errors.go

func (e *Error) Error() string {
	w := new(strings.Builder)

	fmt.Fprintf(w, "%d:%d: error: ", e.ErrorToken.Pos.Line, e.ErrorToken.Pos.Column)
	if e.Err != nil {
		fmt.Fprintf(w, "%w: %q", e.Err, e.ErrorToken.Lit)
	} else {
		if len(e.ExpectedTokens) == 1 {
			// if there's only one expectation, don't describe it in list terms.
			fmt.Fprint(w, "expected: ", e.ExpectedTokens[0])
		} else if len(e.ExpectedTokens) == 2 {
			// for two, use a simple "either" expression
			fmt.Fprint(w, "expected either ", e.ExpectedTokens[0], " or ", e.ExpectedTokens[1])
		} else {
			// for a plurality > 2, use the expression "expected one of" and use an oxford comma to
			// conclude the list.
			fmt.Fprintf(w, "expected one of: ")
			expected := make([]string, len(e.ExpectedTokens))
			for idx, tok := range e.ExpectedTokens {
				switch idx {
				case 0:
					// noop
				default:
					if idx == len(expected) - 1 {
						fmt.Fprint(w, ", or ")
					} else {
						fmt.Fprint(w, ", ")
					}
				}
				fmt.Fprint(w, tok)
			}
		}
		fmt.Fprint(w, "; got: ", string(e.ErrorToken.Lit))
	}
	return w.String()
}