// Is reports whether any error in err's chain matches target. // // The chain consists of err itself followed by the sequence of errors obtained by // repeatedly calling Unwrap. // // An error is considered to match a target if it is equal to that target or if // it implements a method Is(error) bool such that Is(target) returns true. func Is(err, target error) bool { … } // As finds the first error in err's chain that matches target, and if so, sets // target to that error value and returns true. // // The chain consists of err itself followed by the sequence of errors obtained by // repeatedly calling Unwrap. // // An error matches target if the error's concrete value is assignable to the value // pointed to by target, or if the error has a method As(interface{}) bool such that // As(target) returns true. In the latter case, the As method is responsible for // setting target. // // As will panic if target is not a non-nil pointer to either a type that implements // error, or to any interface type. As returns false if err is nil. func As(err error, target interface{ … } // Unwrap returns the result of calling the Unwrap method on err, if err's // type contains an Unwrap method returning error. // Otherwise, Unwrap returns nil. func Unwrap(err error) error { … }