Accept Interfaces Return Structs

Accept Interfaces, Return Structs

The fundamental Go interface design principle: functions should accept interfaces but return concrete types.

The Pattern

CORRECT - Accept interface, return concrete

type Storage interface {
    Save(data []byte) error
}

func NewProcessor(s Storage) *Processor {
    return &Processor{storage: s}
}

func (p *Processor) Process(input string) (*Result, error) {
    // Returns concrete *Result, accepts Storage interface
    return &Result{Value: input}, nil
}

WRONG - Return interface unnecessarily

func NewProcessor(s *FileStorage) Storage {
    // Locks caller into interface, prevents direct method access
    return &Processor{storage: s}
}

Why This Works

Accepting interfaces:

• Caller controls abstraction
• Easy to mock/test
• Flexible composition

Returning concrete types:

• No hidden behaviors
• All methods visible
• Can add methods without breaking compatibility

When to Deviate

Return interface when:

func NewLogger(env string) io.Writer {
    // Valid: stdlib interface, multiple implementations
    if env == "prod" {
        return &fileLogger{}
    }
    return &consoleLogger{}
}

Return interfaces only when:

• Using stdlib interfaces (io.Writer, io.Reader)
• Multiple implementations chosen at runtime
• Interface already well-established in ecosystem

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