[Haskell-cafe] Trapped by the Monads

[Neil Mitchell]

Take a look at unsafePerformIO, it is of type IO a -> a. Its not
particularly "safe" (the name gives a clue), but it does what you
want.

On 9/20/05, Mark Carter <mcturra2000 at yahoo.co.uk> 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.
> 
> 
> 
> 
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