- Haskell-Cafe - Haskell.org

haskell and reflection
by Greg Meredith 14 Sep '07

14 Sep '07

Haskellians, Am i wrong in my assessment that the vast majority of reflective machinery is missing from Haskell? Specifically, - there is no runtime representation of type available for programmatic representation - there is no runtime representation of the type-inferencing or checking machinery - there is no runtime representation of the evaluation machinery - there is no runtime representation of the lexical or parsing machinery Best wishes, --greg -- L.G. Meredith Managing Partner Biosimilarity LLC 505 N 72nd St Seattle, WA 98103 +1 206.650.3740 http://biosimilarity.blogspot.com

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help getting happy
by Greg Meredith 14 Sep '07

14 Sep '07

Haskellians, The code pasted in below causes Happy to return parE when invoked with happy rparse.y -i . Is there anyway to get Happy to give me just a wee bit more info as to what might be causing the parE (which i interpret a 'parse error'). Best wishes, --greg { module Main where } %name rparse %tokentype { Token } %error { parseError } %token '{' { TokenLCurly } '}' { TokenRCurly } '[' { TokenLSquare } ']' { TokenRSquare } '(' { TokenLRound } ')' { TokenRRound } '@' { TokenAt } ',' { TokenComma } ';' { TokenSemi } lquote { TokenLQuote } rquote { TokenRQuote } %% Molecule : '{' '}' { Zero } | Name Reagent { Locate $1 $2 } | '@' Name { Decode $2 } ReagentList : Reagent { [ $1 ] } | ReagentList ';' Reagent { $1 ++ [$3] } Reagent : '?' '(' NameList ')' Mixture { Abstraction $3 $5 } | '[' Mixture ']' { Concretion $2 } Mixture : Molecule { Mix [ $1 ] } | '{' ReagentList '}' { Mix $2 } NameList : Name { [ $1 ] } | NameList ',' Name { $1 ++ [$3] } Name : lquote Mixture rquote { Name $2 } { parseError :: [Token] -> a parseError _ = error "Parse error" data Molecule = Zero | Locate Name Reagent | Decode Name deriving (Eq, Show) data Reagent = Abstraction [Name] Mix | Concretion Mix deriving (Eq, Show) data Mix = Mix [Molecule] deriving (Eq, Show) data Name = Name Mix deriving (Eq, Show) data Token = TokenLQuote | TokenRQuote | TokenLCurly | TokenRCurly | TokenLSquare | TokenRSquare | TokenLRound | TokenRRound | TokenComma | TokenSemi | TokenAt deriving Show lexer :: String -> [Token] lexer [] = [] lexer (c:cs) | isSpace c = lexer cs lexer ('{':cs) = TokenLCurly : lexer cs lexer ('}':cs) = TokenRCurly : lexer cs lexer ('[':cs) = TokenLSquare : lexer cs lexer (']':cs) = TokenRSquare : lexer cs lexer ('(':cs) = TokenLRound : lexer cs lexer (')':cs) = TokenRRound : lexer cs lexer (',':cs) = TokenComma : lexer cs lexer (';':cs) = TokenSemi : lexer cs lexer ('@':cs) = TokenAt : lexer cs lexer ('<':'<':cs) = TokenLQuote : lexer cs lexer ('>':'>':cs) = TokenRQuote : lexer cs main = getContents >>= print . rparse . lexer } -- L.G. Meredith Managing Partner Biosimilarity LLC 505 N 72nd St Seattle, WA 98103 +1 206.650.3740 http://biosimilarity.blogspot.com

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Basic FFI with GHC
by Ronald Guida 13 Sep '07

13 Sep '07

How do I create C-libraries that I can load into GHCi? I am trying to do some basic FFI, and it's not working. Here's the background. I created three files, foo.h, foo.cpp, and test_foo.lhs. (source code below) Note: I am using MinGW/Msys under Windows XP. If I compile foo.cpp and then try to load test_foo into GHCi, I get an error. OTOH, if I compile test_foo with GHC, it works. I just don't understand why it works one way and not the other. What am I doing wrong? -- Ron ------------------------------ Shell commands ------------------------------ $ gcc -c foo.cpp $ ghci test_foo.lhs foo.o ___ ___ _ / _ \ /\ /\/ __(_) / /_\// /_/ / / | | GHC Interactive, version 6.6.1, for Haskell 98. / /_\\/ __ / /___| | http://www.haskell.org/ghc/ \____/\/ /_/\____/|_| Type :? for help. Loading package base ... linking ... done. Loading object (static) foo.o ... done final link ... done [1 of 1] Compiling Main ( test_foo.lhs, interpreted ) During interactive linking, GHCi couldn't find the following symbol: foo This may be due to you not asking GHCi to load extra object files, [snip] $ ghc --make test_foo.lhs foo.o [1 of 1] Compiling Main ( test_foo.lhs, test_foo.o ) Linking test_foo.exe ... $ test_foo.exe Entering main y = 22 Exiting main $ ar rcs libfoo.a foo.o $ ghc --make test_foo.lhs -lfoo $ test_foo.exe Entering main y = 22 Exiting main $ ghci test_foo.lhs -lfoo ___ ___ _ / _ \ /\ /\/ __(_) / /_\// /_/ / / | | GHC Interactive, version 6.6.1, for Haskell 98. / /_\\/ __ / /___| | http://www.haskell.org/ghc/ \____/\/ /_/\____/|_| Type :? for help. Loading package base ... linking ... done. Loading object (dynamic) foo ... failed. Dynamic linker error message was: addDLL: unknown error Whilst trying to load: (dynamic) foo Directories to search are: : user specified .o/.so/.DLL could not be loaded. ------------------------------ Source code ------------------------------ // foo.h extern "C" { __stdcall int foo(int x); } ------------------------------ // foo.cpp #include "foo.h" __stdcall int foo(int x) { return 3 * x + 1; } ------------------------------ test_foo.lhs > {-# OPTIONS_GHC -fglasgow-exts #-} > module Main > where > import Foreign > import Foreign.C > foreign import stdcall unsafe "foo" > c_Foo :: Word32 -> IO Word32 > main = do > putStrLn "Entering main" > let x = 7::Word32 > y <- c_Foo(x) > putStrLn $ "y = " ++ show y > putStrLn "Exiting main"

