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composing-matchers

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Build compound Gomega assertions by combining matchers — And/SatisfyAll (all pass), Or/SatisfyAny (any pass), Not (negate), WithTransform to map the actual before matching, Satisfy for an ad-hoc predicate, HaveValue to dereference pointers/interfaces, HaveField for struct fields and method results, HaveEach for every element, plus the matchers-as-arguments idiom that lets you nest matchers inside ContainElement/ConsistOf/HaveKeyWithValue/Receive. Use when one Expect needs several requirements at once, or you want to assert deep into a value without writing a custom matcher.

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Composing and transforming matchers

Gomega matchers are values, so you can wrap and nest them to express several requirements in a single Expect/Eventually. Start with gomega:matchers for the catalog; reach here when one assertion needs to do more. Docs: https://onsi.github.io/gomega/#composing-matchers.

Logical combinators

Expect(n).To(And(BeNumerically(">", 0), BeNumerically("<", 10)))   // all must pass
Expect(msg).To(Or(Equal("Success"), MatchRegexp(`^Error .+

  
    
    
    composing-matchers — Agent Skill guide | OpenParable
    
    
  
  
    )))    // any may pass
Expect(s).To(Not(BeEmpty()))                                        // negate
  • And(ms ...GomegaMatcher) / SatisfyAll(...) — identical; succeeds only if every matcher passes. Short-circuits on the first failure.
  • Or(ms ...GomegaMatcher) / SatisfyAny(...) — identical; succeeds if any matcher passes. Short-circuits on the first success.
  • Not(matcher GomegaMatcher) — logical negation; usually clearer than the .NotTo(...) form when nested.

These are the building blocks for lightweight named matchers:

func BeBetween(min, max int) GomegaMatcher {
    return SatisfyAll(BeNumerically(">", min), BeNumerically("<", max))
}
Expect(n).To(BeBetween(0, 10))

WithTransform — map the actual, then match

WithTransform(transform any, matcher GomegaMatcher) applies transform to ACTUAL and matches the result.

Expect(element).To(WithTransform(func(e Element) Color { return e.Color }, Equal(BLUE)))

Transform-function rules: it must take exactly one argument (the actual) and return either one value, or (value, error). Returning an error fails the assertion gracefully instead of panicking — useful for transforms that accept several input types:

func HaveSprocketName(name string) GomegaMatcher {
    return WithTransform(func(actual any) (string, error) {
        switch s := actual.(type) {
        case *Sprocket: return s.Name, nil
        case Sprocket:  return s.Name, nil
        default:        return "", fmt.Errorf("expected a Sprocket, got %T", actual)
        }
    }, Equal(name))
}

This is the standard pattern for distilling a WithTransform into a reusable named matcher.

Satisfy — an ad-hoc boolean predicate

Satisfy(predicate any) passes when predicate(ACTUAL) returns true. The predicate takes one argument and returns one bool.

Expect(n).To(Satisfy(func(i int) bool { return i%2 == 0 }))

Reach for Satisfy for a one-off check; WithTransform when you want the inner matcher's richer failure message. For anything reused, write a real matcher → gomega:custom-matchers.

HaveValue — dereference before matching

HaveValue(matcher GomegaMatcher) dereferences ACTUAL through pointers/interfaces (up to 31 levels) and applies matcher to the underlying value, so the same matcher works whether the actual is a value or a pointer. Fails on nil.

i := 42
Expect(&i).To(HaveValue(Equal(42)))
Expect(i).To(HaveValue(Equal(42)))

Not(HaveValue(...)) does not suppress the nil error. To accept nil while rejecting a specific value, use Or(BeNil(), Not(HaveValue(...))).

HaveField — assert on a struct field or method result

HaveField(field string, value any) extracts a field (or method result) from a struct and matches it. If value is a matcher it is used directly; otherwise it is wrapped in Equal.

