May 2009 - Haskell-Cafe

typeclasses comprehension problems: situation classes?
by Belka 16 May '09

16 May '09

Hello, cafe visitors! I'm trying to learn Haskell typeclasses, - about how to use them, - but can't handle some conceptiual problems, which confuses me a lot. I took one real problem (ErrorInfo gragual gathering), to tackle it in my studies: I have a class of situations: there is an object, and it gets cumulatively filled (or updated) with content. The code is in the end - it consists of 3 versions: 1. My first try. Fast written, based on intuitive understanding. Failed. 2. The second try - exploring an open world assumption. I hoped this would set me on the right path. Failure. 3. Surrendered to compiler - statisfied all it's requirements. This code looks absurd to me: parameter-never-to-be-used, unwanted-defaults. Compiler accepted this one though. Problems: 1. How to define *fillerRole* correctly, so that it depends on the type-value of "src_t"? 2. How to define *initFillable* correctly, so that it depends only on the type-value "filled_t", which is specified by the context of evaluation? 3. What are my misconcepts in the use of Haskell typeclasses here? 4. Maybe I should distinguish *situation class* (as this one), as something unavailable in Haskell? This assumption is the last one to make... I'd rather belive, that there is something I'm not aware of (for a considerably long time already) in Haskell. A lack of some programming technique Please, Help! Regards, Belka ==TRY=1===DOESN'T=COMPILE================================================== {-# LANGUAGE MultiParamTypeClasses #-} class FillsConsideringRoles src_t filled_t role_t where initFillable :: filled_t fillerRole :: role_t fill :: src_t -> filled_t -> filled_t ------------------------------------------------------------ data Role = Role1 | Role2 deriving (Show) data FillableObject = FillableObject { foData1 :: Maybe (Int, Role) , foData2 :: Maybe (Int, Role) } deriving (Show) newEmptyFillableObject :: FillableObject newEmptyFillableObject = FillableObject Nothing Nothing data Constructor1 = Constructor1 Int data Constructor2 = Constructor2 Int instance FillsConsideringRoles Constructor1 FillableObject Role where initFillable = newEmptyFillableObject fillerRole = Role1 fill c fo = let (Constructor1 i) = c in fo { foData1 = Just (i, fillerRole) } instance FillsConsideringRoles Constructor2 FillableObject Role where initFillable = newEmptyFillableObject fillerRole = Role2 fill c fo = let (Constructor2 i) = c in fo { foData2 = Just (i, fillerRole) } main = putStrLn $ show $ fill c2 $ fill c1 initFillable where c1 = Constructor1 76 c2 = Constructor2 43 ==TRY=1==[END]=================================================== ==TRY=2===DOESN'T=COMPILE======================================== {-# LANGUAGE MultiParamTypeClasses #-} {-# LANGUAGE FlexibleInstances #-} {-# LANGUAGE OverlappingInstances #-} class FillsConsideringRoles src_t filled_t role_t where