How to cancel an Aff

README.md

How to cancel an Aff

The PureScript purescript-aff library provides launchAff, runAff, and forkAff which all return a Canceler that can be passed into cancel to attempt cancelation of the asynchronous process.

Main.purs

module Main where

import Prelude
import Data.Either (either)
import Control.Monad.Aff (Aff, attempt, cancel, forkAff, later', launchAff)
import Control.Monad.Aff.Console (CONSOLE, log, logShow)
import Control.Monad.Eff.Exception (error)

getUpdate :: forall e. Aff (console :: CONSOLE | e) String
getUpdate = do
  later' 10000 do
    pure "Server update"

main = void $ launchAff $ do
  -- Fork the async showFeedback function and
  -- store its canceler.  This function will
  -- run forever until it's canceled
  feedbackProcess <- forkAff $ later' 2000 showFeedback

  -- Aff e (Either Error a)
  update <- attempt $ getUpdate

  -- Attempt to cancel the feedback process, we
  -- have a response from the server.  Cancel will return
  -- true or false indicating if the cancelation was successful.
  -- I'm not sure what the purpose of the second error argument is -
  -- I'm assuming that's a way to say why the cancelation failed?
  cs <- cancel feedbackProcess (error "Could not cancel showFeedback process.")

  case cs of
    true -> log "feedbackProcess canceled"
    false -> log "feedbackProcess could not be canceled."

  log "(delay)"

  later' 5000 do
    either logShow log update


  where
    showFeedback = do
      log "Fetching update, please wait..."
      later' 2000 showFeedback

output.txt

Fetching update, please wait...
Fetching update, please wait...
Fetching update, please wait...
Fetching update, please wait...
feedbackProcess canceled
(delay)
Server update