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Agent skill

Composing Matchers

by onsi in onsi/gomega

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…

MITAuto-check passedDevelopment

Install Composing Matchers

skills CLI
$ npx skills add onsi/gomega --skill composing-matchers -a claude-code

Project install by default; add -g for ~/.claude/skills/.

GitHub CLI
$ gh skill install onsi/gomega composing-matchers --agent claude-code

Project scope by default; add --scope user for a personal install. Needs GitHub CLI 2.90.0 or later (public preview).

Manual copy
$ git clone --depth 1 https://github.com/onsi/gomega.git skills-src && mkdir -p .claude/skills && cp -r skills-src/plugins/gomega/skills/composing-matchers .claude/skills/composing-matchers && rm -rf skills-src

Use ~/.claude/skills/ instead of .claude/skills for a personal install. The folder must contain SKILL.md.

Claude Code skills documentation · loads skills from .claude/skills/

Facts

Skill name
composing-matchers
GitHub stars
2.4k
Token cost
~1.5k tokens
SKILL.md length
457 words
Files
1
Skills in repo
12
Repo updated
First seen
Licence
MIT

At a glance

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…

  • One Expect needs several requirements at once
  • SKILL.md covers Logical combinators, WithTransform — map the…, Satisfy — an ad-hoc boolean… and HaveValue — dereference before…, plus 4 more sections
  • Instructions only: no scripts, shell commands, URLs or credentials in SKILL.md
  • You want to assert deep into a value without writing a custom matcher

What it does

Composing Matchers is an agent skill from onsi/gomega. 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…

Its SKILL.md is about 1.5k tokens, which your agent loads only when the skill is triggered. It is a single SKILL.md file with no bundled scripts.

It sits in Development. The repository describes itself as: Ginkgo's Preferred Matcher Library. The licence is MIT.

When your agent uses it

  • One Expect needs several requirements at once
  • You want to assert deep into a value without writing a custom matcher

Example prompts

  • “/composing-matchers”

What it can do on your machine

Read from SKILL.md and the folder at commit 37fa900. It shows what the files ask for, not the result of running them.

  • Tool permissions

    Pre-approves nothing: there is no allowed-tools line, so your agent's usual permission prompts apply.

    From allowed-tools in the SKILL.md frontmatter.

  • Runs code

    No scripts in the folder and no shell commands in SKILL.md (its code samples are go).

    From the folder's file list and the shell code blocks in SKILL.md.

  • Network

    Links to these hosts (documentation or services it may open):

    • onsi.github.io

    From URLs in SKILL.md, links to its own repository left out.

  • Credentials

    Names no API keys, tokens, secrets or passwords.

    From names ending in _API_KEY, _TOKEN, _SECRET, _KEY or _PASSWORD in SKILL.md.

Context cost

Composing Matchers loads about 1.5k tokens when it runs. Until then it costs about 150 tokens; SKILL.md has 457 words of instructions outside code blocks.

Always · name and description, kept in context so the agent knows when to use it
~150
When it runs · the whole SKILL.md, loaded when a task matches
~1.5k

Estimates: characters ÷ 4, the usual rule of thumb; real counts depend on the model's tokenizer. Scripts and assets cost tokens only if the agent reads them.

Safety

Auto-check passed

The automated check found no risky patterns in SKILL.md.

Automated static check — not a guarantee. Review scripts before installing. It scans the text of SKILL.md for risky patterns (piping downloads into a shell, reading credential files, hidden Unicode, destructive commands); files beside SKILL.md are not scanned.

SKILL.md

The full file from onsi/gomega at commit 37fa900, republished under its MIT licence (© onsi). 457 words, ~1,523 tokens.

Download SKILL.mdSave it as .claude/skills/composing-matchers/SKILL.md (or your agent's skills folder).
name
composing-matchers
description
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.

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

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

go
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.

go
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:

go
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.

go
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.

go
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(...))).

Show full SKILL.md (195 more words)Show less

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.

go
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.

go
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:

go
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.

© onsi, MIT. Rendered from Markdown: HTML in the file is shown as text, images as links, and headings moved down two levels. Raw file

Files

Just SKILL.md in plugins/gomega/skills/composing-matchers of onsi/gomega.

