- Haskell-Cafe - Haskell.org

Rendering TTF fonts in Haskell and OpenGL
by Jefferson Heard 28 Feb '08

28 Feb '08

In C and in Java, I can use truetype fonts in Haskell using select libraries, and I'd like to be able to do the same in Haskell. Are there any portable libraries out there for loading fonts into OpenGL geometry for Haskell? I can use the vector fonts from GLUT is absolutely neccessary, but I'd like something that looks a little more professional. Are there any suggestions? -- Jeff -- I try to take things like a crow; war and chaos don't always ruin a picnic, they just mean you have to be careful what you swallow. -- Jessica Edwards

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ANNOUNCE: type-level 0.1: Haskell type-level programming library
by Alfonso Acosta 28 Feb '08

28 Feb '08

The goal of the "Type-level" library is to standardize and extend the features offered by the multiple (and heterogeneous) type-level programming implementations already around. To date, type-level Booleans and arbitrary sized Naturals are supported. I implemented a few things I didn't see in any other type-level programming libraries: * Numerals use decimal representation to make error reports friendlier. * Type-level numerals can be intuitively typed in different bases (although everything is internally represented in base 10) by using aliases: D16 represents the type-level decimal 16 B1010 represents the type-level binary 1010 076 represents the type-level octal 76 HFF represents the type-level hexadecimal FF * All basic arithmetic operations are supported (including logarithm and exponentiation in any base) * The library provides reification of Naturals and Booleans (from value-level to type-level) . To get the code: HackageDB page (including haddock documentation): http://hackage.haskell.org/cgi-bin/hackage-scripts/package/type-level-0.1 Darcs repository: http://code.haskell.org/type-level/ Big Thanks to Oleg and Wolfgang for their suggestions and contributions. Actually, most of the implementation of Naturals is based on Oleg's "Binary Type Arithmetic" http://okmij.org/ftp/Haskell/types.html#binary-arithm

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Haskell Jobs
by Tom Hawkins 28 Feb '08

28 Feb '08

Hi, We have an opening for a software engineer, with the potential for a lot of Haskell development. Our group at Eaton (http://www.eaton.com/) develops real-time control software for vehicle and machinery applications. This position is specifically for the design and verification of hydraulic hybrid vehicle systems. Think Toyota Prius, but with a accumulator and hydraulic pump instead of a battery and an electric motor. That, and these vehicles can weight over 30,000 pounds. The primary tasks would include vehicle software design, system verification with simulation and possibly formal analysis, telemetry development for remote diagnostics, and tool development to further automate our design flows. Currently we use Haskell for our in-house data analysis tools, and some vehicles run code partially generated by a Haskell DSL. We hope to expand our use of Haskell, especially for simulation regression suites -- maybe QuickCheck in combination with a system modeling and verification DSL. If interested, send a resume. Thanks! -Tom

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ANNOUNCE: hsparklines 0.1.0 - A sparklines implementation in Haskell
by Hitesh Jasani 28 Feb '08

28 Feb '08

Sparklines are small, word sized graphs that can be interspersed with text to provide context and enhance communication. There are implementations in many languages and even some web services that will generate them on the fly. I was looking for a Haskell solution and finding none, wrote my own. * http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hsparklines-0.1.0 * http://www.jasani.org/2008/02/initial-release-of-hsparklines-010.html On a side note, while writing my blog entry I decided to generate a sparkline for the number of uploads to hackage. If it's any indication, the Haskell community is growing by leaps and bounds. - Hitesh

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where is module System.Posix.Types?
by Galchin Vasili 28 Feb '08

28 Feb '08

Hello, I don't see the types modules in with the other modules in the Posix pacakage, ?? Thanks, Vasya

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Just a quick question
by Imam Tashdid ul Alam 28 Feb '08

28 Feb '08

A few days back, I *think* I stumbled upon the statement "the interaction between GADTs and functional dependencies is not yet well understood". Then I glossed over it. Now that I have (finally) started understanding what GADTs are meant to do, I am somewhat terrified. Did I read that one correctly? Or was it "... between GADTs and *typeclasses*..."? Was it? Gah. That would be disastrous. ____________________________________________________________________________________ Never miss a thing. Make Yahoo your home page. http://www.yahoo.com/r/hs

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Re: [Haskell-cafe] fast integer base-2 log function?
by Jens Blanck 28 Feb '08

28 Feb '08

> {-# LANGUAGE MagicHash #-} > import GHC.Exts > import Data.Bits > > -- experiment with using a LUT here (hint: FFI + static arrays in C) > ilog2i0, ilog2i1, ilog2i2, ilog2i3, ilog2i4 :: Int -> Int -> Int > ilog2i0 k x | x .&. 0xFFFF0000 /= 0 = ilog2i1 (k + 16) (x `shiftR` 16) > | otherwise = ilog2i1 k x > ilog2i1 k x | x .&. 0xFF00 /= 0 = ilog2i2 (k + 8) (x `shiftR` 8) > | otherwise = ilog2i2 k x > ilog2i2 k x | x .&. 0xF0 /= 0 = ilog2i3 (k + 4) (x `shiftR` 4) > | otherwise = ilog2i3 k x > ilog2i3 k x | x .&. 0xC /= 0 = ilog2i4 (k + 2) (x `shiftR` 2) > | otherwise = ilog2i4 k x > ilog2i4 k x | x .&. 0x2 /= 0 = k + 1 + (x `shiftR` 1) > | otherwise = k + x > > log2i :: Integer -> Int -- actually returns bit length, and returns > garbage on negatives, but do you care? > log2i (J# len adr) = ilog2i0 0 (I# (indexIntArray# adr (len -# 1#))) + I# > (32# *# (len -# 1#)) > log2i (S# sml) = ilog2i0 0 (I# sml) > > I tried the above but I got wrong results on 2^31..2^32-1 because in the additions in ilog2i4, the number x was -1 because of sign extension performed by the shifts all the way (thanks for the ghci debugger). (So, yes, I do care somewhat about garbage on negatives :) I modified to the following hoping also to use both on 32 and 64 bit machines. Have I shot myself in the foot anyway? For example, is there a guarantee that the most significant limb is non-zero? Is there any possibility of this or some related function being added to Data.Bits? {-# LANGUAGE MagicHash #-} import GHC.Exts import Data.Bits limbSize = bitSize (0 :: Int) ilog2 k x = case limbSize of 64 -> ilog2i0 k (fromIntegral x) 32 -> ilog2i1 k (fromIntegral x) _ -> undefined -- experiment with using a LUT here (hint: FFI + static arrays in C) ilog2i0, ilog2i1, ilog2i2, ilog2i3, ilog2i4, ilog2i5 :: Int -> Word -> Int ilog2i0 k x | x .&. 0xFFFFFFFF00000000 /= 0 = ilog2i1 (k + 32) (x `shiftR` 32) | otherwise = ilog2i1 k x ilog2i1 k x | x .&. 0xFFFF0000 /= 0 = ilog2i2 (k + 16) (x `shiftR` 16) | otherwise = ilog2i2 k x ilog2i2 k x | x .&. 0xFF00 /= 0 = ilog2i3 (k + 8) (x `shiftR` 8) | otherwise = ilog2i3 k x ilog2i3 k x | x .&. 0xF0 /= 0 = ilog2i4 (k + 4) (x `shiftR` 4) | otherwise = ilog2i4 k x ilog2i4 k x | x .&. 0xC /= 0 = ilog2i5 (k + 2) (x `shiftR` 2) | otherwise = ilog2i5 k x ilog2i5 k x | x .&. 0x2 /= 0 = k + 1 + fromIntegral (x `shiftR` 1) | otherwise = k + fromIntegral x log2i :: Integer -> Int -- actually returns bit length log2i (J# len adr) = ilog2 0 (I# (indexIntArray# adr (len -# 1#))) + I# (32# *# (len -# 1#)) log2i (S# sml) = ilog2 0 (I# sml)

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Trouble finding exception source
by Denis Bueno 28 Feb '08

28 Feb '08

Hi all, I've got some code crashing with "Prelude.foldr1: empty list". In GHCi, the code uses too much memory (I kill it after it consumes 1GB) to be able to use :trace and :history, but I just found out about the -xc RTS option. I tried that, and I get the following: <GHC.List.CAF>dsat: Prelude.foldr1: empty list Now, manually going through GHC.List (http://haskell.org/ghc/docs/latest/html/libraries/base/src/GHC-List.html) I only found one CAF, namely prel_list_str, at the bottom of the file. However, it is late and I quite possibly missed something. This CAF, of course, doesn't seem a likely culprit. I also stumbled on some discussion of the explicit call stack (http://hackage.haskell.org/trac/ghc/wiki/ExplicitCallStack/StackTraceExperi…) which exemplifies the behavior I've found, but doesn't seem to suggest a way to fix it. I'm developing on GHC 6.8.2. I've attached my code, the output of a cabal sdist, which requires parse-dimacs [0] and bitset [1] (both cabalised) to build. If you build the executable (with profiling), you should see my behavior by running: ./dist/build/dsat/dsat -verify +RTS -xc I know that the problem is somewhere in the "Backtracking" section, in the backJump function. But I don't know where. Any help is appreciated. -- Denis [0] http://hackage.haskell.org/cgi-bin/hackage-scripts/package/parse-dimacs-1.0… [1] http://hackage.haskell.org/cgi-bin/hackage-scripts/package/bitset-0.5

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Stack overflow
by Grzegorz Chrupala 28 Feb '08

