Custom Matchers

Writing your own matchers

When no built-in matcher fits and composition (And/Or/WithTransform/SatisfyAll — see gomega:composing-matchers) can't express it cleanly, write a custom matcher. Reach for gcustom.MakeMatcher first — it's the modern, low-boilerplate path. Drop to a hand-written type only when you need full control. Docs: https://onsi.github.io/gomega/#adding-your-own-matchers.

gcustom.MakeMatcher — the recommended path

gcustom.MakeMatcher(matchFunc, ...) builds a full types.GomegaMatcher from one function. The match function must be func(actual T) (bool, error):

import (
	"github.com/onsi/gomega/gcustom"
	"github.com/onsi/gomega/types"

	"encoding/json"
	"fmt"
	"net/http"
	"reflect"
)

func RepresentJSONifiedObject(expected any) types.GomegaMatcher {
	return gcustom.MakeMatcher(func(response *http.Response) (bool, error) {
		ptr := reflect.New(reflect.TypeOf(expected)).Interface()
		if err := json.NewDecoder(response.Body).Decode(ptr); err != nil {
			return false, fmt.Errorf("failed to decode JSON: %w", err)
		}
		decoded := reflect.ValueOf(ptr).Elem().Interface()
		return reflect.DeepEqual(decoded, expected), nil
	}).WithTemplate("Expected:\n{{.FormattedActual}}\n{{.To}} contain the JSON representation of\n{{format .Data 1}}").WithTemplateData(expected)
}

Then Expect(resp).To(RepresentJSONifiedObject(book)) just works.

Typed match funcs get free type-checking. Because the func takes *http.Response, gcustom rejects any other actual with a clear error before calling your code. Use func(actual any) (bool, error) only if you want to handle multiple types or do your own type checks.

Return (false, err) for bad input, not a panic. A non-nil error fails the assertion in both the To and NotTo directions — you can't accidentally pass a negated assertion by feeding garbage.

Messages: WithMessage vs WithTemplate

• .WithMessage("contain the JSON representation") — simplest. Renders Expected:\n<actual>\nto contain the JSON representation (and not to when negated).
• .WithTemplate(tmpl, optionalData) — full control. WithTemplate(tmpl, data) is shorthand for .WithTemplate(tmpl).WithTemplateData(data).
• Omit both and you get a generic Custom matcher failed for:\n<actual> message.

Template variables and functions:

Avoid recompiling templates in hot paths. WithTemplate parses on every constructor call. Precompile once with gcustom.ParseTemplate(str) (use that, not raw text/template, so the format func is registered) and pass it to MakeMatcher(matchFunc, tmpl) or .WithPrecompiledTemplate(tmpl).

The raw GomegaMatcher interface

MakeMatcher returns this; implement it directly only when you need behavior gcustom doesn't expose:

type GomegaMatcher interface {
	Match(actual any) (success bool, err error)
	FailureMessage(actual any) (message string)
	NegatedFailureMessage(actual any) (message string)
}

A hand-rolled version of the matcher above:

func RepresentJSONifiedObject(expected any) types.GomegaMatcher {
	return &representJSONMatcher{expected: expected}
}

type representJSONMatcher struct{ expected any }

func (m *representJSONMatcher) Match(actual any) (bool, error) {
	response, ok := actual.(*http.Response)
	if !ok {
		return false, fmt.Errorf("RepresentJSONifiedObject matcher expects an *http.Response")
	}
	ptr := reflect.New(reflect.TypeOf(m.expected)).Interface()
	if err := json.NewDecoder(response.Body).Decode(ptr); err != nil {
		return false, fmt.Errorf("failed to decode JSON: %w", err)
	}
	decoded := reflect.ValueOf(ptr).Elem().Interface()
	return reflect.DeepEqual(decoded, m.expected), nil
}

func (m *representJSONMatcher) FailureMessage(actual any) string {
	return format.Message(actual, "to contain the JSON representation of", m.expected)
}

func (m *representJSONMatcher) NegatedFailureMessage(actual any) string {
	return format.Message(actual, "not to contain the JSON representation of", m.expected)
}

Conventions:

