What data types can be used as keys?

In Go, keys in a map can only be of types that support the comparison operation (==). This is because, to find a key in a hash table, keys need to be compared for equality.

Types that can be used as keys:

• Primitive types: int, float64, string, bool, etc.
• Pointers
• Structs, if all their fields are comparable
• Arrays, if elements are comparable

Types that cannot be used as keys:

• Slices (slice)
• Maps (map)
• Functions

This is because these types do not support comparison using ==.

Example:

m := make(map[string]int)
m["key"] = 42

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

m2 := make(map[Point]string)
m2[Point{1, 2}] = "A point"

Trying to use a slice or map as a key will result in a compile-time error.