28 Feb '08

Hi all, I have a very simple program which reads a Data.Map from a file using Data.Binary and Data.ByteString.Lazy, which gives stack overflow with files over a certain size. Any ideas of what might be causing it? You can try it with the small file (11M) at: http://computing.dcu.ie/~gchrupala/map.bin import System import Data.Map import qualified Data.ByteString.Lazy as BS import Data.Binary main = do [path] <- getArgs m <- fmap decode (BS.readFile path)::IO (Map (Int,Maybe String) Int) putStrLn . show . findMax $ m -- Grzegorz -- View this message in context: http://www.nabble.com/Stack-overflow-tp15479718p15479718.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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RFC: SAT solver using Cont/callCC for backtracking search
by Denis Bueno 27 Feb '08

27 Feb '08

Hi all, I've recently done a small Haskell port of some OCaml code from a paper entitled "SAT-MICRO: petit mais costaud!" It's a tiny (one emacs buffer for the algorithm, ~160 lines overall) DPLL SAT solver with non-chronological backtracking, implemented using the Cont monad and callCC. If anyone has time and interest, I'm requesting comments on how the code could be improved in any way: style, efficiency, etc. The code in the paper is OCaml which uses exceptions as the main control-flow mechanism for backtracking. If you can read French, you may be interested in the paper, which is available here: http://hal.inria.fr/inria-00202831/en/ Of course, you will likely understand the OCaml code even if you don't understand French. Thanks in advance to anyone who comments. -- Denis

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Re: [Haskell-cafe] Problem with Python AST
by Roel van Dijk 27 Feb '08

27 Feb '08

On Fri, Feb 22, 2008 at 2:42 PM, Daniel Gorín <dgorin(a)dc.uba.ar> wrote: > > On Feb 21, 2008, at 7:55 PM, Roel van Dijk wrote: > > > Your solutions allows a bit more but fails with the equivalent of > > > > def foo(): > > for i in range(10): > > if i == 6: > > return None > > > > The loop context 'overwrites' the function context which makes the > > return statement illegal. I think I need a type level list. > > i see. how about this? > > if_ = If [(IntLit 6, Suite [] [Break])] (Just $ Suite [] [Return]) > while_ = While (IntLit 6) (Suite [] [if_]) Nothing > while2_ = While (IntLit 6) (Suite [] [Return]) Nothing > foo = FunDecl $ Suite [] [while_, while2_] > > p = Program [foo] > > > newtype Ident = Id String > > data BinOp = Add > | Sub > > data Exp = IntLit Integer > | BinOpExp BinOp Exp Exp > > data Ctx reqloop reqfun > data True > data False > > data Statement ctx where > If :: [(Exp, Suite (Ctx reqloop reqfun))] > -> Maybe (Else (Ctx reqloop reqfun)) > -> Statement (Ctx reqloop reqfun) > While :: Exp > -> (Suite (Ctx True reqfun)) > -> Maybe (Else (Ctx True reqfun)) > -> Statement (Ctx reqloop reqfun) > Pass :: Statement (Ctx reqloop reqfun) > Break :: Statement (Ctx True reqfun) > Return :: Statement (Ctx reqloop True) > FunDecl :: Suite (Ctx False reqfun) -> Statement (Ctx False False) > > newtype Global = Global [Ident] > > data Suite ctx = Suite [Global] [Statement ctx] > > type Else ctx = Suite ctx > > newtype Program = Program [Statement (Ctx False False)] Ah, you set a specific context when needed. Very nice. Although I had to remove the Ctx type. Otherwise I need pattern matching at the type level to bind the reqLoop and reqFun type variables (is such a thing even possible?): data Statement (Ctx reqLoop reqFun) where .... Now I simple pass 2 type variables to my statements: data Statement reqLoop reqFun where .... I now have a complete Python AST and pretty printer. Onwards towards a parser! Thanks again, Roel

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Re: [Haskell] Re: [Haskell-cafe] GSoC 2008
by Duncan Coutts 27 Feb '08

27 Feb '08

On Tue, 2008-02-26 at 23:39 +0100, Lemmih wrote: > On Tue, Feb 26, 2008 at 8:43 PM, Johan Tibell <johan.tibell(a)gmail.com> wrote: > > On Mon, Feb 25, 2008 at 3:28 PM, Malcolm Wallace > > <Malcolm.Wallace(a)cs.york.ac.uk> wrote: > > > Google has today announced that they will be running their Summer of > > > Code programme again this year. > > > http://code.google.com/soc/2008/ > > > > > > haskell.org has had many student projects funded by this programme, in > > > 2006 and 2007. Would you like to participate? Either as a student or > > > a mentor? > > > > I'd like to see a Unicode equivalent of ByteString and I'm willing to > > mentor someone who's willing to implement it. > > Doesn't compact-string support unicode? Yes but not everyone agrees that it had the right interface. There was an interesting discussion on #haskell a month or so ago on the question of whether a string type should be parametrised by an encoding type or not. John and I were amongst those of the view that it should not. I also noted to John earlier that a MSc student I'm supervising is expected to complete this project before the GSoC coding phase kicks off so it's probably not the best choice for a GSoC project. Duncan

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fast graph algorithms without object identities
by Henning Thielemann 27 Feb '08

27 Feb '08

It seems that algorithms on graphs can be implemented particularly efficient in low-level languages with pointers and in-place updates. E.g. topological sort needs only linear time, provided that dereferencing pointers requires constant time. I could simulate pointer dereferencings and pointer updates by Map yielding linear logarithmic time for topological sort. I wonder if it is possible to write a linear time topological sort using lazy evaluation, since the runtime system of Haskell implementations is a graph processor based on pointers.

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GSoC 2008
by Malcolm Wallace 27 Feb '08

27 Feb '08

Google has today announced that they will be running their Summer of Code programme again this year. http://code.google.com/soc/2008/ haskell.org has had many student projects funded by this programme, in 2006 and 2007. Would you like to participate? Either as a student or a mentor? Come and meet up in #haskell-soc on freenode.net. We need to discuss and craft our organisation's application to Google, by March 12th. We need to have ideas for projects before students apply to Google on March 24th. We need volunteers for adminstration. This is community stuff! All sorts of people can play a part. Regards, Malcolm

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Trees building
by Krzysztof Skrzętnicki 27 Feb '08

27 Feb '08

Hi everyone I'm trying to write a piece of Haskell code, but for some reason I can't make it working. I'm asking for your help in finding a bug in this code: > import Data.Tree > import Data.List > expandTree :: Int -> [(Int,Int)] -> ((Tree Int),[(Int,Int)]) > expandTree rootLabel [] = ((Node rootLabel []),[]) -- probably obsolete line > expandTree rootLabel nodePairs = ((Node rootLabel forest),unused) > where > (marked,unmarked) = partition (\ (x,y) -> (x==rootLabel) || (y==rootLabel)) nodePairs > (forest,unused) = foldl > (\ (forestAcc, unusedPairs) node -> let (a,b) = expandTree node unused in ((a:forestAcc),b) ) > ([],unmarked) > (map (\(o,p)-> if o == rootLabel then p else o ) marked) It is supposed to create tree with root labeled with rootLabel, while nodePairs is a description of a tree. If a list is not exhausted, the remaining should be returned. It should work so that: expandTree 1 [(1,2)] == ((Node 1 [(Node 2 [])]),[]) expandTree 1 [(2,3)] == ((Node 1 []),[(2,3)]) expandTree 1 [(1,2),(3,4)] == ((Node 1 [(Node 2 [])]),[(3,4)]) However, the code loops. Any clues? Thanks in advance. Regards Christopher Skrzętnicki

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derivation of mapP, a parallel, lazy map
by Dan Zwell 26 Feb '08

26 Feb '08

Hello, everyone. I have just written a tutorial (sort of) on Haskell concurrency, specifically on the derivation of a function mapP, a parallel version of map. I wrote this because I am fairly new to Haskell, and I didn't realize how easy concurrent code is until I wrote this, and because I could not find a tutorial describing a parallel implementation of map. If anybody is curious to see this, the page is http://zwell.net/content/haskell.html#mapP . I would appreciate any criticism, too. I hope somebody finds this helpful. Dan Zwell

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noob question
by Ben 26 Feb '08

26 Feb '08

I'm just starting to learn Haskell, and I'm having some confusion (I think) with how the type inference is working. Can someone explain why, in ghc 6.8.2 this works: *Main> (1/3)^3 3.7037037037037035e-2 But this doesn't *Main> (\k -> (1/k) ^ k) 3 <interactive>:1:8: Ambiguous type variable `t' in the constraints: `Fractional t' arising from a use of `/' at <interactive>:1:8-10 `Integral t' arising from a use of `^' at <interactive>:1:7-15 Probable fix: add a type signature that fixes these type variable(s) TIA, Ben

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"Computational Category Theory" in Haskell
by Dan Weston 26 Feb '08

26 Feb '08

Has anyone already ported to Haskell the ML code in Rydeheard and Burstall, "Computational Category Theory"? Dan

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Graphical graph reduction
by dainichi＠gmail.com 26 Feb '08

26 Feb '08

Hi Haskell-Cafe, I'm relatively new to Haskell, but have a background with SML. One of the things that amaze me about Haskell is lazy graph reduction, e.g. how the graph unfolds during the evaluation of, say, let fibs = 1 : 1 : zipWith (+) fibs (tail fibs) in take 10 fibs Lazy lists can be simulated in SML too, but unless I do something clever with references, I end up taking exponential time to compute the n'th Fibonacci number. Now to the point: Wouldn't it be great if I had a visual tool that visually showed me the graph while the above evaluation unfolded? I could use it to show some of my co-workers to whom laziness is a mystery, what it's all about. Does anybody know if such a tool exists? I'd be grateful for pointers if it does. I very much doubt that I'm the first person who has thoughts like this, but then again, who knows. People who really know Haskell might think this is too trivial a task to really be worth spending time on. If nothing similar exists, I was thinking about creating such a tool (i.e. an interpreter with additional graph-displaying features) for a very, very small subset/dialect of Haskell. I would probably be lazy (no pun intended) and start right away with abstract syntax trees to avoid lexing and parsing and such. My language of implementation would be SML, using references as the edges of the graph. Any ideas/comments would be welcome. Kai

