On Thu, Jul 08, 2010 at 09:30:23PM -0500, Louis Wasserman wrote:
Consider
newtype Foo = Foo Int
lift :: (Int -> a) -> Foo -> a lift f (Foo x) = f x
Now, I'd expect this to compile with -O2 down to something like
lift f = f `cast` (Foo -> a)
but it doesn't.
It seems that GeneralizedNewtypeDeriving assumes that these two things *are* equivalent, and it just directly casts the class dictionary. The implication would be that that GeneralizedNewtypeDeriving gives more efficient instances than you could *possibly* get if you wrote them by hand, which is very sad.
It's true. For more, see this thread: http://www.mail-archive.com/haskell-cafe@haskell.org/msg72300.html Of course that thread is more about how sometimes GND gives you *wrong* code. That's currently being worked on and will hopefully be fixed at some point. But as you point out, it's also worth thinking about the flip side: how to optimize away non-type-class-related functions like your example. -Brent