Mark Carter wrote:
I'm puzzling out how to get a Bool from am IO Bool. I know I'm not
supposed to, but I don't see any way around my predicament.
The basic setup is: I have an edit box, and a panel. If you click the
LMB on the panel when the edit box is checked, this means you want to
move a graphical object around the panel. If it is unchecked, then
clicking the LMB means you want to add a graphical object.
The relevant bits I've managed to put together so far are:
mainFrame = do -- main application frame
streams <- varCreate []
...
cbEdit <- checkBox p1 [text := "Edit Mode", on command ::= onCbEdit
textlog] -- p1 is the panel, ignore textlog
let isEditing = get cbEdit checked -- returns type IO Bool
windowOnMouse p False {- no motion events -} (onMouse p streams
isEditing)
...
where
onMouse w streams isEditChecked mouse = case mouse of
MouseLeftDown pt mods ->
if isEditChecked then
findStream w streams pt
else
addStream w streams pt
other -> skipCurrentEvent --
unprocessed event: send up the window chain
where
-- define findStream and addStream
The problem is that isEditChecked is of type IO Bool, not Bool. I
presume that I should actually be taking a different (non-imperative)
approach, and I'm wondering if anyone could suggest what that approach
should be? Many apologies for being a clueless n00b.
Well, your onMouse function is acutally in the IO monad, so you can just
use the do notation. You can also get rid of the case, like so:
onMouse w streams isEditChecked (MouseLeftDown pt mods) =
do ec <- isEditChecked
if ec then ... else ...
onMouse _ _ _ _ = skipCurrentEvent
_______________________________________________
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
