Why Can't I Define Constant Maps in Go?

Understanding Constant Restrictions in Go

In Go, the use of constants ensures that values remain immutable throughout the program. This immutability extends to core data structures including maps. When defining a constant map, as seen in the code snippet below, it raises a compiler error:

const (
        paths = &map[string]*map[string]string {
            Smith: {
                "theFather": "John",
            },
        }
        paths["Smith"]["theSon"] = paths["Smith"]["theFather"] + " Junior"
)

Why the Compiler Error?

The compiler error stems from the fundamental nature of constants. Constants are expected to remain unchanged, and attempts to modify them are flagged as errors. Go adheres strictly to this principle, unlike some other languages that allow constant modification.

Limitations of Map Constants

Maps are dynamic data structures in Go, allowing key-value pairs to be added or removed after their creation. This dynamic nature conflicts with the immutable characteristic of constants. Thus, Go does not allow the declaration of constant maps.

Allowed Constant Types

The Go specification defines the following types as valid constants: boolean, rune, integer, floating-point, complex, and string.

Workaround

To utilize a map in a constant context, a workaround is to define the map as a variable instead of a constant. This allows the map's contents to be modified even though it is used within a constant declaration.

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