Monad transformer for progress tracking - haskell

Question

I am looking for a monad transformer that can be used to track the progress of a procedure. To explain how it would be used, consider the following code:
procedure :: ProgressT IO ()
procedure = task "Print some lines" 3 $ do
liftIO $ putStrLn "line1"
step
task "Print a complicated line" 2 $ do
liftIO $ putStr "li"
step
liftIO $ putStrLn "ne2"
step
liftIO $ putStrLn "line3"
-- Wraps an action in a task
task :: Monad m
=> String -- Name of task
-> Int -- Number of steps to complete task
-> ProgressT m a -- Action performing the task
-> ProgressT m a
-- Marks one step of the current task as completed
step :: Monad m => ProgressT m ()
I realize that step has to exist explicitly because of the monadic laws, and that task has to have an explicit step number parameter because of program determinism/the halting problem.
The monad as described above could, as I see it, be implemented in one of two ways:
Via a function that would return the current task name/step index stack, and a continuation in the procedure at the point that it left off. Calling this function repeatedly on the returned continuation would complete the execution of the procedure.
Via a function that took an action describing what to do when a task step has been completed. The procedure would run uncontrollably until it completed, "notifying" the environment about changes via the provided action.
For solution (1), I have looked at Control.Monad.Coroutine with the Yield suspension functor. For solution (2), I don't know of any already available monad transformers that would be useful.
The solution I'm looking for should not have too much performance overhead and allow as much control over the procedure as possible (e.g. not require IO access or something).
Do one of these solutions sound viable, or are there other solutions to this problem somewhere already? Has this problem already been solved with a monad transformer that I've been unable to find?
EDIT: The goal isn't to check whether all the steps have been performed. The goal is to be able to "monitor" the process while it is running, so that one can tell how much of it has been completed.

Answer 1

This is my pessimistic solution to this problem. It uses Coroutines to suspend the computation on each step, which lets the user perform an arbitrary computation to report some progress.
EDIT: The full implementation of this solution can be found here.
Can this solution be improved?
First, how it is used:
-- The procedure that we want to run.
procedure :: ProgressT IO ()
procedure = task "Print some lines" 3 $ do
liftIO $ putStrLn "--> line 1"
step
task "Print a set of lines" 2 $ do
liftIO $ putStrLn "--> line 2.1"
step
liftIO $ putStrLn "--> line 2.2"
step
liftIO $ putStrLn "--> line 3"
main :: IO ()
main = runConsole procedure
-- A "progress reporter" that simply prints the task stack on each step
-- Note that the monad used for reporting, and the monad used in the procedure,
-- can be different.
runConsole :: ProgressT IO a -> IO a
runConsole proc = do
result <- runProgress proc
case result of
-- We stopped at a step:
Left (cont, stack) -> do
print stack -- Print the stack
runConsole cont -- Continue the procedure
-- We are done with the computation:
Right a -> return a
The above program outputs:
--> line 1
[Print some lines (1/3)]
--> line 2.1
[Print a set of lines (1/2),Print some lines (1/3)]
--> line 2.2
[Print a set of lines (2/2),Print some lines (1/3)]
[Print some lines (2/3)]
--> line 3
[Print some lines (3/3)]
The actual implementation (See this for a commented version):
type Progress l = ProgressT l Identity
runProgress :: Progress l a
-> Either (Progress l a, TaskStack l) a
runProgress = runIdentity . runProgressT
newtype ProgressT l m a =
ProgressT
{
procedure ::
Coroutine
(Yield (TaskStack l))
(StateT (TaskStack l) m) a
}
instance MonadTrans (ProgressT l) where
lift = ProgressT . lift . lift
instance Monad m => Monad (ProgressT l m) where
return = ProgressT . return
p >>= f = ProgressT (procedure p >>= procedure . f)
instance MonadIO m => MonadIO (ProgressT l m) where
liftIO = lift . liftIO
runProgressT :: Monad m
=> ProgressT l m a
-> m (Either (ProgressT l m a, TaskStack l) a)
runProgressT action = do
result <- evalStateT (resume . procedure $ action) []
return $ case result of
Left (Yield stack cont) -> Left (ProgressT cont, stack)
Right a -> Right a
type TaskStack l = [Task l]
data Task l =
Task
{ taskLabel :: l
, taskTotalSteps :: Word
, taskStep :: Word
} deriving (Show, Eq)
task :: Monad m
=> l
-> Word
-> ProgressT l m a
-> ProgressT l m a
task label steps action = ProgressT $ do
-- Add the task to the task stack
lift . modify $ pushTask newTask
-- Perform the procedure for the task
result <- procedure action
-- Insert an implicit step at the end of the task
procedure step
-- The task is completed, and is removed
lift . modify $ popTask
return result
where
newTask = Task label steps 0
pushTask = (:)
popTask = tail
step :: Monad m => ProgressT l m ()
step = ProgressT $ do
(current : tasks) <- lift get
let currentStep = taskStep current
nextStep = currentStep + 1
updatedTask = current { taskStep = nextStep }
updatedTasks = updatedTask : tasks
when (currentStep > taskTotalSteps current) $
fail "The task has already completed"
yield updatedTasks
lift . put $ updatedTasks

