Saturday, November 21, 2009

Workflow: an Haskell package for transparent support of interruptible computations

A month ago i uploaded the package Workflow to hackage, the repository of Haskell libraries. I added an hopefully self suficient documentations and examples.
Here follows a few remarks, a simple example and a more sophsticated example:

The main features are:

  • Transparent state logging trough a monad transformer: step :: m a -> Workflow m a.
  • Resume the computation state after an accidental o planned program shutdown (restartWorkflows).
  • Event handling (waithFor, waitForData).
  • Monitoring of workflows with state change display and other auxiliary features.
  • Communications with other processes including other workflows trough persistent data objects, inspecttion of intermediate workflow results , Queues so that no data is lost due to shutdowns

Here is a complete example:

This is a counter that shows a sequence of numbers, one a second:

module Main where 
import Control.Concurrent(threadDelay) 
import System.IO (hFlush,stdout)  
count n= putStr (show n ++  ) >> hFlush stdout >> threadDelay 1000000 >> count (n+1) 
main= count 0

This is the same program, with the added feature of remembering the last count after interrupted:

module Main where 
import Control.Workflow 
import Control.Concurrent(threadDelay) 
import System.IO (hFlush,stdout)  
mcount n= step $  putStr (show n ++  ) >> hFlush stdout >> threadDelay 1000000 >> mcount (n+1)  
main= do    
   registerType :: IO ()    
   registerType :: IO Int    
   let start= 0 :: Int    
   startWF  count  start   [(count, mcount)] :: IO ()

This is the execution log:

Worflow-0.5.5demos>runghc sequence.hs 
0 1 2 3 4 5 6 7 sequence.hs: win32ConsoleHandler sequence.hs: sequence.hs: interrupted 
Worflow-0.5.5demos>runghc sequence.hs 
7 8 9 10 11 ....
Here is the complete documentation iof the package
And here is a more sophisticated example, at The code is also documented.