Add better query supports: projection

[gtrefs]

Problem

Currently a query is a fold over all domain events in an eventstore which may result into some boiler plate code. For example, two queries for different events result into two folds which only differ in the actual type of the event.

val registeredUsers: (List<DomainEvent>) -> List<UserRegistered> = { it.fold(emptyList(),
         { s,d -> when(d){ is UserRegistered -> s + d else -> s }}
)} 
val loggedInUsers: (List<DomainEvent>) -> List<UserLoggedIn> = { it.fold(emptyList(),
        {s, d -> when(d){ is UserLoggedIn -> s + d else -> s}} 
)}

Another problem occurs when we want to compose queries. For example, if we want to get all users which are currently viewing a specific page, we need to first query for all logged in users and then filter those which are on the page. However, composing those queries is quite hard with fold.

A little projection algebra

Let's think about query again. Instead using the fold model we can see it as a function.

• A projection is a function from a list of events to a structure.

typealias Projection<T> = (List<DomainEvent>) -> T

• An empty projection does not project anything

val empty:Projection<List<DomainEvent>> = { it } // identity function