Event Sourcing Example

README.md

The rules of the reservation of a train are the following:

• We cannot reserve seats in a train if it bumps up the occupancy over 70%
• All the reserved seats should be in the same coach (we cannot separate families)
• Preferably, we should avoid bumping the occupancy of a coach over 80%

Train.elm

module Trains exposing (..)

type Command
  = ReserveSeat { count : Int }


handle : Command -> Model -> Result String Event
handle cmd model =
  case cmd of
    ReserveSeats { count } ->
      if model.reservedSeats + count > model.availableSeats then
        Err "No enough available seats"
      else
        SeatsReserved { count = count }


type alias Model =
  { availableSeats : Int
  , reservedSeats : Int
  }


type Event
  = SeatsReserved { count : Int }
  

init : Model
init = { availableSeats = 200, reservedSeats = 0 }

        
apply : Event -> Model -> Model
apply event model =
  case event of
    SeatsReserved { count } ->
      { model | reservedSeats = model.reservedSeats + count }