On Sat, Aug 21, 2010 at 1:45 PM, Ian Lynagh <igloo@earth.li> wrote:

On Wed, Aug 18, 2010 at 02:00:39PM +0200, Johan Tibell wrote:
>
> The current API doesn't offer any efficient way to do something simple as
> e.g. summing the values in a map. I suggest we add a foldlWithKey' function:
>
> http://hackage.haskell.org/trac/ghc/ticket/4261

Shouldn't (go z l) be forced too?

It makes sense to me to do so but the core looks worse for some reason:

   foldlWithKey' :: (b -> k -> a -> b) -> b -> Map k a -> b

   foldlWithKey' f z0 m0 = go z0 m0
   where
   go z _ | z `seq` False = undefined
   go z Tip = z
   go z (Bin _ kx x l r) = let z' = go z l

   z'' = f z' kx x

By the way, the problem remains if the unnecessary forcing of z'' is removed or the guard that forces z is removed.

in z' `seq` z'' `seq` go z'' r

{-# INLINE foldlWithKey' #-}