How to Prevent Custom Types from Being Used as Map Keys in Go?

Map Key Restriction for Custom Types

Certain types are inherently unsuitable as map keys, such as maps, slices, and functions. However, some developers may wish to explicitly prevent a specific custom type from being used as a map key.

Initially, relying on private members within the type may seem like a solution, but this approach is ineffective. To address this issue, we need a more fundamental solution that adheres to the language specification.

The key to prohibiting the use of a type as a map key lies in violating the requirements for comparison operators. According to the specification, map key types must have fully defined comparison operators == and !=. Therefore, by including a field with a non-comparable type, we can effectively make the entire type non-comparable.

For instance, consider the following type:

type MyType struct {
    A *A
    b b
    notComparable []int
}

Adding the field notComparable with a slice type renders the entire MyType non-comparable, preventing its use as a map key. This is because slices cannot be compared.

m := map[MyType]int{} // Compile-time error: invalid map key type MyType

It's worth noting that introducing a non-comparable field may have further implications. For example, it will make it impossible to compare values of the type using operators like == and !=. However, by carefully designing the type, it's possible to retain comparability for specific fields while still preventing the type from being used as a map key.

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