Golang游览Switch评估顺序：time.Now（）。Weekday（）+ 2产生运行时错误：索引超出范围

I am learning Golang, was going through the tour where I found a tutorial on Switch evaluation order. I modified it a bit (e.g. Saturday to Sunday), just to play around. It printed Too far away. even for Sunday. So, I modified the code to look like this:

package main

import (
    "fmt"
    "time"
)

func main() {
    day := time.Monday

    fmt.Printf("When's %v?
", day)
    today := time.Now().Weekday()

    switch day {
    case today + 0:
        fmt.Println("Today.")
    case today + 1:
        fmt.Println("Tomorrow.", today + 1)
    case today + 2:
        fmt.Println("In two days.", today + 2)
    default:
        fmt.Println("Too far away.", today + 2)
    }
}

Now, it gives me the output:

When's Monday?
Too far away. %!v(PANIC=runtime error: index out of range)

What can I do to MOD the index, instead of adding it beyond array? Seems to me like some kind of operator overloading. Shouldn't it do MOD, on add operation, by default in case of days, at least?

This is an implementation detail.

In this line

fmt.Println("In two days.", today + 2)

today is of type time.Weekday which has int as its underlying type, 2 is an untyped integer constant, which will be converted to time.Weekday and the addition will be carried out.

The implementation of fmt.Println() will check if values passed to it implement fmt.Stringer, and because time.Weekday does, its String() method will be called whose implementation is:

// String returns the English name of the day ("Sunday", "Monday", ...).
func (d Weekday) String() string { return days[d] }

Where days is an array of 7 elements:

var days = [...]string{
    "Sunday",
    "Monday",
    "Tuesday",
    "Wednesday",
    "Thursday",
    "Friday",
    "Saturday",
  }

There is no range check in Weekday.String() because time.Saturday + 2 for example is not a weekday. Weekday.String() only guarantees to work properly for the constants defined in the time package:

type Weekday int

const (
    Sunday Weekday = iota
    Monday
    Tuesday
    Wednesday
    Thursday
    Friday
    Saturday
)

If you want to make it work, you have to use the remainder after dividing by 7, like this:

switch day {
case (today + 0) % 7:
    fmt.Println("Today.")
case (today + 1) % 7:
    fmt.Println("Tomorrow.", (today+1)%7)
case (today + 2) % 7:
    fmt.Println("In two days.", (today+2)%7)
default:
    fmt.Println("Too far away.", (today+2)%7)
}