- Haskell-Cafe - Haskell.org

isFunction
by john lask 04 Jan '08

04 Jan '08

Oleg provided the following code to test whether a is a function, this code will not type check in hugs, due to the overlapping instances in IsFunction (x->y) f IsFunction x f and the functional dependency | a-> b ERROR "isfunction.lhs":43 - Instances are not consistent with dependencies *** This instance : IsFunction a b *** Conflicts with : IsFunction (a -> b) c *** For class : IsFunction a b *** Under dependency : a -> b is there a way to acheive the same effect in hugs ? > data HTrue > data HFalse > > class IsFunction a b | a -> b > instance TypeCast f HTrue => IsFunction (x->y) f > instance TypeCast f HFalse => IsFunction x f > > -- literally lifted from the HList library > class TypeCast a b | a -> b, b->a where typeCast :: a -> b > class TypeCast' t a b | t a -> b, t b -> a where typeCast' :: t->a->b > class TypeCast'' t a b | t a -> b, t b -> a where typeCast'' :: t->a->b > instance TypeCast' () a b => TypeCast a b where typeCast x = typeCast' () x > instance TypeCast'' t a b => TypeCast' t a b where typeCast' = typeCast'' > instance TypeCast'' () a a where typeCast'' _ x = x _________________________________________________________________ New music from the Rogue Traders - listen now! http://ninemsn.com.au/share/redir/adTrack.asp?mode=click&clientID=832&refer…

1 0

Instance classes and error (also, related to Data.Binary.GET)
by bbrown 04 Jan '08

04 Jan '08

I am using the Data.Binary module and having some issues reading big endian files (actually, just reading the file). I can read the header but not the rest of the data which contains a set of row information. Also, go ahead and make fun my style of coding. Anyway, This is the my code and the error at the bottom. The issue stems from here, it says I didn't define an instance, but I did: instance Binary URLSet where put _ = do BinaryPut.putWord8 0 get = do remainingByteData <- BinaryGet.getRemainingLazyByteString i :: URLInfo <- decode remainingByteData j :: TitleInfo <- decode remainingByteData k :: DescrInfo <- decode remainingByteData x :: KeywordsInfo <- decode remainingByteData return (URLSet {urlinfo=i, titleinfo=j, descrinfo=k, keywordsinfo=x}) ----- module Main where import Data.Word import Data.Binary import qualified Data.ByteString.Lazy.Char8 as BSLC8 import Data.ByteString.Lazy (ByteString) import Data.Binary.Get as BinaryGet import Data.Binary.Put as BinaryPut import IO import Text.Printf import System.Environment import Control.Monad (replicateM, liftM) {- ********************************************************* Define the Database Data Types SpiderDatabase represents a singleton wrapper for an entire database. ********************************************************* -} data SpiderDatabase = SpiderDatabase { magicNumberA :: Word16, magicNumberB :: Word16, majorVers :: Word16, minorVers :: Word16, headerTag :: Word16, poolLen :: Word16, spiderpool :: [URLSet] } data URLSet = URLSet { urlinfo :: URLInfo, titleinfo :: TitleInfo, descrinfo :: DescrInfo, keywordsinfo :: KeywordsInfo } data URLInfo = URLInfo { tag :: Word8, urlid :: Word16, urllen :: Word16, url :: ByteString } data TitleInfo = TitleInfo { titletag :: Word8, titlelen :: Word16, title :: ByteString } data DescrInfo = DescrInfo { descrtag :: Word8, descrlen :: Word16, descr :: ByteString } data KeywordsInfo = KeywordsInfo { keywordstag :: Word8, keywordslen :: Word16, keywords :: ByteString } {- ********************************************************* Class instances ********************************************************* -} instance Show SpiderDatabase where show db = let magicb = (magicNumberB db) header = (headerTag db) poolct = (poolLen db) in "<<<Database Content>>>\n" ++ (((printf "Magic: %X %X\n") (magicNumberA db)) (magicNumberB db)) ++ printf "URL Pool Count: %d\n" poolct ++ "<<<End>>>" instance Binary URLInfo where put _ = do BinaryPut.putWord8 0 get = do urltag <- getWord8 idx <- getWord16be len <- getWord16be strdata <- BinaryGet.getLazyByteString (fromIntegral len) return (URLInfo {tag=urltag, urlid=idx, urllen=len, url=strdata}) instance Binary DescrInfo where put _ = do BinaryPut.putWord8 0 get = do tag <- getWord8 len <- getWord16be strdata <- BinaryGet.getLazyByteString (fromIntegral len) return (DescrInfo {descrtag=tag, descrlen=len, descr=strdata}) instance Binary TitleInfo where put _ = do BinaryPut.putWord8 0 get = do tag <- getWord8 len <- getWord16be strdata <- BinaryGet.getLazyByteString (fromIntegral len) return (TitleInfo {titletag=tag, titlelen=len, title=strdata}) instance Binary KeywordsInfo where put _ = do BinaryPut.putWord8 0 get = do tag <- getWord8 len <- getWord16be strdata <- BinaryGet.getLazyByteString (fromIntegral len) return (KeywordsInfo {keywordstag=tag, keywordslen=len, keywords=strdata}) instance Binary URLSet where put _ = do BinaryPut.putWord8 0 get = do remainingByteData <- BinaryGet.getRemainingLazyByteString i :: URLInfo <- decode remainingByteData j :: TitleInfo <- decode remainingByteData k :: DescrInfo <- decode remainingByteData x :: KeywordsInfo <- decode remainingByteData return (URLSet {urlinfo=i, titleinfo=j, descrinfo=k, keywordsinfo=x}) instance Binary SpiderDatabase where put _ = do BinaryPut.putWord8 0 get = do magicnumbera <- BinaryGet.getWord16be magicnumberb <- BinaryGet.getWord16be major <- BinaryGet.getWord16be minor <- BinaryGet.getWord16be header <- BinaryGet.getWord16be poolct <- BinaryGet.getWord16be -- ******************************* -- Get the remaining byte string data, -- So that we can use lazy bytestring to load to load the -- the data types. -- ******************************* remainingByteData <- BinaryGet.getRemainingLazyByteString -- pool <- (replicate (fromIntegral poolct) (decode remainingByteData)) z :: URLSet <- decode remainingByteData return (SpiderDatabase {magicNumberA=magicnumbera, magicNumberB=magicnumberb, majorVers=major, minorVers=minor, headerTag=header, poolLen=poolct }) main = do putStrLn "Running Spider Database Reader" args <- getArgs db :: SpiderDatabase <- decodeFile (args !! 0) putStrLn $ show db putStrLn "Done" *** *** Error: DbReader.hs:119:22: No instance for (Binary (Get URLInfo)) arising from a use of `decode' at DbReader.hs:119:22-45 Possible fix: add an instance declaration for (Binary (Get URLInfo)) In a 'do' expression: i :: URLInfo <- decode remainingByteData In the expression: do remainingByteData <- getRemainingLazyByteString i :: URLInfo <- decode remainingByteData j :: TitleInfo <- decode remainingByteData k :: DescrInfo <- decode remainingByteData .... In the definition of `get': get = do remainingByteData <- getRemainingLazyByteString i :: URLInfo <- decode remainingByteData j :: TitleInfo <- decode remainingByteData .... DbReader.hs:120:24: No instance for (Binary (Get TitleInfo)) arising from a use of `decode' at DbReader.hs:120:24-47 Possible fix: add an instance declaration for (Binary (Get TitleInfo)) In a 'do' expression: j :: TitleInfo <- decode remainingByteData In the expression: do remainingByteData <- getRemainingLazyByteString i :: URLInfo <- decode remainingByteData j :: TitleInfo <- decode remainingByteData k :: DescrInfo <- decode remainingByteData .... In the definition of `get': get = do remainingByteData <- getRemainingLazyByteString i :: URLInfo <- decode remainingByteData j :: TitleInfo <- decode remainingByteData .... DbReader.hs:121:24: No instance for (Binary (Get DescrInfo)) arising from a use of `decode' at DbReader.hs:121:24-47 Possible fix: add an instance declaration for (Binary (Get DescrInfo)) In a 'do' expression: k :: DescrInfo <- decode remainingByteData In the expression: do remainingByteData <- getRemainingLazyByteString i :: URLInfo <- decode remainingByteData j :: TitleInfo <- decode remainingByteData k :: DescrInfo <- decode remainingByteData .... In the definition of `get': get = do remainingByteData <- getRemainingLazyByteString i :: URLInfo <- decode remainingByteData j :: TitleInfo <- decode remainingByteData .... DbReader.hs:122:27: No instance for (Binary (Get KeywordsInfo)) arising from a use of `decode' at DbReader.hs:122:27-50 Possible fix: add an instance declaration for (Binary (Get KeywordsInfo)) In a 'do' expression: x :: KeywordsInfo <- decode remainingByteData In the expression: do remainingByteData <- getRemainingLazyByteString i :: URLInfo <- decode remainingByteData j :: TitleInfo <- decode remainingByteData k :: DescrInfo <- decode remainingByteData .... In the definition of `get': get = do remainingByteData <- getRemainingLazyByteString i :: URLInfo <- decode remainingByteData j :: TitleInfo <- decode remainingByteData .... DbReader.hs:142:21: No instance for (Binary (Get URLSet)) arising from a use of `decode' at DbReader.hs:142:21-44 Possible fix: add an instance declaration for (Binary (Get URLSet)) In a 'do' expression: z :: URLSet <- decode remainingByteData In the expression: do magicnumbera <- getWord16be magicnumberb <- getWord16be major <- getWord16be minor <- getWord16be .... In the definition of `get': get = do magicnumbera <- getWord16be magicnumberb <- getWord16be major <- getWord16be .... make: *** [dbreader] Error 1 -- Berlin Brown [berlin dot brown at gmail dot com] http://botspiritcompany.com/botlist/?

