go-interfaces
Use when defining or implementing Go interfaces, designing abstractions, creating mockable boundaries for testing, or composing types through embedding. Also use when deciding whether to accept an interface or return a concrete type, or using type assertions or type switches, even if the user doesn't explicitly mention interfaces. Does not cover generics-based polymorphism (see go-generics).
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Install
Works with every agent the skills CLI supports.
npx skills add https://github.com/cxuu/golang-skills --skill go-interfaces- 01
Install
Run the command in your project. The CLI asks which agents to install for.
- 02
Describe the task
Ask your agent for something this skill covers. It loads go-interfaces automatically when relevant.
- 03
Review the result
The agent follows the SKILL.md steps, scripts and references bundled with the skill.
SKILL.md
The instructions your agent reads when the skill is triggered.
Go Interfaces and Composition
Resource Routing
scripts/check-interface-compliance.sh- Run as a heuristic to find exported interfaces that may need compile-time assertions.scripts/check-interface-compliance.go- Implementation helper invoked bycheck-interface-compliance.sh; patch this when changing method-set analysis.references/EMBEDDING.md- Read when embedding interfaces or structs in public APIs.references/RECEIVER-TYPE.md- Read when pointer/value receivers affect interface satisfaction.
Accept Interfaces, Return Concrete Types
Interfaces belong in the package that consumes values, not the package that implements them. Return concrete (usually pointer or struct) types from constructors so new methods can be added without refactoring.
// Good: consumer defines the interface it needs
package consumer
type Thinger interface { Thing() bool }
func Foo(t Thinger) string { ... }
// Good: producer returns concrete type
package producer
type Thinger struct{ ... }
func (t Thinger) Thing() bool { ... }
func NewThinger() Thinger { return Thinger{ ... } }
// Bad: producer defines and returns its own interface
package producer
type Thinger interface { Thing() bool }
type defaultThinger struct{ ... }
func NewThinger() Thinger { return defaultThinger{ ... } }
Do not define interfaces before they are used. Without a realistic example of usage, it is too difficult to see whether an interface is even necessary.
Generality: Hide Implementation, Expose Interface
If a type exists only to implement an interface with no exported methods beyond that interface, return the interface from constructors to hide the implementation:
func NewHash() hash.Hash32 {
return &myHash{} // unexported type
}
Benefits: implementation can change without affecting callers, substituting algorithms requires only changing the constructor call.
Type Assertions: Comma-Ok Idiom
Without checking, a failed assertion causes a runtime panic. Always use the comma-ok idiom to test safely:
str, ok := value.(string)
if ok {
fmt.Printf("string value is: %q\n", str)
}
To check if a value implements an interface:
if _, ok := val.(json.Marshaler); ok {
fmt.Printf("value %v implements json.Marshaler\n", val)
}
Type Switch
It's idiomatic to reuse the variable name (t := t.(type)) — the variable has
the correct type in each case branch. When a case lists multiple types
(case int, int64:), the variable has the interface type.
Embedding
Avoid embedding types in public structs — the inner type's full method set becomes part of your public API. Use unexported fields instead.
Interface Satisfaction Checks
Use a blank identifier assignment to verify a type implements an interface at compile time:
var _ json.Marshaler = (*RawMessage)(nil)
This causes a compile error if *RawMessage doesn't implement json.Marshaler.
Use this pattern when:
- There are no static conversions that would verify the interface automatically
- The type must satisfy an interface for correct behavior (e.g., custom JSON marshaling)
- Interface changes should break compilation, not silently degrade
Don't add these checks for every interface — only when no other static conversion would catch the error.
Validation: After defining interfaces or implementations, run
bash scripts/check-interface-compliance.shto verify all concrete types have compile-timevar _ I = (*T)(nil)checks.
Receiver Type
If in doubt, use a pointer receiver. Don't mix receiver types on a single
type — if any method needs a pointer, use pointers for all methods. Use value
receivers only for small, immutable types (Point, time.Time) or basic types.
Quick Reference
| Concept | Pattern | Notes |
|---|---|---|
| Consumer owns interface | Define interfaces where used | Not in the implementing package |
| Safe type assertion | v, ok := x.(Type) | Returns zero value + false |
| Type switch | switch v := x.(type) | Variable has correct type per case |
| Interface embedding | type RW interface { Reader; Writer } | Union of methods |
| Struct embedding | type S struct { *T } | Promotes T's methods |
| Interface check | var _ I = (*T)(nil) | Compile-time verification |
| Generality | Return interface from constructor | Hide implementation |
Related Skills
- Interface naming: See go-naming when naming interfaces (the
-ersuffix convention) or choosing receiver names - Error types: See go-error-handling when implementing the
errorinterface, custom error types, orerrors.Asmatching - Generics vs interfaces: See go-generics when deciding whether generics are needed or an interface already suffices
- Functional options: See go-functional-options when using an interface-based Option pattern for flexible constructors
- Defensive boundaries: See go-defensive when interface assertions are one part of a broader API-boundary hardening pass
FAQ
What does the go-interfaces skill do?
Use when defining or implementing Go interfaces, designing abstractions, creating mockable boundaries for testing, or composing types through embedding. Also use when deciding whether to accept an interface or return a concrete type, or using type assertions or type switches, even if the user doesn't explicitly mention interfaces. Does not cover generics-based polymorphism (see go-generics).
How do I install go-interfaces?
Run npx skills add https://github.com/cxuu/golang-skills --skill go-interfaces in your project. The skills CLI detects the agents you use and copies the skill into the right folder. For Claude Code you can also place the skill folder at ~/.claude/skills/go-interfaces/ (personal) or .claude/skills/go-interfaces/ (project).
Which AI agents can use go-interfaces?
Any agent that supports the open Agent Skills format (a folder with a SKILL.md file), including Claude Code, Codex, Cursor, GitHub Copilot, Gemini CLI, Windsurf, Cline, OpenCode and more.
Is go-interfaces safe to install?
No partner security audit is available yet. Skills can run commands and scripts, so read the SKILL.md and any bundled files before installing.
Who maintains go-interfaces?
It is published by cxuu in cxuu/golang-skills. It has 1,258 recorded installs.
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