. It returns either a
	// success message, or one or more errors formatted as a string.
	check := func(p Policy) string {
		if err := cuego.Validate(p); err != nil {
			return errors.Details(err, nil)
		}
		return "✅ ok"
	}

	// good is an instance of a Policy that adheres to the type's CUE constraints.
	good := Policy{
		Level:  100,
		Action: "Allow",
	}
	// bad is an instance of a Policy that violates the type's CUE constraints.
	bad := Policy{
		Level:  5,
		Action: "Bypass",
	}

	// Display the validation result for each Policy instance.
	fmt.Printf("good: %v\n", check(good))
	fmt.Printf("bad: %v\n", check(bad))
}

This example code uses cuego to check two instances of a simple struct type (good / bad) against CUE constraints embedded in the type definition (Policy), and prints the validation result for each instance.

3

Add a dependency on cuelang.org/go and ensure the Go module is tidy:

TERMINAL
$ go get cuelang.org/go@v0.9.1
...
$ go mod tidy
...
4

Run the program:

TERMINAL
$ go run .
good: ✅ ok
bad: Action: 2 errors in empty disjunction:
Action: conflicting values "Allow" and "Bypass":
    <field:>:1:2
Action: conflicting values "Deny" and "Bypass":
    <field:>:1:12
Level: invalid value 5 (out of bound >10):
    <field:>:1:1
  • Go API: cuego – package documentation
  • Go API: cue/errors – package documentation
  • Tag: go api – pages explaining and exploring CUE’s Go API