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Haskell and State Monad.
by VinyleEm 13 Sep '07

13 Sep '07

Hello all, I am Vinay Emani, new to this forum. I am doing an academic project of writing an assembler cum linker in Haskell as part of building a compiler set written entirely in Haskell. Since i am new to the language, i thought it would do me good to look at some of the existing haskell code, be it in whatever project and written for whatever purpose. This should help me get started. In particular i need to find some project/program which extensively uses the State Monad. I tried looking at some of the existing open source projects at sourceforge. There are only limited number of projects there, some of them being incomplete. Could you please suggest me some project, from which i can learn the State Monad. Thanks very much Vinay Emani India -- View this message in context: http://www.nabble.com/Haskell-and-State-Monad.-tf4427176.html#a12629170 Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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"Functional dependencies conflict between instance declarations"
by Peter Verswyvelen 13 Sep '07

13 Sep '07

The Haskell mailing list seems to be filled with people requesting information about this error, so I cannot resist to include myself in it ;-) I've read the information about funcdeps in the GHC user guide, and I think I understand how it works, but I get the error a lot, without having a clue why. Sometimes I get the error even when all the types of all the type parameters are completely different, so even when all parameters would be one-to-one dependent, I still don't get why I would get a conflict error. Does a wiki exists with possible reasons why this error occured and how to fix it? Thanks, Peter

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Re: [Haskell-cafe] Re: Is "take" behaving correctly?
by PR Stanley 13 Sep '07

13 Sep '07

>Hi > > > The same should apply to head and tail. head or tail of [] should be []. > > > > What does the list think? > >Disagree, strongly. Its not even possible for head, since [a] -> a. >Wadler's theorems for free states that if head is given an empty list >the _only_ thing it can do is crash. What's the logic behind this? Thanks, Paul

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Haddock/hscolour integration broken on Hackage?
by David Menendez 12 Sep '07

12 Sep '07

I was looking at the Data.Binary documentation[1] on Hackage, and I've noticed some problems with the associated source listings[2]. First, none of the "Source" links work. They all refer to fragment IDs (e.g., "#Binary") that are not defined. I'm not sure if this is a problem with hscolor, haddock, or hackage. Second, the source listing itself[2] is invalid HTML[3]. The W3C validator lists 678 errors. Of these, 677 are have to do with the line number anchors, all of which start with parentheses. This is bad not only because HTML requires fragment ids to start with a letter, but because parentheses are not allowed to occur in URIs. As a result, links to line numbers will come out looking like "Data-Binary.html#%28line100%29". [1] <http://hackage.haskell.org/packages/archive/binary/0.3/doc/html/Data-Binary…> [2] <http://hackage.haskell.org/packages/archive/binary/0.3/doc/html/src/Data-Bi…> [3] <http://validator.w3.org/check?uri=http%3A%2F%2Fhackage.haskell.org%2Fpackag…>

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Re: Tiny documentation request
by manu 12 Sep '07

12 Sep '07

On Sep 11, 2007, Simon Marlow wrote: > > Please, please, someone do this for me. I tried, and failed, to > get the > layout right for the contents list in all browsers at the same > time. The > semantics of CSS is beyond my comprehension. > > Cheers, > Simon Hi Simon, On the page http://www.haskell.org/ghc/docs/latest/html/libraries/ index.html, you only need tables to display the foldable lists of modules (HTML tables were commonly used to display many "things" on a same line), but they can be replaced by nested lists with a bit of CSS : Check this page out : http://la.di.da.free.fr/haddock/ I can help further, if need be. Emmanuel

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Developing XPCOM components (Firefox extensions) in Haskell
by Pasqualino 'Titto' Assini 12 Sep '07

12 Sep '07

Hello, I wondered if anyone could advice on how to develop XPCOM components in Haskell. I am aware that HDirect supports COM and that there are some similarities with XPCOM but I do not have the time to extend/fix HDirect to support XPCOM. Would using FFI directly be simpler? What problems should I expect? Would you anticipate platform specific issues ? The components should work under both Linux and Windows and ideally Mac as well. Thanks in advance, titto

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Re: Tiny documentation request
by manu 12 Sep '07

12 Sep '07

On Sep 11, 2007, at 9:58 PM, Miguel Mitrofanov wrote: > > > Well, I'm not a web designer, but I did work with few of them, and it > seems to me that you either create a table design in two hours or > spend three days trying to create a CSS one and THEN create a table > design in two hours. That's an exaggeration. It's widely accepted than tables can straightforwardly be dropped in most cases. They are intented to be used to present tabular datas, NOT for layout. (This ideas is not new, Zeldman talked about it in 2001) It does require a bit of CSS-Fu, however, as CSS support in browsers varies... But hey, you've got to occupy these HTML-CSS guys ! Emmanuel

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GraphicsMagick binding for Haskell
by Tim Chevalier 12 Sep '07

12 Sep '07

As a Hackathon project, I'm thinking of trying to write a Haskell binding for the GraphicsMagick image manipulation library (http://www.graphicsmagick.org/) Has anyone done this already? If not, would anyone else besides me use it? If you think there's a better library out there that serves a similar purpose, feel free to tell me about it as well. Cheers, Tim -- Tim Chevalier * catamorphism.org * Often in error, never in doubt "Work is there when love is gone" -- Greg Brown

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"Prime" monads?
by Greg Meredith 12 Sep '07

12 Sep '07

Haskellians, Is there a characterization of "prime" monads? Here the notion of factorization i'm thinking about is decomposition into adjoint situations. For example, are there monads for which there are only the Kleisli and Eilenberg-Moore decompositions into adjoint situations? Would this be a characterization of quintessentially "free" or "generative"? Best wishes, --greg -- L.G. Meredith Managing Partner Biosimilarity LLC 505 N 72nd St Seattle, WA 98103 +1 206.650.3740 http://biosimilarity.blogspot.com

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Licenses and Libraries
by H. 12 Sep '07

12 Sep '07

Hello, I have some questions connected more with the licenses and libraries as the language itself: Is it possible to: - publish Haskell source code under the BSD3 license - provide an executable binary together with the code "including" (compiled e.g. with 'ghc --make') "standard" (in the ghc compiler download) libraries of the base and/or other packages - publish Haskell source code under the BSD3 license which will work with GTK2HS - provide an executable binary together with the code compiled with 'ghc -- make' - does there have to be any special flags set to do so - does there have to be included further notes about the libraries and/or there license -- Thanks in advance, H.

