Go Interfaces and Composition

Available Scripts

• scripts/check-interface-compliance.sh — Finds exported interfaces missing compile-time compliance checks (var _ I = (*T)(nil)). Run bash scripts/check-interface-compliance.sh --help for options.

---

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.

Read references/EMBEDDING.md when using struct embedding for composition, overriding embedded methods, resolving name conflicts, applying the HandlerFunc adapter pattern, or deciding whether to embed in public API types.

---

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.sh to verify all concrete types have compile-time var _ 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.

Read references/RECEIVER-TYPE.md when deciding between pointer and value receivers for a new type, especially for types with sync primitives or large structs.

---

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 -er suffix convention) or choosing receiver names
• Error types: See go-error-handling when implementing the error interface, custom error types, or errors.As matching
• 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
• Compile-time checks: See go-defensive when adding var _ I = (*T)(nil) satisfaction checks at API boundaries