Why Can't Constant Maps in Go Be Filled After Declaration?

Constant Maps in Go: Why Can't They Be Filled After Declaration?

In Go, the error "cannot assign to const" can arise when attempting to modify a map after declaring it constant. This behavior stems from the nature of constants and the rules governing their manipulation in the Go language.

Constants are values whose value cannot be altered after initialization. Their primary purpose is to ensure that a particular value remains the same throughout a program's execution. In Go, constants can be of various types, including integers, strings, booleans, and enumeration types. However, maps, unlike these other types, cannot be declared as constants because they are mutable, meaning their key-value pairs can be modified after creation.

The error occurs because the assignment to paths["Smith"]["theSon"] violates the immutability of the map constant. Attempting to modify the map in this way would result in a change to the original constant value, which is forbidden by the Go compiler. Instead, the compiler requires that all constants, including maps, have a fixed value at the point of declaration.

To resolve this issue, one should declare the map as a variable instead of a constant, as illustrated below:

var paths = map[string]*map[string]string{
    Smith: {
        "theFather": "John",
    },
}
paths["Smith"]["theSon"] = paths["Smith"]["theFather"] + " Junior"

By declaring the map as a variable, it becomes mutable and can be modified as desired. It's important to note that, while this workaround allows for the modification of the map, it still requires that all key-value pairs be initialized at the time of declaration.

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