Extending fold/build fusion

[Akio Takano]

Any input on this is appreciated. In particular, I'd like to know: if I
implement the idea as a patch to the base package, is there a chance it is
considered for merge?

-- Takano Akio

On Fri, Jan 3, 2014 at 11:20 PM, Akio Takano <tkn.akio at gmail.com> wrote:

> Hi,
>
> I have been thinking about how foldl' can be turned into a good consumer,
> and I came up with something that I thought would work. So I'd like to ask
> for opinions from the ghc devs: if this idea looks good, if it is a known
> bad idea, if there is a better way to do it, etc.
>
> The main idea is to have an extended version of foldr:
>
> -- | A mapping between @a@ and @b at .
> data Wrap a b = Wrap (a -> b) (b -> a)
>
> foldrW
>   :: (forall e. Wrap (f e) (e -> b -> b))
>   -> (a -> b -> b) -> b -> [a] -> b
> foldrW (Wrap wrap unwrap) f z0 list0 = wrap go list0 z0
>   where
>     go = unwrap $ \list z' -> case list of
>       [] -> z'
>       x:xs -> f x $ wrap go xs z'
>
> This allows the user to apply an arbitrary "worker-wrapper" transformation
> to the loop.
>
> Using this, foldl' can be defined as
>
> newtype Simple b e = Simple { runSimple :: e -> b -> b }
>
> foldl' :: (b -> a -> b) -> b -> [a] -> b
> foldl' f initial xs = foldrW (Wrap wrap unwrap) g id xs initial
>   where
>     wrap (Simple s) e k a = k $ s e a
>     unwrap u = Simple $ \e -> u e id
>     g x next acc = next $! f acc x
>
> The wrap and unwrap functions here ensure that foldl' gets compiled into a
> loop that returns a value of 'b', rather than a function  'b -> b',
> effectively un-CPS-transforming the loop.
>
> I put preliminary code and some more explanation on Github:
>
> https://github.com/takano-akio/ww-fusion
>
> Thank you,
> Takano Akio
>
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