PhantomData 1/4: De-duplicating Same Data & Semantics

The newtype pattern can sometimes come up against the DRY principle, how do we solve this?

pub struct UserId(u64);
impl ChatUser for UserId { /* ... */ }

pub struct PatronId(u64);
impl ChatUser for PatronId { /* ... */ }

pub struct ModeratorId(u64);
impl ChatUser for ModeratorId { /* ... */ }
impl ChatModerator for ModeratorId { /* ... */ }

pub struct AdminId(u64);
impl ChatUser for AdminId { /* ... */ }
impl ChatModerator for AdminId { /* ... */ }
impl ChatAdmin for AdminId { /* ... */ }

// And so on ...
fn main() {}

  • Problem: We want to use the newtype pattern to differentiate permissions, but we’re having to implement the same traits over and over again for the same data.

  • Ask: Assume the details of each implementation here are the same between types, what are ways we can avoid repeating ourselves?

    Expect:

    • Make this an enum, not distinct data types.
    • Bundle the user ID with permission tokens like struct Admin(u64, UserPermission, ModeratorPermission, AdminPermission);
    • Adding a type parameter which encodes permissions.
    • Mentioning PhantomData ahead of schedule (it’s in the title).