Add check for no progress

[adityagupta1089]

No description provided.

[roelvandijk]

Why is this check necessary?
There is already a guard against progressDone progress <= 0:

terminal-progress-bar/lib/src/System/ProgressBar.hs

Line 572 in 1035be4

| progressDone progress <= 0 = altMsg

Setting progressFraction to 1 when progressDone progress <= 0 would prevent the alternative message from being shown in some circumstances.

[adityagupta1089]

@roelvandijk seems about right. I missed that part. But I am not sure why I was getting a divide by zero error: I was working on a project where the total tasks was say 300, but it's depth-first search, so each task can add more tasks, I tried to use updateProgressBar to set progressTodo += n. So in this case progress can move from 0/300 to 0/(300+n) which makes estimatedtotalTime = recip 0 giving an exception. Let me know if there would've been a better way to handle this.

[adityagupta1089]

@roelvandijk This is the MWE that I came up with.

module Main where

import Control.Concurrent
import Control.Monad
import Control.Monad.IO.Class
import Control.Monad.Trans.State
import System.ProgressBar
import System.Random

main :: IO ()
main = evalStateT go 0
  where
    go :: StateT Int IO ()
    go = do
        pb <- liftIO $ newProgressBar pbStyle 10 (Progress 0 300 ())
        forM_ [0 .. 10] $ \i -> do
            when (mod i 3 /= 2) $ do
                liftIO $ threadDelay 1000000 -- some time spent in getting child task count
                (extra, _) <- liftIO $ randomR (0, 10) <$> getStdGen -- children tasks count
                liftIO $ updateProgress pb $ \p ->
                    p { progressTodo = extra + progressTodo p } -- add to work todo
                modify (+ extra) -- store tasks 
            when (mod i 3 == 2) $ do
                v <- get -- get child tasks 
                liftIO $ threadDelay 1000000 -- complete them
                liftIO $ incProgress pb (3 + v) -- update progress for both
                put 0 -- reset pending child tasks count