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Tiny documentation request
by Andrew Coppin 12 Sep '07

12 Sep '07

I have the following page bookmarked: http://haskell.org/ghc/docs/latest/html/libraries/ I'd like to ask 2 things. 1. Would it be possible to make the *huge* list of package names at the top collapsable? (That way I don't have to scroll through several pages of uninteresting text to get to the bit I actually want.) 2. Could we make is so all items are collapsed initially? (Currently they're all expended initially - which makes it take rather a long time to find anything.) Just thought I'd ask...

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Can somebody give any advice for beginners?
by clisper 12 Sep '07

12 Sep '07

haskell is greate but i don't know how to start. 2007-09-11 clisper

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Re: [Haskell-cafe] haskell and reflection
by Reinier Lamers 12 Sep '07

12 Sep '07

Op 11-sep-2007, om 18:43 heeft Greg Meredith het volgende geschreven: > Thanks for these comments. i wouldn't judge Haskell solely on the > basis of whether it embraced reflection as an organizing > computational principle or as a toolbox for programmers. Clearly, > you can get very far without it. And, it may be that higher-order > functional gives you enough of the 'programs that build programs' > capability that 80% of the practical benefits of reflection are > covered -- without having to take on the extra level of complexity > that reflection adds to typing. i was really just seeking information. Template Haskell [1] is a system that lets you write programs that get executed at *compile time*, and that produce parts of the Haskell program to be compiled by manipulating a representation of the program as structured data. It's a form of reflection restricted to compile time, if you'd ask me. Regards, Reinier [1] http://www.haskell.org/haskellwiki/Template_Haskell

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Comments and/or Criticisms
by PR Stanley 11 Sep '07

11 Sep '07

Hi Any comments and/or criticisms would be most appreciated: --count the occurrences of char in string countC :: Char -> [Char] -> Int countC x xs = sum [1 | c <- xs, c == x] --count occurrences of chars in string countCS :: [Char] -> [(Char, Int)] countCS xs = [(x, (countC x xs)) | x <- [' '..'z'], (countC x xs) > 0] Can anyone come up with a better alternative? Thanks, Paul

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Custom unary operator extension?
by Peter Verswyvelen 11 Sep '07

11 Sep '07

I find it unfortunate that one can't (I guess) define custom unary operators in Haskell. Is this correct? If so, is this just because eg (* 100) declares a function that partially applies the * operator, so this syntax disallows unary operators? Could this be fixed by introducing a different syntax for partial operator application? E.g. (? * 100) instead of just (*100)? This would also fix the ambiguity between (-10) as being either the number minus 10 or the "subtract by 10' function. Cheers, Peter

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turning an imperative loop to Haskell
by Axel Gerstenberger 11 Sep '07

11 Sep '07

Hi all, I am completely stuck with a problem involving a loop construct from imperative programming, that I want to translate to Haskell code. The problem goes as follows: Based on a value u_n, I can calculate a new value u_{n+1} with a function f(u). u_n was calculated from u_{n-1} and so on down to some initial value u_0. So far it looks like a standard recursion to me. The main goal is to write each result u_{n+1} to a file or screen. The problem arises (for me), when u is an array of doubles or a complex data object instead of a simple Double/Integer value, so that I can store only maybe the last or the two last steps in memory. I never need older values. In C, I would write a for loop, calculate the new u and write it to the file. Then I update the old values to the new ones and do the next step in the for loop. Here is what I did in Haskell: I create an infinite list and tried to print the n-th value to the screen/file. But it always calculates all values in the list "all_results", before it starts printing values to screen. On the other side, the function f is called exactly 50 times as the loop suggests. The result is correct, however, it would prohibitive much memory for more complex data and more steps. Can anyone help in explaining me, how I can print to screen and still keep only the last needed values in memory? I can only find imperative solutions, but maybe it is an imperative problem anyway.? Thanks for your time. Best Axel My approach: module Main where import System.IO import Text.Printf main :: IO () main = do let all_results1 = take 20000 $ step [1] --print $ length all_results1 -- BTW: if not commented out, -- all values of all_results -- are already -- calculated here loop [1..50] $ \i -> do let x = all_results1!!i putStrLn $ show i ++ " " ++ show x -- create an infinite list with values u_{n+1} ++ [u_n,u_{n-1},...,u_1] -- where u_{n+1} = f (u_n) step history = case history of [] -> error "no start values" xs -> xs ++ (step [ f (head $ reverse (xs) )]) f u = u + 1 + (sqrt u) -- some arbitrary complex function -- copied from some blog, not sure if this is a good way loop ns stuff = mapM_ stuff ns

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Where would I find fromInt?
by PR Stanley 11 Sep '07

11 Sep '07

Hi Where would I find the fromInt function, please? Better still, would anyone know how to write a func for converting int to float and vice versa? Thanks, Paul

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simple parsing (parsec) intro
by Tim Newsham 11 Sep '07

11 Sep '07

I wrote a small intro about how to write a parser in haskell. Its basically about parsec and how it works, but its written without directly referencing parsec and aimed towards beginners (basically wrote it for some friends). Any feedback is appreciated. http://www.thenewsh.com/%7Enewsham/formal/parse/parser.lhs Tim Newsham http://www.thenewsh.com/~newsham/

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Second Request for code review - Knuth Morris Pratt for Data.Sequence
by Justin Bailey 11 Sep '07

11 Sep '07

Using the code developed for ByteStrings by myself, Christ Kuklewicz and Daniel Fischer, I've implemented Knuth-Morris-Pratt substring searching on Data.Sequence "Seq" values. Attached you'll find the library in kmp.zip.safe. The algorithm is implemented in the module Data.Sequence.KMP. At the root, SpeedTest.hs can be compiled on Windows with the "prof_compile.bat" file included (you'll need to install the "regex-dfa" and "regex-base" packages from Hackage to build). SpeedTest searches for a known value in the 7 MB file "endo.dna" (which can be downloaded from http://www.icfpcontest.org/endo.zip) using several different algorithms and methods: strict and lazy bytestrings, regular expressions, and the KMP algorithm for "Seq" values. SpeedTest is pretty fast but I worry about its space usage. It may just be the nature of Seq values, but I cannot get it to run in constant space when I think it should be. I'm especially interested in help here. All comments and feedback are welcome. Since the zip file includes a darcs context file, feel free to send patches. Justin

