Because the pattern of having a result inside a promise is so common (e.g. an HTTP request that might fail), Async provides a “Deferred.Result” monad where the type is a Result.t Deferred.t, with the associated infix operators and let-syntax. And if it’s awkward to open that module, then there are also infix bind and map operators >>=? and >>|? exposed in Async itself. So one option would be to implement the equivalent of those functions for Lwt, assuming they don’t exist already.