4 3

Consensus about databases / serialization
by Peter Verswyvelen 04 Jan '08

04 Jan '08

As I'm a selfmade man, I never really studied relational databases in detail. My intuition told me that the "relational" part was not really suitable for the 3D data, 2D images, animation curves, state machines, and other data I encountered in the videogame and animation business. I could always get away with files, and for the applications I needed to deploy, plugging in a couple of extra gigabytes of RAM and serializing the "object" state to disk was more practical, cheaper and faster. However, a couple of years ago I started studying computer science (I seem to do the theory after the practice, weird behavior ;-) at the Open University, and one of the exams I did was about databases. Initially this course convinced me that databases are actually very nice, but the course ended with a topic on object oriented databases, which were designed to make storing data like "3D models, graphs, networks, and complex datastructures" more practical. Duh. Since then, I did deploy a few commercial applications for customers using databases, which worked fine for the typical "simple/flat" database data. I hated embedding a dynamic untyped language like SQL, as much as I hated embedding code in HTML or XML. IMHO it feels UGLY and unsafe. Regarding the other popular data storage format - XML - I did use that a lot, but it seems like going back to the stone ages, when hierarchical stores/databases got invented (and ditched?) Now, initially after an introduction to Microsoft's LINQ, and recently having read a very brief overview of HAppS, it seems I'm not the only one with those "feelings". Ouch, this introduction got way to long, sorry about that ;-) Finally some practical questions: . regarding Haskell and databases, the page http://haskell.org/haskellwiki/Libraries_and_tools/Database_interfaces describes a few, but which are the ones that are stable and practical? Any user experiences? . HApps is not listed in the page above, because it does not use databases? Is HApps reliable or experimental, and does it scale well? Any success stories? . regarding Haskell and serialization, I don't think that implementing Read/Show is a good way for real serialization, so what other options exist? I could find some libraries at http://hackage.haskell.org/packages/archive/pkg-list.html#cat:Data, but again which are the most practical and stable? When programming in C++/MFC and C#/.NET, I tended to develop my own serialization frameworks because I used that for many things, like logging commands to disk, performing undo/redo, intra and inter process cut/copy/paste, save/load, etc. . Regarding serialization, I'm kinda curious how ADTs and even GADTs are stored and retrieved in a relational database? I guess it could be done using BLOBs and serialization to ByteStrings, so bypassing a lot of the database table structures? . If I would want to experiment with say HAppS, the way I understand it, I first would first have to study "Scratch your boilerplate" and Template Haskell, and maybe some other language features? I'm still new to Haskell, and the road to understanding all language elements and extensions is very long, so sequentially learning it would be insane I guess. I have no practical experience with TH, but I spent a long time trying to do "aspect oriented programming" in C# without success, so TH looks uber to me. Thanks a lot and best wishes for 2008? Peter

9 14

Supporting both cabals?
by Magnus Therning 04 Jan '08

04 Jan '08

GHC 6.8 has just made it into Debian in a usable form. (w00t!) Due to the library split my old cabal files don't work any longer. updating them isn't the problem, the problem is keeping them compatible with both versions of Cabal. I searched the wiki (quickly) but didn't find anything on this topic. Is there a cookbook out there for it? /M -- Magnus Therning (OpenPGP: 0xAB4DFBA4) magnus＠therning．org Jabber: magnus．therning＠gmail．com http://therning.org/magnus What if I don't want to obey the laws? Do they throw me in jail with the other bad monads? -- Daveman

3 2

Importing Data.Char speeds up ghc around 70%
by Joost Behrends 04 Jan '08

04 Jan '08

Hi, while still working on optimizing (naively programmed) primefactors i watched a very strange behavior of ghc. The last version below takes 2.34 minutes on my system for computing 2^61+1 = 3*768614,336404,564651. Importing Data.Char without anywhere using it reduces this time to 1.34 minute - a remarkable speed up. System is WindowsXP on 2.2GHZ Intel, 512MB Ram. I give the complete code here - hopefully all tabs are (4) blanks. Can this be reproduced ? I compile just with --make -O2. module Main where import IO import System.Exit import Data.Char main = do hSetBuffering stdin LineBuffering putStrLn "Number to decompose ?" s <- getLine if s == [] then exitWith ExitSuccess else do putStrLn (show$primefactors$read s) main data DivIter = DivIter {dividend :: Integer, divisor :: Integer, bound :: Integer, result :: [Integer]} intsqrt m = floor (sqrt $ fromInteger m) primefactors :: Integer -> [Integer] primefactors n | n<2 = [] | even n = o2 ++ (primefactors o1) | otherwise = if z/=1 then result res ++[z] else result res where res = divisions (DivIter {dividend = o1, divisor = 3, bound = intsqrt(o1), result = o2}) z = dividend res -- is 1 sometimes (o1,o2) = twosect (n,[]) twosect :: (Integer,[Integer]) -> (Integer,[Integer]) twosect m |odd (fst m) = m |even (fst m) = twosect (div (fst m) 2, snd m ++ [2]) found :: DivIter -> DivIter found x = x {dividend = xidiv, bound = intsqrt(xidiv), result = result x ++ [divisor x]} where xidiv = (dividend x) `div` (divisor x) d2 :: DivIter -> DivIter d2 x |dividend x `mod` divisor x > 0 = x { divisor = divisor x + 2} |otherwise = found x d4 :: DivIter -> DivIter d4 x |dividend x `mod` divisor x > 0 = x { divisor = divisor x + 4} |otherwise = found x d6 :: DivIter -> DivIter d6 x |dividend x `mod` divisor x > 0 = x { divisor = divisor x + 6} |otherwise = found x divisions :: DivIter -> DivIter divisions y |or[divisor y == 3, divisor y == 5] = divisions (d2 y) |divisor y <= bound y = divisions (d6$d2$d6$d4$d2$d4$d2$d4 y) |otherwise = y

7 20

Escape Codes
by H. Mayer 03 Jan '08

03 Jan '08

Hello, Some days ago I needed escape codes on Win32. I didn't find any library for that, so I wrote a simple one. I thought, perhaps someone else will have the same problem, so I put it on my website:http://0hc.net/?what=downloads_sup_AWin32Console I've tested it with the ghc 6.6.1, and everything works as I expected it. I would be happy if someone else could test it and/or report if some important escape codes are missing. --Sincerely H. _________________________________________________________________ Erweitere dein digitales Leben. Hol dir jetzt gratis das neue Windows Live. http://get.live.com

3 2

Basic question concerning data constructors
by Joost Behrends 03 Jan '08

03 Jan '08

Hello, perhaps i will make a wishlist of topics not dealt in the tutorials. Here is something i miss in each of them: notes at the semantics of data constructors. We read data Pair a b = Pair a b in YetAnotherHaskellTutorial. And that is all ! If we omit "data" here, this would be a silly pleonasm. And no single word about this strange behavior of "data" in every tutorial i read. A similar point: The tutorials teach, that "=" has a similar meaning than "=" in mathematics. But there is a big difference: it is not reflexive. The the right side is the definition of the left. Thus "x=y" has still some kind of temporality, which mathematics doesn't have. Wadler himself describes bunches of lazily computed equations as "dataflows" somewhere. Ok, so much on theory. Here a concrete question: For adapting hws (one of the reasons for me to be here, not many languages have a native web server) to Windows i must work on time. In System.Time i found data ClockTime = TOD Integer Integer 2 questions arise here: Does this define "TOD" (which i do not find elsewhere) together with ClockTime also ? And: Why is this not: data ClockTime Integer Integer = TOD Integer Integer ? Is it just an abbreviation for the first? Or is there a connection to ClockTime as an "abstract data type" (a notion, which would have a subtle different meaning than in OOP - since "instance" is such different thing here). Thanks for your attention, Joost

16 31

regex package for yhc?
by Thomas Hartman 03 Jan '08

03 Jan '08

Is there some way to use any of the various regex packages on hackage via yhc? Has anyone installed one them successfully? I'd like regex-tdfa, but would settle for regex-posix, or really, anything that brings the convenience of regex to yhc. In general, is there a straightforward way to install "extralibs" type packages for universal "import" availability in yhc? Is there a cabal equivalent for yhc? Documentation pointer would be appreciated; I searched but couldn't find anything. Maybe not using the right keywords... t. --- This e-mail may contain confidential and/or privileged information. If you are not the intended recipient (or have received this e-mail in error) please notify the sender immediately and destroy this e-mail. Any unauthorized copying, disclosure or distribution of the material in this e-mail is strictly forbidden.

