Dynamic stdout in Haskell

Question

I have simple command line interface with insertion records in DB and now it writes a lot of info to stdout, like this:

...    
record 856/1000: 85%
record 857/1000: 85%
record 858/1000: 85%
...

but I want to have 1 dynamic line with updating current string parameters

 status         |T    | C   | A   | E
 ---------------------------------------
   inserting    |1000 | 857 | 85% | 96

How can I achieve that?

ncurses or some higher-level progressbar library like this will do, I think. — fjarri
– fjarri, Commented Nov 20, 2015 at 1:24
Just print a CR - see this SO question: stackoverflow.com/q/31900566/866915 — ErikR
– ErikR, Commented Nov 20, 2015 at 1:38

Lambda Fairy · Accepted Answer · 2015-11-20 01:37:44Z

11

If it's just one row, you can use \r to rewind the cursor to the beginning of the line.

Here's an example:

import Control.Concurrent
import Control.Monad
import Text.Printf

main :: IO ()
main = do
    forM_ [10, 9 .. 1] $ \seconds -> do
        printf "\rLaunching missiles in %2d..." (seconds :: Int)
        threadDelay $ 1 * 1000 * 1000
    putStrLn "\nBlastoff!"

answered Nov 20, 2015 at 1:37

Lambda Fairy

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drquicksilver · Accepted Answer · 2015-11-20 09:03:18Z

3

Joey Hess's concurrent-output library is designed for progress output like this (and more complex variations).

http://hackage.haskell.org/package/concurrent-output

https://joeyh.name/blog/entry/a_tiling_region_manager_for_the_console/

answered Nov 20, 2015 at 9:03

drquicksilver

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