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WinHugs shortcut keys
by Peter Verswyvelen 10 Sep '07

10 Sep '07

Winhugs seems to have some shortcut keys, like F5 for run. It does not seem to have shortcut keys for reload, edit, etc... These would be really handy. It is possible to customize the shortcuts assigned to the commands? If not, can it be build from source? Thanks, Peter

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Installation of GLUT package
by Ronald Guida 10 Sep '07

10 Sep '07

About a month ago, I asked about how to install freeglut and get GHC to recognize it. I was side-tracked; I finally decided to try to get it working, and I'm just stuck. I have GHC 6.6.1 and freeglut 2.4.0. Freeglut successfully compiled "out of the box" in MS Visual Studio.Net 2003 on my Windows XP box. GHC won't recognize freeglut; I found out that I need to recompile the GLUT package to get things working. Today I downloaded the GLUT package from hackage.haskell.org and attempted to install it. The installation blows up. According to Cabal/How_to_install_a_Cabal_package, I need to download the package, unzip it, and then run the following three commands. 1. runghc Setup.hs configure 2. runghc Setup.hs build 3. runghc Setup.hs install The first command, "runghc Setup.hs configure", fails, reporting that it can't find "sh". Naturally, I'm on a Windows box, so I don't have the Unix shell on my system. So now I'm stuck... * Is there an easy way to work around the absence of "sh"? * Is there an easy way to get "sh" on my Windows box? * <joke> GHC was available pre-compiled for Windows. Is there an easy way to get someone else to compile the GLUT package for me? :-) </joke> * Is it feasible to walk past a cut and "upgrade" to Linux? Most Windows programs can be replaced by open-source equivalents, but my sticky issue is that there's no Linux version of iTunes. -- Ron PS, Here's the output of configure. Do I really need all this stuff just for the GLUT package? C:\GLUT\GLUT-2.0>runghc Setup.hs configure Configuring GLUT-2.0... configure: Dependency base-any: using base-2.1.1 configure: Dependency OpenGL-any: using OpenGL-2.2.1 configure: Using install prefix: C:\Program Files configure: Binaries installed in: C:\Program Files\Haskell\bin configure: Libraries installed in: C:\Program Files\Haskell\GLUT-2.0\ghc-6.6.1 configure: Private binaries installed in: C:\Program Files\GLUT-2.0 configure: Data files installed in: C:\Program Files\Common Files\GLUT-2.0 configure: Using compiler: C:\ghc\ghc-6.6.1\bin\ghc.exe configure: Compiler flavor: GHC configure: Compiler version: 6.6.1 configure: Using package tool: C:\ghc\ghc-6.6.1\bin\ghc-pkg.exe configure: Using ar found on system at: C:\ghc\ghc-6.6.1\bin\ar.exe configure: No haddock found configure: No pfesetup found configure: No ranlib found configure: Using runghc found on system at: C:\ghc\ghc-6.6.1\bin\runghc.exe configure: No runhugs found configure: No happy found configure: No alex found configure: Using hsc2hs: C:\ghc\ghc-6.6.1\bin\hsc2hs.exe configure: No c2hs found configure: No cpphs found configure: No greencard found Setup.hs: Cannot find: sh

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ANN: The Monad.Reader - Issue 8
by Wouter Swierstra 10 Sep '07

10 Sep '07

I am pleased to announce that the latest issue of The Monad.Reader is now available: http://www.haskell.org/haskellwiki/The_Monad.Reader Issue 8 consists of the following two articles: * Brent Yorgey Generating Multiset Partitions * Conrad Parker Type-Level Instant Insanity The Monad.Reader is a quarterly magazine about functional programming. It is less-formal than journal, but somehow more enduring than a wiki page or blog post. If you'd like to submit something to the next issue of The Monad.Reader, please get in touch. I haven't fixed the deadline for the next issue just yet; I hope to send out an official call for copy in the coming weeks. I'd also like to point out that you can now order TMR apparel from: http://www.cafepress.com/TheMonadReader. Enjoy! Wouter This message has been checked for viruses but the contents of an attachment may still contain software viruses, which could damage your computer system: you are advised to perform your own checks. Email communications with the University of Nottingham may be monitored as permitted by UK legislation.

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Re: [Haskell-cafe] Tiny documentation request
by manu 10 Sep '07

10 Sep '07

Neil Mitchell wrotes : > Replicating actual tables with CSS is a nightmare - you shouldn't use > table's for lots of things, I agree > but there are sometimes when it really is > the best option. Which isn't the case here ! Nested lists would easily do the trick... > Fixing up the CSS and still keeping tables is a > perfectly valid option. yes it's true... E.D

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Re: [Haskell-cafe] Speed of character reading in Haskell
by Miguel Mitrofanov 10 Sep '07

10 Sep '07

> I started with the obvious > main = getContents >>= print . tokenise > where tokenise maps its way down a list of characters. This is very > simple, very pleasant, and worked like a charm. > However, the language has an INCLUDE directive, so I'm going to have > to call readFile or something in the middle of tokenising, so the > main tokeniser loop can't be a pure String -> [Token] function any > more. What about > tokenise :: [String] -> ([Token],[FilePath]) > main = print . fst =<< mfix process where > process (tokens,paths) = do > mainContents <- getContents > includes <- mapM readFile paths > return $ tokenise $ mainContents : includes I guess, it would be useful to replace ([Token],[FilePath]) with Writer [FilePath] [Token] > Method 1A (pure list processing) > main = getContents >>= print . doit 0 > doit n ('\n':cs) = doit (n+1) cs > doit n ( _ :cs) = doit n cs > doit n [] = n I think, you should use something like (doit $! n+1) cs here. > In *retrospect*, it is really obvious why this was > necessary, but I must say that in *prospect* I wasn't expecting it. In fact, I was expecting this to be an issue even for 1A. I suppose, GHC is smart enough to suppress lazyness in the first method.

