[Xmonad] fixing warnings with stricter type?

[David Roundy]

Both reasons.

I know the existing code is safe, but in my opinion pattern match warnings
usually (often?) show that there's a better way to do things.  In this
case, having a type for non-empty stacks seems like a good thing, so you
can then write functions that expicitely rely on this property, the type
checker (rather than Catch) will ensure that they are never used unsafely.
Plus, I like the Maybe monad and the pretty things you can do with it.
Stack+modify looks basically like a poor reimplementation of parts of the
Maybe module.

e.g. finding all non-empty stacks (from a list of stacks) would just be a
catMaybes.  It's an easy function that's already written.

David

P.S. A more accurate translation of (modify d f x) would be ((fmap f x)
`mplus` d)... but probably less useful than (Just . maybe d . fmap f).

On Wed, May 23, 2007 at 09:22:38PM +0100, Neil Mitchell wrote:
> Hi David,
> 
> I assume the warnings you are referring to in the subject are
> incomplete pattern matches? Catch has generated a proof that the
> pattern-match warnings are completely spurious. Also note that things
> like "head []" don't generate pattern-match warnings, despite being
> completely _|_. Perhaps {-# OPTIONS_GHC -fnowarn-pattern-match #-} is
> sensible to add, given you know its safe anyway.
> 
> If the reason is to make the code cleaner, then its still a good idea,
> of course.
> 
> Thanks
> 
> Neil
> 
> 
> >type Stack a = Maybe (NonEmptyStack a)
> >data NonEmptyStack a = Node { focus  :: !a        -- focused thing in this 
> >set
> >                            , left   :: [a]       -- clowns to the left
> >                            , right  :: [a] }     -- jokers to the right
> >
> >Then (modify Empty f) could just be (fmap f), and (modify d f) becomes
> >(Just . maybe d . fmap f), which seems cleaner to me.  This also allows
> >nice use of pattern matching in the Maybe monad.
> >
> >I like Maybe.