5 6

[16/16] SBM: Discussion and Conclusion
by Peter Firefly Brodersen Lund 03 Jan '08

03 Jan '08

General ------- Bytestrings are faster than haskell strings. This is good and expected. But their memory use, ouch! There seems to be a grave memory bug in 6.9. Lazy bytestrings are slower than strict bytestrings -- this was completely unexpected for me. The strictness analyzer works *very* well. It rarely made a difference to mark arguments as strict (and when it did, it was very easy to use). I/O --- The I/O patterns seen in the various programs are: 1) strict bytestrings = read increasingly large chunks (they double every time). I don't know if the already-read data have to be copied around or if the memory allocator can handle extending existing blocks (if they are at the "front" of the heap, say) like realloc() can. Even if the blocks do get copied around, that's not where the performance limiter is (speed-wise). It does seem to be a bit unwise in terms of memory pressure. Does the gc kick in every time the heap has to expand? Does it do a full collection then? If no other allocations have happened since the last collection than the allocation that caused the heap to expand, then perhaps skip the collection? (or some criteria similar to that) 2) lazy bytestrings = read blocks of 32K-8 bytes, as expected. The C benchmarks show that there's no penalty for reading 32K-8 vs. 32K. 3) native strings = read blocks 8K. The C benchmarks show that it barely matters if the block size is 4K, 32K, or 32K-8 bytes. In any case, the small differences due to block size are completely in the noise. Reading very large blocks, though, as the strict bytestrings do, actually has a cost (38-70% slower, depending on the CPU and kernel and RAM and busspeeds, etc -- see email 10 in the series). It is still peanets compared for almost all the benchmarks. Backend ------- The backend turns out to be rather bad. It is generally very easy to improve the generated code by hand without doing anything in terms of sophisticated analysis. One can rewrite inner loops using heroic code (load four characters at a time together into a 32-bit register or even use MMX to handle eight characters in parallel) but it isn't really necessary to do that to gain a good improvement and start being competitive with C. The backend is probably what is costly on some of the lazy bytestring and native string code because there are too many tests to see if the buffer has been exhausted (I think). Some simple hoisting and common subexpression elimination would go far here. For the simpler strict bytestring benchmarks, heroic backend optimizations are not necessary because they give less of an improvement over merely competent register use than sensible I/O and buffer management (cache effects start to be important once the compiler generates sensible code). I would have liked to examine backend performance further by playing with the generated C-- code but unfortunately, the C-- that ghc emits won't compile. Dissecting the generated machine code for the more complicated benchmarks was so hard that I skipped it -- I can't even recognize the labels! It's one thing if the backend generates lots of extra labels but another if the expected labels simply are not there! (this is why tools/cut.pl complains that it can't find the main loop for some of the programs -- if the */*.discut file is unexpectedly short, then that's the reason.) Quality of measurements ----------------------- The speed measurements are of extremely high quality, much more than it turns out is needed at this moment. I probably spent a bit too much effort there. The memory measurements are unusual. I don't recall having seen anything like them elsewhere because one seldom has to work with uncooperative programs when benchmarking (the preloading trick works with any program, as long as it is dynamically linked on Linux and uses the standard loader). They are probably the biggest single contribution from this work, together with the visualization provided by the bar charts. Thoroughness ------------ It really is necessary to benchmark more than one ghc/library combination, otherwise the bad memory bugs wouldn't stand out so clearly or I would have believed, as Don Stewart did, that he had fixed the memory allocation bug. It also matters what microarchitectures one use, as pointed out by email 10 in this series. And you gotta have many benchmarks, otherwise you might not really be measuring what you think you measure. I could probably have gotten away with fewer benchmarks in this case but that would have left a nagging feeling at the back of my head that there was a benchmark I /could/ have included that would have showed up a spectacular performance bug. Using so big input files (150MB) made the programs run slow enough that the timer granularity (1ms) was small compared to their runtime. It also made the memory performance problems really stand out (and be unignorable). I think that was a good idea. Having automatic feedback on the timing measurement quality helped a lot during the development of the testing harness because it made it easier and faster for me to eliminate the causes of imprecision. That's why I wrote the eatmem tool and that's why the test harness does a dd of the input file and the compiled program before running the tests. It should probably also dd the C library (and the few other libraries linked in: math, GNU multiprecision, and TLS stuff) but the quality estimators show that any improvement would be very small. Using top together with huge input files convinced me that -sstderr was untrustworthy so I developed the pause-at-end preloading hack. Paranoia paid off big time there. Visual presentation ------------------- The bar charts (with UTF-8) turned out to be so much more readably than raw numbers. This is a trick I learned from the Cairo team, which created a performance suite very early on. The bar is drawn with '%' if the measurement is bad. That turned out to be a much clearer visual indicator than my previous '*' and '!' flag characters. Really, there's no good reason not to use UTF-8 these days. I also discovered that the program names used for the benchmarks really matter. I had started out with names like 'cntspaces12' and 'cntbytes34' and kept getting confused about what program measured what. If the names are good, you don't need to put a description of each benchmark on the report. The order the benchmarks are presented in also matters. It is important that they are grouped and it is also important that the order is as stable as possible, even when not all runs contain exactly the same benchmarks. Separation of the time and memory bar charts + the separation of byte and space counting benchmarks also turned out to be useful. I had neither at first. And finally, merging of reports. I had no idea it would be so useful before I wrote the scripts for it. This is what makes the memory bugs in 6.9 really stand out and this is what shows the improvements of bytestring 0.9.0.2 (in particular with ghc 6.8.2). It also shows how wildly the performance of four different microarchitectures can differ. Scripting --------- Scripting everything is key to the success of an effort like this. I am VERY happy that scripted not just the running of the benchmarks but also the report handling and even the installation of the ghc/bytestring combinations. (I also scripted the sending of these 17 emails -- let's see if the script works :) ) The predictions elicited by my teaser email ------------------------------------------- As expected, predicting the performance of haskell programs is really hard, even modulo performance bugs. And, no, strings are not "silly". Action Items ------------ Suggested short-term actions: o Can the allocator expand blocks? If not, see if it can be implemented. o Find out why lazy bytestrings "by hand" are that slow - is it due to lack of hoisting in the backend? o Improve backend, mainly by not hiding information from register allocator. Alternatively, by using an assembly-to-assembly transformator that improves the register allocation. o Fix -sstderr. o perhaps even let the RTS print out /proc/self/maps or /proc/self/status or "peak RSS" returned by getrusage() in the ru_maxrss field? o Find memory leaks in ghc 6.9 code. o ghc should allow round-tripping of the C-- code it generates. o would be awfully nice if the backend didn't sometimes throw the recognizable labels away. o can the core/stg code be made easier to read? -Peter

3 3

ANN: Dimensional 0.7.2
by Bjorn Buckwalter 03 Jan '08

03 Jan '08

Dear all, I've released version 0.7.2 of the Dimensional library. The only change from version 0.7.1 is that the CGS module has been re-enabled. Unless you use the CGS module there is no need to upgrade from 0.7.1. Note that this version is incompatible with GHC 6.8.1 due to bug #1919 (introduced in GHC 6.8.1 and fixed in 6.8.2). For GHC 6.8.1 use version Dimensional 0.7.1 which lacks the CGS module. Thanks, Bjorn Buckwalter http://dimensional.googlecode.com http://hackage.haskell.org/cgi-bin/hackage-scripts/package/dimensional On Nov 23, 2007 8:32 AM, Bjorn Buckwalter <bjorn.buckwalter(a)gmail.com> wrote: > Dear all, > > The Dimensional library has been ported to GHC 6.8.1 (it remains > backwards-compatible with GHC 6.6.1, and also with Cabal 1.1.6 I > believe). The new version number is 0.7.1. > > Due to a GHC 6.8.1 bug (#1919) the CGS module will not compile and has > been disabled. > > Thanks, > Bjorn Buckwalter > > http://dimensional.googlecode.com > http://hackage.haskell.org/cgi-bin/hackage-scripts/package/dimensional > http://hackage.haskell.org/trac/ghc/ticket/1919 >

1 0

FW: [Haskell-cafe] Is there anyone out there who can translate C# generics into Haskell?
by Peter Verswyvelen 03 Jan '08