• A constructor function (here RepresentJSONifiedObject) returns the matcher; the struct itself stays unexported. Take concrete types in the constructor where you can; use any only when you genuinely must.
• FailureMessage/NegatedFailureMessage are always called after Match — stash anything you computed in Match on the struct and reuse it in the messages.
• Lean on reflect to interpret generic actual/expected inputs.

format package helpers

Use these for messages instead of hand-rolling fmt.Sprintf so output matches every other Gomega matcher:

• format.Message(actual, "to <verb>", expected) → Expected\n<actual>\n<message>\n<expected>. Omit expected for a one-sided message.
• format.Object(obj, indent) → the pretty-printed block Gomega uses everywhere (indent is a uint).
• format.MessageWithDiff(actual, "to equal", expected) for string-diff highlighting.

MatchMayChangeInTheFuture — stopping Eventually/Consistently early

Eventually/Consistently (see gomega:async) re-invoke your matcher each poll. If a match result can become permanently fixed (e.g. a closed channel, an exited process), implement the optional OracleMatcher method so polling can short-circuit:

func (m *exitMatcher) MatchMayChangeInTheFuture(actual any) bool {
	session := actual.(*gexec.Session)
	return session.ExitCode() == -1 // true while still running; false once exited
}

This is not part of GomegaMatcher — most matchers don't need it. When present, Gomega calls it after each Match; returning false stops polling and fails/passes as appropriate (so Eventually(session).Should(Exit(0)) fails the instant a wrong exit code lands instead of waiting out the timeout). gcustom matchers don't expose this; hand-write the type if you need it.

Only consulted for bare values. If Eventually is polling a function, Gomega can't assume the result is stable, so MatchMayChangeInTheFuture is skipped.

Aborting from inside a matcher with StopTrying

A more direct, modern alternative: have Match return a StopTrying(...) error (or call StopTrying(...).Now()) to abort Eventually/Consistently immediately and fail with your message.

func (m *thing) Match(actual any) (bool, error) {
	if irrecoverable {
		return false, StopTrying("the resource was deleted; it will never appear")
	}
	return check(actual), nil
}

You can .Wrap(err) and .Attach("desc", obj) onto the signal. Full polling-signal semantics (including TryAgainAfter) live in gomega:async.

Testing your custom matchers

Drive the matcher under test with ordinary assertions, and for the error paths call Match directly (an erroring matcher would otherwise just fail your assertion):

var _ = Describe("RepresentJSONifiedObject", func() {
	var book Book
	var response *http.Response

	BeforeEach(func() {
		book = Book{Title: "Les Miserables", Author: "Victor Hugo"}
		j, err := json.Marshal(book)
		Expect(err).NotTo(HaveOccurred())
		response = &http.Response{Body: io.NopCloser(bytes.NewBuffer(j))}
	})

	It("matches an http.Response carrying the object's JSON", func() {
		Expect(response).To(RepresentJSONifiedObject(book))
	})

	It("does not match a different payload", func() {
		response.Body = io.NopCloser(strings.NewReader(`{}`))
		Expect(response).NotTo(RepresentJSONifiedObject(book))
	})

	It("errors on the wrong actual type", func() {
		_, err := RepresentJSONifiedObject(book).Match("not a response")
		Expect(err).To(HaveOccurred())
	})
})

Assert on FailureMessage(actual) / NegatedFailureMessage(actual) when the wording matters.

Packaging & contributing

• Keep matchers in their own package; export only the constructor.
• To contribute a matcher upstream, mimic Gomega's house style: format output via the format package, put the matcher and its tests in the matchers package, and add the constructor to matchers.go in the top-level package. Issues and PRs welcome at https://github.com/onsi/gomega.

Wrapping the whole Gomega (advanced)

You can replace gomega.Default with a type implementing both gomega.Gomega and an Inner() Gomega method that returns the real default, delegating every call through. Useful for cross-suite logging, call counting, or injecting delays to surface timing dependencies — rarely needed, but the seam exists.

See also: gomega:matchers (catalog — check before building), gomega:composing-matchers (build matchers without code), gomega:async (polling signals).