Why Can't You Have Constant Maps in Go?

Understanding Constant Maps in Go

In Go, constant variables are immutable values that can be initialized at compile-time. However, unlike other data types, Go does not allow constant maps.

Why Const Maps Are Not Allowed

According to the Go language specification, only certain data types can be declared as constants: runes, integers, floating-point numbers, imaginary numbers, strings, and constants identifiers. Arrays, slices, and maps do not fall under these permitted types.

Underlying Reason

Constant values in Go require a definitive representation during compilation. Maps, being dynamic and mutable data collections, cannot guarantee this immutable property at compile-time. The elements within a map can change, which would violate the principle of constants.

Alternative Approaches

While constant maps are not directly supported, there are alternative ways to achieve similar functionality:

• Use iota-based Enums: Enumerations provide a way to represent constant values with predefined identifiers.
• Define Constants-Only Functions: Create a function that returns the desired key-value pairs. For example:

const (
    One   = 1
    Two   = 2
    Three = 3
)

func ConstantsMap() map[int]string {
    return map[int]string{
        One:   "ONE",
        Two:   "TWO",
        Three: "THREE",
    }
}

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