isn't this what zippers are for? b On Aug 30, 2013, at 1:04 PM, Clark Gaebel wrote:

I don't think a really smart compiler can make that transformation. It looks like an exponential-time algorithm would be required, but I can't prove that.

GHC definitely won't...

For this specific example, though, I'd probably do:

darle :: [a] -> (a, [a]) darle xs = case reverse xs of [] -> error "darle: empty list" (x:xs) -> (x, reverse xs)

- Clark

On Fri, Aug 30, 2013 at 2:18 PM, Lucas Paul wrote: Suppose I need to get an element from a data structure, and also modify the data structure. For example, I might need to get and delete the last element of a list:

darle xs = ((last xs), (rmlast xs)) where rmlast [_] = [] rmlast (y:ys) = y:(rmlast ys)

There are probably other and better ways to write rmlast, but I want to focus on the fact that darle here, for lack of a better name off the top of my head, appears to traverse the list twice. Once to get the element, and once to remove it to produce a new list. This seems bad. Especially for large data structures, I don't want to be traversing twice to do what ought to be one operation. To fix it, I might be tempted to write something like:

darle' [a] = (a, []) darle' (x:xs) = let (a, ys) = darle' xs in (a, (x:ys))

But this version has lost its elegance. It was also kind of harder to come up with, and for more complex data structures (like the binary search tree) the simpler expression is really desirable. Can a really smart compiler transform/optimize the first definition into something that traverses the data structure only once? Can GHC?

- Lucas

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Am 31.08.13 14:35, schrieb Petr Pudlák:

One solution would be to fold over a specific semigroup instead of a recursive function:

|import Data.Semigroup import Data.Foldable(foldMap) import Data.Maybe(maybeToList)

data Darle a =Darle {getInit :: [a],getLast ::a } deriving Show instance Semigroup (Darle a)where ~(Darle xs1 l1) <> ~(Darle xs2 l2) =Darle (xs1 ++ [l1] ++ xs2) l2

darle :: [a] ->Darle a darle = foldr1 (<>) . map (Darle [])|

It's somewhat more verbose, but the core idea is clearly expressed in the one line that defines |<>|, and IMHO it better shows /what/ are we doing rather than /how/. It's sufficiently lazy so that you can do something like |head . getInit $ darle [1..]|.

I am wondering why you put the Semigroup instance there and what the other imports are for. Doesn't this work just as well? data Darle a = Darle {getInit :: [a], getLast :: a} deriving Show ~(Darle xs1 l1) <> ~(Darle xs2 l2) = Darle (xs1 ++ [l1] ++ xs2) l2 darle :: [a] ->Darle a darle = foldr1 (<>) . map (Darle []) Seems to work here. I am still puzzled, though, if this is really a good idea performance-wise. I am afraid I don't understand it well enough. Harald -- Harald Bögeholz (PGP key available from servers) Redaktion c't Tel.: +49 511 5352-300 Fax: +49 511 5352-417 http://www.ct.de/ int f[9814],b,c=9814,g,i;long a=1e4,d,e,h; main(){for(;b=c,c-=14;i=printf("%04d",e+d/a),e=d%a) while(g=--b*2)d=h*b+a*(i?f[b]:a/5),h=d/--g,f[b]=d%g;} (Arndt/Haenel) Affe Apfel Vergaser /* Heise Zeitschriften Verlag GmbH & Co. KG * Karl-Wiechert-Allee 10 * 30625 Hannover * Registergericht: Amtsgericht Hannover HRA 26709 * Persönlich haftende Gesellschafterin: Heise Zeitschriften Verlag * Geschäftsführung GmbH * Registergericht: Amtsgericht Hannover, HRB 60405 * Geschäftsführer: Ansgar Heise, Dr. Alfons Schräder */