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Elevator pitch for Haskell.
by Paul Johnson 10 Sep '07

10 Sep '07

This page (http://www.npdbd.umn.edu/deliver/elevator.html) has a template for an "elevator pitch". This is what you say to someone when you have 30 seconds to explain your big idea, for instance if you find yourself in an elevator with them. I thought I'd try instantiating it for Haskell. For software developers who need to produce highly reliable software at minimum cost, Haskell is a pure functional programming language that reduces line count by 75% through reusable higher order functions and detects latent defects with its powerful static type system. Unlike Ada and Java, Haskell allows reusable functions to be combined without the overhead of class definitions and inheritance, and its type system prevents the hidden side effects that cause many bugs in programs written in conventional languages. Comments? Paul.

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Re: [Haskell-cafe] Where would I find fromInt?
by Tom Harper 10 Sep '07

10 Sep '07

Hi Paul -- The function you want is called fromIntegral, and works for all Integral types. Using it, you can add a type signature to specify what you want to change the number to (Float, Double, other Integral type, etc. Example: > fromIntegral (4 :: Int) :: Double 4.0 Hope this helps! -- Tom Harper rtharper(a)aftereternity.co.uk +1 949 235 0185 Public Key: http://aftereternity.co.uk/rth.asc On 9/9/07, PR Stanley <prstanley(a)ntlworld.com> wrote: > Hi > Where would I find the fromInt function, please? > Better still, would anyone know how to write a func for converting > int to float and vice versa? > Thanks, Paul > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > -- Tom Harper rtharper(a)aftereternity.co.uk +1 949 235 0185 Public Key: http://aftereternity.co.uk/rth.asc

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Help understanding type error
by Levi Stephen 10 Sep '07

10 Sep '07

Hi, I'm after some help understanding either what I'm doing wrong, or why this error occurs. I have a data type: > data T a = forall b. (Show b) => T b a and I want to use/extract 'b' from this. > extShow (T b _) = b This gives the following compilation error: extest.hs:5:0: Inferred type is less polymorphic than expected Quantified type variable `b' escapes When checking an existential match that binds $dShow :: {Show b} b :: b The pattern(s) have type(s): T t The body has type: b In the definition of `extShow': extShow (T b _) = b I tried adding a type signature: > extShow' :: (Show b) => T a -> b > extShow' (T b _) = b Now I get a different error: extest.hs:8:19: Couldn't match expected type `b' (a rigid variable) against inferred type `b1' (a rigid variable) `b' is bound by the type signature for `extShow'' at extest.hs:7:18 `b1' is bound by the pattern for `T' at extest.hs:8:10-14 In the expression: b In the definition of `extShow'': extShow' (T b _) = b It seems (to newbie me ;) ) like it should be possible to extract the first part of T, and deduce that it is an instance of the class 'Show'. The following seems ok: > doShow (T b _) = putStrLn (show b) Thanks, Levi

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Hackage and GHC 6.8
by Neil Davies 09 Sep '07

09 Sep '07

Given that GHC 6.8 is just around the corner and, given how it has re-organised the libraries so that the dependencies in many (most/all) the packages in the hackage DB are now not correct. Is there a plan of how to get hackage DB up to speed with GHC 6.8 ? Cheers Neil

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((a -> b) -> c) -> (a -> m b) -> m c
by Stuart Cook 09 Sep '07

09 Sep '07

(Inspired by this[1] reddit thread.) When combining monadic and non-monadic code, I've often wished for a magical combinator of type (Monad m) => ((a -> b) -> c) -> (a -> m b) -> m c which would let me inject a monadic function into a pure one, then wrap the ultimate result to ensure that no nastiness escapes. Now, I'm pretty sure that writing such a combinator as-is is impossible. My question is, what restrictions/changes would be necessary for a similar combinator to be definable and well-behaved? (Obviously it would be impossible to ensure that the (m b)s get chained in any particular order, for order-sensitive monads, but it would be the caller's responsibility to ensure order-irrelevance in this case.) As a thought experiment, I imagined an implementation for IO using unsafePerformIO. This makes it easy to give the injected function the right type, and it's possible to use return to give the result the correct type too. Unfortunately, the semantics fail: if the injected function is used lazily, there's no way to ensure that the unsafe IO actions actually take place before the rest of the IO-based program, and it's possible for them to show up later than expected. I suspect that even in a strict language, lambda-abstractions would cause similar trouble. [1] http://programming.reddit.com/info/2n6eh/comments Stuart Cook

3 3

ICMP in Haskell (was Elevator pitch for Haskell)
by Dominic Steinitz 09 Sep '07

09 Sep '07

> > Does it enable you to, say, send raw ICMP packets? > > AFAIK, Haskell supports TCP, and nothing else. (A while back I wanted to > write an automated pinging program. But the only way I could figure out > how to do it is to call the OS "ping" utility and attempt to parse what > it writes to stdout. Oh, did I mention that's different on WinNT, WinXP > and UNIX?) > Andrew, I thought this was relatively straightforward. Look in http://www.haskell.org/networktools/src for multi-threaded versions. Beware I wrote ping and traceroute about 5 years ago. They're pretty much straight from Stevens' book. On the other hand, I quite liked this: sequenceWhile_ :: Monad m => (a -> Bool) -> [m a] -> m () sequenceWhile_ p [] = return () sequenceWhile_ p (x:xs) = x >>= \c -> if (p c) then sequenceWhile_ p xs else return () I'd forgotten about them and I'm not particularly proud of them. I guess I could put them in hackage so that they are at least a starting point for someone. Dominic.