Expect(book).To(HaveField("Title", "Les Miserables"))
Expect(book).To(HaveField("Title", ContainSubstring("Les Mis")))   // value can be a matcher
Expect(book).To(HaveField("Author.Name", "Victor Hugo"))           // nested with "."
Expect(book).To(HaveField("Author.DOB.Year()", BeNumerically("<", 1900))) // "()" calls a method

The () suffix invokes a method that takes no arguments and returns exactly one value. A missing field is an error, not a failed match. To guard, pair with HaveExistingField(field): And(HaveExistingField("X"), HaveField("X", VALUE)) — handy when reusing HaveField as a ContainElement filter.

HaveEach — every element must match

HaveEach(element any) passes when every element of an array, slice, map (values), or iter.Seq/iter.Seq2 matches. A plain value is wrapped in Equal; pass a matcher for richer checks.

Expect([]string{"Foo", "FooBar"}).To(HaveEach(ContainSubstring("Foo")))

An empty (or nil) collection is an error — it is ambiguous whether "each" holds. If empty is acceptable, use Or(BeEmpty(), HaveEach(...)).

The big idea: matchers as arguments

Nearly every collection/channel matcher accepts matchers where it accepts values, so you compose by nesting rather than learning new operators:

Expect(xs).To(ContainElement(HaveField("Name", "gomega")))
Expect(xs).To(ContainElement(HaveField("Name", BeKeyOf(names)), &found)) // matcher + capture pointer
Expect(books).To(ConsistOf(
    HaveField("Author.Name", "Victor Hugo"),
    And(HaveField("Pages", BeNumerically(">", 100)), HaveField("InPrint", BeTrue())),
))
Expect(m).To(HaveKeyWithValue("user", HaveField("Active", BeTrue())))
Eventually(ch).Should(Receive(HaveField("Kind", "sesame")))

This composes cleanly with Eventually/Consistently too → gomega:async.

When to switch to gstruct

For deep, partial matching across nested structs, slices, and maps, hand-rolled HaveField/And/HaveEach trees get noisy. gstruct's MatchFields/MatchAllElements/MatchKeys are purpose-built for that shape → gomega:gstruct.