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Using template haskell to create pseudo columns
by Justin Bailey 26 Feb '08

26 Feb '08

Adding pseudo columns to a projection is cumbersome, to say the list. For example, to add a field named "hidden" to a query I have to write: data Hide = Hide instance FieldTag Hide where fieldName _ = "hide" hideField = mkAttr Hide I wrote a little Template haskell that reduces this to: $(mkField "hide") If you want to customize the name of the column in the generated SQL, another function is used: $(mkFieldWithName "NumPullingParts" "num_pulling_parts") The code is below. Some questions: * Does the code look reasonable and useful? * Would the haskelldb maintainers want to introduce a dependency on template haskell to haskelldb for this kind of code? * Otherwise, would an "extensions" package make sense? If the reaction is positive, I'll work on extending this for a patch. My heartfelt thanks to Bulat for his excellent template haskell tutorials. I really wish that library had better documentation! Justin mkField :: String -> Q [Dec] mkField [] = error "Can't generate field from an empty string." mkField f = let fieldType = if isUpper (head f) then f else (toUpper (head f)) : tail f colName = if isLower (head f) then f else (toLower (head f)) : tail f in mkFieldWithName fieldType colName -- ^ Creates a compile time field declaration using the given -- arguments for the type and column name of the field. The fieldType argument -- is used to produce the "<field name>Field" function which can be used to -- add the field to a projection. -- -- Note that an error will occur if fieldType is not a proper type/constructor name. mkFieldWithName :: String -> String -> Q [Dec] mkFieldWithName [] _ = error "Can't create a field with an empty type." mkFieldWithName _ [] = error "Can't create a field with an empty column name." mkFieldWithName fieldType colName = let ty = mkName fieldType fieldName = toLower (head fieldType) : tail fieldType fieldD = [DataD [] ty [] [NormalC ty []] []] fieldI = [InstanceD [] (AppT (ConT (mkName "Database.HaskellDB.HDBRec.FieldTag")) (ConT ty)) [FunD (mkName "fieldName") [Clause [WildP] (NormalB (LitE (StringL colName))) []]]] fieldF = [ValD (VarP (mkName (fieldName ++ "Field"))) (NormalB (AppE (VarE (mkName "Database.HaskellDB.DBLayout.mkAttr")) (ConE ty))) []] in return (fieldD ++ fieldI ++ fieldF)

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hoogle & gtk2hs
by Gour 26 Feb '08

26 Feb '08

Hi! Emacs haskell-mode has haskell-hoogle function to search Haskell API, but it, unfortunately, does not work with gtk2hs API which can be accessed via web at http://haskell.org/hoogle/?package=gtk Any plan to integrate Gtk2Hs so it becomes accessible via haskell-hoogle function? Sincerely, Gour -- Gour | Zagreb, Croatia | GPG key: C6E7162D ----------------------------------------------------------------

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placeholders in hsql
by Roman Cheplyaka 26 Feb '08

26 Feb '08

Is there an ability to use placeholders in SQL statement using hsql? (Actually I'm interested in SQLite.) E.g. "INSERT INTO sales (product_code, qty, price) VALUES (?, ?, ?)" and then supply values that will be escaped and inserted in place of each '?'. -- Roman I. Cheplyaka :: http://ro-che.info/ ...being in love is totally punk rock...

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haskellwiki and Project Euler
by Daniel Fischer 25 Feb '08

25 Feb '08

Hi all, I try not to be too rude, although I'm rather disgusted. I know there are several sites out on the web where solutions to PE problems are given. That is of course absolutely against the sporting spirit of Project Euler, but hey, not all people are sporting. I've found http://www.haskell.org/haskellwiki/Euler_problems irritating for a while, but wasn't overly annoyed by it while it only contained code for solving a few dozen problems. Today I learnt that it now contains code for all problems. Really bad! On top of that, the code for many problems isn't even Haskell, but C, WTF! Other code was submitted without consent of the author, copied from the PE fora, which are restricted access and so, even if perhaps not legally, but in spirit, do not fall under the legitimate resources for haskellwiki: "You are also promising us that you wrote this yourself, or copied it from a public domain or similar free resource. DO NOT SUBMIT COPYRIGHTED WORK WITHOUT PERMISSION!" To make matters worse still, there was a page containing nothing but the answers. That was changed, but Cale chose to reintroduce that crap. I just removed it again. Your turn, Cale. I call on the Haskell community to vote for immediate removal of these pages from the wiki! Show that you're a sporting bunch. Daniel Fischer

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parsing c
by Evan Martin 25 Feb '08

25 Feb '08

I'd like to parse some C headers for a toy project. It seems like c2hs probably has the functionality I want, but it's not packaged up as a library. So two questions: Is there an alternative C-parsing library? Has anyone looked into librarifying c2hs's parser?

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Re: static constants -- ideas?
by Don Stewart 25 Feb '08

25 Feb '08

jay: > Don Stewart dons(a)galois.com: > >jay: > >> I also have constants that are too large to compile. I am resigned to > >> loading them from data files--other solutions seem even worse. > ... > >> Data.Binary eases the irritation somewhat. > > > >Did you try bytestring literals (and maybe parsing them in-memory with > >Data.Binary)? > > That didn't occur to me, since neither of my large constants includes > strings.... I think you're suggesting that each constant could appear in > the source as a long bytestring and be deserialized into the data > structure. If that works, it should improve the startup time, but it's > still not as nice as simply compiling it straight up. > > I'll try it. Here's an example, which stores a Data.Map in a gzip-compressed bytestring literal (a C string literal in the compiled code). The Map is reconstructed on startup. {-# LANGUAGE OverloadedStrings #-} import Data.Binary import qualified Data.Map as M import qualified Data.ByteString.Char8 as S import Data.ByteString.Lazy import Codec.Compression.GZip -- -- this is a gzip compressed literal bytestring, storing a binary-encoded Data.Map -- mytable = "\US\139\b\NUL\NUL\NUL\NUL\NUL\NUL\ETXEN\219\SO\194 \f\197\224\188\196\CAN\227\US\224\171~\NAKc\GS4ce\161`\178\191\215(\176\190\180\167\231\210\n\241\171\203\191\ti\157\217\149\249< \ENQ\214\&9>\202\162\179a\132X\233\ESC=\231\215\164\SYN\157\DC2D\226*\146\174o\t\167\DLE\209\"i_\240\193\129\199<W\250nC\CAN\212\CAN\162J\160\141C\178\133\216;\\@4\144-W\203\209x\205\140\166\RS\163\237]9f\170\143\ACK\163g\223\STX\184\&7\rH\222\FSW\130\&7D\197\NUL\164\&0U\193\186\t\186o\228\180~\NUL\a6\249\137#\SOH\NUL\NUL" main = print =<< M.lookup "ghc" m where -- build the table from the bytestring: m :: M.Map String (Maybe String) m = decode . decompress . fromChunks . return $ mytable Running it: $ ./A Just "dinosaur!" :) Important to use a bytestring, since that gets compiled to a C string literal (and not messed with by the simplifier). -- Don

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A little toy of Haskell Trivia
by Steve Lihn 25 Feb '08

25 Feb '08

I proudly announce a little toy that lists the frequency of modules being imported by other modules. Do you know Control.Monad is the most frequently imported module? I did not! Currently it only includes GHC 6.8 core library. If you have any idea how to parse through HackageDB code, please let me know. http://haskell.ecoin.net/cgi-bin/modules.pl Disclaimer: This is entirely for fun. It is by no means accurate or complete. I only spent a couple hours on a little perl script and a mysql table...

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how to catch keyboard interrupts?
by Uwe Hollerbach 25 Feb '08

25 Feb '08

Hi, all, I am continuing to mess with my little scheme interpreter, and I decided that it would be nice to be able to hit control-C in the middle of a long-running scheme computation to interrupt that and return to the lisp> prompt; hitting control-C and getting back to the shell prompt works, but is a little drastic. So I looked at System.Posix.Signals and after a bit of messing about got the following: > mysighandler = > Catch (do hPutStrLn stderr "caught a signal!" > fail "Interrupt!") > > runREPL :: IO () > runREPL = > do getProgName >>= writeHdr > env <- setupBindings [] True > runInit env > installHandler sigINT mysighandler Nothing > installHandler sigQUIT mysighandler Nothing > doREPL env This compiles just fine, the interpreter runs as usual... but the added code doesn't seem to do anything. You can probably guess already... the print statement in mysighandler is there to see if it actually caught a signal. It does: I see "caught a signal!" just fine, in fact I see dozens of them as I lean on the control-C; but now my scheme calculation doesn't get interrupted at all! I see in the System.Posix.Signals documentation that the signal handler gets invoked in a new thread; is this the source of the problem? If so, what should I do to fix it? I'm afraid that sort of stuff is still beyond my haskell-fu... many thanks! Uwe

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Binary IEEE floating point format for AMQP
by Paul Johnson 24 Feb '08

24 Feb '08

I'm working on an implementation of the framing layer for AMQP (www.amqp.org) I almost had 0.9 finished when I found they had released the spec for 0.10, so I have to redo quite a lot of work. Amongst the new features of 0.10 are wire formats for floating point. These are the 4 and 8 byte IEEE formats. * Is there a principled way of converting a Haskell Float or Double (or Rational, for that matter) to and from IEEE format? * Since many computers use IEEE format natively, is there a quicker way of doing this on those architectures? Paul.

