Simple webservice to validate data with CUE

sample.go

package sample

import (
	"bytes"
	"fmt"
	"io"
	"io/ioutil"
	"net/http"
	"net/http/httptest"
	"testing"

	"cuelang.org/go/cue"
	"cuelang.org/go/cue/cuecontext"
)

// DataProduct is a structure that holds the definition as a CUE value
type DataProduct struct {
	definition cue.Value
	// ...
}

// ServeHTTP to make the *DataProduct a http handler
// This is an example, we do not handle the method properly nor we check the content type
// This methods reads the payload from the request an calls the ExtractData method for validation
func (d *DataProduct) ServeHTTP(w http.ResponseWriter, r *http.Request) {
	b, _ := ioutil.ReadAll(r.Body)
	defer r.Body.Close()
	_, err := d.ExtractData(b)
	if err != nil {
		http.Error(w, err.Error(), http.StatusBadRequest)
		return
	}
	fmt.Fprint(w, "ok")
}

// ExtractData tries to reconstruct a data from the payload b, and
// unifies it with the data definition.
// then it validates the resulting value
func (d *DataProduct) ExtractData(b []byte) (cue.Value, error) {
	data := d.definition.Context().CompileBytes(b)
	unified := d.definition.Unify(data)
	opts := []cue.Option{
		cue.Attributes(true),
		cue.Definitions(true),
		cue.Hidden(true),
	}
	return data, unified.Validate(opts...)
}

sample_test.go

// TestHandler is triggering a http test server with the DataProduct's handler,
// send a post request with various payload and check the result
func TestHandler(t *testing.T) {
	ctx := cuecontext.New()
	val := ctx.CompileString(`
	 {
		// first name of the person
		first: =~ "[A-Z].*"
		// Last name of the person
		Last: =~ "[A-Z].*"
		// Age of the person
		Age?: number & < 130
	}
	`)
	ts := httptest.NewServer(&DataProduct{
		definition: val,
	})
	defer ts.Close()
	tests := []struct {
		name           string
		payload        io.Reader
		expectedStatus int
	}{
		{
			name: "valid",
			payload: bytes.NewBufferString(`
			{
				"first": "John",
				"Last": "Doe",
				"Age": 40
			}
`),
			expectedStatus: http.StatusOK,
		},
		{
			name: "invalid",
			payload: bytes.NewBufferString(`
			{
				"first": "John",
				"Last": "Doe",
				"Age": 140
			}
`),
			expectedStatus: http.StatusBadRequest,
		},
	}
	for _, tt := range tests {
		t.Run(tt.name, func(t *testing.T) {
			req, _ := http.NewRequest("POST", ts.URL, tt.payload)
			res, err := http.DefaultClient.Do(req)
			if err != nil {
				t.Error(err)
			}
			if res.StatusCode != tt.expectedStatus {
				t.Errorf("expected %v, got %v", tt.expectedStatus, res.StatusCode)
			}
		})
	}

}