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"Cofunctor" as a name
by Conal Elliott 09 Sep '07

09 Sep '07

I've heard contravariant functors referred to as "cofunctors", and I've also heard objections to that term. As I'm preparing to release some libraries using contravariant functors, I'd appreciate hearing preferences and/or reasons. For instance, another name would be "Contrafunctor". -- | Contravariant functors. often useful for /acceptors/ (consumers, -- sinks) of values. class Cofunctor acc where cofmap :: (a -> b) -> (acc b -> acc a) Thanks, - Conal

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Serial Communications in Haskell
by Ronald Guida 08 Sep '07

08 Sep '07

I'm on a Windows box and I'm looking for a way to talk to a serial port (for example, RS-232) from Haskell. I couldn't find a library to do this, so I am wondering how to create one. I have a fairly thorough understanding of how to open and use a serial port with the Windows API. In particular, to open a serial port, I have to use CreateFile, which is the same API call that opens files. In fact, if I call openFile from GHC, and pass "COM1:" as the filename, then I can get a writable serial port. > module Serial > where > import System.IO > > main = do > h <- openFile "COM1:" ReadWriteMode > hPutStrLn h "Hello World" I can't read from the port (I always get an immediate EOF), and I have no way of configuring things like the baud rate or the parity settings. Nevertheless, this demonstrates that openFile can at least open the serial port. What I would like to do is create some functions that would allow me to open and configure a serial port, and get a Handle back so that I can use the existing IO functions like hGetChar and hPutChar. I am assuming that hGetChar eventually calls win32::ReadFile and hPutChar eventually calls win32::WriteFile. These same two API calls would work for serial ports. In Windows, there are 23 API functions that apply specifically to serial ports. Out of these 23 functions, only a few of them are actually necessary if I just want to send and receive data. Of course, I don't know how to call Windows API functions from Haskell, and I have no idea how to hook things to the IO library so that I can use a Handle for a serial port. I'm looking for some advice on how to proceed. -- Ron

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generate a news summary page from web services
by brad clawsie 08 Sep '07

08 Sep '07

i recently worked up a little example program i use to generate a general news summary page using yahoo web services if you are interested in an example of XHT and Text.XHtml.Strict in action, you can see it here http://www.b7j0c.org/content/haskell-newspage.html

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Block-wise lazy sequences in Haskell
by Henning Thielemann 08 Sep '07

08 Sep '07

I want to have a data structure like Data.ByteString.Lazy, that is block-wise lazy, but polymorphic. I could use a lazy list of unboxed arrays (UArray) but the documentation says, that the element types are restricted. But I will need (strict) pairs of Double and the like as elements. It seems that I need generalized instances in order to use these arrays. I thought it must be possible to define an unboxed array type with Storable elements. I cannot find such a type in the standard libraries. Further on, I wonder why pairs are not instances of Storable.

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Extending the idea of a general Num to other types?
by Peter Verswyvelen 08 Sep '07

08 Sep '07

I like the fact that Haskel treats numbers in a generic way, so you can lift them to any other datatype by instantiating Num. Can the same be done on other builtin constructs? For example, if I have [a], can this list be lifted to other types? I guess not, because no type class exists for the list type? Does this make sense? I'm still searching for correct terminology... Thanks, Peter

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Learning advice
by Chris Saunders 08 Sep '07

08 Sep '07

I wish to learn Haskell. I purchased a book - An Introduction to Functional Programming Systems Using Haskell but I think this might be too outdated. I'm using GHC and it seems that some of the functions used in the book are no longer a part of Haskell. I'm looking for advice on books or tutorials that might be more up to date. So far I'm finding Haskell difficult - I may be too thick. Regards Chris Saunders

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Fwd: [Haskell-cafe] turning an imperative loop to Haskell
by Stephen Dolan 07 Sep '07

07 Sep '07

Sorry, forgot to cc list > I think this is called taking a good thing too far, but cool too: > > > f1 u = u + 1 > f2 u v = u + v > f3 u v w = u + v + w > > -- functions renamed for consistency) > zipWith1 = map > zipWith2 = zipWith > > -- and hey presto! > us1 = 3 : zipWith1 f1 us1 > us2 = 2 : 3 : zipWith2 f2 (drop 1 us2) us2 > us3 = 2 : 3 : 4 : zipWith3 f3 (drop 2 us3) (drop 1 us3) us3 How about this: import Data.List -- for transpose f = sum zipWithN fn = map fn . transpose us k = s where s = [2..k+1] ++ (zipWithN f $ x $ map (flip drop s) [0..k-1]) take 10 (us 1) [2,2,2,2,2,2,2,2,2,2] take 10 (us 2) [2,3,5,8,13,21,34,55,89,144] take 10 (us 3) [2,3,4,9,16,29,54,99,182,335] take 10 (us 4) [2,3,4,5,14,26,49,94,183,352] We can always take it further :D Stephen

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Typeclasses and implicit parameters
by ajb＠spamcop.net 07 Sep '07

07 Sep '07

{-# OPTIONS -fglasgow-exts #-} -- G'day everyone. -- This is okay. f1 :: (?foo :: String) => String f1 = ?foo -- So is this. f2 :: (Show a, ?foo :: a) => a -> String f2 _ = show ?foo -- Hugs allows this. GHC rejects it on the grounds that "a" is unused -- on the right-hand side of the (=>). I think this is arguably a bug -- in GHC. f3 :: (Show a, ?foo :: a) => String f3 = show ?foo -- GHC rejects this. Hugs compiles it, but I can't call it as -- let ?foo = "Hello" in show Foo -- -- Is there a good reason to disallow this? data Foo = Foo instance (?foo :: String) => Show Foo where showsPrec _ Foo = showString ?foo . showString "Foo" -- Cheers, -- Andrew Bromage

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ANNOUNCE: xmonad 0.3
by dons＠cse.unsw.edu.au 07 Sep '07