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Haskell + Windows Mobile?
by Bulat Ziganshin 24 Feb '08

24 Feb '08

Hello haskell-cafe, is there any haskell implementation for Windows Mobile? does they are support creation of GUI apps and internet networking features? -- Best regards, Bulat mailto:Bulat.Ziganshin@gmail.com

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Haskell w/ delimited continuations
by oleg＠okmij.org 24 Feb '08

24 Feb '08

Call-by-name lambda-calculus is strictly more expressive (in Felleisen sense) than call-by-value lambda-calculus, and the call-by-need (aka, lazy) lambda-calculus is observationally equivalent to the call-by-name. One can add shift/reset to any of these calculi (CBV shift/reset is most known; there are several CBN shift/reset, including the one I'm particularly attached to; in principle one can add shift/reset to call-by-need). Adding control effects (shift/reset) changes the expressivity results. Now all three calculi are distinct and none subsumes the other. For example, the expression reset( (\x -> 1) (abort 2)) evaluates to 1 in call-by-name and evaluate to 2 in call-by-value. The expression reset ((\x -> x + x) (shift f f)) has the type int->int in call-by-need (it is a function \x -> x + x) and it has the type int->int->int in call-by-name (and it is the curried addition function). The fact that call-by-need is no longer observationally equivalent to call-by-name and sharing becomes observable is the most distressing. It disables many optimizations GHC is allowed to do. Types help: there are call-by-name calculi with shift/reset with effect typing; one can look at the type and see what control effect an expression may make. That will still permit GHC optimize pure expressions and leave effectful expressions as they are. Alas, that type system is complex and I don't think inference is decidable there due to the presence of subtyping (one must annotate at least some of the binders with types, in particular, the binders of shift). It seems the simplest solution is to confine shift/reset to a monad. Regarding purity: the obligatory reference is Amr Sabry. What is a Purely Functional Language? In J. Functional Programming, 8(1), 1-22, Jan. 1998. http://www.cs.indiana.edu/~sabry/papers/purelyFunctional.ps Please see the definition 4.7. As Matthias Blume said, a bit informally, evaluation of a pure expression should not depend on CBN or CBV or some other such strategy. By this definition, an expression that involves shift/reset is not pure, as the above examples demonstrate.

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Haskell Weekly News: February 23, 2008
by Don Stewart 24 Feb '08