03 Jan '08

I'm also new to Haskell, but I'm a C++/C# veteran, so I'll give it a shot. C#'s interfaces look a bit like Haskell's type classes. Although not exactly the same, you could try something like this: -- C#: interface IX1 { String foo1(int); } class IX1 obj where foo1 :: Int -> obj -> String -- C#: interface IX2<A> { String foo2(A); } class IX2 obj a where foo2 :: a -> obj -> String --C#: interface IX3<A> where A : IY { String foo3(A); } class IY a where {- ... -} class IY a => IX3 obj a where foo3 :: a -> obj -> String --C#: interface IX4<A> : IZ where A : IY class IZ a where {- ... -} class (IY a, IZ obj) => IX4 obj a where foo4 :: a -> obj -> String The C# "class" is called an "instance" in Haskell, so to implement an interface, you would do e.g. data X2 = X2 { x2Bla :: String, ... } instance IX2 X2 String where foo2 s o = (x2Bla o) ++ s This assumes your "objects" are immutable, otherwise you would have to return (obj,String) instead of just String and then you most likely want to use the state monad and "do notation" to make functional programming look more like imperative programming. You really have to drop the OO way of thinking, which I find the hardest :) Haskell's type classes are more powerful in some sense than C# interfaces; for example, in C# you can't attach an interface to any class (take for example the Point struct), it has to be your own class, while in Haskell, you can implement an instance of a type class for any datatype! Hope this helps a bit. As I'm new myself to Haskell, so take this with a "grain of salt". Once you bite the bullet, I found Haskell a lot more fun than C#, although C# of course comes with a gigantic .NET framework and tools... Peter -----Original Message----- From: haskell-cafe-bounces(a)haskell.org [mailto:haskell-cafe-bounces@haskell.org] On Behalf Of Nicholls, Mark Sent: Wednesday, January 02, 2008 5:41 PM To: haskell-cafe(a)haskell.org Subject: [Haskell-cafe] Is there anyone out there who can translate C# generics into Haskell? I'm trying to translate some standard C# constucts into Haskell... some of this seems easy.... Specifically 1) Interface IX { } 2) Interface IX<A> { } 3) Interface IX<A> Where A : IY { } 4) Interface IX<A> : IZ Where A : IY { } I can take a punt at the first 2....but then it all falls apart _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

1 0

I am looking for the source code for STM implementation ...
by Galchin Vasili 03 Jan '08

03 Jan '08

Hello, I found STM.c(lower layer), but cannot find the top layer code written Haskell.?? Vasya

2 1

Newbie question: can laziness lead to space compression?
by Brian Hurt 03 Jan '08

03 Jan '08

My apologies if this has been beat to death before, I'm still new to Haskell. But I was wondering if it is possible that lazy evaluation could lead to space compression, especially under heavily persistant usage patterns? Here's the argument I'm making. Say we have a tree-based Set with, say, 1024 values in it. For ease of math we'll assume that it's perfectly balanced. Say each node in the tree takes 5 words of memory. So in an eager language (for example, Ocaml), adding a new node to this tree requires the allocation of 10 new nodes, or 50 words of memory. In Haskell, what would happen (as I understand it) is that just a new lazy thunk would be allocated- say, 10 words of memory. Conceptually, we could think of the returned value as the original tree plus a small delta. The lazy implementation is using 40 fewer words of memory than the eager implementation. Note that the benefit isn't *big*- we're talking about 40 words of memory when the main data structure is taking up 5K plus words of memory- so it's less than 1% different. But there is a (small) upside in memory usage at least occassionally, right? Brian

3 2

[RFC] benchmarks of bytestrings, teaser
by Peter Lund 02 Jan '08

02 Jan '08

What do you think the relative speeds are of the six small haskell programs at the end of this email? All they do is read from stdin and count the number of spaces they see. There are two that use strict bytestrings, two that use lazy bytestrings, and two that use the standard Haskell strings. Three use a recursive function with an accumulator parameter and three use a foldl with a lambda function. Say the fastest one takes the time 1. How much time will the others take? And how about memory? How much memory do you think they require? Let's say we feed a 150MB(*) file into each of them, how many megabytes do you think they end up using (as seen from the OS, not in terms of how big the live heap is)? I'm going to post full benchmarks + analysis on Wednesday. -Peter *) hardddisk megabytes. The file is 150000034 bytes ≈ 143 mebibytes. PS: For extra credit, what do you think is the peak memory use for this program when given an input file of 150MB? {-# LANGUAGE BangPatterns #-} import qualified Data.ByteString.Lazy.Char8 as B import GHC.Int (Int64) -- note that D.BS.Lazy.Char8.length is ByteString -> Int64 -- D.BS.C8.length is ByteString -> Int cnt :: B.ByteString -> Int64 cnt bs = B.length (B.filter (== ' ') bs) main = do s <- B.getContents print (cnt s) ============================== hs/space-bs-c8-acc-1.hs: {-# LANGUAGE BangPatterns #-} import qualified Data.ByteString.Char8 as B cnt :: Int -> B.ByteString -> Int cnt !acc bs = if B.null bs then acc else cnt (if B.head bs == ' ' then acc+1 else acc) (B.tail bs) main = do s <- B.getContents print (cnt 0 s) ============================== hs/space-bslc8-acc-1.hs: {-# LANGUAGE BangPatterns #-} import qualified Data.ByteString.Lazy.Char8 as B cnt :: Int -> B.ByteString -> Int cnt !acc bs = if B.null bs then acc else cnt (if B.head bs == ' ' then acc+1 else acc) (B.tail bs) main = do s <- B.getContents print (cnt 0 s) ============================== hs/space-xxxxx-acc-1.hs: {-# LANGUAGE BangPatterns #-} cnt :: Int -> String -> Int cnt !acc bs = if null bs then acc else cnt (if head bs == ' ' then acc+1 else acc) (tail bs) main = do s <- getContents print (cnt 0 s) ============================== hs/space-bs-c8-foldlx-1.hs: {-# LANGUAGE BangPatterns #-} import qualified Data.ByteString.Char8 as B cnt :: B.ByteString -> Int cnt bs = B.foldl' (\sum c -> if c == ' ' then sum+1 else sum) 0 bs main = do s <- B.getContents print (cnt s) ============================== hs/space-bslc8-foldlx-1.hs: {-# LANGUAGE BangPatterns #-} import qualified Data.ByteString.Lazy.Char8 as B cnt :: B.ByteString -> Int cnt bs = B.foldl' (\sum c -> if c == ' ' then sum+1 else sum) 0 bs main = do s <- B.getContents print (cnt s) ============================== hs/space-xxxxx-foldl.hs: {-# LANGUAGE BangPatterns #-} cnt :: String -> Int cnt bs = foldl (\sum c -> if c == ' ' then sum+1 else sum) 0 bs main = do s <- getContents print (cnt s) ==============================

15 33

Problem building hdbc-sqlite3 with ghc 6.8.2
by manu 02 Jan '08

02 Jan '08

hello, has anybody managed to build hdbc-sqlite3 with ghc 6.8.2 ? I get the following error : Macintosh:HDBC-sqlite3-1.1.3.0 manu$ runhaskell Setup.lhs build Preprocessing library HDBC-sqlite3-1.1.3.0... ghc-6.8.2: unrecognised flags: -F/Users/manu/Library/Frameworks Usage: For basic information, try the `--help' option. compiling dist/build/Database/HDBC/Sqlite3/Statement_hsc_make.c failed command was: /usr/local/bin/ghc -c -package base-3.0.1.0 -package mtl-1.1.0.0 -package HDBC-1.1.3 -I. -F/Users/manu/Library/Frameworks dist/build/Database/HDBC/Sqlite3/Statement_hsc_make.c -o dist/build/ Database/HDBC/Sqlite3/Statement_hsc_make.o if somebody has an idea... Thanks E.D

2 1

Knowledge
by jlw501 02 Jan '08

02 Jan '08

I'm new to functional programming and Haskell and I love its expressive ability! I've been trying to formalize the following function for time. Given people and a piece of information, can all people know the same thing? Anyway, this is just a bit of fun... but can anyone help me reduce it or talk about strictness and junk as I'd like to make a blog on it? contains :: Eq a => [a]->a->Bool contains [] e = False contains (x:xs) e = if x==e then True else contains xs e perfectcomm :: Bool perfectcomm = undefined knowself :: Bool knowself = undefined allKnow :: Eq a => [a]->String->Bool allKnow _ "" = True allKnow [] k = False allKnow (x:[]) k = knowself allKnow (x:xs) k = comm x xs k && allKnow xs k where comm p [] k = False comm p ps k = if contains ps p then knowself else perfectcomm -- View this message in context: http://www.nabble.com/Knowledge-tp14423007p14423007.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

4 6

Missing join and split
by Mitar 02 Jan '08

02 Jan '08

Hi! I am really missing the (general) split function built in standard Haskell. I do not understand why there is something so specific as words and lines but not a simple split? The same goes for join. Yes, I can of course define them but ... in an interactive mode it would be quite handy to have them there. Or am I wrong and are those hidden somewhere? So what are common ways to get around this? What are elegant definitions? Inline definitions? Mitar

