func（e * errorString）FormatError（p打印机）（下一个错误）

Question

https://github.com/golang/xerrors/blob/master/errors.go#L29:47

func (e *errorString) FormatError(p Printer) (next error) {
    p.Print(e.s)
    e.frame.Format(p)
    return nil
}

If am not mistaken this always returns nil right ? What is then the purpose of next if it's always nil ?

Answer 1

What is then the purpose of next?

---

The FormatError(p Printer) (next error) method satisfies an interface.

// A Formatter formats error messages.
type Formatter interface {
    error

    // FormatError prints the receiver's first error and returns the next error in
    // the error chain, if any.
    FormatError(p Printer) (next error)
}

Sometimes we do return a non-nil error.

func (e *noWrapError) FormatError(p Printer) (next error) {
    p.Print(e.msg)
    e.frame.Format(p)
    return e.err
}