go-interfaces

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 by check-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: