- Haskell-Cafe - Haskell.org

Records and associated types
by Taru Karttunen 22 Dec '08

22 Dec '08

Hello What is the correct way to transform code that uses record selection with TypeEq (like HList) to associated types? I keep running into problems with overlapping type families which is not allowed unless they match. The fundep code: class Select rec label val | rec label -> val instance TypeEq label label True => Select (Label label val :+: rest) label val instance (Select tail field val) => Select (any :+: tail) field val And a conversion attempt: class SelectT rec label where type S rec label instance TypeEq label label True => SelectT (Label label val :+: rest) label where type S (Label label val :+: rest) label = val instance (SelectT tail field) => SelectT (any :+: tail) field where type S (any :+: tail) field = S tail field which fails with: Conflicting family instance declarations: type instance S (Label label val :+: rest) label -- Defined at t.hs:19:9 type instance S (any :+: tail) field -- Defined at t.hs:23:9 How is it possible to get the TypeEq constraint into the type family? Attached is a complete example that illustrates the problem. - Taru Karttunen

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ANN: Data.List.Split
by Brent Yorgey 21 Dec '08

21 Dec '08

I am pleased to announce the release of Data.List.Split, which provides a wide range of strategies and a unified combinator framework for splitting lists with respect to some sort of delimiter. It strives to be flexible yet simple. If you've ever wished there was a simple 'split' function you could grab from Data.List, this is the package for you---no matter which of the seventeen* slightly different ways to split a list you actually need! Just use one of the prepackaged splitting strategies, or build your own with the combinator library. Get it from Hackage: cabal install split http://hackage.haskell.org/cgi-bin/hackage-scripts/package/split Get the source: darcs get http://code.haskell.org/~byorgey/code/split Get excited: http://byorgey.wordpress.com/2008/12/21/datalistsplit/ -Brent * Actual number may vary.

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Logos of Other Languages
by Ashley Yakeley 21 Dec '08

21 Dec '08

I browsed around a bit for logos from other languages... Python http://www.python.org/images/python-logo.gif The "snake pair" is visually interesting while still remaining simple. The typeface is unusual and yet clean and humanistic. The logo is only slightly marred by the "TM". Overall, elegant and appealing. A. Caml http://caml.inria.fr/styles/modern/title-en.gif This is actually more of a heading than a logo: it continues to the right edge of the page to close the oval. The typeface is readable, but the text is a bit verbose, spelling out "The Caml Language" with a visual nod to its ML roots. B+. Ruby http://www.ruby-lang.org/images/logo.gif An ordinary book typeface with an ordinary picture of an ordinary ruby. And the strapline "A Programmer's Best Friend" is vague and uninspired. Bland, but at least inoffensive. C. Perl http://upload.wikimedia.org/wikipedia/en/e/e0/Programming-republic-of-perl.… I'm not sure if this is "the Perl logo" or "O'Reilly's Perl logo", but it's ugly, busy, difficult to read and a bit obscure. The title "PROGRAMMING REPUBLIC OF" also lends an air of snotty pretension. D. Apple Dylan http://osteele.com/projects/images/Dylan.logo-thumb.png Arty, abstract, vaguely Hermitian, but a bit corporate. Easy to make a website favicon from, but the actual word "Dylan" is not strictly included. A-. All of these get one thing right that the current and most of the proposed Haskell logos do not: they don't make any reference to the syntax of the language itself. Doing so seems to miss the point of a logo: it's supposed to appeal visually, rather than semantically. So I'd like to see some submissions that don't use lambdas. -- Ashley Yakeley Seattle, WA

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Re: [Haskell] ANN: Hoogle with more libraries
by Neil Mitchell 21 Dec '08