13 19

Fwd: Attempt at defining typeful primary key/foreign key relationships
by Justin Bailey 01 Jan '08

01 Jan '08

Small correction - the extensions listed at the bottom weren't necessary. I had attempted to define a relationship among tables automatically as: instance (HasField fk child, HasField pk parent, HasRelation pk fk) => HasRelation parent child But that was redundant with the signature on relation. Removing it allowed me to remove all extensions except MultiParameterTypeClasses. Justin ---------- Forwarded message ---------- From: Justin Bailey <jgbailey(a)gmail.com> Date: Dec 31, 2007 12:25 PM Subject: Attempt at defining typeful primary key/foreign key relationships To: haskelldb-users(a)lists.sourceforge.net All, As part of my data model, I'd like to say there is a primary key/foreign key relationship between two tables/queries. Right now I only want to do this to capture some extra-query information that I'm using for code generation - I don't intend it to actually affect queries generated, though I'd like to go there eventually. I define a class which says there is a relation: class HasRelation pk fk I define a type which holds relation values: data Relation a pk fk p c = Relation { ... } Relation values can only be constructed by a function with appropriate constraints on it: relation :: (HasRelation pk fk, HasField pk parent, HasField fk child) => Attr pk a -> Attr fk a -> Rel parent -> Rel child -> Relation a pk fk parent child relation primaryKey foreignKey prnt chld = Relation .... As an example, if I have tables Orders and LineItems, both with field order_id, I can establish a relationship by first defining an instance: instance HasRelation Orders.Order_id LineItems.Order_id I can then create a relation value between the two: orders <- table Orders.orders lineItems <- table LineItems.lineItems let r1 = relation Orders.order_id LineItems.order_id orders lineitems Note these types would be defined using DBDirect, so Order_id as a type with one constructor, lineItems has type "Table ...", etc. If I try to define an incorrect relation (wrong fields, fields in the wrong order, etc), then the above will not compile, which is just what I want. If you've read this far, I've not done any type-level programming or even played much with classes and instances before so I'd like to know if the approach makes sense. I took most of my inspiration from the HasField class in the HDBRec module. 1) Could this be simpler? Any obvious deficiencies? 2) I had to enable a number of extensions (though all of these are also enabled by haskelldb) - FlexibleInstances, FlexibleContexts, UndecidableInstances and OverlappingInstances. Should I be worried? 3) It's possible to define relationships among any two types. How can I limit that to 1) attributes with 2) the same type? Would I have to define an Attr typeclass, parallel to the Attr type? Thanks for any and all feedback. Happy New Year! Justin

1 0

Trying to fix space leak
by Ben Challenor 01 Jan '08

01 Jan '08

Hi I'm learning Haskell through writing a compiler. I'm seeing huge memory use in a function which converts the dataflow graph to the form required by Data.Graph. It needs to return a map from dataflow nodes to Vertexs, a map in the other direction, and the list of edges (as Vertex pairs). > total time = 18.78 secs (939 ticks @ 20 ms) > total alloc = 6,616,297,296 bytes (excludes profiling overheads) > > COST CENTRE MODULE %time %alloc > > j Dataflow 87.6 99.9 > graphvizCompile Dataflow 6.4 0.0 > f Dataflow 2.9 0.0 > con2tag_DFReal# Representation 2.1 0.0 I assume the allocation is being garbage-collected pretty quickly, because a) 6,616,297,296 bytes is stupid (!) and b) Process Explorer informs me that the peak private bytes of the program is not more than a couple of MB. The offending code looks like this: > -- (next Vertex available for use, edges so far, map so far, map so far) > type DependencyEdgesAcc = (Vertex, [Edge], Map.Map DFNode Vertex, Map.Map Vertex DFNode) > > -- Process a list of DFNodes in the same context. > dependencyEdges :: DependencyEdgesAcc -> [DFNode] -> DependencyEdgesAcc > dependencyEdges acc [] = {-# SCC "a" #-} acc > dependencyEdges acc@(_, _, mnv1, _) (n:ns) = > -- Check whether this node has been done before. > {-# SCC "j" #-} case {-# SCC "i" #-} Map.lookup n mnv1 of > Just _ -> {-# SCC "b" #-} dependencyEdges acc ns > Nothing -> > -- Find the list of dependencies. > let ndeps = {-# SCC "c" #-} nodeDependencies n in > -- Process them first. > let (v2, es2, mnv2, mvn2) = {-# SCC "d" #-} dependencyEdges acc ndeps in > -- Now we can claim v2 as the label for this node n. > -- Look up vdep for each dependency ndep and add an edge from the vdep to v2. > let {(v4, es4, mnv4, mvn4) = {-# SCC "e" #-} List.foldl' ( > \(v3, es3, mnv3, mvn3) ndep -> > case {-# SCC "f" #-} Map.lookup ndep mnv3 of > Just vdep -> (v3, (vdep, v2):es3, mnv3, mvn3) > Nothing -> error $ "this node should have been added already: " ++ show ndep > ) (v2, es2, mnv2, mvn2) ndeps } in > assert (v2 == v4) > assert (mvn2 == mvn4) > assert (mnv2 == mnv4) > -- Finally, add this node to the accumulator and then recurse. > dependencyEdges (v2+1, es4, {-# SCC "g" #-} Map.insert n v2 mnv2, {-# SCC "h" #-} Map.insert v2 n mvn2) ns with the following profile: > dependencyGraph Dataflow 4395 1 0.0 0.0 93.4 99.9 > getRootDFNodes Dataflow 4397 5 0.0 0.0 0.0 0.0 > dependencyEdges Dataflow 4396 282 0.0 0.0 93.4 99.9 > a Dataflow 4407 104 0.0 0.0 0.0 0.0 > j Dataflow 4399 178 87.6 99.9 93.4 99.9 > b Dataflow 4417 75 0.0 0.0 0.0 0.0 > g Dataflow 4405 103 0.1 0.0 0.2 0.0 > con2tag_DFBool# Representation 4419 1972 0.0 0.0 0.0 0.0 > con2tag_DFReal# Representation 4414 186890 0.1 0.0 0.1 0.0 > con2tag_DFNode# Representation 4413 1204 0.0 0.0 0.0 0.0 > h Dataflow 4404 103 0.0 0.0 0.0 0.0 > e Dataflow 4403 103 0.0 0.0 4.8 0.0 > f Dataflow 4412 174 2.9 0.0 4.8 0.0 > con2tag_DFBool# Representation 4420 245794 0.0 0.0 0.0 0.0 > con2tag_DFReal# Representation 4416 25772482 1.9 0.0 1.9 0.0 > con2tag_DFNode# Representation 4415 2068 0.0 0.0 0.0 0.0 > d Dataflow 4402 103 0.0 0.0 0.0 0.0 > c Dataflow 4401 103 0.0 0.0 0.0 0.0 > nodeDependencies Dataflow 4406 103 0.0 0.0 0.0 0.0 > i Dataflow 4400 178 0.6 0.0 0.7 0.0 > con2tag_DFBool# Representation 4418 48230 0.0 0.0 0.0 0.0 > con2tag_DFReal# Representation 4410 5045646 0.1 0.0 0.1 0.0 > con2tag_DFNode# Representation 4409 1928 0.0 0.0 0.0 0.0 Is this a space leak, or is my algorithm just silly? (It could well be the latter...) I've tried adding random bangs, to no effect. So, can anyone help me? :) Cheers Ben

4 4

research topics
by Henrique Ferreiro 01 Jan '08

01 Jan '08

Hi haskellers! I am a postgraduate student interested in functional programming and Haskell. I would appreciate if someone could elaborate on which are the current hot topics of research in this area, and related to Haskell, of course. In particular, I am interested in compiler implementation. Thanks in advance. -- Henrique Ferreiro <henrique.ferreiro(a)gmail.com>

1 0

Re: [Haskell-cafe] How to convert number types.
by L.Guo 01 Jan '08

01 Jan '08

Ah, thanks, it is my fault, for mis-understanding the hint from GHC. And what is the difference between fromIntegral and fromInteger ? ------------------ L.Guo 2008-01-01 ------------------------------------------------------------- From: Mark T.B. Carroll At: 2008-01-01 01:42:37 Subject: Re: [Haskell-cafe] How to convert number types. Perhaps fromIntegral does what you want? Mark

2 1

How to convert number types.
by L.Guo 01 Jan '08

01 Jan '08

Hi MailList Haskell-Cafe: I am a new haskeller. And was farmilar with C. When tring to do some calculate, like this: input = 5 :: Int factor = 1.20 :: Float output = factor ** (toFloat input) I found that I do not know any function could do just what 'toFloat' should do. What should I do then ? Regards -------------- L.Guo 2008-01-01

4 3

A "commutative diagram" conjecture about applicative functors
by Robin Green 31 Dec '07

31 Dec '07

I am proving various statements relating to applicative functors, using the Coq proof assistant (I am considering only Coq terms, which always terminate so you don't have to worry about _|_). However, I'm not sure how to go about proving a certain conjecture, which, translated back into Haskell and made more specific to make it easier to think about, looks like this (assuming Control.Applicative and Control.Arrow are imported): "For all applicative functors: \f x -> fmap second f <*> fmap ((,) (0::Int)) x is equivalent to \f x -> fmap ((,) (0::Int)) (f <*> x)" McBride and Patterson give four laws for applicative functors in their paper "Functional Pearl: Applicative programming with effects". Can these laws be used to prove this conjecture for all applicative functors satisfying those laws? -- Robin P.S. I realise this might not look like a "commutative diagram conjecture", but as I said, I've made it more specific - my actual conjecture is more general, but hopefully if I can understand how to prove the specific one, I'll be able to prove my real one in short order.

