Aggregate.fs

[<RequireQualifiedAccess>]
module Aggregate

type Aggregate<'TState, 'TCommand, 'TEvent> = {    
    zero  : 'TState;
    apply : 'TState -> 'TEvent -> 'TState;
    exec  : 'TState -> 'TCommand -> 'TEvent;
}

type Id = System.Guid

/// Creates a persistent command handler for an aggregate give load and commit functions.
let makeHandler (aggregate:Aggregate<'TState, 'TCommand, 'TEvent>) (load:Id -> 'TEvent seq, commit:Id * int -> 'TEvent -> unit) =
    fun (id,version) command ->
        let state = load id |> Seq.fold aggregate.apply aggregate.zero
        let event = aggregate.exec state command
        event |> commit (id,version)