{-# OPTIONS -fglasgow-exts -farrows -fno-monomorphism-restriction #-}

import FG

-- A Widget with three buttons "Inc", "Dec" and "Reset".  "Dec" is
-- disabled when the count is 0.   Does not actually display the count.
-- The output value is the current value of the counter.
counter :: Widget WidgetP Int
counter = proc p -> hbox [] (proc _ -> do
    rec inc <- tag (+1) <<< button [text "Inc"] -< def
        dec <- tag (+(-1)) <<< button [text "Dec"] -< [enabled (c > 0)]
        reset <- tag (const 0) <<< button [text "Reset"] -< def
        cs@(_,c) <- hold 0 -< onEvent (\f -> Just $ f c) Nothing
                                      (inc >< dec >< reset)
    returnA -< cs) -< (p, ())

-- The main FG.  Connects the value of the counter to a Label.
mainFG :: Container () ()
mainFG = vbox [spacing 2] $ proc _ -> do
    (_,c) <- counter -< def
    label [] -< [text $ show c]
    returnA -< ()

main :: IO ()
main = runFG mainFG