- Haskell-Cafe - Haskell.org

Re: Left fold enumerator - a real pearl overlooked?
by John Lato 06 Mar '09

06 Mar '09

> John A. De Goes schrieb: > >> Elsewhere, laziness can be a real boon, so I don't understand your >> question, "Why have laziness in Haskell at all?" > > As I have written, many libaries process their data lazily (or could be > changed to do so without altering their interface) but their interface > can forbid application to data that is fetched from the outside world. > Say you are used to 'map', 'filter', 'foldr' - you cannot use them on > data fetched by the iteratee/enumerator approach. > Thank you for replying to this; it's good to see what features people would like to make iteratees more useful. Where did you get the idea that you can't use 'map'? IterateeGM is a Monad and Functor, so fmap is available for the resultant value. If you want to map over the elements of the stream, use the 'mapStream' function ('map_stream' in Oleg's writings). I never thought about writing a general iteratee fold because that's all iteratees are anyway; it's simple enough to make a foldl. It should be possible to write foldr for seekable resources by enumerating the stream backwards. That certainly wouldn't work for network I/O, unfortunately. Note that this is essentially the approach taken by lazy bytestrings, for example. There are other elements I'd want to address before adding this, however. There are also complications dealing with nested streams. I have some ideas, but the way forward is far from clear. filter should be fairly simple to implement with peek and drop. I'll look into it. Cheers, John

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Theory about uncurried functions
by Peter Verswyvelen 05 Mar '09

05 Mar '09

Lambda calculus is a nice theory in which every function always has one input and one output. Functions with multiple arguments can be simulated because functions are first class and hence a function can "return" a function. Multiple outputs cannot be done, one must embed these outputs in some data type, e.g. using a tuple, or one must use continuation passing style. Now, does a similar theory exist of functions that always have one input and one output, but these inputs and outputs are *always* tuples? Or maybe this does not make any sense?

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missing support to Data.Binary for uvector package
by Manlio Perillo 05 Mar '09

05 Mar '09

Hi. I'm still having problems with the uvector package. I have an IntMap (UArr xxx) data type, and I want to serialize it to disk, in binary format. I'm using the uvector package from http://patch-tag.com/repo/pumpkin-uvector/home The problem is with missing instance declarations, for Hyperstrict data type and Data.Array.Vector.UArr.UPrim. For the former, the instance is simple, but for the latter I have no idea about what to do. The full error message (GHC 6.8.2): No instance for (uvector-0.2:Data.Array.Vector.UArr.UPrim (Word32 uvector-0.2:Data.Array.Vector.Prim.Hyperstrict.:*: Word8)) arising from a use of `encode' at bin/process-data-1.hs:72:18-36 Possible fix: add an instance declaration for (uvector-0.2:Data.Array.Vector.UArr.UPrim (Word32 uvector-0.2:Data.Array.Vector.Prim.Hyperstrict.:*: Word8)) Thanks Manlio Perillo

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monadic MapReduce
by Manlio Perillo 05 Mar '09

05 Mar '09

Hi. I have a function that do some IO (take a file path, read the file, parse, and return some data), and I would like to parallelize it, so that multiple files can be parsed in parallel. I would like to use the simple mapReduce function, from Real Word Haskell: mapReduce :: Strategy b -- evaluation strategy for mapping -> (a -> b) -- map function -> Strategy c -- evaluation strategy for reduction -> ([b] -> c) -- reduce function -> [a] -- list to map over -> c mapReduce mapStrat mapFunc reduceStrat reduceFunc input = mapResult `pseq` reduceResult where mapResult = parMap mapStrat mapFunc input reduceResult = reduceFunc mapResult `using` reduceStrat Is this possible? Thanks Manlio Perillo I

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Re: [Haskell-cafe] Re: [Haskell] Lazy IO breaks purity
by Svein Ove Aas 05 Mar '09

05 Mar '09

On Thu, Mar 5, 2009 at 2:08 PM, Simon Marlow <marlowsd(a)gmail.com> wrote: > > So the argument is something like: we can think of the result of a call to > unsafeInterleaveIO as having been chosen at the time we called > unsafeInterleaveIO, rather than when its result is actually evaluated. This > is on dodgy ground, IMO: either you admit that the IO monad contains an > Oracle, or you admit it can time-travel. I don't believe in either of > those things :-) > As it turns out, time-travel as a vehicle for computation has a long tradition in the more advanced systems; see http://www.frc.ri.cmu.edu/~hpm/project.archive/general.articles/1991/TempCo… for an example.

