Re: Proposal: Add a strict version of foldlWithKey to Data.Map

22 Aug 2010


      On Sun, Aug 22, 2010 at 9:45 AM, Johan Tibell wrote:
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On Sat, Aug 21, 2010 at 1:45 PM, Ian Lynagh  wrote:
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On Wed, Aug 18, 2010 at 02:00:39PM +0200, Johan Tibell wrote:
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The current API doesn't offer any efficient way to do something simple
as
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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' #-}