wxhaskell-users

[wxhaskell-users] Testing to see if a display is available

[Mads <mad...@ya...>]

Hi

I want to develop an application which opens a GUI if there is a display
available. If no display is available I want the application to
gracefully switch to console mode.

This leads me to the following question: How does one test if a display
is available using WxHaskell?

I have tried the catch (IO a -> (IOError -> IO a) -> IO a) function in
the IO monad like:

  main =3D catch someForm ( const $ putStr "No display available\n" )

  someForm =3D start $ do f <- frame []
                        set f [ layout :=3D widget $ label "Foo" ]

but WxHaskell do not call the exception handler function. WxHaskell Just
prints something like:

  Gtk-WARNING**: Cannot open display

I would appreciate any help I can get.

/Mads Lindstr=F8m

Re: [wxhaskell-users] Testing to see if a display is available

[Arjan v. I. <af...@cs...>]

Hello Mads,

> This leads me to the following question: How does one test if a  
> display
> is available using WxHaskell?

I don't know, but you might check the wxWidgets documentation first  
to see whether this framework supports it. If not, there is no chance  
of finding a wxHaskell method that does it. I couldn't find one after  
quickly scanning the available classes.

Alternatively, is there maybe an environment variable that tells you  
whether a display is available?

Cheers, Arjan

Re: [wxhaskell-users] Testing to see if a display is available

[Lennart K. <kol...@dt...>]

Arjan van IJzendoorn wrote:

> Hello Mads,
>
>> This leads me to the following question: How does one test if a  display
>> is available using WxHaskell?
>
>
> I don't know, but you might check the wxWidgets documentation first  
> to see whether this framework supports it. If not, there is no chance  
> of finding a wxHaskell method that does it. I couldn't find one after  
> quickly scanning the available classes.
>
> Alternatively, is there maybe an environment variable that tells you  
> whether a display is available?

Usually the environment variable DISPLAY is set if a display is 
available. This is of course no guaranty that the display can actually 
be opened.

Cheers,
  Lennart

>
> Cheers, Arjan

Re: [wxhaskell-users] Testing to see if a display is available

[Mads <mad...@ya...>]

Hi Arjan and Lennart

> Arjan van IJzendoorn wrote:
>=20
> > Hello Mads,
> >
> >> This leads me to the following question: How does one test if a  dis=
play
> >> is available using WxHaskell?
> >
> >
> > I don't know, but you might check the wxWidgets documentation first =20
> > to see whether this framework supports it. If not, there is no chance=
 =20
> > of finding a wxHaskell method that does it. I couldn't find one after=
 =20
> > quickly scanning the available classes.
> >
> > Alternatively, is there maybe an environment variable that tells you =
=20
> > whether a display is available?
>=20
> Usually the environment variable DISPLAY is set if a display is=20
> available. This is of course no guaranty that the display can actually=20
> be opened.

I created the following function:

-- WARNING: This function has only been tested on Linux (Debian).
displayAvailable :: IO Bool
displayAvailable =3D do isWindows <- isSet "SystemDrive"
                      if isWindows
                         then return True
                         else isSet "DISPLAY" -- We are geussing it's Lin=
ux.
                             =20
    where
    isSet environmentVariable =3D catch (do getEnv environmentVariable
                                          return True
                                      ) (\_ -> return False)

However, it is not really cross-platform. It does work on Linux and
maybe on MS Windows. It also assumes MS Windows always has a graphical
display (is this true?). However shouldn't a cross platform solution be
part of WxHaskell?

Another problem with the solution is that if the DISPLAY variable is
set, but the user is not allowed to use the display which the DISPLAY
variable refers to, the function will return (IO True) even though it
should not. This can happen after an su-command, if you have not done a
"xhost localhost".

Thank you both for the replies.


/Mads Lindstr=F8m

>=20
> Cheers,
>   Lennart
>=20
> >
> > Cheers, Arjan