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Finding the Convex Hull (Problem 12 of Real World Haskell)
by Rob Crowther 05 Mar '09

05 Mar '09

I wrote a "solution" to this problem, but it appears to return incorrect results. There's a pastebin of the code at http://hpaste.org/fastcgi/hpaste.fcgi/view?id=2121 and a picture of the inputs, outputs, and expected results graphed at http://img510.imageshack.us/img510/9971/resultsg.jpg I'm wondering if this is a flaw in my code, my understanding of the problem, or both. Any ideas on how to track this one down would be very much appreciated. Thank you! -- ヽ(^o^)ノ -rob

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possible memory leak in uvector 0.1.0.3
by Manlio Perillo 05 Mar '09

05 Mar '09

Hi. In the "help optimizing memory usage for a program" I discovered some interesting things: 1) Using lazy ByteStrings to read files it not a good choice, since the garbage collector is not able to proper garbage cleanup. Running with -F1 RTS flag, however, keeps memory usage down. 2) UArr from uvector leaks memory. I'm rather sure about this. In fact the memory usage of my program is 815 MB, when: - I use lazy ByteString with -F1 flag or - if I use strict ByteString Using UArray and strict ByteString, memory usage is 660 MB. In total I have 17770 arrays, so there is a leak of about 9146 bytes per array. In another program, where I do a lot of array concatenations, memory leak is much more evident. Even using -F1 flag, memory usage grows too much. Is this a know bug? How can I verify if it is really a memory leak bug? pumpkin_ (on #haskell.it) suggested me to use a more recent version of uvector package, from http://patch-tag.com/publicrepos/pumpkin-uvector Using this version memory usage is, finally, 643 MB! (and execution if a bit faster, too). The other program, with a lot of array concatenations, still eats a lot of memory... Thanks Manlio Perillo

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help optimizing memory usage for a program
by Manlio Perillo 05 Mar '09

05 Mar '09

Hi. After some work I have managed to implement two simple programs that parse the Netflix Prize data set. For details about the Netflix Prize, there was a post by Kenneth Hoste some time ago. I have cabalized the program, and made available here: http://haskell.mperillo.ath.cx/netflix-0.0.1.tar.gz The process-data-1 program parse the training data set, grouping ratings by movies. The process-data-2 program, instead, groups ratings by users. The structure of the two programs is similar: I create singletons IntMap and then use foldl + union to merge them together. The data structure used is: type Rating = Word32 :*: Word8 -- id, rating type MovieRatings = IntMap (UArr Rating) UArr is from uvector package. The training data set contains about 100 million ratings, so, in theory, the amount of memory required to store pairs of (id, rating) is about 480 MB. The process-data-1 program parse the entire dataset using about 1.4 GB of memory (3x increment). This is strange. The memory required is proportional to the number of ratings. It may be IntMap the culprit, or the garbage collector than does not release the memory to the operating system (or, worse, does not deallocate all used temporary memory). The process-data-2 has much more problems. When I try to parse *only* 500 movie ratings, the memory usage is 471 MB; 780 MB after all data has been fully evaluated (array concatenations). I'm using ghc 6.8.2 on Debian Etch. I would like to do some tests with recent GHC versions, but it may be a problem. Thanks Manlio Perillo

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binary serialization
by FFT 05 Mar '09

05 Mar '09

Is there a way to do binary serialization of Haskell values (in GHC, at least)? If you propose a method, what are its type safety and portability properties?

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ANN: Happstack 0.2 Released
by Matthew Elder 05 Mar '09

05 Mar '09

Happstack 0.2 has been released; it is available on Hackage. A lot of community effort has gone into it! For details, please see this post: http://blog.happstack.com/2009/03/04/happstack-02-released Regards, Matthew Elder -- Need somewhere to put your code? http://patch-tag.com Want to build a webapp? http://happstack.com

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