Grade Switch

Go's switch is the same keyword you have seen elsewhere with two of its worst habits removed. Cases do not fall through, so no break is needed and forgetting one cannot cost you an afternoon. And a case can list several values, which turns a stack of near-identical branches into one line.

switch day {
case "sat", "sun":
	return "weekend"
default:
	return "unknown"
}

Leave the expression out and the switch compares against true, so every case is a condition of its own. That is the form you want for ranges:

switch {
case score >= 90:
	return "A"
case score >= 80:
	return "B"
}

Falling through is still possible when you actually want it, with an explicit fallthrough as the last statement in a case. It jumps to the next case's body without testing its condition.

Task

Finish the four functions in main.go.

1. Grade(score int) string maps a percentage to a letter: 90+ is "A", then "B", "C", "D" every ten points, and below 60 is "F". A score outside 0..100 is not a score at all: return "?".
2. DayKind(day string) string answers "weekend" for "sat" and "sun", "weekday" for the other five abbreviations, and "unknown" for anything else, including "Mon" with its capital.
3. Perks(tier string) []string lists a membership tier's perks, most exclusive first. Each tier includes everything below it: gold gets ["lounge", "priority", "points"], silver drops the lounge, bronze is just points. An unrecognised tier gets nothing.
4. Season(month int) string names the meteorological season of a month number, with December, January and February together in "winter", and anything outside 1..12 "unknown".

Hints

• Grade wants the expressionless form, and the out-of-range case has to come first. Put it last and 101 matches score >= 90 on the way past.
• The cases in an expressionless switch are tested top to bottom, exactly like an if/else if chain, so score >= 80 only ever sees scores under 90.
• DayKind and Season want the value form with several values per case. No break anywhere; a Go case ends where the next one begins.
• Perks is the fallthrough exercise. Three cases, each appending its own perk and then falling through to the one below, and no default at all so an unknown tier leaves the slice untouched.
• fallthrough must be the last statement in its case, and it cannot appear in the final case. It ignores the next case's condition entirely, which is why it is rare and why Go makes you write it.
• Appending to a var perks []string that is still nil works and returns a slice of one. A nil slice has length zero and ranges over nothing, so an unknown tier needs no special handling.