24 Feb '08

--------------------------------------------------------------------------- Haskell Weekly News http://sequence.complete.org/hwn/20080223 Issue 70 - February 23, 2008 --------------------------------------------------------------------------- Welcome to issue 70 of HWN, a newsletter covering developments in the [1]Haskell community. One hundred unique new and updated libraries and applications in the past two weeks, including mutable arrays, compression, games, web frameworks, data structures, a file system, Haskell tools, concurrency, graphics, cryptography, systems administration, signal processing, new guis and several audio libraries 1. http://haskell.org/ Hackage New and updated libraries in [2]the Hackage library database. 2. http://hackage.haskell.org/ * ArrayRef 0.1.2. Uploaded by Gwern Branwen. [3]ArrayRef: Unboxed references, dynamic arrays and more. * zlib 0.4.0.4. Uploaded by Duncan Coutts. [4]zlib: Compression and decompression in the gzip and zlib formats. * hetris 0.1. Uploaded by Gwern Branwen. [5]hetris: Text Tetris. * bzlib 0.4.0.3. Uploaded by Duncan Coutts. [6]bzlib: Compression and decompression in the bzip2 format. * HAppS-Server 0.9.2. Uploaded by David Himmelstrup. [7]HAppS-Server: Web related tools and services.. * HAppS-State 0.9.2. Uploaded by David Himmelstrup. [8]HAppS-State: Event-based distributed state.. * HAppS-Data 0.9.2. Uploaded by David Himmelstrup. [9]HAppS-Data: HAppS data manipulation libraries. * HAppS-IxSet 0.9.2. Uploaded by David Himmelstrup. [10]HAppS-IxSet: Added by DavidHimmelstrup, Fri Feb 22 15:18:20 PST 2008.. * HAppS-Util 0.9.2. Uploaded by David Himmelstrup. [11]HAppS-Util: Web framework. * Ranged-sets 0.2.0. Uploaded by Paul Johnson. [12]Ranged-sets: Ranged sets for Haskell. 3. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/ArrayRef-0.1.2 4. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/zlib-0.4.0.4 5. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hetris-0.1 6. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/bzlib-0.4.0.3 7. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/HAppS-Server-0.9… 8. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/HAppS-State-0.9.2 9. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/HAppS-Data-0.9.2 10. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/HAppS-IxSet-0.9.2 11. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/HAppS-Util-0.9.2 12. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/Ranged-sets-0.2.0 * halfs 0.2. Uploaded by Gwern Branwen. [13]halfs: Haskell File System. * sessions 2008.2.22. Uploaded by Matthew Sackman. [14]sessions: Session Types for Haskell. * infix 0.1. Uploaded by Gwern Branwen. [15]infix: Infix expression re-parsing (for HsParser library). * reify 0.1. Uploaded by Gwern Branwen. [16]reify: Serialize data. * highWaterMark 0.1. Uploaded by Gwern Branwen. [17]highWaterMark: Memory usage statistics. * hinvaders 0.1. Uploaded by Gwern Branwen. [18]hinvaders: Space Invaders. * baskell 0.1. Uploaded by Gwern Branwen. [19]baskell: An interpreter for a small functional language. * control-event 0.2. Uploaded by Thomas DuBuisson. [20]control-event: Event scheduling system.. * nymphaea 0.1. Uploaded by Gwern Branwen. [21]nymphaea: An interactive GUI for manipulating L-systems. * hopenssl 1.0. Uploaded by Peter Simons. [22]hopenssl: FFI bindings to OpenSSL's EVP digest interface. 13. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/halfs-0.2 14. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/sessions-2008.2.… 15. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/infix-0.1 16. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/reify-0.1 17. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/highWaterMark-0.1 18. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hinvaders-0.1 19. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/baskell-0.1 20. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/control-event-0.2 21. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/nymphaea-0.1 22. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hopenssl-1.0 * Monadius 0.91. Uploaded by Gwern Branwen. [23]Monadius: 2-D arcade scroller. * postmaster 0.1. Uploaded by Peter Simons. [24]postmaster: Postmaster ESMTP Server. * hsyslog 1.2. Uploaded by Peter Simons. [25]hsyslog: FFI interface to syslog(3) from POSIX.1-2001.. * hsemail 1.2. Uploaded by Peter Simons. [26]hsemail: Internet Message Parsers. * hsdns 1.3. Uploaded by Peter Simons. [27]hsdns: Asynchronous DNS Resolver. * funcmp 1.1. Uploaded by Peter Simons. [28]funcmp: Functional MetaPost. * streamproc 1.1. Uploaded by Peter Simons. [29]streamproc: Stream Processer Arrow. * pugs-HsSyck 0.41. Uploaded by Gwern Branwen. [30]pugs-HsSyck: Fast, lightweight YAML loader and dumper. * HsSyck 0.42. Uploaded by Gwern Branwen. [31]HsSyck: Fast, lightweight YAML loader and dumper. * mohws 0.1. Uploaded by Gwern Branwen. [32]mohws: Modular Haskell Web Server. 23. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/Monadius-0.91 24. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/postmaster-0.1 25. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hsyslog-1.2 26. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hsemail-1.2 27. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hsdns-1.3 28. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/funcmp-1.1 29. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/streamproc-1.1 30. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/pugs-HsSyck-0.41 31. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/HsSyck-0.42 32. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/mohws-0.1 * HsJudy 0.1. Uploaded by Gwern Branwen. [33]HsJudy: Judy bindings, and some nice APIs. * probability 0.2.1. Uploaded by Henning Thielemann. [34]probability: Probabilistic Functional Programming. * dsp 0.2.1. Uploaded by Henning Thielemann. [35]dsp: Haskell Digital Signal Processing. * pugs-hsregex 1.0. Uploaded by Gwern Branwen. [36]pugs-hsregex: Haskell PCRE binding. * ListLike 1.0.1. Uploaded by John Goerzen. [37]ListLike: Generic support for list-like structures. * SDL-gfx 0.5.2. Uploaded by David Himmelstrup. [38]SDL-gfx: Binding to libSDL_gfx. * SDL-ttf 0.5.2. Uploaded by David Himmelstrup. [39]SDL-ttf: Binding to libSDL_ttf. * SDL-mixer 0.5.2. Uploaded by David Himmelstrup. [40]SDL-mixer: Binding to libSDL_mixer. * SDL-image 0.5.2. Uploaded by David Himmelstrup. [41]SDL-image: Binding to libSDL_image. * SDL 0.5.2. Uploaded by David Himmelstrup. [42]SDL: Binding to libSDL. 33. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/HsJudy-0.1 34. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/probability-0.2.1 35. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/dsp-0.2.1 36. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/pugs-hsregex-1.0 37. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/ListLike-1.0.1 38. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/SDL-gfx-0.5.2 39. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/SDL-ttf-0.5.2 40. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/SDL-mixer-0.5.2 41. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/SDL-image-0.5.2 42. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/SDL-0.5.2 * DeepArrow 0.2. Uploaded by Conal Elliott. [43]DeepArrow: Arrows for "deep application". * GuiTV 0.4. Uploaded by Conal Elliott. [44]GuiTV: GUIs for Tangible Values. * Shellac-compatline 0.9. Uploaded by Robert Dockins. [45]Shellac-compatline: "compatline" backend module for Shellac. * WordNet 0.1.2. Uploaded by Max Rabkin. [46]WordNet: Haskell interface to the WordNet database. * lazyarray 0.1.3. Uploaded by Milan Straka. [47]lazyarray: Efficient implementation of lazy monolithic arrays (lazy in indexes).. * GenI 0.16.1. Uploaded by Eric Kow. [48]GenI: A natural language generator (specifically, an FB-LTAG surface realiser). * libGenI 0.16.1. Uploaded by Eric Kow. [49]libGenI: A natural language generator (specifically, an FB-LTAG surface realiser). * alsa-midi 0.3.1. Uploaded by Henning Thielemann. [50]alsa-midi: Bindings for the ALSA sequencer API (MIDI stuff). * midi 0.0.5. Uploaded by Henning Thielemann. [51]midi: Handling of MIDI messages and files. * event-list 0.0.6. Uploaded by Henning Thielemann. [52]event-list: Event lists with relative or absolute time stamps. 43. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/DeepArrow-0.2 44. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/GuiTV-0.4 45. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/Shellac-compatli… 46. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/WordNet-0.1.2 47. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/lazyarray-0.1.3 48. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/GenI-0.16.1 49. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/libGenI-0.16.1 50. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/alsa-midi-0.3.1 51. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/midi-0.0.5 52. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/event-list-0.0.6 * numeric-quest 0.1.1. Uploaded by Henning Thielemann. [53]numeric-quest: Math and quantum mechanics. * markov-chain 0.0.2. Uploaded by Henning Thielemann. [54]markov-chain: Markov Chains for generating random sequences with a user definable behaviour.. * hmp3 1.5.1. Uploaded by Don Stewart. [55]hmp3: An ncurses mp3 player written in Haskell. * TypeIlluminator 0.0. Uploaded by Gwern Branwen. [56]TypeIlluminator: TypeIlluminator is a prototype tool exploring debugging of type errors/. * Takusen 0.7. Uploaded by Don Stewart. [57]Takusen: Database library with left-fold interface, for PostgreSQL, Oracle, SQLite, ODBC.. * carray 0.1.2. Uploaded by Jed Brown. [58]carray: A C-compatible array library.. * jack 0.5. Uploaded by Henning Thielemann. [59]jack: Bindings for the JACK Audio Connection Kit. * non-negative 0.0.1. Uploaded by Henning Thielemann. [60]non-negative: Non-negative numbers. * RJson 0.3.3. Uploaded by Alex Drummond. [61]RJson: A reflective JSON serializer/parser.. * clevercss 0.1.1. Uploaded by Georg Brandl. [62]clevercss: A CSS preprocessor. 53. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/numeric-quest-0.… 54. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/markov-chain-0.0… 55. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hmp3-1.5.1 56. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/TypeIlluminator-… 57. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/Takusen-0.7 58. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/carray-0.1.2 59. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/jack-0.5 60. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/non-negative-0.0… 61. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/RJson-0.3.3 62. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/clevercss-0.1.1 * fft 0.1.1. Uploaded by Jed Brown. [63]fft: Bindings to the FFTW library.. * storable-complex 0.1. Uploaded by Jed Brown. [64]storable-complex: Storable instance for Complex. * winerror 0.1. Uploaded by Felix Martini. [65]winerror: Error handling for foreign calls to the Windows API.. * linkchk 0.0.2. Uploaded by Gwern Branwen. [66]linkchk: linkchk is a network interface link ping monitor.. * popenhs 1.0.0. Uploaded by Gwern Branwen. [67]popenhs: popenhs is a popen-like library for Haskell.. * Flippi 0.0.3. Uploaded by Gwern Branwen. [68]Flippi: Wiki. * DisTract 0.2.5. Uploaded by Gwern Branwen. [69]DisTract: Distributed Bug Tracking System. * goa 3.0. Uploaded by Gwern Branwen. [70]goa: GHCi bindings to lambdabot. * hinstaller 2008.2.16. Uploaded by Gwern Branwen. [71]hinstaller: Installer wrapper for Haskell applications. * GeoIp 0.1. Uploaded by Stephen Cook. [72]GeoIp: Pure bindings for the MaxMind IP database.. 63. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/fft-0.1.1 64. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/storable-complex… 65. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/winerror-0.1 66. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/linkchk-0.0.2 67. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/popenhs-1.0.0 68. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/Flippi-0.0.3 69. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/DisTract-0.2.5 70. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/goa-3.0 71. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hinstaller-2008.… 72. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/GeoIp-0.1 * hpodder 1.1.2. Uploaded by John Goerzen. [73]hpodder: Podcast Aggregator (downloader). * wxcore 0.10.2. Uploaded by Eric Kow. [74]wxcore: wxHaskell core. * wx 0.10.2. Uploaded by Eric Kow. [75]wx: wxHaskell. * flow2dot 0.3. Uploaded by Dmitry Astapov. [76]flow2dot: Generates sequence diagrams from textual descriptions. * strict-concurrency 0.2. Uploaded by Donald Stewart. [77]strict-concurrency: Strict concurrency abstractions. * TV 0.4. Uploaded by Conal Elliott. [78]TV: Tangible Values -- composable interfaces. * geniconvert 0.15. Uploaded by Eric Kow. [79]geniconvert: Conversion utility for the GenI generator. * ctemplate 0.1. Uploaded by Adam Langley. [80]ctemplate: Binding to the Google ctemplate library. * arrows 0.4. Uploaded by Ross Paterson. [81]arrows: Arrow classes and transformers. * lhs2tex 1.13. Uploaded by Andres Loeh. [82]lhs2tex: Preprocessor for typesetting Haskell sources with LaTeX. 73. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hpodder-1.1.2 74. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/wxcore-0.10.2 75. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/wx-0.10.2 76. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/flow2dot-0.3 77. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/strict-concurren… 78. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/TV-0.4 79. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/geniconvert-0.15 80. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/ctemplate-0.1 81. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/arrows-0.4 82. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/lhs2tex-1.13 * NGrams 1.1. Uploaded by Justin Bailey. [83]NGrams: Simple application for calculating n-grams using Google.. * lambdabot 4.1. Uploaded by Gwern Branwen. [84]lambdabot: A multi-talented IRC bot. * HsOpenSSL 0.4. Uploaded by Masatake Daimon. [85]HsOpenSSL: (Part of) OpenSSL binding for Haskell. * network-minihttp 0.1. Uploaded by Adam Langley. [86]network-minihttp: A very minimal webserver. * ZFS 0.0. Uploaded by Gwern Branwen. [87]ZFS: Oleg's Zipper FS. * fst 0.9. Uploaded by Gwern Branwen. [88]fst: Finite state transducers. * haskell-in-space 0.1. Uploaded by Gwern Branwen. [89]haskell-in-space: 'Asteroids' arcade games.. * unix-pty-light 0.1. Uploaded by Stuart Cook. [90]unix-pty-light: POSIX pseudo-terminal support. * bot 0.1. Uploaded by Conal Elliott. [91]bot: bots for functional reactive programming. * Hedi 0.1. Uploaded by Paolo Veronelli. [92]Hedi: Line oriented editor. 83. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/NGrams-1.1 84. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/lambdabot-4.1 85. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/HsOpenSSL-0.4 86. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/network-minihttp… 87. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/ZFS-0.0 88. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/fst-0.9 89. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/haskell-in-space… 90. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/unix-pty-light-0… 91. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/bot-0.1 92. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/Hedi-0.1 * network-bytestring 0.1.1.2. Uploaded by Johan Tibell. [93]network-bytestring: Fast and memory efficient low-level networking. * leksah 0.1.1. Uploaded by Juergen NicklischFranken. [94]leksah: Haskell IDE written in Haskell. * nano-hmac 0.2.0. Uploaded by Hitesh Jasani. [95]nano-hmac: Bindings to OpenSSL HMAC.. * monadenv 0.0-2005-02-14. Uploaded by Gwern Branwen. [96]monadenv: Added by GwernBranwen, Sun Feb 10 20:15:11 PST 2008.. * blockio 0.0-2006-02-03. Uploaded by Gwern Branwen. [97]blockio: Block-oriented I/O Driver. * child 0.0-2005-02-14. Uploaded by Gwern Branwen. [98]child: Added by GwernBranwen, Sun Feb 10 19:35:20 PST 2008.. * highlighting-kate 0.2.1. Uploaded by John MacFarlane. [99]highlighting-kate: Syntax highlighting. 93. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/network-bytestri… 94. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/leksah-0.1.1 95. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/nano-hmac-0.2.0 96. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/monadenv-0.0-200… 97. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/blockio-0.0-2006… 98. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/child-0.0-2005-0… 99. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/highlighting-kat… Blog noise [100]Haskell news from the [101]blogosphere. 100. http://planet.haskell.org/ 101. http://haskell.org/haskellwiki/Blog_articles * [102]Unit testing is not a substitute for static typing * [103]Misunderstandings about Erlang (and functional programming) * [104]Simple UNIX tools in OCaml * [105]The Evolution of the Imperative Programmer * [106]Onageristic speculation * [107]Rotating args in Haskell and Ruby block style programming * [108]Terse and verbose variable names in Haskell * [109]Pysec: Monadic Combinatoric Parsing in Python (aka Parsec in Python) * [110]Haskell and C structures 102. http://pinderkent.blogsavy.com/archives/154 103. http://tim-watson.blogspot.com/2008/02/ok-so-there-seem-to-be-few.html 104. http://fhtr.blogspot.com/2008/02/simple-unix-tools-in-ocaml.html 105. http://matburt.net/?p=33 106. http://pozorvlak.livejournal.com/91293.html 107. http://www.jasani.org/2008/02/rotating-args-in-haskell-and-ruby-block.html 108. http://www.dougalstanton.net/blog/index.php/2008/02/12/measuring-the-challe… 109. http://www.valuedlessons.com/2008/02/pysec-monadic-combinatoric-parsing-in.… 110. http://therning.org/magnus/archives/315 * [111]Haskell on Windows * [112]Haskell, HDBC and Sqlite * [113]Parsec and zippers for interpreters * [114]Playing with monad transformers * [115]Code CAN be beautiful * [116]Elegance and Power * [117]Why don't you use Haskell? * [118]Functional control flow * [119]Programs as functions and how I/O can fit in nicely * [120]Elegance and power 111. http://therning.org/magnus/archives/338 112. http://www.jasani.org/2008/02/haskell-hdbc-and-sqlite.html 113. http://sabbatical-year.blogspot.com/2008/01/brainfuck-its-last-one-i-promis… 114. http://sabbatical-year.blogspot.com/2008/02/playing-with-monad-transformers… 115. http://basildoncoder.com/blog/2008/02/22/code-can-be-beautiful/ 116. http://research.swtch.com/2008/02/elegance-and-power.html 117. http://reddit.com/info/69jbs/comments/ 118. http://neilmitchell.blogspot.com/2008/01/functional-flow-control.html 119. http://qplqyer.wordpress.com/2008/02/23/programs-as-functions-and-how-io-ca… 120. http://research.swtch.com/2008/02/elegance-and-power.html * [121]How important is elegance? * [122]Types, named (was: 'Name that type!'), plus two more questions * [123]Code CAN Be Beautiful * [124]True unions * [125]The craft of functional programming :: review 121. http://progjournal.blogspot.com/2008/02/how-important-is-elegance.html 122. http://blog.jbapple.com/2008/02/types-named-was-name-that-type-plus-two.html 123. http://basildoncoder.com/blog/2008/02/22/code-can-be-beautiful/ 124. http://calculist.blogspot.com/2008/02/true-unions.html 125. http://clipboarded.blogspot.com/2008/02/craft-of-functional-programming-rev… Quotes of the Week * Erik Engbrecht: The key to language success is making it powerful enough for a couple cowboys to do the work of an entire team in a shorter period of time. Selling fast and cheap is easy. If you have enough fast and cheap, the business people won't care if you are making it out of bubble-gum and duct-tape, because you are giving them what they want. * pozorvlak: I'm going to make what should be an uncontroversial statement: if you don't understand and use monads, you are at best a quarter of a Haskell programmer. A corollary of this is that, since using monad transformers is the only (or at least the approved) way to use two or more monads together, if you don't understand and use monad transformers you are at best half a Haskell programmer. * mrevelle: As Lisp to lists and Smalltalk to objects: Haskell to computation * Paul: I would use Haskell to build a product or service, and I mean that in the sense that I can see how to train a team and build processes (prototyping, implementation, quality, deployment, support) around Haskell. * Yegge: Haskell, OCaml and their ilk are part of a 45-year-old static-typing movement within academia to try to force people to model everything. About the Haskell Weekly News New editions are posted to [126]the Haskell mailing list as well as to [127]the Haskell Sequence and [128]Planet Haskell. [129]RSS is also available, and headlines appear on [130]haskell.org. Headlines are available as [131]PDF. To help create new editions of this newsletter, please see the [132]contributing information. Send stories to dons at galois.com. The darcs repository is available at darcs get [133]http://code.haskell.org/~dons/code/hwn/ 126. http://www.haskell.org/mailman/listinfo/haskell 127. http://sequence.complete.org/ 128. http://planet.haskell.org/ 129. http://sequence.complete.org/node/feed 130. http://haskell.org/ 131. http://code.haskell.org/~dons/code/hwn/archives/20080223.pdf 132. http://haskell.org/haskellwiki/HWN 133. http://code.haskell.org/~dons/code/hwn/

