"evaluation" of promises for debugging purposes

I'm just making a first attempts using promises in shiny.
How do I force the evaluation of a promises when debugging? Something similar to value(future(1))?

i.e When debugging a snipped like the following. Let's say I'm interested in the output of the "head(inpiu$n)" step. How would I do that?

future({ expensive_operation() }) %...>%
      ... %...>%
      head(input$n) %...>%

I could use value() on the future and replace all the %...>% by %>% to get the desired result. But this seems like a lot of work for debugging and I guess there's a much simpler way?

Sorry for pushing. Coming to such situations all the time when using promies. Any best practise advice for debugging them?