Promise chains and interrupts

promise_interrupts.R

library(promises)
library(later)

# Regular version ===========
# Interrupts during execution of a step in the promise chain cause the chain to stop
# without running further steps, the catch, or finally.
# Ideally, the finally would run, but not the others.
{
  p <- promise_resolve(TRUE)
  
  p <- p$then(function(value) {
    message("step 2")
    Sys.sleep(5)
  })
  p <- p$then(function(value) {
    message("done")
  })
  p <- p$catch(function(err) {
    message("Caught error")
  })
  
  p_is_finished <- FALSE
  p <- p$finally(function() {
    message("finally")
    p_is_finished <<- TRUE
  })
  
  while (!p_is_finished) {
    run_now()
  }
  # Press Esc/Ctrl-C when it's in step 2
  # The catch and the finally do not execute
}


# trycatch version ===========
# This is a modified version of the previous example, with tryCatch.
# The catch and the finally both execute, although we want to have only
# the finally execute.
{
  p <- promise_resolve(TRUE)
  
  p <- p$then(function(value) {
    tryCatch({
        message("step 2")
        Sys.sleep(4)
      },
      interrupt = function(e) stop("interrupted")
    )
  })
  p <- p$then(function(value) {
    message("done")
  })
  p <- p$catch(function(err) {
    message("Caught error: ", err)
  })
  
  p_is_finished <- FALSE
  p <- p$finally(function() {
    message("finally")
    p_is_finished <<- TRUE
  })
  
  while (!p_is_finished) {
    run_now()
  }
  # Press Esc/Ctrl-C when it's in step 2
  # The catch and the finally both execute
}



# Interrupts while waiting for a timeout ==============
# In this example, the interrupt happens when the promise code is not executing.
# The done and finally execute, but only after control returns to the console and
# later is running the event loop in the background via R's input handler.
{
  p <- promise(function(resolve, reject) {
    message("step 1")
    later(function() resolve(TRUE), 4)
  })
  
  p <- p$then(function(value) {
    message("done")
  })
  p <- p$catch(function(err) {
    message("Caught error: ", err)
  })
  
  p_is_finished <- FALSE
  p <- p$finally(function() {
    message("finally")
    p_is_finished <<- TRUE
  })
  
  
  while (!p_is_finished) {
    run_now()
  }
  # Press Esc/Ctrl-C when it's in step 2
  # The done and finally will execute after exiting
}

# Note: Wrapping this examples in with_private_loop() would show more clearly that
# the done and finally are executed by the input handler.