https://pkg.go.dev/fmt#State

https://pkg.go.dev/fmt#Formatter

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// Format formats the frame according to the fmt.Formatter interface.
//
//    %s    source file
//    %d    source line
//    %n    function name
//    %v    equivalent to %s:%d
//
// Format accepts flags that alter the printing of some verbs, as follows:
//
//    %+s   function name and path of source file relative to the compile time
//          GOPATH separated by \n\t (<funcname>\n\t<path>)
//    %+v   equivalent to %+s:%d
func (f Frame) Format(state fmt.State, verb rune) {
	switch verb {
	case 's':
		switch {
		case state.Flag('+'): // Flag reports whether the flag is present
			io.WriteString(state, f.name()) // state has the Write function -> Writer interface
			io.WriteString(state, "\n\t")
			io.WriteString(state, f.file())
		default:
			io.WriteString(state, path.Base(f.file()))
		}
	case 'd':
		io.WriteString(state, strconv.Itoa(f.line()))
	case 'n':
		io.WriteString(state, funcname(f.name()))
	case 'v':
		f.Format(state, 's')
		io.WriteString(s, ":")
		f.Format(state, 'd')
	}
}