1 0

A beginners question
by Harri Kiiskinen 24 Feb '08

24 Feb '08

Dear All, banging my head against Haskell, but liking the feeling of hurting brains. Just a simple question: If fmap (^4) [1,2,3] >>= \i -> shows i " " gives "1 16 81 " then why does let i = fmap (^4) [1,2,3] in shows i " " give "[1,16,81] " Probably very simple, but there must be a delicate difference between these two expressions. I just don't get it. -- Harri Kiiskinen <harri.kiiskinen(a)utu.fi>

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Re: [Haskell] *** JOB OFFER *** [...]
by Paul Johnson 24 Feb '08

24 Feb '08

Peter Verswyvelen wrote: > > PS: This job offer was already placed in the Haskell Café a while ago, > but I was advised there to place it in the main Haskell list. I hope > this is not considered as spam. > I certainly don't see it as spam. One day there will be a dozen jobs for Haskell programmers on Monster or Jobserve, and at that point another such announcement *would* be spam. But at the moment it has rarity value. Paul.

1 0

Database relations mapping
by Radosław Grzanka 23 Feb '08

23 Feb '08

Hi, I'm developing toy application to learn HDBC. I have "problem" with doing relations mapping. Actually I don't know is it a problem or "feature". ;) Anyway, I have two tables with relation between them: (this example is simplified to the whole structure of database, you can imagine) CREATE TABLE TD_set ( setId INTEGER PRIMARY KEY, setName TEXT NOT NULL ) CREATE TABLE TO_card ( cardId INTEGER PRIMARY KEY AUTOINCREMENT, setId INTEGER NOT NULL," -- Relation to TD_set setSeq INTEGER NOT NULL ) (no foreign key as sqlite3 does not care anyway - at least AFAIK) And in Haskell: > type CardSet = String > > data Card = Card { > set :: CardSet, > seqNo :: Integer, > } There is no problem with filling the structure Card with "seqNo" but I can't fill "set". I would have to put there IO (CardSet) but I don't want to do that as whole structure returned from my mapping function is already inside IO monad. Can I do it? Here is my (perfect) function - http://hpaste.org/5839 . See "getCards" and some helper functions. This does not compile as (I believe) getElement wants to return IO (CardSet) Thank you in advance for all your input. Radek.

2 1

Haskell w/ delimited continuations
by Taral 23 Feb '08

23 Feb '08

My understanding of these things is limited, but what would stop me, theoretically speaking, of making a version of ghc with these primitives added: type Prompt r reset :: (Prompt r -> r) -> r shift :: Prompt r -> ((a -> _) -> r) -> a (Where _ is either r or forall b. b) -- Taral <taralx(a)gmail.com> "Please let me know if there's any further trouble I can give you." -- Unknown

4 9

haddock as a markdown preprocessor
by Conal Elliott 23 Feb '08

23 Feb '08

There was a chat today on #haskell<http://tunes.org/%7Enef/logs/haskell/08.02.20> (15:08 to 16:10) about evolving haddock. I'd like to get comments. The goal is to get the full functionality of a general purpose, programmer-friendly markup language like markdown. One example is image embedding. Another is friendly links (no visible URL). The idea is to make a future haddock be a *preprocessor* that generates pandoc's extended markdown (or some such). Documentation would be mostly markdown, with very few extensions for code documentation ('foo' and " Foo.Bar", maybe a bit more). Most of the doc would simply be passed through untouched. The code-doc extensions would get rewritten into standard markdown and mixed in with the rest. Pandoc could then take the generated markdown and produce HTML, LaTeX, DocBoook XML, etc. Perhaps there will be ways in which markdown falls short in expressiveness. If so, I'm guessing the shortcomings wouldn't be specific to the task of code documentation, and so could be approached as improvements to markdown/pandoc (which is written in Haskell). Since the old and new doc languages would be quite incompatible, we might want to specify in a .cabal file which language to use. Reactions? - Conal

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Re: [Haskell] Re: Re: RE: Extensible records: Static duck typing
by Mark P Jones 23 Feb '08

23 Feb '08

[Redirecting to haskell-cafe] Ben Franksen wrote: > TREX seems to be generally agreed to be too complicated to implement and > explain. What evidence do you have for this? Speaking as somebody who implemented Trex for Hugs (and who also witnessed Ben Gaster build an independent implementation), I'd have to disagree about the first part of this. As I recall, the implementation was reasonably straightforward, and wasn't any more complicated than other common type system extensions that I've worked on. I'm also not sure why you think it is complicated to explain, but I suppose that's even more subjective. One of the most difficult things about the Trex implementation was finding a syntax that meshed nicely with the rest of the Haskell syntax. This is likely to be a problem for any record system extension of Haskell---unless you're prepared to accept a more unconventional syntax---because many of the symbols that you might want to use ({, }, ., |, \, for example) have already been adopted for other purposes. Ah, syntax! All the best, Mark

2 1

Support for Transaction Memory by Sun Microsystems ..
by Galchin Vasili 23 Feb '08

23 Feb '08

Hello, Found this: http://www.theregister.co.uk/2007/08/21/sun_transactional_memory_rock/ http://research.sun.com/spotlight/2007/2007-08-13_transactional_memory.html vasya

1 0

Fwd: [Haskell-cafe] Repeated function application
by Thomas Hartman 23 Feb '08

23 Feb '08

This was easier for me to understand when written so, with the start value explicit times3 :: (a -> a) -> Int -> (a -> a) times3 f n !start | n == 0 = start | otherwise = times3 f (n-1) (f start) -- no stack overflow :) tTimes3 = times3 (+1) 1000000 0 Here, only the third arg, the start value, needs to be "bangified/strictified", and it's pretty clear why. Without the bang pattern, it stack overflows. What I'm not sure of is whether this version is in fact completely equivalent to Dan's version above. I hope it is. 2008/2/21, Dan Weston <westondan(a)imageworks.com>: > Ben Butler-Cole wrote: > > Hello > > > > I was surprised to be unable to find anything like this in the standard libraries: > > > > times :: (a -> a) -> Int -> (a -> a) > > times f 0 = id > > times f n = f . (times f (n-1)) > > > > Am I missing something more general which would allow me to repeatedly apply a function to an input? Or is this not useful? > > > Invariably, this seems to invite a stack overflow when I try this (and > is usually much slower anyway). Unless f is conditionally lazy, f^n and > f will have the same strictness, so there is no point in keeping nested > thunks. > > If you apply f immediately to x, there is no stack explosion and faster > runtime: > > > times :: (a -> a) -> Int -> (a -> a) > > times f !n !x | n > 0 = times f (n-1) (f x) > | otherwise = x > > > Dan > > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe >

2 2

Tutorial for using the state monad or a better suggestion?
by Jefferson Heard 23 Feb '08

