eventually-core 0.4.0

Foundation crate for using Event Sourcing in Rust applications
Documentation 
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//! Contain support for [`Projection`], an optimized read model
//! of an [`Aggregate`] or of a number of `Aggregate`s.
//!
//! More information about projections can be found here:
//! https://eventstore.com/docs/getting-started/projections/index.html
//!
//! [`Projection`]: trait.Projection.html
//! [`Aggregate`]: ../aggregate/trait.Aggregate.html

use futures::future::BoxFuture;

use crate::store::Persisted;

/// A `Projection` is an optimized read model (or materialized view)
/// of an [`Aggregate`] model(s), that can be assembled by left-folding
/// its previous state and a number of ordered, consecutive events.
///
/// The events passed to a `Projection` have been persisted onto
/// an [`EventStore`] first.
///
/// [`Aggregate`]: ../aggregate/trait.Aggregate.html
/// [`EventStore`]: ../store/trait.EventStore.html
pub trait Projection {
    /// Type of the Source id, typically an [`AggregateId`].
    ///
    /// [`AggregateId`]: ../aggregate/type.AggregateId.html
    type SourceId: Eq;

    /// Event to be stored in the `EventStore`, typically an [`Aggregate::Event`].
    ///
    /// [`Aggregate::Event`]: ../aggregate/trait.Aggregate.html#associatedtype.Event
    type Event;

    /// Type of the possible error that might occur when projecting
    /// the next state.
    type Error;

    /// Updates the next value of the `Projection` using the provided event value.
    fn project<'a>(
        &'a mut self,
        event: Persisted<Self::SourceId, Self::Event>,
    ) -> BoxFuture<'a, Result<(), Self::Error>>;
}