What "Accept Interfaces, Return Structs" Means and Why

The principle "Accept Interfaces, Return Structs" means functions should take interfaces as parameters to allow flexible callers but return concrete structs to prevent callers from depending on implementation details. This design keeps the API stable and decouples the caller from the specific type used internally.

// Accept an interface for flexibility
func Process(r Reader) error {
    // ... implementation ...
    return nil
}

// Return a concrete struct to hide implementation
func NewProcessor() *Processor {
    return &Processor{}
}

The "Accept Interfaces, Return Structs" rule means your functions should be flexible about what they receive but specific about what they give back. By accepting an interface, you allow any type that fits the bill to be used, making your code easier to test and reuse. By returning a concrete struct, you ensure users of your code don't accidentally rely on internal details that might change later.