Part 2 · 1 chapters · ~8 min

Traits and Generics

Defining and implementing traits, generics with trait bounds and where clauses, monomorphisation and static dispatch, trait objects and dynamic dispatch, derive, standard traits (From, Display, Iterator, Default), closures and the Fn traits, iterators, newtypes and making invalid states unrepresentable.

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Traits and newtypes

code
#[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord)]
pub struct Kobo(i64);
impl std::ops::Add for Kobo { type Output = Kobo; fn add(self, o: Kobo) -> Kobo { Kobo(self.0 + o.0) } }
impl std::fmt::Display for Kobo {
    fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result { write!(f, "₦{}.{:02}", self.0 / 100, self.0 % 100) }
}

pub trait Rail { async fn submit(&self, payout: &Payout) -> Result<RailStatus, RailError>; }   // async fn in traits (1.75+)

fn total<I: IntoIterator<Item = Kobo>>(xs: I) -> Kobo { xs.into_iter().fold(Kobo(0), |a, b| a + b) }

// state machine where illegal transitions do not compile: typestate
struct Payout<S> { id: String, state: S }
struct Reserved; struct Submitted; struct Completed;
impl Payout<Reserved> { fn submit(self) -> Payout<Submitted> { Payout { id: self.id, state: Submitted } } }
impl Payout<Submitted> { fn complete(self) -> Payout<Completed> { Payout { id: self.id, state: Completed } } }
// Payout<Reserved>::complete() does not exist: skipping submission is a compile error
TRAITS, GENERICS AND ZERO-COST ABSTRACTION
behaviour without inheritance
traitsShared behaviour: trait Notifier {fn send(&self, to: &str) ->Result<()>; }generics + boundsfn alert<N: Notifier>(n: &N):monomorphised, no runtime cost.trait objects&dyn Notifier / Box<dyn Notifier>:dynamic dispatch through a vtable.derive#[derive(Debug, Clone, PartialEq,Serialize)] generatesimplementations.iteratorsmap, filter, fold compile to thesame loops you would write byhand.newtypesstruct Kobo(i64); distinct typesfor distinct meanings, zeroruntime cost.
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traits
Traits define shared behaviour, like interfaces, but implemented separately from the type (impl Notifier for SmsClient), even for types from other crates within the orphan rules.
behaviour defined separately from dataimpl Trait for Type