3 4

Getting links to Haddock documentation in Hackage
by Adam Langley 31 Dec '07

31 Dec '07

Some packages[1] have links on their Hackage pages to the haddock generated documentation for each exported module[2]. However, many[3] don't. What's the secret to getting this generated documentation to work with Hackage? Even packages for which `runhaskell Setup.lhs haddock` runs don't (always) get the magic links. Cheers, AGL [1] http://hackage.haskell.org/cgi-bin/hackage-scripts/package/bzlib-0.4.0.1 [2] http://hackage.haskell.org/packages/archive/bzlib/0.4.0.1/doc/html/Codec-Co… [3] http://hackage.haskell.org/cgi-bin/hackage-scripts/package/tar-0.1.1.1 -- Adam Langley agl(a)imperialviolet.org http://www.imperialviolet.org 650-283-9641

2 1

The semantics of constructor patterns
by Cristian Baboi 31 Dec '07

31 Dec '07

In section 4.3.3., chapter 4: Structured types and the semantics of pattern-matching, by S.Peyton Jones and Philip Wadler, there is this equation: Eval[[\(s p1 p2 ... pt).E]] (s a1 a2 ...at) = Eval[[\p1 ... \pt.E]] a1 ... at The text say: "To apply \(s p1 ... pt).E to an argument A we first evaluate A to find out what sort of object it is." "Finally, if A was built with the same constructor as the patern, then the first rule applies." What I don't get it : (s a1 a2 ... at) must be the value of A in the semantic domain. Let call that value a. Then how can one know if a was built with (s a1 a2 ... at) and not with (egg b1 b2) ?

3 4

easy cabal tasks
by Duncan Coutts 30 Dec '07

30 Dec '07

People are often unsure about where they can help out with Cabal. One thing we can do to make that easier is to point out smaller simpler tasks that people might like to have a go at. We have a list of tasks that are marked as easy or very easy: http://hackage.haskell.org/trac/hackage/report/13 This list is also linked from the hackage trac web page ("Easy tickets") http://hackage.haskell.org/trac/hackage/ For people reporting bugs and working with the hackage/cabal trac, look out for tasks that you think could be done by a new or relatively new contributor and mark the bug's difficulty appropriately. Please also give plenty of detail on what the solution should involve and pointers to other documentation as appropriate. Duncan

1 0

Type-system fun: a type-safe embedding of System F Lambda Calculus into Haskell
by Ryan Ingram 30 Dec '07

30 Dec '07

(The full code used for this message is available at http://ryani.freeshell.org/haskell/systemf.hs) System F is the polymorphically typed lambda calculus. It is strongly typed but allows polymorphic functions like "id". id is represented as follows: > eId = EGamma "A" (ELam "x" TVar EVar) which represents > \A -> \x:A -> x that is, a function that takes a type and a single argument of that type and returns its argument. This embedding only allows well-typed terms to be constructed; the Haskell typechecker verifies that a term is well-typed. I include a full compiler which converts terms into Haskell functions (albeit not-very-efficient ones). A sample session with GHCi follows: GHCi, version 6.8.1: http://www.haskell.org/ghc/ :? for help Loading package base ... linking ... done. Ok, modules loaded: SystemF. Prelude SystemF> eId \A a:A -> a Prelude SystemF> :t eId eId :: NExpr (a :-> (a -> a)) Prelude SystemF> :t eBottom eBottom :: NExpr (a :-> a) Prelude SystemF> eBottom \A -> fix (\a:A -> a) Prelude SystemF> putStrLn $ showExprExplicit eBottom (EGamma "A" (EFix (ELam "a" TVar EVar))) Prelude SystemF> tInt I# Prelude SystemF> :t (TPrim "I#" (undefined :: Integer)) (TPrim "I#" (undefined :: Integer)) :: TypRep Integer ts Prelude SystemF> :t tInt tInt :: TypRep Integer ts Prelude SystemF> eK \A B a:A b:B -> a Prelude SystemF> eFact fix (\rec:(I#->I#) z:I# -> (\A -> if#) I# (eq# z 0) 1 (times# z (rec (minus# z 1)))) Prelude SystemF> :t eFact eFact :: NExpr (Integer -> Integer) Prelude SystemF> compile eFact 5 120 Prelude SystemF> putStrLn $ showExprExplicit eFat <interactive>:1:28: Not in scope: `eFat' Prelude SystemF> putStrLn $ showExprExplicit eFact (EFix (ELam "rec" (TArrow (TPrim I#) (TPrim I#)) (ELam "z" (TPrim I#) (EAp (EAp (EAp (ETypAp (EGamma "A" (EPrim if#)) (TPrim I#)) (EAp (EAp (EPrim eq#) EVar) (EPrim 0))) (EPrim 1)) (EAp (EAp (EPrim times#) EVar) (EAp (EPush EVar) (EAp (EAp (EPrim minus#) EVar) (EPrim 1)))))))) Prelude SystemF> :t (whnf (EAp eFact (EPrim "6" 6))) (whnf (EAp eFact (EPrim "6" 6))) :: Expr Integer NDecl NDecl Prelude SystemF> (whnf (EAp eFact (EPrim "6" 6))) if#*** Prelude SystemF> extractPrim (whnf (EAp eFact (EPrim "6" 6))) Just 720

1 0

Specializing classes with classes
by alex 30 Dec '07

30 Dec '07

Hi, I am new to everything Haskell and am stumped on an aspect of Type Classes. I would like to do this: class Foo t where hi :: t -> Bool class Foo t => Bar t where hi x = True But using GHC 6.8.1 on PPC I get this error: `hi' is not a (visible) method of class `Bar' The actual problem is more like this: class Num a => Alg a where (<>) :: Mat m => m -> a -> a -- matrix multiply class Alg v => Vec v where toRow, toColumn :: Mat m => v -> m fromRow, fromColumn :: Mat m => m -> v fromRow = fromColumn . transpose toRow = transpose . toColumn (<>) m v = fromColumn (m <> (toColumn v)) class Alg m => Mat m where transpose :: m -> m (I hope that's readable...) I'm having trouble finding what I need with Google, and SOE doesn't really go into specifics. Thanks for any help... Alex Make the switch to the world's best email. Get the new Yahoo!7 Mail now. www.yahoo7.com.au/worldsbestemail

5 6

encountering another STM problem .. "teething problems" ; ^)
by Galchin Vasili 30 Dec '07

30 Dec '07

Hello, Here is a code snippet: type Resource = TVar Int putR :: Resource -> Int -> STM () putR r i = do { v <- readTVar r; writeTVar r (v + i) } getR :: Resource -> Int -> STM () getR r i = do { v <- readTVar r; if (v < i) then retry else writeTVar r (v-i)} createR :: Int -> STM (Resource) createR i = newTVar i main = do {atomic (r <- (createR 1); putR r 1)} Strangely I am not getting a type error rather a syntax on the "<-" in main. ?? Kind regards, Vasya

2 1

More newbie typeclass confusion...
by alex 30 Dec '07

30 Dec '07

Hi there. If someone can tell me why I am getting type ambiguity in the following code: class (Ord s, Num s) => Scalar s where zero :: s class Metric m where delta :: Scalar s => m -> m -> s (=~) :: m -> m -> Bool (/~) :: m -> m -> Bool (=~) a b = (delta a b) <= zero (/~) a b = not (a =~ b) I will scream. The error I get compiling is: Ambiguous type variable `s' in the constraint: `Scalar s' arising from a use of `delta' at test.hs:13:23-31 Probable fix: add a type signature that fixes these type variable(s) If I change "Scalar" to "Num", and "zero" to "0" within the class "Metric", then it works. I don't get it. "zero" is defined in the "Scalar" class, and "delta" returns a Scalar, and Scalar derives "Ord", so where's the ambiguity? Thanks. I will understand Haskell eventually... =) Make the switch to the world's best email. Get the new Yahoo!7 Mail now. www.yahoo7.com.au/worldsbestemail

3 2

Re[6]: [Haskell-cafe] Re: Wikipedia on first-class object
by Bulat Ziganshin 30 Dec '07

