What keys can a map in Go have?

In Go, map keys can only be of types that support the comparison operation (operator ==). This means that keys must be comparable.

Suitable key types:

• Basic types: bool, int, float, string, etc.
• Pointers
• Structs, if all their fields are comparable
• Interfaces, if the concrete values inside are comparable

Keys that cannot be used:

• Slices
• Maps
• Functions

Example:

m := make(map[string]int) // string is a valid key

// Struct with comparable fields
 type Point struct {
     X, Y int
 }

m2 := make(map[Point]string)

// Cannot use a slice
// m3 := make(map[[]int]string) // compilation error

This is because keys must have a defined and stable behavior when compared for the hash table to work correctly.