let addService<'a when 'a : not struct and 'a :> IHostedService> services = services.AddHostedService<'a>()

app {
    add_service <'ServiceType>
}

type AppBuilder internal () =

    [<CustomOperation("service_config")>]
    member _.ServiceConfig(state, config) =
        { state with ServicesConfigs = config::state.ServicesConfigs }
    
    [<CustomOperation("hosted_service")>]
    member this.HostedService<'service> state =
        this.ServiceConfig (state, (fun cfg -> cfg.AddHostedService<'service>()))

let application = AppBuilder()