initFillable :: filled_t fillerRole :: role_t fill :: src_t -> filled_t -> filled_t ---------------------------------------------------------------------- data Role = DefaultRole | Role1 | Role2 deriving (Show) data FillableObject = FillableObject { foData1 :: Maybe (Int, Role) , foData2 :: Maybe (Int, Role) } deriving (Show) newEmptyFillableObject :: FillableObject newEmptyFillableObject = FillableObject Nothing Nothing data Constructor1 = Constructor1 Int data Constructor2 = Constructor2 Int instance FillsConsideringRoles filler_t filled_t Role where fillerRole = DefaultRole instance FillsConsideringRoles Constructor2 filled_t Role where fillerRole = Role2 instance FillsConsideringRoles Constructor1 filled_t Role where fillerRole = Role1 instance FillsConsideringRoles filler_t FillableObject role_t where initFillable = newEmptyFillableObject instance FillsConsideringRoles Constructor1 FillableObject Role where fill c fo = let (Constructor1 i) = c in fo { foData1 = Just (i, fillerRole) } instance FillsConsideringRoles Constructor2 FillableObject Role where fill c fo = let (Constructor2 i) = c in fo { foData2 = Just (i, fillerRole) } main = putStrLn $ show $ fill c2 $ fill c1 initFillable where c1 = Constructor1 76 c2 = Constructor2 43 ==TRY=2==[END]=================================================== ==TRY=3===WORKS================================================= {-# LANGUAGE MultiParamTypeClasses #-} class FillsConsideringRoles src_t filled_t role_t where initFillable :: ((),src_t, role_t) -> filled_t fillerRole :: ((),src_t, filled_t) -> role_t fill :: ((),role_t) -> src_t -> filled_t -> filled_t ------------------------------------------------------------ data Role = DefaultRole | Role1 | Role2 deriving (Show) data FillableObject = FillableObject { foData1 :: Maybe (Int, Role) , foData2 :: Maybe (Int, Role) } deriving (Show) newEmptyFillableObject :: FillableObject newEmptyFillableObject = FillableObject Nothing Nothing data Constructor1 = Constructor1 Int data Constructor2 = Constructor2 Int instance FillsConsideringRoles Constructor1 FillableObject Role where initFillable _ = newEmptyFillableObject fillerRole _ = Role1 fill _ c fo = let (Constructor1 i) = c in fo { foData1 = Just (i, fillerRole ((), c, fo)) } instance FillsConsideringRoles Constructor2 FillableObject Role where initFillable _ = newEmptyFillableObject fillerRole _ = Role2 fill _ c fo = let (Constructor2 i) = c in fo { foData2 = Just (i, fillerRole ((), c, fo)) } main = putStrLn $ show $ fill ((), DefaultRole) c2 $ fill ((), DefaultRole) c1 ((initFillable ((), Constructor1 (-1), DefaultRole)) :: FillableObject) where c1 = Constructor1 76 c2 = Constructor2 43 ==TRY=3==[END]=================================================== -- View this message in context: http://www.nabble.com/typeclasses-comprehension-problems%3A-situation-class… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Foldable for BNFC generated tree
by Deniz Dogan 16 May '09

