Example of using sentinel values with github.com/google/go-cmp

ignore_test.go

package example_test

import (
	"encoding/json"
	"net/http"
	"net/http/httptest"
	"testing"

	"github.com/google/go-cmp/cmp"
)

type Object struct {
	String string  `json:"string,omitempty"`
	Int    int     `json:"int,omitempty"`
	Object *Object `json:"object,omitempty"`
}

// Handler writes a {"string": "a", "object": {"string": "b"}} in response.
func Handler(w http.ResponseWriter, req *http.Request) {
	w.Header().Set("Content-Type", "application/json")
	json.NewEncoder(w).Encode(&Object{
		String: "A",
		Object: &Object{
			String: "B",
			Object: &Object{
				String: "C",
			},
		},
	})
}

var (
	// ignore is our sentinel value for ignoring map fields.
	ignore = struct{}{}
	// ignoreOpt is how we tell cmp.Diff and cmp.Equal to discount ignore values.
	ignoreOpt = cmp.FilterValues(
		func(a, b interface{}) bool {
			return a == ignore || b == ignore
		},
		cmp.Ignore(),
	)
)

// TestHandler demonstrates comparing interface{} values which we might do if
// we're checking the contents of a JSON body.
func TestHandler(t *testing.T) {
	var actual interface{}

	// Get the parsed value from the handler response.
	r := httptest.NewRecorder()
	Handler(r, httptest.NewRequest("POST", "/", nil))
	json.NewDecoder(r.Result().Body).Decode(&actual)

	// Our expected template.
	expected := map[string]interface{}{
		"string": "A",
		"object": map[string]interface{}{
			"string": ignore,
			"object": map[string]interface{}{
				"string": "C",
			},
		},
	}

	if d := cmp.Diff(expected, actual, ignoreOpt); d != "" {
		t.Errorf("Unexpected diff:\n%s", d)
	}
}