07 Sep '07

The xmonad dev team is pleased to announce the 0.3 release of xmonad. xmonad: a tiling window manager http://xmonad.org About: xmonad is a tiling window manager for X. Windows are arranged automatically to tile the screen without gaps or overlap, maximising screen use. All features of the window manager are accessible from the keyboard: a mouse is strictly optional. xmonad is written and extensible in Haskell. Custom layout algorithms, and other extensions, may be written by the user in config files. Layouts are applied dynamically, and different layouts may be used on each workspace. Xinerama is fully supported, allowing windows to be tiled on several screens. Features: * Very stable, fast, small and simple. * Automatic window tiling and management * First class keyboard support: a mouse is unnecessary * Full support for tiling windows on multi-head displays * Full support for floating windows * XRandR support to rotate, add or remove monitors * Per-workspace layout algorithms * Per-screens custom status bars * Easy, powerful customisation and reconfiguration * Large extension library * Extensive documentation and support for hacking Since xmonad 0.2, the following notable features and bug fixes have appeared: New features: * floating layer support: transients windows are not tiled by default, and windows may be dragged to and from a traditional floating layer (which allows mouse-resizing, and overlapping windows). * improved Xinerama support: workspace switching reuses multiple displays more effectively. * huge new extension library. Over 50 extensions to xmonad have been contributed by users, and are available all in a standard library, with documentation. More information, screenshots, documentation and community resources are available from: http://xmonad.org Xmonad is available from hackage, and via darcs. Happy hacking! The Xmonad Team: Spencer Janssen Don Stewart Jason Creighton Xmonad has also received contributions from at least: Alec Berryman Andrea Rossato Chris Mears Daniel Wagner David Glasser David Lazar David Roundy Hans Philipp Annen Joachim Fasting Joe Thornber Kai Grossjohann Karsten Schoelzel Michael Sloan Miikka Koskinen Neil Mitchell Nelson Elhage Nick Burlett Peter De Wachter Robert Marlow Sam Hughes Shachaf Ben-Kiki Shae Erisson Simon Peyton Jones Stefan O'Rear as well as many others on the IRC channel and mailing list. Thanks to everyone!

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Speed of character reading in Haskell
by ok 07 Sep '07

07 Sep '07

I'm writing a tokeniser for a legacy programming language in Haskell. I started with the obvious main = getContents >>= print . tokenise where tokenise maps its way down a list of characters. This is very simple, very pleasant, and worked like a charm. However, the language has an INCLUDE directive, so I'm going to have to call readFile or something in the middle of tokenising, so the main tokeniser loop can't be a pure String -> [Token] function any more. It occurred to me to wonder whether it would be better to try to stick with the list of characters style as much as possible (and use readFile) or to switch over to character at a time processing (with openFile, hIsEOF, hGetChar, hClose). So I decided on an experiment: count the lines both ways. Method 1A (pure list processing) main = getContents >>= print . doit 0 doit n ('\n':cs) = doit (n+1) cs doit n ( _ :cs) = doit n cs doit n [] = n Method 1B (doit is in the IO monad) main = getContents >>= doit 0 doit n ('\n':cs) = doit (n+1) cs doit n ( _ :cs) = doit n cs doit n [] = print n Method 2 (character at a time) main = doit 0 doit n = isEOF >>= \eof -> if eof then print n else getChar >>= \c -> doit (if c == '\n' then n+1 else n) I tried this using GHC 6.4 on a 2.8 GHz PC running linux 2.6.20-1.2933.fc6 I don't know whether gcc was involved at all, but in case it was, the gcc version was 4.1.1. All test programs were compiled using ghc -o foobar -O foobar.hs The input was a 3.8 MB file. wc: 0.08 seconds total Method 1A: 0.30 seconds total Method 1B: 0.47 seconds total Method 2: ran out of stack Method 2': 2.21 seconds total. When I got the stack overflow, I suspected that this might be due to lazy evaluation of the integer argument to doit, so I changed the last line to doit $! (if c == '\n' then n+1 else n) and that worked. In *retrospect*, it is really obvious why this was necessary, but I must say that in *prospect* I wasn't expecting it. Other things being equal, I would have expected Method 2' to be faster. I'm now wondering if there is some other consequence of laziness that I am just not seeing. I'm also a little puzzled that there is a measurable difference between method 1A and method 1B. Things that I am not asking about: - ghc version; that's what's installed on the machine and if I want something different I may have to wait many days. - style comments on the list processing versions; yes I know how to write getContents >>= print . length . filter (=='\n') and that really has no relevance to the full tokeniser and in any case is slower than Method 1A.

1 0

wanted: HAppS example combining state and io
by Thomas Hartman 07 Sep '07

07 Sep '07

In the latest happs (darcs pulled, updated head is 0.9.1 iirc), I am experimenting with the example file in src/HAppS/Examples/HTTP1.hs. I would like to combine state with io. Eventually io will mean stuff like reading from a database, but for now I'm just reading a file. The example file HTTP1.hs has an example that demonstrates state with macid. I added an example that allows you to execute arbitrary io. I tried, but was unable to, add a handler that combines state and io. , h ["iohandler"] GET $ ioReadFileHandler , h ["statehandler"] GET $ stateHandler --, h ["ioandstatehandler"] GET $ ioAndStateHandler ..... -- displays contents of HAPPS.hs in current directory ioReadFileHandler = iohandler $ readFile "./HAppS.hs" -- displays incremented state counter stateHandler = ok $ \() () -> modify (+(1::Int)) >> get >>= respond . show -- should combine effect of iohandler with statehandler -- specifically, should display contents of HAppS.hs, and under that an incremented state handler -- is this possible ioAndStateHandler = undefined undefined Is mixing state and io possible with HAppS? If so, an example showing how to do it would be extremely helpful. best, thomas. --- 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.

2 2

let and fixed point operator
by Peter Hercek 07 Sep '07

07 Sep '07

Hi, I find the feature that the construct "let x = f x in expr" assigns fixed point of f to x annoying. The reason is that I can not simply chain mofifications a variable like e.g. this: f x = let x = x * scale in let x = x + transform in g x When one is lucky then it results in a compile error; in worse cases it results in stack overflow in runtime. The annoying part is figuring out new and new variable names for essentially the same thing to avoid the search/evaluation of the fixed point. I suppose Haskell was designed so that it makes sense. The only usage I can see is like this: let fact = \x -> if x == 0 then 1 else x * fact (x-1) in ... but that is not any shorter than: let fact x = if x == 0 then 1 else x * fact (x-1) in So the question is what am I missing? Any nice use cases where fixed point search is so good that it is worth the trouble with figuring out new and new variable names for essentially the same stuff? Peter.