16 May '09

Hi I have a bunch of data types which are used to represent a JavaScript program. The data types and a lexer and a parser have all been generated using BNFC. So basically an entire JavaScript program is represented as a tree using these data types. Ideally I'd like to be able to "fold" over this data structure, but I can't instantiate Foldable for my data types, since the data types all have kind *, if I'm not completely lost. Here's an example data type: data Statement = StmtFunDecl JIdent [JIdent] ExprOrBlock | StmtVarDecl [VarDecl] | StmtLetDecl [VarDecl] | StmtWhile Expr Statement | ... So, basically I'd like some sort of folding functionality for these data types, without having to hack the lexer/parser myself (parameterising the data types), because as I said they're being generated by BNFC. I noticed that you can make BNFC generate GADTs instead of normal ADTs, which would allow me to instantiate Foldable, but I'm not entirely sure that this is the best way to do this. Any help is appreciated, Deniz Dogan

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Building 'text-icu-0.1' on Windows
by Serge LE HUITOUZE 16 May '09

16 May '09

Hi there, I fail to build text-icu-0.1 on Windows, and would appreciate some help... Thanks in advance! Let me describe what I've done so for, and what results I got: First, I got a version of ICU on my machine, specifically unzipping the file "icu4c-4_0_1-Win32-msvc8.zip" from <http://icu-project.org/download/4.0.html> http://icu-project.org/download/4.0.html. This made me having ICU stuff at location "C:\HaskellStuff\icu\icu", as witnessed below: >> C:\HaskellStuff\icu\icu>dir << 30/04/2009 16:08 <REP> . << 30/04/2009 16:08 <REP> .. << 30/04/2009 16:51 <REP> bin << 30/04/2009 16:08 <REP> include << 30/04/2009 16:36 <REP> lib << 14/01/2009 22:48 2 032 license.html After downloading and untaring "text-icu-0.1", I tried to configure, typing in: >> C:\HaskellStuff\pkg\text-icu-0.1>runhaskell Setup configure << Configuring text-icu-0.1... << Setup: Missing dependencies on foreign libraries: << * Missing C libraries: icui18n, icuuc, icudata << This problem can usually be solved by installing the system packages that << provide these libraries (you may need the "-dev" versions). If the libraries << are already installed but in a non-standard location then you can use the << flags --extra-include-dirs= and --extra-lib-dirs= to specify where they are. OK, let's restart with appropriate values for said flags! >> C:\HaskellStuff\pkg\text-icu-0.1>runhaskell Setup configure >> --extra-include-dirs=C:\HaskellStuff\icu\icu\include >> --extra-lib-dirs=C:\HaskellStuff\icu\icu\lib << Configuring text-icu-0.1... And then proceed with the building per se: >> C:\HaskellStuff\pkg\text-icu-0.1>runhaskell Setup build << Preprocessing library text-icu-0.1... << In file included from C:/HaskellStuff/icu/icu/include/unicode/umachine.h:47, << from C:/HaskellStuff/icu/icu/include/unicode/utypes.h:36, << from Data\Text\ICU\Error.hsc:148: << C:/HaskellStuff/icu/icu/include/unicode/pwin32.h:120: << error: redefinition of typedef 'int8_t' << C:/ghc/ghc-6.10.1/include/mingw/stdint.h:27: << error: previous declaration of 'int8_t' was here << ... << ... (a bunch of similar messages for other type aliases, e.g. 'uint8_t') << ... << compiling dist\build\Data\Text\ICU\Error_hsc_make.c failed << command was: C:\ghc\ghc-6.10.1\gcc.exe -c -BC:\ghc\ghc-6.10.1\gcc-lib << -IC:\ghc\ghc-6.10.1\include\mingw -D__GLASGOW_HASKELL__=610 << -IC:\HaskellStuff\icu\icu\include << -IC:\ghc\ghc-6.10.1\bytestring-0.9.1.4\include << -IC:\ghc\ghc-6.10.1\base-4.0.0.0\include << -IC:\ghc\ghc-6.10.1/include << -IPAPI_INCLUDE_DIR dist\build\Data\Text\ICU\Error_hsc_make.c << -o dist\build\Data\Text\ICU\Error_hsc_make.o << << C:\HaskellStuff\pkg\text-icu-0.1> The "mingw/stdint.h" file contains a series of lines of this style: << << typedef signed char int8_t; Whereas, the "unicode/pwin32.h" file contains contains stuff like: << << /* Define whether inttypes.h is available */ << #ifndef U_HAVE_INTTYPES_H << #define U_HAVE_INTTYPES_H 0 << #endif << ... << << /* Determines whether specific types are available */ << #ifndef U_HAVE_INT8_T << #define U_HAVE_INT8_T U_HAVE_INTTYPES_H << #endif << ... << << /* If your platform does not have the <inttypes.h> header, you may << need to edit the typedefs below. */ << #if U_HAVE_INTTYPES_H << #include <inttypes.h> << #else /* U_HAVE_INTTYPES_H */ << << #if ! U_HAVE_INT8_T << typedef signed char int8_t; << #endif << ... << #endif So, do I need to explicitely change the icu include file? And in which way? Any other solution/suggestion? Best regards. --Serge

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Re: [Haskell-cafe] Decoupling OpenAL/ALUT packages from OpenGL
by Tillmann Vogt 16 May '09

