Use traits to keep event methods dry

book/08-proof-of-work.md

@@ -44,7 +44,7 @@ pub mod pow;
 
 use sha2::{Digest, Sha256};
 
-use crate::events::{Event, HashedEvent, OwnedEvent};
+use crate::events::{HasId, HasTags, HashedEvent, OwnedEvent};
 use crate::tags::{Tag, Tags};
 ```
 
@@ -119,31 +119,31 @@ commitment was made.
 
 ### Methods on event types
 
-Both `HashedEvent` and `Event` carry an id and tags, so both get a
-`get_pow` method that delegates to the free function. This lets callers
-write `event.get_pow() >= 20` regardless of whether the event has been
-signed yet.
+Proof of work is the first query that needs the event id as well as its
+tags, so its extension trait is bounded on both `HasId` and `HasTags` from
+the events chapter. The method body delegates to the free function through
+those accessors, and the blanket impl puts `get_pow` on every type that
+satisfies the bound — both `HashedEvent` and `Event` do. Callers write
+`event.get_pow() >= 20` regardless of whether the event has been signed yet,
+as long as the trait is in scope (via `coracle_lib::prelude::*`).
 
 ```rust {file=coracle-lib/src/pow.rs}
-impl HashedEvent {
+/// Proof-of-work queries on any event that has an id and tags.
+pub trait EventExtensionProofOfWork: HasId + HasTags {
     /// Return the validated proof-of-work difficulty of this event.
     ///
     /// The result is the minimum of the actual leading zero bits in the
     /// event id and the difficulty claimed in the nonce tag.
-    pub fn get_pow(&self) -> u8 {
-        get_pow(&self.id, &self.tags)
+    fn get_pow(&self) -> u8 {
+        get_pow(self.id(), self.tags())
     }
 }
 
-impl Event {
-    /// Return the validated proof-of-work difficulty of this event.
-    ///
-    /// The result is the minimum of the actual leading zero bits in the
-    /// event id and the difficulty claimed in the nonce tag.
-    pub fn get_pow(&self) -> u8 {
-        get_pow(&self.id, &self.tags)
-    }
-}
+impl<T: HasId + HasTags> EventExtensionProofOfWork for T {}
+```
+
+```rust {file=coracle-lib/src/prelude.rs}
+pub use crate::pow::EventExtensionProofOfWork;
 ```
 
 ## Mining