F# event-sourcing with monoids

EventSourcingMonoids.fs

type Monoid<'a> = {
    unit : 'a
    op   : 'a -> 'a -> 'a
}

let endo<'a> =
  { unit = id 
    op   = fun (f:'a -> 'a) (g:'a -> 'a) -> f << g }



type Aggregate<'State, 'Input, 'Output> = {   
    zero  : 'State
    apply : 'Output -> 'State -> 'State
    exec  : 'State -> 'Input -> Async<'Output>
}

/// note it is more efficient to just do Seq.fold
let currentState (aggregate:Aggregate<'State, 'Input, 'Output>) : seq<'Output> -> 'State =
  (Seq.foldMap aggregate.apply endo |> flip) aggregate.zero