What data types can be used as keys in a map in Go?

Keys in a Go map can be types for which the equality operation == is defined. These include:

• Numeric types: int, int8, int16, int32, int64, uint, uint8, uint16, uintptr, float32, float64, complex64, complex128.
• Boolean type: bool.
• String type: string.
• User-defined types based on the above (e.g., type MyInt int).
• Pointers: *T (for any type T).
• Channels: chan T (for any type T).
• Interfaces: interface{} or a named interface, if its dynamic type satisfies the comparability condition.
• Structs: struct, if all fields are comparable.
• Arrays: [n]T, if the element type T is comparable.

Types that cannot be used as map keys:

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

The reason is that these types do not have a defined == operation. Slices, functions, and maps are not comparable.

Example usage:

// Valid keys
var m1 map[int]string
var m2 map[string]int
var m3 map[*byte]bool
var m4 map[struct{X, Y int}]float64 // Fields are comparable

// Invalid keys
// var m5 map[]int // Slice
// var m6 map[func()]int // Function
// var m7 map[map[int]string]int // Map