16 May '09

Sven Panne schrieb: > > * a tiny "ObjectName" package, consisting only of OpenGL's ObjectName class > (In "Data.ObjectName"? I'm not very sure about a good place in the hierarchy > here.) How about Data.GraphicsObjects ? > > * a package containing most of the data types/newtypes in OpenGL's > VertexSpec module (Vertex{2,3,4}, Color{3,4}, etc.) plus instances for the > base classes like Eq, Ord, Show, etc. I really don't know a good name for this > package and what a good place in the hierarchy would be (probably something > like "Data.Foo", but what is Foo?) Now that you mention it. I just looked through haskell hierachical libraries and there seems to be no standard data types for vector math. Am I getting something wrong or is every library that is using linear algebra stuff using its own data types? So if I use a numeric library for matrices inside HOpenGL I have to convert around? I think it would be nice to have data types and functions for dot produkt, scalar product, norms, ... together with HOpenGL types. Currently I am trying to embed a triangulation library from a ten year old diploma thesis (http://www.dinkla.net/fp/cglib.html) in my libary (I know that glu has tesselation). The author has developed a quite big and abstract type structure for all sorts of computer graphics algorithms and I didn't wanted to copy this into my library just for triangulation. But it is reasonable. It could be combined with HOpenGL types to maybe Data.VectorMath or Data.LinearAlgebra . I would favour the second. > > The point is: The first two package would be very small. Would this be OK? > Does anybody see other alternatives? What would be good names for those > packages and where in the naming hierarchy should they really go? > > Cheers, > S. > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe

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conflicting variable definitions in pattern
by Martin Hofmann 16 May '09

16 May '09

It is pretty clear, that the following is not a valid Haskell pattern: foo (x:x:xs) = x:xs My questions is _why_ this is not allowed. IMHO, the semantics should be clear: The pattern is expected to succeed, iff 'x' is each time bound to the same term. Isn't this allowed, because this would require a strict evaluation of the 'x' variables? Thanks, Martin

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help :type class?
by yu yang 16 May '09

16 May '09

Does anyone have an idea how to use an instance to list all the functions of type? Thank you! ___________________________________________________________ 好玩贺卡等你发，邮箱贺卡全新上线！ http://card.mail.cn.yahoo.com/

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Data.FiniteMap deprecrated
by Vasili I. Galchin 16 May '09

16 May '09

Hello, http://hackage.haskell.org/cgi-bin/hackage-scripts/package/FiniteMap ... since this is deprecated what is the orthodox way to implement "finite map"?? Thanks, Vasili

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Re: [Haskell-cafe] Example code won't compile
by michael rice 15 May '09

15 May '09

OK. Thanks, everyone. Michael --- On Fri, 5/15/09, Krzysztof Skrzętnicki <gtener(a)gmail.com> wrote: From: Krzysztof Skrzętnicki <gtener(a)gmail.com> Subject: Re: [Haskell-cafe] Example code won't compile To: "michael rice" <nowgate(a)yahoo.com> Cc: "Lennart Augustsson" <lennart.augustsson(a)gmail.com>, "haskell-cafe(a)haskell.org" <haskell-cafe(a)haskell.org> Date: Friday, May 15, 2009, 9:55 PM The one important line is > import Control.Monad.Error It adds dependancy on mtl package, which is not used by default (contrary to 'base' package, which includes Monad, System and IO modules). With --make GHC adds it automatically. Therefore $ ghc -package mtl ex14.hs compiles fine. I'd recommend using --make. In rare occasions when there is a namespace clash between packages one can simply hide offending packages or specify preferred ones. Best regards Christopher Skrzętnicki

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Re: [Haskell-cafe] Example code won't compile
by michael rice 15 May '09

