[Haskell-cafe] Trapped by the Monads

[Piyush P Kurur]

On Tue, Sep 20, 2005 at 04:30:25PM +0100, Neil Mitchell wrote:
> 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.
> 
	I dont think you would ever need to do unsafePerformIO unless
you are writing some lib calls or some such thing

> >     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      --

In your case the approach should be some thing along these lines


	onMouse w streams isEditChecked mouse
		= do
			ischecked <- isEditChecked
			case mouse of
				if ischecked then	
					findStream w streams pt
			...

			
	I am assuming that findStream w streams pt is of type  IO a for
some a. otherwise you might have to use something like 

	return $ findStream w streams pt
	
Also the function onMouse will return some IO something. Remember there is
no real reason to use unsafePerformIO unless you are writing  some new 
IO library call.


ppk