))) // any may pass\nExpect(s).To(Not(BeEmpty())) // negate\n```\n\n- `And(ms ...GomegaMatcher)` / `SatisfyAll(...)` — identical; succeeds only if every matcher passes. Short-circuits on the first failure.\n- `Or(ms ...GomegaMatcher)` / `SatisfyAny(...)` — identical; succeeds if any matcher passes. Short-circuits on the first success.\n- `Not(matcher GomegaMatcher)` — logical negation; usually clearer than the `.NotTo(...)` form when nested.\n\nThese are the building blocks for lightweight named matchers:\n\n```go\nfunc BeBetween(min, max int) GomegaMatcher {\n return SatisfyAll(BeNumerically(\">\", min), BeNumerically(\"\u003c\", max))\n}\nExpect(n).To(BeBetween(0, 10))\n```\n\n## WithTransform — map the actual, then match\n\n`WithTransform(transform any, matcher GomegaMatcher)` applies `transform` to `ACTUAL` and matches the result.\n\n```go\nExpect(element).To(WithTransform(func(e Element) Color { return e.Color }, Equal(BLUE)))\n```\n\n**Transform-function rules:** it must take exactly one argument (the actual) and return either one value, or `(value, error)`. Returning an error fails the assertion gracefully instead of panicking — useful for transforms that accept several input types:\n\n```go\nfunc HaveSprocketName(name string) GomegaMatcher {\n return WithTransform(func(actual any) (string, error) {\n switch s := actual.(type) {\n case *Sprocket: return s.Name, nil\n case Sprocket: return s.Name, nil\n default: return \"\", fmt.Errorf(\"expected a Sprocket, got %T\", actual)\n }\n }, Equal(name))\n}\n```\n\nThis is the standard pattern for distilling a `WithTransform` into a reusable named matcher.\n\n## Satisfy — an ad-hoc boolean predicate\n\n`Satisfy(predicate any)` passes when `predicate(ACTUAL)` returns `true`. The predicate takes one argument and returns one `bool`.\n\n```go\nExpect(n).To(Satisfy(func(i int) bool { return i%2 == 0 }))\n```\n\nReach for `Satisfy` for a one-off check; `WithTransform` when you want the inner matcher's richer failure message. For anything reused, write a real matcher → `gomega:custom-matchers`.\n\n## HaveValue — dereference before matching\n\n`HaveValue(matcher GomegaMatcher)` dereferences `ACTUAL` through pointers/interfaces (up to 31 levels) and applies `matcher` to the underlying value, so the same matcher works whether the actual is a value or a pointer. Fails on `nil`.\n\n```go\ni := 42\nExpect(&i).To(HaveValue(Equal(42)))\nExpect(i).To(HaveValue(Equal(42)))\n```\n\n**`Not(HaveValue(...))` does not suppress the nil error.** To accept `nil` while rejecting a specific value, use `Or(BeNil(), Not(HaveValue(...)))`.\n\n## HaveField — assert on a struct field or method result\n\n`HaveField(field string, value any)` extracts a field (or method result) from a struct and matches it. If `value` is a matcher it is used directly; otherwise it is wrapped in `Equal`.\n\n```go\nExpect(book).To(HaveField(\"Title\", \"Les Miserables\"))\nExpect(book).To(HaveField(\"Title\", ContainSubstring(\"Les Mis\"))) // value can be a matcher\nExpect(book).To(HaveField(\"Author.Name\", \"Victor Hugo\")) // nested with \".\"\nExpect(book).To(HaveField(\"Author.DOB.Year()\", BeNumerically(\"\u003c\", 1900))) // \"()\" calls a method\n```\n\nThe `()` suffix invokes a method that takes no arguments and returns exactly one value. **A missing field is an error, not a failed match.** To guard, pair with `HaveExistingField(field)`: `And(HaveExistingField(\"X\"), HaveField(\"X\", VALUE))` — handy when reusing `HaveField` as a `ContainElement` filter.\n\n## HaveEach — every element must match\n\n`HaveEach(element any)` passes when every element of an array, slice, map (values), or `iter.Seq`/`iter.Seq2` matches. A plain value is wrapped in `Equal`; pass a matcher for richer checks.\n\n```go\nExpect([]string{\"Foo\", \"FooBar\"}).To(HaveEach(ContainSubstring(\"Foo\")))\n```\n\n**An empty (or nil) collection is an error** — it is ambiguous whether \"each\" holds. If empty is acceptable, use `Or(BeEmpty(), HaveEach(...))`.\n\n## The big idea: matchers as arguments\n\nNearly every collection/channel matcher accepts matchers where it accepts values, so you compose by nesting rather than learning new operators:\n\n```go\nExpect(xs).To(ContainElement(HaveField(\"Name\", \"gomega\")))\nExpect(xs).To(ContainElement(HaveField(\"Name\", BeKeyOf(names)), &found)) // matcher + capture pointer\nExpect(books).To(ConsistOf(\n HaveField(\"Author.Name\", \"Victor Hugo\"),\n And(HaveField(\"Pages\", BeNumerically(\">\", 100)), HaveField(\"InPrint\", BeTrue())),\n))\nExpect(m).To(HaveKeyWithValue(\"user\", HaveField(\"Active\", BeTrue())))\nEventually(ch).Should(Receive(HaveField(\"Kind\", \"sesame\")))\n```\n\nThis composes cleanly with `Eventually`/`Consistently` too → `gomega:async`.\n\n## When to switch to gstruct\n\nFor deep, partial matching across nested structs, slices, and maps, hand-rolled `HaveField`/`And`/`HaveEach` trees get noisy. `gstruct`'s `MatchFields`/`MatchAllElements`/`MatchKeys` are purpose-built for that shape → `gomega:gstruct`.\n"}],"versionEndpoint":"/skill/api/version"}