23 Feb '08

So the reason I keep pinging the list so much of late is I'm currently writing a GLUT program to visualize a heirarchical clustering of 18,000+ protein-protein interaction pairs (and associated gene-ontology terms). Thanks for the help on reading CSVs, those who wrote me back... my program intitializes and displays its first image within 6 seconds, about 10 times faster and in 10 times less memory than the Java program the guy was using. Now I'm to the point of making this thing interactive, and I I'm trying to figure out the Haskell way of doing this. Last time I wrote a program like this, I made a record data type with all the state and placed it into an IORef and curried it into the GLUT callback functions. I'm going to do the same thing now if there aren't cringes and wailings from people with a better sense of pure-functional aesthetics out there on the list with a willingness to either point me towards a tutorial that would help me do this better. Keep in mind that Graphics.UI.GLUT callbacks all want to return an IO (), and thus leftover state monads at ends of functions aren't going to be acceptable to the standard library... Any ideas? Oh, currently my program state includes: The geometry I'm rendering (Ptr GLfloat vertex and color arrays), The same geometry as a display list for rendering into the selection buffer An indexed and named tree that represents the clustering A tree of text containing tooltips to display The previous current mouse position (for dragging purposes) A couple of histograms as Array.IArray.Diff.DiffArrays Various parameters for constructing rendered data out of the indexed tree (for reconstruction after a node is collapsed/expanded) So I'm carrying around some pretty bulky state; should give you some understanding as to why I thougt the record data type would be the sanest way to do this without polluting my parameter list with individual IORrefs. Oh, and again, it's not that I don't know that I can make the IORef solution work, I can and I've done it before. It's just that I thought there might be a prettier way to do this. Thanks in advance! -- Jeff

4 4

Tips of Conditional Expression
by TOPE KAREM 23 Feb '08

23 Feb '08

I know the following: [1] That the general form of conditional expression is: if * Boolean_expression* then *exp1* else *exp2 *[2] That a conditional expression must always have both a then and an elseexpression. [3] That both *exp1* and *exp2* must have the same type, which is the type of the entire conditional expression. [4] That *exp1* must be <true-value> and *exp2* must be <false-value> Reference: Discrete Mathematics Using Computer by John O'Donnell and et al. (Second Edition) My question: Is it possible that exp1 and exp 2 be different function calls of another functions (separately) elsewhere within the same program? Note: My Boolean_expression is Boolean My *exp1* is a function call elsewhere within the same program (more like a subroutine) My *exp2* is another function call also elsewhere within the same program. Tope South Africa

3 5

The Proliferation of List-Like Types
by John Goerzen 22 Feb '08

22 Feb '08

Hi folks, Before I started using Haskell, I used OCaml for a spell. One of my biggest annoyances with OCaml was that it had two list types: the default list (strict), and a lazy list (known as a stream). This led to all sorts of annoyances. Libraries were always written to work with one list or the other. If you wanted to use two libraries, one that assumed one list and the other that assumed another, you had a difficult task ahead of you. I am concerned that the same thing is happening in Haskell. We know have three common list-like types: the regular list, strict ByteString, and lazy ByteString. This has created some annoying situations. For instance, a ByteString is great for reading data fast, but Parsec doesn't work on ByteStrings. I am glad that someone wrote a Parsec equivolent that does[1], which answers a real need. But this means that all the combinators in the hsemail library that implement standard RFC conventions won't be usable in my ByteString code, for instance. Similarly, we have another annoying situation relating to character encodings: * The iconv library works only on lazy ByteStrings, and does not handle Strings or strict ByteStrings * The utf8-string library doesn't support UTF-16, doesn't require an external library, and works only on Strings -- no support for ByteStrings. * Data.Encoding.* is native haskell, supports UTF-*, but works only on ByteSting.Lazy again. Now, to help solve this problem, I wrote ListLike[2], providing a set of typeclasses that make list operations generic. I also provided default instances of ListLike for: ListLike Data.ByteString.ByteString Word8 ListLike Data.ByteString.Lazy.ByteString Word8 ListLike [a] a (Integral i, Ix i) => ListLike (Array i e) e (Ord key, Eq val) => ListLike (Map key val) (key, val) These instances use the native underlying calls where appropriate (for instance, ByteString and Data.List both provide a 'head'). The typeclass also contains large numbers of default implementations, such that only four functions must be implemented to make a type a member of ListLike. API ref is at http://software.complete.org/listlike/static/doc/ListLike/Data-ListLike.htm… Now, the questions: 1) Does everyone agree with me that we have a problem here? 2) Would it make sense to make ListLike, or something like it, part of the Haskell core? 3) Would it make sense to base as much code as possible in the Haskell core areound ListLike definitions? Here I think of functions such as lines and words, which make sense both on [Char] as well as ByteStrings. 4) We are missing one final useful type: a Word32-based ByteString. When working in the Unicode character set, a 32-bit character can indeed be useful, and I could see situations in which the performance benefit of a ByteString-like implementation could be useful combared to [Char]. [1] Yes, I have read about Parsec 3 being imminent, which is also great [2] http://software.complete.org/listlike

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Re: [Haskell-cafe] Can't seem to get `par` working appropriately with lists
by Luke Andrew 22 Feb '08

22 Feb '08

Felipe Lessa wrote: > On Thu, Feb 21, 2008 at 8:58 AM, Luke Andrew <luke.haskell-cafe(a)la.id.au> wrote: > >> test2.hs: >> >> import Control.Parallel >> >> fib1 n = if n == 0 then 0 else if n == 1 then 1 else fib1 (n-1) + >> fib1 (n-2) >> fib2 n = if n == 0 then 0 else if n == 1 then 1 else fib2 (n-1) + >> fib2 (n-2) >> fiblist1 n = [fib1 x| x <- [1..n]] >> fiblist2 n = [fib2 x| x <- [1..n]] >> >> main = do print $ zipWith (+) (fiblist2 37 `par` fiblist1 37) >> (fiblist2 37) >> > > Besides what Jules Bean said, note also that 'par' works a bit like > 'seq', only evaluating until WHNF. So even if you said > > main = > let f1 = fiblist1 37 > f2 = fiblist2 37 > in print $ zipWith (+) (f2 `par` f1) f2 > > you still wouldn't get what you want. Why? It only evaluates f1 and f2 > until it reaches [] or (_:_), and nothing more, it doesn't even try to > figure out that f2 is _:_:_:_:···:[] nor it tries to see that f2 is > 1:_, but you wanted that the parallel computation went until f2 was > 1:1:2:3:···:[]. > > To do that, force the list (i.e. do a "deep seq"). There's > Control.Parallel.Strategies to help you doing this, but if you want to > reimplement it, try > > force [] = () > force (x:xs) = x `seq` force xs > > main = > let f1 = fiblist1 37 > f2 = fiblist2 37 > in print $ zipWith (+) (force f2 `par` f1) f2 > > 'par' will try to see that 'force f2' is really (), but to do that > 'force' will go all the way down thru the list forcing its spine and > its values. > > HTH, > > Thanks Felipe, works as advertised. Guest I'm going to have to read up on WHNF, I just ignored it after my initial success & hoped it would continue to work. Guess I should have RTFM :)

1 0

Re: [Haskell-cafe] Selecting Array type
by Ryan Ingram 22 Feb '08

22 Feb '08

On 2/21/08, Jeff φ <jeff1.61803(a)gmail.com> wrote: > Thanks again. I'm not familiar with type equality constraints. I could not > find information on this in the GHC users guide. Is it documented > somewhere? Section 7.3 here talks about equality constraints. http://www.haskell.org/haskellwiki/GHC/Indexed_types There's an extremely good paper talking about the low-level mechanisms for equality constraints in GHC Core here: http://research.microsoft.com/%7Esimonpj/papers/ext%2Df/ GHC Core -is- System Fc as described in that paper; the type equality constraints amount to a "coercion" function that is passed around the core language to convert between the types and which is statically guaranteed to be the identity function. > It is interesting that you were able to drop the IArray > constraint from the instances of SmartArraySelector. During compilation from Haskell to Core, the compiler implicitly adds calls to any available conversion functions it knows about as required to convert between types. This is why the IArray declaration can be dropped; the compiler looks at the coercions currently available, finds (a ~ UArray), and inserts a call that converts the existing "IArray UArray Bool" declaration into an "IArray a Bool" declaration. Determining that such a path exists is a search problem and much of the work with associated types and type equality constraints involves proving that the search is decidable. -- ryan

1 0

Can't seem to get `par` working appropriately with lists
by Luke Andrew 22 Feb '08

22 Feb '08

Hopefully an easy one here; after reading Don Stewart's blog posts about parallel Haskell and with a shiny new quad-core cpu begging for a workout, i thought I'd give Haskell a try. I've also been meaning to write a ray-tracer, so I started with that. I've got the initial ray-tracer working, and am now looking to parallize it. I tried using the `par` function to evaluate things in parallel, but I couldn't get it to work with lists. I simplified my problem down into the following 2 test cases: (there are two fibonacci functions to ensure haskell treats them as 2 independent computations to be spread across 2 cores) test1.hs: import Control.Parallel fib1 n = if n == 0 then 0 else if n == 1 then 1 else fib1 (n-1) + fib1 (n-2) fib2 n = if n == 0 then 0 else if n == 1 then 1 else fib2 (n-1) + fib2 (n-2) main = do print $ (fib2 37 `par` fib1 37) + (fib2 37) compilation & testing: luke@Sonata:~/mcls$ ghc -O2 -threaded --make test1 [1 of 1] Compiling Main ( test1.hs, test1.o ) Linking test1 ... luke@Sonata:~/mcls$ time ./test1 +RTS -N1 48315634 real 0m5.856s user 0m5.816s sys 0m0.004s luke@Sonata:~/mcls$ time ./test1 +RTS -N2 48315634 real 0m3.450s user 0m6.734s sys 0m0.024s As expected, running it with 2 cores is substantially faster. Doing almost the same thing, but with lists, doesn't seem to have any significant speed difference: test2.hs: import Control.Parallel fib1 n = if n == 0 then 0 else if n == 1 then 1 else fib1 (n-1) + fib1 (n-2) fib2 n = if n == 0 then 0 else if n == 1 then 1 else fib2 (n-1) + fib2 (n-2) fiblist1 n = [fib1 x| x <- [1..n]] fiblist2 n = [fib2 x| x <- [1..n]] main = do print $ zipWith (+) (fiblist2 37 `par` fiblist1 37) (fiblist2 37) compilation & testing: luke@Sonata:~/mcls$ ghc -O2 -threaded --make test2 luke@Sonata:~/mcls$ time ./test2 +RTS -N1 [2,2,4,6,10,16,26,42... ...405774,18454930,29860704,48315634] real 0m15.294s user 0m15.196s sys 0m0.013s luke@Sonata:~/mcls$ time ./test2 +RTS -N2 [2,2,4,6,10,16,26,42... ...405774,18454930,29860704,48315634] real 0m15.268s user 0m15.169s sys 0m0.013s I've tried using bang patterns in various places, but to no avail. Any ideas? Luke Andrew.

