SKILL.md
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