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Re: [Haskell] (no subject)
by Scott Williams 06 Sep '07

06 Sep '07

[bcc haskell, cc haskell-cafe] On 9/5/07, Tomi Owens <t.owens(a)hautlieu.sch.je> wrote: > > Hi there. I'm a teacher of Maths and am working my way through the Euler > Project problems for fun. I have mostly been using Basic, but have read up > about Haskell and think it looks like a sensible way to solve many of the > problems. > > OK, so I've downloaded GHCi and am trying to teach myself. > > So far I have done this: > > ___ ___ _ > / _ \ /\ /\/ __(_) > / /_\// /_/ / / | | GHC Interactive, version 6.6.1, for Haskell 98. > / /_\\/ __ / /___| | http://www.haskell.org/ghc/ > \____/\/ /_/\____/|_| Type :? for help. > > Loading package base ... linking ... done. > Prelude> let f (a,b) = a * floor (100000/b) > Prelude> f(2,5) > 40000 Here you can find out type ghci has inferred for this function. > :t f f :: (RealFrac b, Integral b1) => (b1, b) -> b1 This function works just as I want it to. > > Now I try creating a list: > > Prelude> [(a2+b2,a)| a <- [1..4] , b<- [1..4], a2+b2<20, b<=a] > [(2,1),(5,2),(8,2),(10,3),(13,3),(18,3),(17,4)] Let's assign this to an intermediate variable so we can query it's type: Prelude> let lst = [(a ^ 2 + b ^ 2, a) | a <- [1..4], b <- [1..4], a^2 + b^2 < 20, b <= a] Prelude> lst [(2,1),(5,2),(8,2),(10,3),(13,3),(18,3),(17,4)] Prelude> :t lst lst :: [(Integer, Integer)] aha; here's the source of the type mismatch: Prelude> :t floor floor :: (RealFrac a, Integral b) => a -> b Floor has to take a RealFrac. According to hoogle[1], we can use various floating-point approximations (Float, Double, CFloat, etc) or we can use the exact Rational type. [1] http://haskell.org/hoogle/?q=RealFrac You can get your types to match by declaring your list to be of type [(Rational, Rational)] either by explicitly typing one of the untyped variables or the entire expression: Prelude> let lst = [(a ^ 2 + b ^ 2, a) | (a::Rational) <- [1..4], b <- [1..4], a^2 + b^2 < 20, b <= a] Prelude> :t lst lst :: [(Rational, Rational)] Prelude> let lst :: [(Rational, Rational)] = [(a ^ 2 + b ^ 2, a) | a <- [1..4], b <- [1..4], a^2 + b^2 < 20, b <= a] Prelude> :t lst lst :: [(Rational, Rational)] and this works > So now I try to apply the function to the list: > > Prelude> map (f) [(a2+b2,a)| a <- [1..4] , b<- [1..4], a2+b2<20, b<=a] > > and I get this result: > > <interactive>:1:5: > Ambiguous type variable `t' in the constraints: > `Integral t' arising from use of `f' at <interactive>:1:5 > `RealFrac t' arising from use of `f' at <interactive>:1:5 > Probable fix: add a type signature that fixes these type variable(s) I'm sorry, but I don't quite get how to set the type signature and how it > will apply to my function... > > Thanks, Hope this helps Tomi > -------------------------------------------------------------------------------- > > > Department for Education, Sport and Culture E Mail > This message is for the named person's use only. It may contain > confidential, proprietary or legally privileged information. No > confidentiality or privilege is waived or lost by any mistransmission. > If you receive this message in error, please immediately delete it and all > copies of it from your system, destroy any hard copies of it and notify the > sender. You must not, directly or indirectly, use, disclose, distribute, > print, or copy any part of this message if you are not the intended > recipient. The Department for Education, Sport and Culture and any of its > establishments each reserve the right to monitor all e-mail communications > through its networks. > > Any views expressed in this message are those of the individual sender, > except where the message states otherwise and the sender is authorised to > state them to be the views of any such entity. > > The Department for Education, Sport and Culture shall not be liable to > the recipient or any third party for any loss or damage, however it appears, > from this e-mail or its content. This includes loss or damage caused by > viruses. It is the responsibility of the recipient to ensure that the > opening of this message and its attachments shall not adversely affect > systems or data. > > > -------------------------------------------------------------------------------- > > _______________________________________________ > Haskell mailing list > Haskell(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell > > -- Scott Williams

3 3

Code from "Why Functional Programming Matters"
by Andrew Wagner 06 Sep '07

06 Sep '07

I've been reading the classic "Why functional programming matters" paper [1] lately, particularly looking at the alpha beta stuff. I've ported all his code to haskell, but I have a question. His algorithm takes a board position, creates a gametree out of it, maps a static evaluation function over the gametree, then uses alpha beta to find the real evaluation from a Tree Int. But of course, in an actual application, what you actually want is the best MOVE from the current position (or, even more ideally, the so-called "principal variation", which is the best series of moves from the current position). Is there a good way to collect this, without mapping some sort of function over the tree that puts a list of moves on every node too? Hughes seems to completely ignore this, and I wonder if it's because it gets ugly to implement.

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Re: [Haskell-cafe] Re: [Off topic] Proving an impossibility
by Miguel Mitrofanov 05 Sep '07

05 Sep '07

> It depends on arbitrary restrictions on what constitutes an > (boolean) expression, something that is anathema to > functional programmers :-) Spot the language: > while if E > then S; F > else False > fi > do T > od > > It reminds me of a paper by Knuth, where he states that > > "goto" statement is necessary; don't remember the title, > > however. > I don't remember needing a goto in Haskell... Well, for imperative languages, of course.

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[Off topic] Proving an impossibility
by Vimal 05 Sep '07

05 Sep '07

Hi In my Paradigms of Programming course, my professor presents this piece of code: while E do S if F then break end T end He then asked us to *prove* that the above programming fragment cannot be implemented just using if and while statement, even if S and T can be duplicated a finite number of times He also asked us to state our assumptions if any. 1. We have assignment statements (to create a flag and check) 2. The execution of E or F does not affect execution of S or T... Well, learning Haskell, I quickly realized that even if S and T have to be expressions and came up with this, assuming that 1. There are boolean operations 2. Boolean expressions are evaluated from Left to Right 3. Boolean expressions can be short-circuited while E and (S or 1) and F and (T or 1) do (* Nothing *) end But I would also like to consider an option of proving that its impossible, under the given constraints... Any thoughts? Vimal

4 3

ANN: Finance-Quote-Yahoo 0.3
by brad clawsie 05 Sep '07

05 Sep '07

http://hackage.haskell.org/cgi-bin/hackage-scripts/package/Finance-Quote-Ya… i know minor point releases do not merit a list announcement but yahoo discontinued a url i was using to download data, so users of this package must upgrade. sorry for the hassle. thanks brad

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