What types of data cannot be directly compared to each other in programming?

In Go, comparing data directly is not possible if they contain incomparable types. For example, slices ([]), maps (map), functions (func), and structs containing such non-comparable fields cannot be compared using ==.

This is because these types do not have a defined comparison method at the language level, as they may contain pointers, internal structures, or dynamic data.

Example:

var a = []int{1, 2, 3}
var b = []int{1, 2, 3}

// a == b // compilation error: invalid operation: a == b (slice can only be compared to nil)

var m1 = map[string]int{"x": 1}
var m2 = map[string]int{"x": 1}

// m1 == m2 // compilation error: invalid operation: m1 == m2 (map can only be compared to nil)

To compare such types, you usually use special functions like reflect.DeepEqual or write custom comparison functions.

Comparable types include numbers, strings, pointers, interfaces (if they contain comparable values), and arrays (if their elements are comparable).