Open the folder on GitHubat commit 37fa900

Compare with similar skills

Composing Matchers next to the 5 skills that share the most tags, products or categories with it. Stars are the repository's; “used in” counts other GitHub owners with a copy.

Composing Matchers compared with similar skills
SkillStarsUsed inTokensAuto-checkLicenceRepo updated
Composing Matchers this skillonsi/gomega2.4k—~1.5kAutomated safety check: PassMIT
Vercel Composition Patternssupabase/supabase111k59 repos~726Automated safety check: PassMIT
Finishing a Development Branchobra/superpowers296k5 repos~1.9kAutomated safety check: PassMIT
Typescript Advanced Typesrolling-scopes/rsschool-app10k25 repos~4.2kAutomated safety check: PassMPL-2.0
PR Babysitteropeninterpreter/openinterpreter69k3 repos~4.2kAutomated safety check: PassApache-2.0
Code Review ChecklistshareAI-lab/learn-claude-code78k5 repos~1.1kAutomated safety check: PassMIT

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Categories

Questions about Composing Matchers

What does Composing Matchers do?

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…. Composing Matchers is an agent skill from onsi/gomega. 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.

When should I use Composing Matchers?

Composing Matchers fits situations like: one Expect needs several requirements at once; you want to assert deep into a value without writing a custom matcher.

How do I install Composing Matchers in Claude Code?

Run `npx skills add onsi/gomega --skill composing-matchers -a claude-code`. Or copy the skill folder (plugins/gomega/skills/composing-matchers in onsi/gomega) into .claude/skills/composing-matchers in your project. Claude Code loads it when a task matches its description.

How do I install Composing Matchers in Codex?

Run `npx skills add onsi/gomega --skill composing-matchers -a codex`. Or copy the skill folder (plugins/gomega/skills/composing-matchers in onsi/gomega) into .agents/skills/composing-matchers in your project. Codex loads it when a task matches its description.

Can I use Composing Matchers in Cursor, Gemini CLI or GitHub Copilot?

Cursor, Gemini CLI, GitHub Copilot and OpenCode also load SKILL.md folders. With the skills CLI, run `npx skills add onsi/gomega --skill composing-matchers -a cursor` (or -a gemini-cli, github-copilot or opencode for the others). To copy it by hand, put the folder in .cursor/skills/composing-matchers, .gemini/skills/composing-matchers, .github/skills/composing-matchers and .opencode/skills/composing-matchers in your project.

What does Composing Matchers need to run?

SKILL.md names no scripts, command-line tools or credentials: Composing Matchers is instructions for the agent only.

Does Composing Matchers access the network?

SKILL.md names 1 domain. As links in the text: onsi.github.io. This is read from the text; nothing was executed.

Is Composing Matchers safe to install?

Our automated static check of SKILL.md found no risky patterns, such as piping downloads into a shell, reading credential files or hidden Unicode. It is not a guarantee. Review the folder before installing.

What licence does Composing Matchers use?

Composing Matchers is published under the MIT licence (the repository's licence). It allows redistribution, so the full SKILL.md is shown on this page.

How many tokens does Composing Matchers use?

About 1.5k tokens (SKILL.md is roughly 6.1k characters). Agents keep only the skill's name and description in context until a task matches; then they load SKILL.md in full.

What are the alternatives to Composing Matchers?

Skills that share tags, products or a category with Composing Matchers: Vercel Composition Patterns (supabase/supabase, 111k stars), Finishing a Development Branch (obra/superpowers, 296k stars), Typescript Advanced Types (rolling-scopes/rsschool-app, 10k stars) and PR Babysitter (openinterpreter/openinterpreter, 69k stars). The comparison table on this page puts their stars, adoption, token cost, safety result and licence side by side.

Who maintains Composing Matchers?

onsi (a GitHub user) maintains it in onsi/gomega, which has 2,357 GitHub stars. The repository holds 12 skills in this directory. The repository was last updated on September 24, 2026.

Source: onsi/gomega on GitHub. Facts on this page come from the repository at the commit we read; the author's words are quoted as theirs.