Source: adaptor.go in package golang.org/x/xerrors


package xerrors

import (
	"bytes"
	"fmt"
	"io"
	"reflect"
	"strconv"
)

FormatError calls the FormatError method of f with an errors.Printer configured according to s and verb, and writes the result to s.

Assuming this function is only called from the Format method, and given that FormatError takes precedence over Format, it cannot be called from any package that supports errors.Formatter. It is therefore safe to disregard that State may be a specific printer implementation and use one of our choice instead.

limitations: does not support printing error as Go struct.


	var (
		sep    = " " // separator before next error
		p      = &state{State: s}
		direct = true
	)

	var err error = f

Note that this switch must match the preference order for ordinary string printing (%#v before %+v, and so on).


	case 'v':
		if s.Flag('#') {
			if stringer, ok := err.(fmt.GoStringer); ok {
				io.WriteString(&p.buf, stringer.GoString())
				goto exit

proceed as if it were %v

		} else if s.Flag('+') {
			p.printDetail = true
			sep = "\n  - "
		}
	case 's':

Use an intermediate buffer in the rare cases that precision, truncation, or one of the alternative verbs (q, x, and X) are specified.

		direct = false

	default:
		p.buf.WriteString("%!")
		p.buf.WriteRune(verb)
		p.buf.WriteByte('(')
		switch {
		case err != nil:
			p.buf.WriteString(reflect.TypeOf(f).String())
		default:
			p.buf.WriteString("<nil>")
		}
		p.buf.WriteByte(')')
		io.Copy(s, &p.buf)
		return
	}

loop:
	for {
		switch v := err.(type) {
		case Formatter:
			err = v.FormatError((*printer)(p))
		case fmt.Formatter:
			v.Format(p, 'v')
			break loop
		default:
			io.WriteString(&p.buf, v.Error())
			break loop
		}
		if err == nil {
			break
		}
		if p.needColon || !p.printDetail {
			p.buf.WriteByte(':')
			p.needColon = false
		}
		p.buf.WriteString(sep)
		p.inDetail = false
		p.needNewline = false
	}

exit:
	width, okW := s.Width()
	prec, okP := s.Precision()

Construct format string from State s.

		format := []byte{'%'}
		if s.Flag('-') {
			format = append(format, '-')
		}
		if s.Flag('+') {
			format = append(format, '+')
		}
		if s.Flag(' ') {
			format = append(format, ' ')
		}
		if okW {
			format = strconv.AppendInt(format, int64(width), 10)
		}
		if okP {
			format = append(format, '.')
			format = strconv.AppendInt(format, int64(prec), 10)
		}
		format = append(format, string(verb)...)
		fmt.Fprintf(s, string(format), p.buf.String())
	} else {
		io.Copy(s, &p.buf)
	}
}

var detailSep = []byte("\n    ")

state tracks error printing state. It implements fmt.State.

type state struct {
	fmt.State
	buf bytes.Buffer

	printDetail bool
	inDetail    bool
	needColon   bool
	needNewline bool
}

func (s *state) Write(b []byte) (n int, err error) {
	if s.printDetail {
		if len(b) == 0 {
			return 0, nil
		}
		if s.inDetail && s.needColon {
			s.needNewline = true
			if b[0] == '\n' {
				b = b[1:]
			}
		}
		k := 0
		for i, c := range b {
			if s.needNewline {
				if s.inDetail && s.needColon {
					s.buf.WriteByte(':')
					s.needColon = false
				}
				s.buf.Write(detailSep)
				s.needNewline = false
			}
			if c == '\n' {
				s.buf.Write(b[k:i])
				k = i + 1
				s.needNewline = true
			}
		}
		s.buf.Write(b[k:])
		if !s.inDetail {
			s.needColon = true
		}
	} else if !s.inDetail {
		s.buf.Write(b)
	}
	return len(b), nil
}

printer wraps a state to implement an xerrors.Printer.

type printer state

func (s *printer) Print(args ...interface{}) {
	if !s.inDetail || s.printDetail {
		fmt.Fprint((*state)(s), args...)
	}
}

func (s *printer) Printf(format string, args ...interface{}) {
	if !s.inDetail || s.printDetail {
		fmt.Fprintf((*state)(s), format, args...)
	}
}

func (s *printer) Detail() bool {
	s.inDetail = true
	return s.printDetail