The arrow way combined with

DeferredResult

from Spring would be to have your program in terms of

IO

or

Effect

so you can run it non blocking. Something along the lines of:

import <http://arrow.effects.IO|arrow.effects.IO>

/** transforms IO into a deferred result **/
fun <A> IO<A>.toDeferredResult(): DeferredResult<A> { 
  val deferredResult = new DeferredResult<ResponseEntity<A>>()
  unsafePerformAsync {
     it.fold(deferredResult.setResult, deferredResult.setResult)
  }
  return deferredResult
}

fun getCustomer(customerId: String): DeferredResult<ResponseEntity<CustomerResponse>> =
  IO { getCustomerService().getCustomer(customerId) }
    .attempt()
    .map {
      it.fold({ 
        ResponseEntity(ErrorResponse(), HttpStatus.INTERNAL_SERVER_ERROR)
      }, { 
        ResponseEntity(CustomerResponse(it) , HttpStatus.OK) 
       })
    }.toDeferredResult()