Formatter

Since we can have subject identifiers of different specified types, we need to have a way to display the format name of any given identifier.

Implementation:

• format method > Associated Constants
• Format Constants

format method

Define format method on identifier that says which format it is.

Static String

All the format names defined in the registry are simple strings and enum variants are a sufficient way of knowing the format programmatically (for compiler). For human friendly representation of the format names, string literals can be used.

impl Id {
    fn format(&self) -> &'static str {
        match self {
            Id::Account(_) => "account",
            Id::Email(_) => "email",
        }
    }
}

Global Constants

Use the globally defined constants as format name return value. This is more developer friendly than using string literals (say, for checking equality).

impl Id {
    fn format(&self) -> &'static str {
        match self {
            Id::Account { .. } => ACCOUNT,
            Id::Email { .. } => EMAIL,
        }
    }
}

Associated constants

Each of the constants can be made associated with the subject identifier, thus no cluttering global namespace (module-level).

impl Id {
    const ACCOUNT: &'static str = "account";
    const EMAIL: &'static str = "email";
    fn format(&self) -> &'static str {
        match self {
            Id::Account { .. } => Id::ACCOUNT,
            Id::Email { .. } => Id::EMAIL,
        }
    }
}

Format Constants

For every format name defined in the registry, declare a constant corresponding to it. The format method implementation returns the constants instead of literal strings.

const ACCOUNT: &'static str = "account";
const EMAIL: &'static str = "email";