On 10/31/2011 10:31 PM, Daniel Fischer wrote:
On Monday 31 October 2011, 23:12:03, Hugo Ferreira wrote:
You mean, I have to use something like:
where (correct,_) = Z.cursor z
instead of
where (correct,_) = Z.cursor z
Yes.
I googled the rules and found http://en.wikibooks.org/wiki/Haskell/Indentation. Doesn't seem to be a requirement.
Hence the ";)".
I see. B-)
Been a bugger to hunt down, but I finally found it. The culprit is *drumroll, ba-dum tish*: a one-character typo in Data.List.Zipper. foldlz' isn't. The recursive call is to foldlz, not foldlz'.
Now I am baffled B-(
The package docs state: foldlz' is foldlz with a strict accumulator
Why would/should one not use the strict version here?
Because there's a typo in the implementation (notified the maintainer).
What do you mean when you say "recursive call"?
Let's look at lists first.
The specification in the standard is
foldl f z [] = z foldl f z (x:xs) = foldl f (f z x) xs ^^^^^ recursive call to foldl
Very similar for the list zipper, we have
foldlz :: (b -> Zipper a -> b) -> b -> Zipper a -> b foldlz f x z | endp z = x | otherwise = foldlz f (f x z) (right z) ^^^^^^
And for foldlz', the intention was
foldlz' :: (b -> Zipper a -> b) -> b -> Zipper a -> b foldlz' f x z | endp z = x | otherwise = acc `seq` foldlz' f acc (right z) where acc = f x z
but in the otherwise branch, the ' has been omitted, so instead of recursively calling itself with the new accumulator, it calls foldlz.
Ok, I though the typo was on my side. So you have found a bug. My hat is off to you. Thanks, Hugo F.