Nil interfaces

Canonical guidance

• an interface is nil only if it holds no dynamic type and no dynamic value
• a typed nil inside an interface is not the same as a nil interface
• be careful returning concrete pointer types as error

Use when

• debugging surprising err != nil
• reviewing interface-heavy APIs
• explaining nil behavior in Go

Avoid

• assuming a nil pointer assigned to an interface makes the interface nil
• returning typed nil errors accidentally

Preferred pattern

if err == nil {
	return nil
}
return err

Anti-pattern

var e *MyError = nil
return e

• if returned as error, the interface now has dynamic type *MyError

Explanation: This anti-pattern is common because a nil pointer looks nil in the debugger, but once wrapped in an interface it can change control flow unexpectedly.

Why

• interface values are represented as type plus value
• nil confusion here is a common correctness and API bug