21 Dec '08

Hi Bob, > 1. Searching using a package name that isn't all lower case results in > nothing (e.g. (a -> b) -> f a -> f b +InfixApplicative gives no results, > while (a -> b) -> f a -> f b +infixapplicative gives 2). Yes, if you do +InfixApplicative it assumes you mean only in the module InfixApplicative, not the package. Module is upper case, package starts with lowercase. > 2. Searching with the +package syntax seems to remove all other possible > results. Perhaps it would be nice to show results from the default library > set below. Add +default to search the default too. > If you get the package name wrong (i.e. specify a package that hoogle can't > see), it would be nice for it to report something like google does -- maybe > you meant xyz. Definately! The whole area needs an overhall. Thanks Neil

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Windows vs. Linux x64
by Bartosz Wójcik 21 Dec '08

21 Dec '08

Hi Everybody, while working on my resent project I've noticed that my code seems to be faster under Windows than under Linux x64. More exactly this was an AI game evaluator that ran on given parameters. There was no IO performed. I've run 3 lots of test on both systems and stored some figures. It was physicaly the same PC. 1st lot WinXP total time = 27.18 secs (1359 ticks @ 20 ms) total alloc = 5,788,242,604 bytes (excludes profiling overheads) Linux total time = 34.44 secs (1722 ticks @ 20 ms) total alloc = 11,897,757,176 bytes (excludes profiling overheads) 2nd lot WinXP total time = 63.96 secs (3198 ticks @ 20 ms) total alloc = 13,205,507,148 bytes (excludes profiling overheads) Linux total time = 80.76 secs (4038 ticks @ 20 ms) total alloc = 27,258,694,888 bytes (excludes profiling overheads) 3rd lot WinXP total time = 207.10 secs (10355 ticks @ 20 ms) total alloc = 44,982,716,780 bytes (excludes profiling overheads) Linux total time = 267.58 secs (13379 ticks @ 20 ms) total alloc = 92,307,482,416 bytes (excludes profiling overheads) I've used the same compile and runtime options for both. I've tried to run with -H option, but this didn't improve anything. Is this common behaviour? Does anybody know what can be the reason? regards, Bartek

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Line noise
by Andrew Coppin 21 Dec '08

21 Dec '08

I hang out on another forum that is populated by various kinds of computer geeks. There's a fair few programmers in there, as well as nerds of every degree. And yet, every time I post anything written in Haskell, everybody complains that it "looks like line noise". What do "normal" programmers find so puzzling about Haskell? You might guess some of the following: - High-order functions. - Lambda functions. - Algebraic data types. - Lazy evaluation. - Recursion. - Monads. Actually, none of these things were mentioned. The things people have *actually* complained to me about are: - Haskell expressions are difficult to parse. - Several standard library elements have unhelpful names such as "elem", "Eq" and "fst". (My favourite has to be "Ix". I mean, WTH?) - Several standard library functions have names which clash badly with the usual meanings of those names - e.g., "break", "return", "id". - Variable names such as "x" and "f" aren't fabulously helpful to lost programmers trying to find their way. If you take a "typical" lump of C or Java code, there's so much redundancy and noise in it that you can *usually* figure out vaguely what's going on, even if it doesn't completely make sense. When presented with a typical Haskell fragment... uh... well, good luck with that. Let's take Haskell's most celebrated example: qsort [] = [] qsort (x:xs) = qsort (filter (x <) xs) ++ [x] ++ qsort (filter (x >=) xs) To a novice, it's not actually real clear what the arguments to qsort are. You and I know that function application binds much more tightly than (++), but a novice isn't going to know that. You might also not immediately realise that "xs" is also an argument to filter, not just (x <) or whatever. The people I spoke to also seemed pretty confused about the usage of (.) and ($), even after I explained it a few times. Several people also voiced being puzzled about Haskell's layout rules. I'm not sure what we *do* with all this data, but I found it interesting so I thought I'd share. ;-) I've spent the last few years trying to convert a few people to The Haskell Way(tm), but so far I haven't succeeded in the slightest. I just get yelled at for pointing white noise. Heh. Oh well!

