Unable to capture some modified key strokes.

[AndrewRademacher]

I have a small demo using the following event loop

event :: State -> Event -> EventM (Next State)
event st e = do
    liftIO $ print e
    continue st

and the following draw function

draw :: State -> [Widget]
draw st = [str "Hello World!"]

There are several cases like Ctrl+,, Ctrl+., and Ctrl+/ that do not fire an event. Additionally there is at least one case of sending the wrong event. For example you would expect Ctrl+m to read...

EvKey (KChar 'm') [MCtrl]

however it actually reads...

EvKey KEnter []

[jtdaugherty]

Unfortunately this behavior depends almost entirely on your terminal emulator of choice and the active termcap settings; to see what I mean you might consider trying different terminal emulators and different modifier key combinations.

In the particular case of C-m and Enter, the value for C-m is equivalent to Enter so there isn't a way to distinguish. This is also true of Tab and C-i. So from that perspective, seeing KEnter when C-m is pressed is expected behavior.

As examples of this kind of behavior, these pages might be illuminating:

http://askubuntu.com/questions/441744/pressing-enter-produces-m-instead-of-a-newline
http://stackoverflow.com/questions/598113/can-terminals-detect-shift-enter-or-control-enter

Sadly, terminal application programmers typically need to select control-key combinations that are likely to work on a wide variety of terminal emulators to avoid this problem.

[AndrewRademacher]

Sad indeed. Thanks for the heads up.

[hramrach]

Thanks for looking into this.

Regarding the key mapping - aren't key mappings in some database so applications know what to expect on which terminal?

If not how is anything ever supposed to work on a terminal?

Or is that why vi uses key sequences rather than modifiers most of the time?

Because the modifiers break?

[jtdaugherty]

@hramrach - yes, termcap is the database you mentioned. Still, that is not necessarily enough to make this work for all emulators and all modifier key combinations. It might be something that @coreyoconnor could address by making changes to vty (which brick uses to handle keyboard input events), though.