Re: [wxhaskell-users] really, really basic newbie question

[Daan L. <da...@cs...>]

Hi James,

James Mitchell wrote:
> In order to better learn Haskell, I've set myself a
> project that happens to include GUI work (I figure if
> I can't have a GUI w/ Haskell, then it's back to C++),

Great!

> so I downloaded wxHaskell, read the example, and coded
> up a little window with a "File" menu.  That worked
> out ok, so I added code to correctly enable & disable
> menu attributes, such a disabling "Close" when there's
> no opened file.  This also worked ok, but has left me
> with an organizational challenge.  I have a bundle of
> lines like this:
> 
> mNew      <- menuItem mFile [text := "&New"]
> 
> ... which isn't a big deal, but then when I want to
> code the actual event handlers, I end up with these
> huge parameter lists, with each of the items in
> question an individual variable.  If this were C or
> C++, I'd have each of these guys as a property of a
> structure or class, and then just pass around the
> structure as a whole.  What's the "right" way to do
> this with wxHaskell?

This is one of the main troubles with wxHaskell: the
library  doesn't offer a standard abstraction mechanism
and all state is passed explicitly. However, there are
some ways to make this less painful. I think that
the Dazzle application uses a big state record.
Something like:

data State = State { mNew :: MenuItem ()
                    , ...
                    , mainWindow :: Window ()
                    , ...
                    }

Each event handler gets the state passed. By using a record,
we can easily change the State while keeping all the code
that doesn't rely on the change the same.

Now, you can write a function "updateMenus :: State -> IO ()"
that just updates all the menus by looking at the "State" -- this
also guarantees that everything is done in one place. (Actually,
I just read wxWidgets has an "wxUpdateUIEvent" especially to
allow this kind of programming in C++ too where all the menu
disabling/enabling is in one spot -- I'll add this event to the next
release of wxHaskell).

I believe that Arjan van IJzendoorn has released a utility
library "PersistentDocument" that manages file open/close
stuff for document oriented applications. I couldn't find it
on the web but maybe you can send him an email and I am sure
he tells you his tricks.

I hope this helps,
-- Daan.



> 
> thanks in advance,
> 
> James
> 
> 
> 
> |-----------------------------------------------|
> |   James Mitchell     | http://www.jimdesu.us  |
> |-----------------------------------------------|
> |When the shoe fits, the foot is forgotten; when|
> |the belt fits, the belly is forgotten; when the|
> |heart is right, "for" and "against" are        |
> |forgotten. -- Chuang Tzu                       |
> |-----------------------------------------------|
> 
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