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Trouble with FFI on Windows2000...
by Serge LE HUITOUZE 21 Dec '08

21 Dec '08

Hi there! I'm trying to FFI-bind a MS Visual dll (under Windows2000) to a Haskell program. I have this little C(++) dll: ******* FILE inc.h: START ******* extern "C" { int pten(int i); } ******* FILE inc.h: END ******* ******* FILE inc.cpp: START ******* #include "inc.h" int pten(int i) { return i+10; } ******* FILE inc.cpp: END ******* ******* FILE cproj1.def: START ******* LIBRARY cproj1 EXPORTS pten ******* FILE cproj1.def: END ******* And this little Haskell program: ******* FILE testFFI_2.hs: START ******* {-# LANGUAGE ForeignFunctionInterface #-} module Main where import Foreign.C -- get the C types foreign import ccall unsafe "pten" c_pten :: CInt -> CInt plus10 :: Int -> Int plus10 i = fromIntegral (c_pten (fromIntegral i)) main = print (map plus10 [1..10]) ******* FILE testFFI_2.lhs: END ******* Using information found at paragraph 1.4.3.4 of http://www.haskell.org/haskellwiki/GHC:FAQ#GHC_on_Windows I do the following: > dlltool -d cproj1.def -l libcproj1.a > ghc --make testFFI_2.hs -optl-lcproj1 -optl-L. This seems fine, since it produces a "testFFI_2.exe". However, executing it, I get a MSWindows error box with a message that looks like (approximate translation): "DLL (null) not found on specified path" At first, I didn't have the "LIBRARY" directive in the ".def" file, so I thought that was the reason for the "(null)" appearing in the message. But adding said directive didn't change anything... [I've checked that my MSVisual dll was in current dir, and that this dir was in the search paths showed in the error box] Can anyone enlighten me? Thanks in advance. --Serge

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Graham Hutton's calculator example for win32
by Ahn, Ki Yung 21 Dec '08

21 Dec '08

In Graham Hutton's "Programming in Haskell" there is an interactive calculator example using ANSI code to implement the UI on the terminal. This example doesn't work on MS Windows XP or other MS OSes based on NT kernel, since their command line does not support ANSI very well. But, thanks to ansi-terminal on Hackage, I was able to extract minimal code from the package to make a win32 version of the calculator example in Hutton's book. calculatorWin32.lhs and Win32ANSI.hs is an implementation for win32. I extracted the win32 console API bindings for setting cursor positions from ansi-terminal project and put them in Win32ANSI.hs. I had to put this in a separate file because I had an issue with ghci. To run this, I had to compile the console API bindings with ghc first and then run ghci as follows C:\> ghc -c Win32ANSI.hs C:\> ghci calculatorWin32.lhs Without compiling the object code, ghci cannot find the proper link for win32 console API FFI bindings. C:\> ghci calculatorWin32.lhs GHCi, version 6.10.1: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer ... linking ... done. Loading package base ... linking ... done. [1 of 3] Compiling Parsing ( Parsing.lhs, interpreted ) [2 of 3] Compiling Win32ANSI ( Win32ANSI.hs, interpreted ) During interactive linking, GHCi couldn't find the following symbol: GetConsoleScreenBufferInfo@8 This may be due to you not asking GHCi to load extra object files, archives or DLLs needed by your current session. Restart GHCi, specifying the missing library using the -L/path/to/object/dir and -lmissinglibname flags, or simply by naming the relevant files on the GHCi command line. Alternatively, this link failure might indicate a bug in GHCi. If you suspect the latter, please send a bug report to: glasgow-haskell-bugs(a)haskell.org Is this a bug or a natural behavior of ghci? This is strange to me since ghci finds the proper link for the functions in the other C libraries such as getch in conio.h. In addition, I am attaching a patched calculator.lhs which works for Unix/Linux on both GHC 6.8.x and GHC 6.10.1. The one currently on the book homepage only works for GHC 6.8.x but not GHC 6.10.1. This is due to the bug fix of hSetBuffering in GHC 6.10.1. To run these calculator example you will also need Parsing.lhs from the book hompage. http://www.cs.nott.ac.uk/~gmh/Parsing.lhs -- Ahn, Ki Yung

