Structuring Haskell (gtk2hs) GUI's

Question

I'm trying to build medium sized GUI with Gtk2Hs and I'm not all that sure what would be the best way to structure the system. I'm looking for a way to develop subcomponents in isolation and in general to end up with a structure that doesn't leave me pulling out my hair later.

The main difficulty is caused by components such as cameras for which the API is continuation based (ie. I need to wrap the block using the cameras with withVideoMode :: Camera Undefined -> (Camera a -> IO ()) -> IO ()). I would like to separate these also, but I haven't found a reasonable way to do this.

Most components that I need to add require initialization, such as setting camera parameters or building widgets, catching events that are triggered by other components and cleanup, such as disconnecting hardware, at the end.

Thus far, I've thought of using ContT for the cps parts and something like snaplets for the components and hiding them in some State somewhere. First seems awfully heavyweight and the second seems nasty since I can't elegantly use transformers in gtk2hs callbacks.

(For some reason gists do not work for me today, so apologizes for posting the whole huge code here)

{-#LANGUAGE ScopedTypeVariables#-}
{-#LANGUAGE DataKinds #-}

import CV.CVSU
import CV.CVSU.Rectangle
import CV.Image as CV
import CV.Transforms
import CV.ImageOp 
import CV.Drawing as CV
import CVSU.PixelImage
import CVSU.TemporalForest
import Control.Applicative
import Control.Applicative
import Control.Concurrent
import Control.Monad
import Data.Array.MArray
import Data.IORef
import Data.Maybe
import Data.Word
import Utils.Rectangle
import Foreign.Ptr
import Graphics.UI.Gtk

import System.Camera.Firewire.Simple

convertToPixbuf :: CV.Image RGB D8 -> IO Pixbuf
convertToPixbuf cv = withRawImageData cv $ \stride d -> do
    pixbufNewFromData (castPtr d) ColorspaceRgb False 8 w h stride
   where (w,h) = getSize cv


initializeCamera dc e = do 
    putStrLn $ "Initializing camera "++show e
    cam <- cameraFromID dc e
    setOperationMode cam B
    setISOSpeed  cam ISO_800
    setFrameRate cam Rate_30
    setupCamera cam 20 defaultFlags
    return cam

handleFrame tforest image = do
  pimg    <- toPixelImage (rgbToGray8 image)
  uforest <- temporalForestUpdate tforest pimg
  uimg    <- temporalForestVisualize uforest
  --uimage  <- expectByteRGB =<< fromPixelImage uimg
  temporalForestGetSegments uforest

  --mapM (temporalForestGetSegmentBoundary uforest) ss

createThumbnail img = do 
     pb     <- convertToPixbuf $ unsafeImageTo8Bit $ scaleToSize Linear True (95,95) (unsafeImageTo32F img)
     imageNewFromPixbuf pb


main :: IO ()
main = withDC1394 $ \dc -> do
    -- ** CAMERA Setup **
    cids <- getCameras dc
    cams <- mapM (initializeCamera dc) $ cids

    -- ** Initialize GUI ** 
    initGUI
    pp <- pixbufNew ColorspaceRgb False 8 640 480
    window <- windowNew

    -- * Create the image widgets 
    images <- vBoxNew True 3
    image1  <- imageNewFromPixbuf pp
    image2  <- imageNewFromPixbuf pp
    boxPackStart images image1 PackGrow 0 
    boxPackEnd   images image2 PackGrow 0 

    -- * Create the Control & main widgets
    screen     <- hBoxNew True 3
    control    <- vBoxNew True 3
    info       <- labelNew (Just "This is info")
    but        <- buttonNewWithLabel "Add thumbnail"
    thumbnails <- hBoxNew True 2
    boxPackStart screen images PackGrow 0 
    boxPackStart screen control PackGrow 0 
    boxPackStart control info PackGrow 0 
    boxPackStart control but PackRepel 0 
    boxPackStart control thumbnails PackGrow 0 
    but `onClicked` (do
        info<- labelNew (Just "This is info")
        widgetShowNow info
        boxPackStart thumbnails info PackGrow 0)

    set window [ containerBorderWidth := 10
                   , containerChild := screen ]

    -- ** Start video transmission **
    withVideoMode (cams !! 0) $ \(c :: Camera Mode_640x480_RGB8) -> do
--     withVideoMode (cams !! 1) $ \(c2 :: Camera Mode_640x480_RGB8) -> do
        -- ** Start cameras ** --
        startVideoTransmission c
--        startVideoTransmission c2
        -- ** Setup background subtraction ** --
        Just f <- getFrame c 
        pimg <- toPixelImage (rgbToGray8 f)
        tforest <- temporalForestCreate 16 4 10 130 pimg

        -- * Callback for gtk
        let grabFrame = do
            frame <- getFrame c 
--            frame2 <- getFrame c2 
            maybe (return ()) 
                  (\x -> do
                          ss <- handleFrame tforest x
                          let area = sum [ rArea r | r <- (map segToRect ss)]
                          if area > 10000 
                                then return ()
                                 --putStrLn "Acquiring a thumbnail"
                                 --tn <- createThumbnail x
                                 --boxPackStart thumbnails tn PackGrow 0 
                                 --widgetShowNow tn
                                 --containerResizeChildren thumbnails
                                else return ()
                          labelSetText info ("Area: "++show area)
                          pb <- convertToPixbuf
                                    --  =<< CV.drawLines x (1,0,0) 2 (concat segmentBoundary)
                                    (x <## map (rectOp (1,0,0) 2) (map segToRect ss) )
                          pb2 <- convertToPixbuf x
                          imageSetFromPixbuf image1 pb
                          imageSetFromPixbuf image2 pb2
                          )
                  frame
--            maybe (return ()) 
--                  (convertToPixbuf >=> imageSetFromPixbuf image2)
--                  frame2
            flushBuffer c 
--            flushBuffer c2 
            return True

        timeoutAddFull grabFrame priorityDefaultIdle 20

        -- ** Setup finalizers ** 
        window `onDestroy` do
                    stopVideoTransmission c
                    stopCapture c
                    mainQuit

        -- ** Start GUI **
        widgetShowAll window
        mainGUI

Answer 1

So your requirements are:

• CPS style API
• Resource initialization, and finalization
• probably a monad transformer, for IO
• modularity and composability

it seems like one of the iterator libraries is perfect for you. In particular conduit has the most mature resource finalization, but the theoretical elegance and composability of pipes might interest you as well. If your code is only IO based, then the newly released io-streams would also be a good choice.

pipes: http://hackage.haskell.org/packages/archive/pipes/3.1.0/doc/html/Control-Proxy-Tutorial.html

conduit: https://www.fpcomplete.com/school/pick-of-the-week/conduit-overview

io-streams: http://hackage.haskell.org/packages/archive/io-streams/1.0.1.0/doc/html/System-IO-Streams-Tutorial.html

If you provide a small snippet or description of what you're trying to accomplish, I could try to write it using pipes (the library I'm most familiar with)