Re: [Haskell-beginners] Example needed

15 Jun 2011

      On Jun 14, 2011, at 8:16 PM, David Place wrote:
...
On Jun 14, 2011, at 10:01 PM, Zhi-Qiang Lei wrote:
...
I wonder why haskell do not take foldl' as default foldl if foldl is not used in production often.
It would be quite interesting to grep over the ghc sources to find uses of foldl.
From containers/Data/Tree.hs:
-- takes a sequence (queue) of seeds
-- produces a sequence (reversed queue) of trees of the same length
unfoldForestQ :: Monad m => (b -> m (a, [b])) -> Seq b -> m (Seq (Tree a))
unfoldForestQ f aQ = case viewl aQ of
	EmptyL -> return empty
	a :< aQ' -> do
		(b, as) <- f a
		tQ <- unfoldForestQ f (Prelude.foldl (|>) aQ' as)
		let (tQ', ts) = splitOnto [] as tQ
		return (Node b ts <| tQ')
  where splitOnto :: [a'] -> [b'] -> Seq a' -> (Seq a', [a'])
	splitOnto as [] q = (q, as)
	splitOnto as (_:bs) q = case viewr q of
		q' :> a -> splitOnto (a:as) bs q'
		EmptyR -> error "unfoldForestQ"

haskell2010/Array.hs:
accum                 :: (Ix a) => (b -> c -> b) -> Array a b -> [(a,c)] -> Array a b
accum f               =  foldl (\a (i,v) -> a // [(i,f (a!i) v)])

From pretty/Text/PrettyPrint/HughesPJ.hs:
Version 3.0     28 May 1997
  * Cured massive performance bug.  If you write

        foldl <> empty (map (text.show) [1..10000])

    you get quadratic behaviour with V2.0.  Why?  For just the same
    reason as you get quadratic behaviour with left-associated (++)
    chains.

    This is really bad news.  One thing a pretty-printer abstraction
    should certainly guarantee is insensivity to associativity.  It
    matters: suddenly GHC's compilation times went up by a factor of
    100 when I switched to the new pretty printer.

    I fixed it with a bit of a hack (because I wanted to get GHC back
    on the road).  I added two new constructors to the Doc type, Above
    and Beside:

         <> = Beside
         $$ = Above

    Then, where I need to get to a "TextBeside" or "NilAbove" form I
    "force" the Doc to squeeze out these suspended calls to Beside and
    Above; but in so doing I re-associate. It's quite simple, but I'm
    not satisfied that I've done the best possible job.  I'll send you
    the code if you are interested.

From random/System/Random.hs
{-
 If we cannot unravel the StdGen from a string, create
 one based on the string given.
-}
stdFromString         :: String -> (StdGen, String)
stdFromString s        = (mkStdGen num, rest)
	where (cs, rest) = splitAt 6 s
              num        = foldl (\a x -> x + 3 * a) 1 (map ord cs)

time/Data/Time/Format/Parse.hs -- many instances

Re: [Haskell-beginners] Example needed

Sean Perry