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Haskell as a religion
by Hugo Pacheco 21 Dec '08

21 Dec '08

I just found this on the web. Do you like the description of Haskell? It is not that far from true :P http://www.aegisub.net/2008/12/if-programming-languages-were-religions.html -- www.di.uminho.pt/~hpacheco

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Beginner's TH question
by Jeff Heard 20 Dec '08

20 Dec '08

Two things... can I add fields to records using Template Haskell, like: data T = T { $fields, myfield :: Field, ... } I assume the answer there is no, and then what's wrong with this? I get: Illegal instance declaration for `UIState t' (All instance types must be of the form (T a1 ... an) where a1 ... an are type *variables*, and each type variable appears at most once in the instance head. Use -XFlexibleInstances if you want to disable this.) In the instance declaration for `UIState t' In the expression: [d| instance UIState t where { setSizeY v a = setSizeY v . uist $ a setSizeX v a = setSizeX v . uist $ a setDrawing v a = setDrawing v . uist $ a setKey v a = setKey v . uist $ a .... } |] In the definition of `deriveUIState': deriveUIState uist t = [d| instance UIState t where { setSizeY v a = setSizeY v . uist $ a setSizeX v a = setSizeX v . uist $ a setDrawing v a = setDrawing v . uist $ a .... } |] in this module: -# LANGUAGE TemplateHaskell #-} module Graphics.Rendering.Thingie.TH where import Language.Haskell.TH import Graphics.Rendering.Thingie.UIState import qualified Graphics.Rendering.Thingie.BasicUIState as S deriveUIState uist t = [d| instance UIState t where mousePosition a = S.mousePosition . uist $ a mouseLeftButtonDown a = S.mouseLeftButtonDown . uist $ a mouseRightButtonDown a = S.mouseRightButtonDown . uist $ a mouseMiddleButtonDown a = S.mouseMiddleButtonDown . uist $ a mouseLeftButtonClicked a = S.mouseLeftButtonClicked . uist $ a mouseRightButtonClicked a = S.mouseRightButtonClicked . uist $ a mouseMiddleButtonClicked a = S.mouseMiddleButtonClicked . uist $ a mouseWheel a = S.mouseWheel . uist $ a keyCtrl a = S.keyCtrl . uist $ a keyShift a = S.keyShift . uist $ a keyAlt a = S.keyAlt . uist $ a key a = S.key . uist $ a drawing a = S.drawing . uist $ a sizeX a = S.sizeX . uist $ a sizeY a = S.sizeY . uist $ a setMousePosition v a = setMousePosition v . uist $ a setMouseLeftButtonDown v a = setMouseLeftButtonDown v . uist $ a setMouseRightButtonDown v a = setMouseRightButtonDown v . uist $ a setMouseMiddleButtonDown v a = setMouseMiddleButtonDown v . uist $ a setMouseLeftButtonClicked v a = setMouseLeftButtonClicked v . uist $ a setMouseRightButtonClicked v a = setMouseRightButtonClicked v . uist $ a setMouseMiddleButtonClicked v a = setMouseMiddleButtonClicked v . uist $ a setMouseWheel v a = setMouseWheel v . uist $ a setKeyCtrl v a = setKeyCtrl v . uist $ a setKeyShift v a = setKeyShift v . uist $ a setKeyAlt v a = setKeyAlt v . uist $ a setKey v a = setKey v . uist $ a setDrawing v a = setDrawing v . uist $ a setSizeX v a = setSizeX v . uist $ a setSizeY v a = setSizeY v . uist $ a |]

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