Answer 2

The most obvious way to do this is with StateT.
import Control.Monad.State
type ProgressT m a = StateT Int m a
step :: Monad m => ProgressT m ()
step = modify (subtract 1)
I'm not sure what you want the semantics of task to be, however...
edit to show how you'd do this with IO
step :: (Monad m, MonadIO m) => ProgressT m ()
step = do
modify (subtract 1)
s <- get
liftIO $ putStrLn $ "steps remaining: " ++ show s
Note that you'll need the MonadIO constraint to print the state. You can have a different sort of constraint if you need a different effect with the state (i.e. throw an exception if the number of steps goes below zero, or whatever).

Answer 3

Not sure if this is exactly what you want, but here is an implementation that enforces the correct number of steps and requires there to be zero steps left at the end. For simplicity, I'm using a monad instead of a monad transformer over IO. Note that I am not using the Prelude monad to do what I'm doing.
UPDATE:
Now can extract the number of remaining steps. Run the following with -XRebindableSyntax
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE FunctionalDependencies #-}
module Test where
import Prelude hiding (Monad(..))
import qualified Prelude as Old (Monad(..))
-----------------------------------------------------------
data Z = Z
data S n = S
type Zero = Z
type One = S Zero
type Two = S One
type Three = S Two
type Four = S Three
-----------------------------------------------------------
class Peano n where
peano :: n
fromPeano :: n -> Integer
instance Peano Z where
peano = Z
fromPeano Z = 0
instance Peano (S Z) where
peano = S
fromPeano S = 1
instance Peano (S n) => Peano (S (S n)) where
peano = S
fromPeano s = n `seq` (n + 1)
where
prev :: S (S n) -> (S n)
prev S = S
n = fromPeano $ prev s
-----------------------------------------------------------
class (Peano s, Peano p) => Succ s p | s -> p where
instance Succ (S Z) Z where
instance Succ (S n) n => Succ (S (S n)) (S n) where
-----------------------------------------------------------
infixl 1 >>=, >>
class ParameterisedMonad m where
return :: a -> m s s a
(>>=) :: m s1 s2 t -> (t -> m s2 s3 a) -> m s1 s3 a
fail :: String -> m s1 s2 a
fail = error
(>>) :: ParameterisedMonad m => m s1 s2 t -> m s2 s3 a -> m s1 s3 a
x >> f = x >>= \_ -> f
-----------------------------------------------------------
newtype PIO p q a = PIO { runPIO :: IO a }
instance ParameterisedMonad PIO where
return = PIO . Old.return
PIO io >>= f = PIO $ (Old.>>=) io $ runPIO . f
-----------------------------------------------------------
data Progress p n a = Progress a
instance ParameterisedMonad Progress where
return = Progress
Progress x >>= f = let Progress y = f x in Progress y
runProgress :: Peano n => n -> Progress n Zero a -> a
runProgress _ (Progress x) = x
runProgress' :: Progress p Zero a -> a
runProgress' (Progress x) = x
task :: Peano n => n -> Progress n n ()
task _ = return ()
task' :: Peano n => Progress n n ()
task' = task peano
step :: Succ s n => Progress s n ()
step = Progress ()
stepsLeft :: Peano s2 => Progress s1 s2 a -> (a -> Integer -> Progress s2 s3 b) -> Progress s1 s3 b
stepsLeft prog f = prog >>= flip f (fromPeano $ getPeano prog)
where
getPeano :: Peano n => Progress s n a -> n
getPeano prog = peano
procedure1 :: Progress Three Zero String
procedure1 = do
task'
step
task (peano :: Two) -- any other Peano is a type error
--step -- uncommenting this is a type error
step -- commenting this is a type error
step
return "hello"
procedure2 :: (Succ two one, Succ one zero) => Progress two zero Integer
procedure2 = do
task'
step `stepsLeft` \_ n -> do
step
return n
main :: IO ()
main = runPIO $ do
PIO $ putStrLn $ runProgress' procedure1
PIO $ print $ runProgress (peano :: Four) $ do
n <- procedure2
n' <- procedure2
return (n, n')