What property should a key have in a Map data structure?

The key in a Go map data structure must be comparable.

Typical comparable types in Go:

• Boolean (bool)
• Numeric types (int, float64, complex128, etc.)
• Strings (string)
• Pointers (*T)
• Channels (chan T)
• Interfaces (interface{}) (if the value inside is comparable)
• Structs (struct) (if all fields are comparable)
• Arrays (array) (if element type is comparable)

Examples of non-comparable types:

• Slices (slice)
• Some interfaces containing non-comparable types
• Functions (func)
• Maps (map)

Example of using a comparable type (string) as a key:

// Creating a map with string keys and int values
m := make(map[string]int)

// Adding elements
m["apple"] = 1
m["banana"] = 2

// Accessing an element
value := m["apple"] // value == 1

Example of attempting to use a non-comparable type (slice) as a key (will cause a compile-time error):

/*
// Compile error: invalid map key type []int
m := make(map[[]int]string)
*/