30 Dec '07

Hello Miguel, (i returned to cafe) >> as i've said in other letter, >> both computers and humans may be described using mathematics > Erm... What do you really mean by "may"? "can" > I'm pretty sure you can't > give such description right now - otherwise you should already have > several Nobel prizes. for description of computer? :) i mean that if humans are *physical* creatures then their behavior should obey physic laws which is described in mathematical language. so we should either recognize that mathematics can't be used to describe physical world (i.e. so-called Nature Laws are not exist) or that human, as any other physical object, obeys to these mathematical equations but according to your logic, anyone who says that Nature obeys to the laws, should publish all these laws immediately? > It seems reasonable that even if such > description is theoretically possible, no human would ever read it - > despite that it can be dropped to the Earth from UFO. it's true for whole Science - we can't read so-called Nature Laws from original, we can just believe in their existence and make assumptions about them based on experiments. but all our experiments (facts) are just finite of points - how we can draw whole function by several points? assumptions, assumptions... and we already know beautiful examples of scientists being wrong in their assumptions. to be exact, the whole history of science is history of replacing old wrong assumptions with new ones. even most well-known "laws" of Newtonian physics become obsolete just 300 years later so... my opponents use double-thinking at 100% - they believe in Nature Laws and free will, they love Science but sure that it cannot describe human. i don't mean personalities - it's just set of beliefs that society gives to each of us. moreover, every man should believe that he is thinking, but he should never really think and draw his own conclusion different from orthodox ones. it's double-thinking again. but society learn us that double-thinking is "inferior" (noone knows that is "inferiority" but we should avoid it). here set of beliefs obviously makes self-contradict and because we are teached to not be self-contradict, this at last end requires some personal selection. are you prefer to be self-contradict or not think, or something else - fortunately, society doesn't programmed us ho to make this choice. it's up to every one of us :) -- Best regards, Bulat mailto:Bulat.Ziganshin@gmail.com

2 1

Sending bottom to his room
by Cristian Baboi 30 Dec '07

30 Dec '07

In The Implementation of Functional Programming Languages by S.P. Jones, section 2.5.3, page 32 it is written: Eval [[*]] a b = a x b Eval [[*]] _|_ b = _|_ Eval [[*]] a _|_ = _|_ but in section 2.5.2 it is said that _|_ is an element of the value domain. What business does it have on the left side of the '=' ? Can you help me send him to his room ?

5 8

FFI question -- was: [Haskell-cafe] New slogan for haskell.org
by Steve Lihn 30 Dec '07

30 Dec '07

On Dec 11, 2007 11:16 PM, Don Stewart <dons(a)galois.com> wrote: > > 2. It offers strong support for integration with other languages and tools > > (FFI? Is the support strong?) > > 2. The FFI in Haskell is perhaps the most powerful out there. > You can import C or export Haskell to C with a single line FFI decl. > Don, I am a newbie on FFI, but have been interested in looking into it. If I google on "haskell FFI", what I find is typically: 1. The official Haskell 98 FFI 1.0 2. The FFI preprocessor -- greencard on haskell.org, where several other preprocessors are listed (in the Link section): HaskellDirect, C to Haskell, hsc2hs (included in GHC distributions), QForeign, KDirect. The FFI 1.0 API is certainly the most robust tool. However, the existence of preprocessors seem to suggest (at least very enticing) that the use of a preprocessor would make life easier. But when I dig deeper, several of them have not had any release in (more than) 3 years. The only exception seem to be "C to Haskell", which is part of gtk2hs, and hsc2hs, being part of GHC. It immediately becomes confusing which tool I should use if I were to work on a FFI project... Can you shed some light on this? Thanks, Steve

4 4

Interesting data structure
by Tim Docker 30 Dec '07

30 Dec '07

I'm using a control structure that's a variation of a monad and I'm interested in whether - it's got a name - it deserves a name (!) - anything else similar is used elsewhere Please excuse the longer post... I have two programs that need to interact with the outside world, and I want to constrain the nature of these interactions. I don't want to just sprinkle IO throughout the code. In the first program, I am reading on-demand from a database - just reading, not making any changes. In the second, I am requesting computations to be evaluated externally, in order to take advantage of a grid of machines. In both of these cases, the external requests don't change the state of the world, and the programs can be consided "pure" as long as the world isn't being changed by some other means. Hence the requests can be reordered, performed in parallel, and optimised in various ways without affecting the result. To make this concrete, if an external request has a type like: a -> m b where a is the input to the request, b is the response, and m is some monad, probably IO. Then a data structure capturing calculations over these requests, with result type c can be: data SCalc a b c = SCResult c | SCStage { sc_req :: a, sc_calc :: b -> SCalc a b c } The idea is that we either have a result, or we need to make a external request, whose response is used to generate a new calculation. "Running" such a calculation is straightforward: runSC :: (Monad m ) => SCalc a b c -> (a -> m b) -> m c runSC (SCResult v) _ = return v runSC (SCStage a cf) reqf = do b <- reqf a runSC (cf b) reqf and calculations can be sequence by making a monad instance: instance Monad (SCalc a b) where return = SCResult (>>=) (SCResult v) cf = cf v (>>=) (SCStage a cf1) cf = SCStage a (\b -> cf1 b >>= cf) Where it gets interesting, however, is that when they don't depend on each other, we can run these calculations in parallel. We need the ability to merge requests, and split responses. Hence, class Req a where merge :: a -> a -> a class Resp b where split :: b -> (b,b) par :: (Req a, Resp b) => SCalc a b c -> SCalc a b d -> SCalc a b (c,d) The par primitive above can be used to define other parallel operations, such as parList :: (Req a, Resp b) => [SCalc a b c] -> SCalc a b [c] I found it worthwhile to try and visualise what's going on here. Let's say I have 4 calculations that I want to run in parallel. The first doesn't need a request; the second needs to make a single request (A1); the third needs to make two requests where the second (B2) depends on the result of the first (B1), etc. The resulting parallel operations will be done in 3 batches, looking like: batch1 batch2 batch3 result calc1 ------------------------> V0 calc2 A1 ----------------> V1 calc3 B1 B2 -------> V2 calc4 C1 C2 C3 --> V3 (excuse the ascii art). batch1 will consist of A1,B1,C1 merged together; batch2 of B2,C3 merged; etc. In practice, I've used the above data types to abstract out database access in some reporting code. It works quite well, as use of the parallel primitive above means that the haskell code talking to the database sees all of the information in each batch simultaneously, so it can optimise the queries, remove redundant requests etc. It also makes the reporting code pure despite the fact that information is being loaded on demand from the db (without any unsafe calls behind the scene). I guess the use of the term "pure" here should be qualified: the impure code has been factored out to a single function in a different module that has a limited and well defined interface. I haven't implemented the grid calculation example described above, though I see that it ought to be able to work similarly, potentially removing duplicate calculation requests, etc. So my questions are: does this sort of "monad allowing parallel evaluation" structure have a name? Is it an existing design pattern in fp somewhere that I haven't seen? thanks, Tim

5 4

Doing some things right
by Don Stewart 29 Dec '07

29 Dec '07

"A Wake Up Call for the Logic Programming Community" Or what the logic programming community can learn from the Haskell community (in particular): http://www.cs.kuleuven.ac.be/%7Edtai/projects/ALP//newsletter/dec07/content… Interesting read! -- Don

13 23

Haskell-cafe reply-to etiquette
by Justin Bailey 29 Dec '07

29 Dec '07

When I joined the haskell-cafe mailing list, I was surprised to see the "reply-to" header on each message was set to the sender of a given message to the list, rather than the list itself. That seemed counter to other mailing lists I had been subscribed to, but I didn't think too much about it. Well, the comment below prompted me to ask the question - do people care? Personally, I like to respond to the "list", keeping discussions open by default and reducing (potential) spam on someone's inbox. As impersonal as email can be, it also seems a bit intrusive to write someone directly who I don't have a prior relationship with except for responding to some message they happened to post. Thoughts? ---------- Forwarded message ---------- From: Claus Reinke <claus.reinke(a)talk21.com> Date: Dec 26, 2007 2:07 PM Subject: Re: [Haskell] Re: ANNOUNCE: GHC version 6.8.2 To: "Benjamin L. Russell" <dekudekuplex(a)yahoo.com>, haskell(a)haskell.org [if this thread has to keep running, could you please drop me of the cc? and how did it manage to end up on haskell@ - that is the wrong list ] ....

6 5

Re: [Haskell-cafe] Doing some things right
by Miguel Mitrofanov 29 Dec '07

29 Dec '07

> > ? Kay's definition of OOP necessarily implies imperative behaviour. > OCaml has purely functional object update IIRC. That's OK, but it doesn't seem to agree with Kay's definition. Erlang definitely does.

