wxHaskell - Minimal example demonstrating "concurrent" event occurrences

gistfile1.hs

import Graphics.UI.WX

main = start $ do
    -- create widgets
    f  <- frame [ text := "Example"  ]
    b  <- button f [ text := "Test"  ]
    e  <- entry  f [ text := "1" ]
    status <- staticText f [ size := sz 40 20 ]

    -- populate initial values
    set f [ layout := margin 10 $
        column 10 [ widget e, widget status, widget b]]
    set status [ text := "Ready." ]
    focusOn e

    invariant <- variable [ value := True ]

    let
        onButtonClick = do
            -- disable the text field when the button is clicked
            set invariant [ value := False ]
            set e [ text := "Button clicked", enabled := False ]
            set invariant [ value := True ]
        onLoseFocus   = do
            -- print the invariant when the text fields loses focus
            b <- get invariant value
            set status [ text := "Invariant " ++ if b then "good." else "broken!" ]

    set e [ on focus := \b -> if b then return () else onLoseFocus ]
    set b [ on command := onButtonClick ]

Discussion: http://apfelmus.nfshost.com/blog/2011/10/11-frp-concurrent-events.html