4 3

Re: [Haskell-cafe] Selecting Array type
by Jeff φ 22 Feb '08

22 Feb '08

On 2/19/08, Ryan Ingram <ryani.spam(a)gmail.com> wrote: > > Oleg's done a lot of work here; there's a bunch of magic that can be > done with TypeCast. I took my inspiration from here: > http://okmij.org/ftp/Haskell/typecast.html#ambiguity-resolution > . . . > > The trick is to represent whether a type is boxed or not via a > type-level boolean, which you can then use to affect the instance > selecton. > . . . > > -- Magic toolbox that solves everything! Thanks Oleg! > > data HTrue > data HFalse > > 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 > Thanks for showing me this technique. I studied your code for several hours. And, I've read Oleg's "Strongly Typed Heterogeneous Collections." As a learning exercise, I modified your code. I managed to shorten it a bit, but I had a couple of surprises. Please see my comments in the code below. {-# OPTIONS_GHC -fglasgow-exts -fbreak-on-exception -fallow-undecidable-instances -fallow-overlapping-instances #-} module SmartArray where import IO import Data.Ix import Data.Array.Unboxed import Data.Complex type SmartArray i e = (Ix i, SmartArraySelector a e) => (a i e) -- smartArray is similar to array function from Data.Array. But, -- it will return a UArray if e can be unboxed. Otherwise, it -- returns an Array. smartArray :: (i, i) -> [(i, e)] -> SmartArray i e smartArray bnd eLst = array bnd eLst class ArrTypeCast a b | a -> b, b->a where arrTypeCast :: a i e -> b i e instance ArrTypeCast x x where arrTypeCast = id -- SURPRISE 1: If function, arrTypeCast, is removed, (from both -- the class and instance) GHC assumes the kind of a and b are *, -- instead of * -> * -> * and produce . . . -- -- report3.hs:37:24: -- `UArray' is not applied to enough type arguments -- Expected kind `*', but `UArray' has kind `* -> * -> *' -- In the type `(ArrTypeCast a UArray, IArray a Bool) => -- SmartArraySelector a Bool' -- In the instance declaration for `SmartArraySelector a Bool' -- -- It appears that functions defined in a class can constrain the -- type variables of the class. To me, this seems a bit magical -- and unexpected. class (IArray a e) => SmartArraySelector a e | e -> a -- instances of SmartArraySelector for all boxed types (For -- breivity, not all unboxed types are listed.) instance (ArrTypeCast a UArray, IArray a Bool) => SmartArraySelector a Bool instance (ArrTypeCast a UArray, IArray a Char) => SmartArraySelector a Char instance (ArrTypeCast a UArray, IArray a Double) => SmartArraySelector a Double instance (ArrTypeCast a UArray, IArray a Float) => SmartArraySelector a Float instance (ArrTypeCast a UArray, IArray a Int) => SmartArraySelector a Int -- SURPRISE 2: The class SmartArraySelector has the type -- assertion, (IArray a e). It seems like adding an additional -- IArray assertion to each instance is redundant. However, -- if I remove the assertion (IArray a Int) above, GHC -- reports . . . -- -- report3.hs:37:24: -- `UArray' is not applied to enough type arguments -- Expected kind `*', but `UArray' has kind `* -> * -> *' -- In the type `(ArrTypeCast a UArray, IArray a Bool) => -- SmartArraySelector a Bool' -- In the instance declaration for `SmartArraySelector a Bool' -- -- Why is this second type assertion required? instance (ArrTypeCast a Array, IArray a b) => SmartArraySelector a b test :: SmartArraySelector a e => e -> a Int e test e = smartArray (0,10) [ (i,e) | i <- [0..10]] I'd love to find a good article that describes the ins and outs of multi parameter types, functional dependencies, and type assertions, in enough detail to resolve these surprises. A step-by-step walk through showing how the compiler resolve a type and selects an instance would be awesome. Usually, when I'm having trouble getting Haskell's type system to do what I want, I resort to trial and error tactics. I wish I had a better foundation so I could take a more intelligent approach to type hacking.

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Repeated function application
by Ben Butler-Cole 22 Feb '08

22 Feb '08

Hello I was surprised to be unable to find anything like this in the standard libraries: times :: (a -> a) -> Int -> (a -> a) times f 0 = id times f n = f . (times f (n-1)) Am I missing something more general which would allow me to repeatedly apply a function to an input? Or is this not useful? I thought this looked a bit like a fold, so I tried expressing it like this: times f n x = foldl (flip ($)) x (replicate n f) ... which horrifies me, but I can't help feeling that there must be a way to get rid of the explicit recursion. How would you implement times? Ben

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Problem with Python AST
by Roel van Dijk 22 Feb '08

22 Feb '08

Hello everyone, I am trying to create an AST for Python. My approach is to create a data type for each syntactic construct. But I am stuck trying to statically enforce some constraints over my statements. A very short example to illustrate my problem: newtype Ident = Id String data BinOp = Add | Sub data Exp = IntLit Integer | BinOpExp BinOp Exp Exp data NormalCtx data LoopCtx data Statement ctx where Compound :: Compound -> Statement ctx Pass :: Statement ctx Break :: Statement LoopCtx newtype Global = Global [Ident] data Suite ctx = Suite [Global] [Statement ctx] type Else = Suite NormalCtx data Compound = If [(Exp, Suite NormalCtx)] (Maybe Else) | While Exp (Suite LoopCtx) (Maybe Else) newtype Program = Program [Statement NormalCtx] The "global" statement makes an identifier visible in the local scope. It holds for the entire current code block. So it also works backwards, which is why I didn't make it a statement but part of a suite (= block of statements). Some statements may occur in any context, such as the "pass" statement. But others are only allowed in certain situations, such as the "break" statement. This is why I defined the Statement as a GADT. I just supply the context in which the statement may be used and the typechecker magically does the rest. Feeling very content with this solution I tried a slightly more complex program and discovered that my AST can not represent this Python program: for i in range(10): if i == 6: break The compound if statement is perfectly valid nested in the loop because the Compound constructor of Statement allows any context. But the suites inside the clauses of the if statement only allow normal contexts. Since Break has a LoopCtx the typechecker complains. Is there some other way to statically enforce that break statements can only occur _nested_ inside a loop? There is a similar problem with return statements that may only occur in functions. These nested statements should somehow 'inherit' a context, if that makes any sense :-) I know I can simply create separate data types statements that can occur inside loops and function bodies. But that would make the AST a lot more complex, something I try to avoid. Python's syntax is already complex enough! Most of these constraints are not in the EBNF grammar which can be found in the language reference, but they are specified in the accompanying text. The cpython interpreter will generate SyntaxError's when you violate these constraints. See also Python's language reference: http://docs.python.org/ref/ref.html (see sections 6 and 7)

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stream/bytestring questions
by Chad Scherrer 22 Feb '08

22 Feb '08

ByteStrings have given a real performance boost to a lot of Haskell applications, and I'm curious why some of the techniques aren't more abstracted and widely available. If it's "because it's a big job", that's certainly understandable, but maybe there's something I'm overlooking that some of the abstractions aren't really desirable. My first question is about the stream fusion. This seems (based on the ByteString paper) to speed things up quite a lot, but for some reason this isn't necessarily the case for lists. Is this just a matter of the existing fusion for lists doing such a good job in many cases that it's hard to beat? The streams paper suggests that some contructors aren't optimized away during fusion, but wouldn't this also be a problem for fusion in the bytestring context? Are there many cases where it helps performance to program to streams directly, rather than letting the rules go back and forth between them and lists? I tried to do this, but kept getting hung up on the Unlifted stuff; it's not exposed (pardon the pun) in the API, and I don't understand why the types are different than the usual (), Either a b, (a,b), etc. Second, the idea of representing a sequence as a lazy list of strict arrays is very appealing. But why represent it so concretely? Wouldn't it make sense to do something like data ArrayList a i e = Empty | Chunk !(a i e) (ArrayList a i e) ? Then array fusion could be written for IArray types in general, and the ByteString library would become a set of instances with some specialization pragmas. Presumably a more general StorableVector could be represented as an IArray, and the NDP stuff seems a good fit too, so this would make it easy to leverage future work. Seems to separate the various concerns a little better, too, but again maybe there's a performance issue I'm missing. Sorry for the barrage of questions; I guess there's just a lot I'm trying to understand better. Thanks, Chad

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question about STM and IO
by John Lato 22 Feb '08

22 Feb '08

Hello, I was recently looking at the STM library, and I have a question about the function "unsafeIOToSTM". Can anyone explain to me what is unsafe about it, and what sort of use would be considered "safe"? Thanks, John

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