[Haskell-beginners] A better way?

[Daniel Fischer]

Am Samstag, 21. Februar 2009 23:58 schrieb Daniel Fischer:
> Am Samstag, 21. Februar 2009 23:29 schrieb Thomas Davie:
> > How about this:
> >
> > maxTableColumnWidths :: [[String]] -> [Int]
> > maxTableColumnWidths = map (maximum . map length)
>
> That's not what he needs,
>
> maxTableColumnWidths = map (maximum . map length) . transpose
>
> would be it. But I'm afraid that wouldn't solve his memory problems.
>
> Regarding the memory problems: String is rather a memory hog anyway.
> Keith, have you considered using ByteStrings? That might solve your memory
> problems with a straightforward algorithm.
>
> Also, if the rows can contain many columns, it is wasteful to calculate the
> length of prevMaxValues for every row. You could either have that as a
> parameter, or use a custom zipWith:
>
> zipWithD :: (a -> a -> a) -> [a] -> [a] -> [a]
> zipWithD f (x:xt) (y:yt) = f x y:zipWithD f xt yt
> zipWithD _ [] ys = ys
> zipWithD _ xs [] = xs
>
> Then maxRowFieldWidths would become
>
> maxRowFieldWidths row prev = zipWithD max (map length row) prev
>
> or, pointfree:
>
> maxRowFieldWidths = zipWithD max . map length
>
> seqList can also be written as
>
> seqList = foldr seq False
>
> That would make
>
> maxTableColumnWidths = foldr ((seqList .) . zipWithD max) []

Ouch! Of course not seqList, but an analogous function that returns the list 
itself.

evalList xs
	| seqList xs = undefined
	| otherwise = xs

maxTableColumnWidths = foldr ((evalList .) . zipWithD max) []

>
> I'm not sure if that is strict enough, though.
>
> Also, you might try
>
> maxTCWs = foldr seq [] . foldr (zipWithD max) []

Oops! No good either. I must be too tired :(

maxTCWs = evalList . foldr (zipWithD max) []
>
> , but I expect that to be a bad memory citizen.
>
> > Bob
>
> Cheers,
> Daniel
>