1 0

Unbuffered character IO under Windows XP?
by Peter Schmitz 29 Dec '07

29 Dec '07

module Main(main) where import System.IO main = do b1 <- hGetBuffering stdin print b1 b2 <- hGetBuffering stdout print b2 -- not sure if these help, or are needed hSetBuffering stdin NoBuffering hSetBuffering stdout NoBuffering b1 <- hGetBuffering stdin print b1 b2 <- hGetBuffering stdout print b2 putStr "0" c <- getChar -- echoes during input by default putStr "1" -- want this output w/o hitting Enter hFlush stdout -- adding this does not help putStrLn [c] {------------------- Output: E:\ghcTest>ghc --version The Glorious Glasgow Haskell Compilation System, version 6.8.1 E:\ghcTest>ghc --make main [1 of 1] Compiling Main ( main.hs, main.o ) Linking main.exe ... E:\ghcTest>main LineBuffering LineBuffering NoBuffering NoBuffering 0a 1a E:\ghcTest> ----------------- Question: Is it possible to have unbuffered character IO under Windows XP? I would like to be able to type a single character and have the processing and IO continue without having to hit Enter. I.e., rather than this (had to hit Enter after typing the 'a' in '0a'): 0a 1a I would like to have this (without having to hit Enter after typing the char): 0a1a I have tried a few combinations of hSetBuffering and put/get Str/Char functions, without success. NOTE: I need to run under Windows XP, non-administrator account. This test was run using the normal XP shell cmd.exe. Thanks much in advance. -- Peter --------}

3 2

Re: [Haskell-cafe] Re: Wikipedia on first-class object
by Miguel Mitrofanov 29 Dec '07

29 Dec '07

> It sounds like a limit. xn --> x for n --> :-) Yeah, that's right. > How can I get that maximal value when I start from a non maximal one ? Not a problem. For example, "const 1" maps each value (even (_|_)) to the maximal value "1". > [1 .. ] and x=1:x are maximal ? Yes.

3 6

Re: [Haskell-cafe] Re: Wikipedia on first-class object
by Miguel Mitrofanov 29 Dec '07

29 Dec '07

> How can I test that partial order in Haskell ? You can't. It's kinda internal. More specifically, if you try to compute a value, which is not maximal in this order, you'll get an error or loop forever. But you still can use such values in your programs if you don't try to compute them. The point is, maximal values are the only interesting ones, and all others are just approximations. In fact, you only have to worry about non-maximal values when you use recursion. The fundamental property of Haskell is that it computes the least fixed point. For example, if f maps (_|_) to (_|_), then "let x = f x in x" will evaluate to (_|_) - and you loop forever, see above.

4 3

Re: [Haskell-cafe] Wikipedia on first-class object
by Cristian Baboi 29 Dec '07

29 Dec '07

On Thu, 27 Dec 2007 14:02:36 +0200, Lennart Augustsson <lennart.augustsson(a)gmail.com> wrote: > Comparing functions is certainly possible in Haskell, but there's no > standard function that does it. > If course, it might not terminate, but the same is true for many other > comparable objects in Haskell, e.g., infinite lists (which are > isomorphic to > Nat->T). The list [1 .. ] is a single value in Haskell ? ________ Information from NOD32 ________ This message was checked by NOD32 Antivirus System for Linux Mail Servers. part000.txt - is OK http://www.eset.com

12 38

Function Stores
by Cristian Baboi 29 Dec '07

29 Dec '07

Here is what I was able to come up with: module Store where data FStore1 a b = Empty1 | FS1 ( a->b , FStore1 a b ) store1 :: (a->b) -> (FStore1 a b) store1 f = let x = FS1 (f, x) in x data FStore2 a b = Empty2 | FS2 ( a->b , FStore2 a b, FStore2 a b ) store2 :: (a->b) -> (FStore2 a b) store2 f = let x = FS2 (f,x,x) in x First I tryed store1, but after a while it went out of business. Second I tryed other business model with store2. Same result. Then I tryed to make a street of stores. After a while, they all went out of business. Then I tryed to make several streets of stores. After a while they all went out of business. Then I tryed to make an infinite street of stores. After a while they all went out of business. Then, following some rumors that there is a store on that street with some function in stock that might be willing to give it away, I noticed that the street disappeared, and I cannot go to that store. Can you help me find it ?

3 2

OT: ghc 6.8 in Debian?
by Magnus Therning 29 Dec '07

29 Dec '07

It seems my emails to the Debian Haskell list (CC'd on this email as well) are silently dropped :-( Since I suspect the readers of /that/ list are likely to be on this one as well I thought I'd give this a try. What's happening with GHC 6.8 in Debian? There was an email about a month ago announcing the availability of the core package. Since then it seems very little has happened. What can I do to help, if anything? /M -- Magnus Therning (OpenPGP: 0xAB4DFBA4) magnus＠therning．org Jabber: magnus．therning＠gmail．com http://therning.org/magnus What if I don't want to obey the laws? Do they throw me in jail with the other bad monads? -- Daveman

2 1

nice simple problem for someone struggling....
by Nicholls, Mark 29 Dec '07

29 Dec '07

I'm just trying to pick up the basics....and I've managed to write this code...which remarkably works...... module Main where data SquareType = SquareConstructor Int class ShapeInterface shape where area :: shape->Int data ShapeType = forall a. ShapeInterface a => ShapeType a instance ShapeInterface SquareType where area (SquareConstructor sideLength) = sideLength * sideLength main = do putStrLn (show (area (SquareConstructor 4))) name <- getLine putStrLn "" But my next iteration was to try to parametise SquareType.... So something like data SquareType a = Num a => SquareConstructor a but of course doing this breaks everything.......sepecifically the instance declaration `SquareType' is not applied to enough type arguments Expected kind `*', but `SquareType' has kind `* -> *' In the instance declaration for `ShapeInterface SquareType' And I can't seem to get it to work.....

10 26

FFI"pointer" data types question
by Galchin Vasili 28 Dec '07

28 Dec '07

Hi, If I am calling a ANSI function that requires a pointer to a C struct, which FFI pointer type should use? Kind regards, Vasya

2 1

Re: [Haskell-cafe] Re: Wikipedia on first-class object
by Miguel Mitrofanov 28 Dec '07

28 Dec '07

> >> take 5 [1,2,3,4,5,undefined] > (What would GHCi print? [1,2,3,4,5] > Then please square that with your comment that we don't care how much of > the list gets evaluated.) I don't care if it evaluates it, reads it's answer from some internal table, or consult it's lawyer. I just want it to agree with my reasoning about complete partial orders; that's all I need. I don't know how to explain this clearer.

2 1

Re: [Haskell-cafe] Re: Wikipedia on first-class object
by Miguel Mitrofanov 28 Dec '07

28 Dec '07

> I'll bite. Please, don't. > So what do you expect: > take 5 [1,2,3,4,5,undefined] > to do? Nothing! It's a value, not an instruction! > So it does seem to matter how much of the list it evaluates... No, it's implementation details. I'm reasoning about values, not computations, and I use complete orders for reasoning. What really matters is that implementation gives me results that agree with this reasoning - until we consider performance issues. > > Well, that depends of your notion of interesting. > I defined it - I'm interested in things that are observationally > equivalent. That's your choice, and I certainly can't argue, but I can't help mentioning that this is a bit weird.

2 1

Re: [Haskell-cafe] Re: Wikipedia on first-class object
by Miguel Mitrofanov 28 Dec '07

28 Dec '07

> Right, so when I say to GHCi: > Prelude> take 5 [1..] > and it says: > [1,2,3,4,5] > then it really has computed the entirety of the infinite sequence > [1..], and not some approximation? Of course not! In fact, it doesn't even compute the value "1". It just juggles with the bytes somehow, so that it seems that it computes the whole sequence and then applies "take 5". But does it really matter if it computes 5 integers, or 10, or the whole sequence, at least if we are not concerned about performance? > Perhaps it is better to say that in a lazy language both > approximations and maximal values (least upper bounds of a chain) are > interesting. Well, that depends of your notion of interesting.

3 2

Re: [Haskell-cafe] Re: Wikipedia on first-class object
by Miguel Mitrofanov 28 Dec '07

28 Dec '07

> >> It sounds like a limit. xn --> x for n --> :-) > > > > Yeah, that's right. > If xn is monotone, then I only have to worry when it is not bounded, right > ? Sorry, don't understand what you mean. Yes, the monotone sequence has a limit. No, it's not necessarily maximal. > >> [1 .. ] and x=1:x are maximal ? > Can you give me some examples of non maximal ones ? let x = x+1 in x Or even let x = x in x In both cases, (_|_) is the least fixed point, and is non-maximal.

1 0

Re: [Haskell-cafe] Re: Wikipedia on first-class object
by Miguel Mitrofanov 28 Dec '07

28 Dec '07

> Questions: > The fact that Haskell functions are monotone continuous functions on > pointed complete partial orders imply this ? > - every domain in Haskell is a "pointed complete partial order", including > domains of functions ? Right. > - the "structure" of a domain is preserved in the result when you apply a > Haskell function to it ? Right, if your "structure" means "complete partial order". Note that (_|_) (the least element in this order) is not necessarily preserved. > - every domain can be enumerated ? Ehm... No. Well, it depends on what you mean by "enumeration".

2 1