gstruct

gstruct: matching complex data types

gstruct builds composite matchers that apply a separate matcher to each field of a struct, each element of a slice, each key of a map, or the target of a pointer. It is the tool for fuzzy-matching large, deeply nested values. Docs: https://onsi.github.io/gomega/#gstruct-testing-complex-data-types.

Dot-import gomega; import gstruct normally (or dot-import it too):

import (
	. "github.com/onsi/gomega"
	"github.com/onsi/gomega/gstruct"
)

The values in Fields/Elements/Keys are themselves matchers. That is the whole point: compose with the core catalog (gomega:matchers) and combinators like And/Or/Not/WithTransform (gomega:composing-matchers). Examples below assume gstruct is dot-imported for brevity.

If you find yourself building an overly complex gstruct matcher, consider if you should be building a custom matcher (gomega:custom-matchers) instead.

Structs — Fields

Fields is map[string]types.GomegaMatcher keyed by field name.

actual := struct {
	A int
	B bool
	C string
}{5, true, "foo"}

Expect(actual).To(MatchAllFields(Fields{
	"A": BeNumerically("<", 10),
	"B": BeTrue(),
	"C": Equal("foo"),
}))

MatchAllFields requires an exact 1:1 mapping — every field must have a matcher and every matcher must map to a field. This is great for maintainability: adding or removing a struct field breaks the test until you update it.

To match a subset/superset, use MatchFields(options, Fields{...}):

// IgnoreExtras: ignore struct fields with no matcher
Expect(actual).To(MatchFields(IgnoreExtras, Fields{
	"A": BeNumerically("<", 10),
	"B": BeTrue(), // no entry for C — fine
}))

// IgnoreMissing: ignore matchers with no corresponding field
Expect(actual).To(MatchFields(IgnoreMissing, Fields{
	"A": BeNumerically("<", 10),
	"B": BeTrue(),
	"C": Equal("foo"),
	"D": Equal("bar"), // ignored — actual has no field D
}))

IgnoreUnexportedExtras is a middle ground: it ignores only unexported extra fields (gstruct can't read them via reflect anyway) while still requiring all exported fields to be matched.

Slices — Elements + an id function

Elements is map[string]types.GomegaMatcher keyed by a string id. You supply an Identifier (func(element any) string) that maps each element to its key.

actual := []string{"A: foo bar baz", "B: once upon a time", "C: the end"}
id := func(element any) string { return string(element.(string)[0]) }

Expect(actual).To(MatchAllElements(id, Elements{
	"A": Not(BeZero()),
	"B": MatchRegexp("[A-Z]: [a-z ]+"),
	"C": ContainSubstring("end"),
}))

MatchAllElements requires a 1:1 mapping. Use MatchElements(id, options, Elements{...}) with IgnoreExtras/IgnoreMissing to relax, exactly as with fields. AllowDuplicates lets several elements share one key/matcher (all of them must pass).

Index-based variants MatchAllElementsWithIndex/MatchElementsWithIndex take an IdentifierWithIndex (func(index int, element any) string); the built-in IndexIdentity just uses the index as the key.

Maps — Keys

The *Fields API has a *Keys mirror for maps: MatchAllKeys(Keys{...}) and MatchKeys(options, Keys{...}).

actual := map[string]string{"A": "correct", "B": "incorrect"}

// MatchAllKeys requires every key be matched (this would fail without B):
Expect(actual).To(MatchAllKeys(Keys{
	"A": Equal("correct"),
	"B": Equal("incorrect"),
}))

// IgnoreMissing tolerates matchers for absent keys:
Expect(actual).To(MatchKeys(IgnoreMissing, Keys{
	"A": Equal("correct"),
	"B": Equal("incorrect"),
	"C": Equal("whatever"), // ignored — actual has no key C
}))

Pointers — PointTo

PointTo(matcher) dereferences a pointer and applies matcher to the pointed-to value. It fails if the pointer is nil.

foo := 5
Expect(&foo).To(PointTo(Equal(5)))
var bar *int
Expect(bar).NotTo(PointTo(BeNil())) // nil pointer fails PointTo

Ignore() and Reject()

Ignore() always succeeds, Reject() always fails — use them as entries to skip a field/element or to assert one must never appear.

Putting it all together

The matchers nest arbitrarily, mixing struct/slice/map/pointer matchers with the core catalog:

coreID := func(element any) string { return strconv.Itoa(element.(CoreStats).Index) }

Expect(actual).To(MatchAllFields(Fields{
	"Name":      Ignore(),
	"StartTime": BeTemporally(">=", time.Now().Add(-100*time.Hour)),
	"CPU": PointTo(MatchAllFields(Fields{
		"Time":                 BeTemporally(">=", time.Now().Add(-time.Hour)),
		"UsageNanoCores":       BeNumerically("~", 1e9, 1e8),
		"UsageCoreNanoSeconds": BeNumerically(">", 1e6),
		"Cores": MatchElements(coreID, IgnoreExtras, Elements{
			"0": MatchAllFields(Fields{
				"Index":                Ignore(),
				"UsageNanoCores":       BeNumerically("<", 1e9),
				"UsageCoreNanoSeconds": BeNumerically(">", 1e5),
			}),
		}),
	})),
	"Memory": PointTo(MatchAllFields(Fields{
		"Time":            BeTemporally(">=", time.Now().Add(-time.Hour)),
		"AvailableBytes":  BeZero(),
		"UsageBytes":      BeNumerically(">", 5e6),
		"WorkingSetBytes": BeNumerically(">", 5e6),
	})),
	"Rootfs": Ignore(),
	"Logs":   Ignore(),
}))

Gotchas

• MatchAllFields/MatchAllElements/MatchAllKeys fail on missing OR extra entries. They demand an exact 1:1 mapping. The instant you want a partial match, switch to MatchFields/MatchElements/MatchKeys and pass IgnoreExtras, IgnoreMissing, or both (IgnoreExtras|IgnoreMissing).
• The id function must produce unique, stable keys. Two elements mapping to the same key collide and fail unless you pass AllowDuplicates. Don't derive keys from data that changes between runs.
• Elements keys are strings. The id function returns a string, so numeric ids must be stringified (e.g. strconv.Itoa(i)), and the keys in your Elements{} literal must match those strings exactly.
• PointTo rejects nil pointers — a nil actual fails the match outright, before the inner matcher ever runs.
• Field/element/key matcher values are ordinary matchers, so compose freely with gomega:matchers and gomega:composing-matchers.