15 May '09

This example compiles fine without --make. What's the difference? Michael ================== {- Author: Jeff Newbern Maintainer: Jeff Newbern <jnewbern(a)nomaware.com> Time-stamp: <Wed Jul 2 18:11:36 2003> License: GPL -} {- DESCRIPTION Example 13 - Using the List monad Usage: Compile the code and execute the resulting program with various arguments. Each argument will produce a list of possible parses of the argument as a decimal number, a hexadecimal number or a word. Try: ./ex13 34 f3 bean ./ex13 food f00d ./ex13 beef 10 "n!" ./ex13 "why?" "(punctuation)" -} import Monad import System import Char -- we can parse three different types of terms data Parsed = Digit Integer | Hex Integer | Word String deriving Show -- attempts to add a character to the parsed representation of a hex digit parseHexDigit :: Parsed -> Char -> [Parsed] parseHexDigit (Hex n) c = if isHexDigit c then return (Hex ((n*16) + (toInteger (digitToInt c)))) else mzero parseHexDigit _ _ = mzero -- attempts to add a character to the parsed representation of a decimal digit parseDigit :: Parsed -> Char -> [Parsed] parseDigit (Digit n) c = if isDigit c then return (Digit ((n*10) + (toInteger (digitToInt c)))) else mzero parseDigit _ _ = mzero -- attempts to add a character to the parsed representation of a word parseWord :: Parsed -> Char -> [Parsed] parseWord (Word s) c = if isAlpha c then return (Word (s ++ [c])) else mzero parseWord _ _ = mzero -- tries to parse the digit as a hex value, a decimal value and a word -- the result is a list of possible parses parse :: Parsed -> Char -> [Parsed] parse p c = (parseHexDigit p c) `mplus` (parseDigit p c) `mplus` (parseWord p c) -- parse an entire String and return a list of the possible parsed values parseArg :: String -> [Parsed] parseArg s = do init <- (return (Hex 0)) `mplus` (return (Digit 0)) `mplus` (return (Word "")) foldM parse init s -- show the original string and all possible parses for the string showResult :: String -> IO () showResult s = do putStr s putStr ": " print (parseArg s) -- prints possible parsed values for command-line arguments main :: IO () main = do args <- getArgs mapM_ showResult args -- END OF FILE --- On Fri, 5/15/09, Lennart Augustsson <lennart.augustsson(a)gmail.com> wrote: From: Lennart Augustsson <lennart.augustsson(a)gmail.com> Subject: Re: [Haskell-cafe] Example code won't compile To: "michael rice" <nowgate(a)yahoo.com> Cc: "haskell-cafe(a)haskell.org" <haskell-cafe(a)haskell.org> Date: Friday, May 15, 2009, 7:25 PM --make -- Lennart (iPhone)

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Example code won't compile
by michael rice 15 May '09

15 May '09

Why won't this code compile? Michael ======================= {- Author: Jeff Newbern Maintainer: Jeff Newbern <jnewbern(a)nomaware.com> Time-stamp: <Thu Aug 14 09:53:53 2003> License: GPL -} {- DESCRIPTION Example 14 - Using the IO monad Usage: Compile the code to produce a poor replacement for the standard Unix "tr" command. Try: cat file | ./ex14 "aeiou" "X" cat file | ./ex14 " " "_" ./ex14 "abc" -} import Monad import System import IO import Control.Monad.Error -- translate char in set1 to corresponding char in set2 translate :: String -> String -> Char -> Char translate [] _ c = c translate (x:xs) [] c = if x == c then ' ' else translate xs [] c translate (x:xs) [y] c = if x == c then y else translate xs [y] c translate (x:xs) (y:ys) c = if x == c then y else translate xs ys c -- translate an entire string translateString :: String -> String -> String -> String translateString set1 set2 str = map (translate set1 set2) str usage :: IOError -> IO () usage e = do putStrLn "Usage: ex14 set1 set2" putStrLn "Translates characters in set1 on stdin to the corresponding" putStrLn "characters from set2 and writes the translation to stdout." -- translates stdin to stdout based on commandline arguments main :: IO () main = (do [set1,set2] <- getArgs contents <- hGetContents stdin putStr $ translateString set1 set2 contents) `catchError` usage -- END OF FILE ===================================== [michael@localhost ~]$ ghc ex14.hs -o ex14 ex14.o: In function `rF8_info': (.text+0x48): undefined reference to `mtlzm1zi1zi0zi2_ControlziMonadziError_zdf17_closure' ex14.o: In function `sGp_info': (.text+0x861): undefined reference to `mtlzm1zi1zi0zi2_ControlziMonadziError_zdf17_closure' ex14.o: In function `sGp_info': (.text+0x91d): undefined reference to `__stginit_mtlzm1zi1zi0zi2_ControlziMonadziError_' ex14.o: In function `rF8_info': (.text+0x50): undefined reference to `mtlzm1zi1zi0zi2_ControlziMonadziErrorziClass_zdp1MonadError_info' ex14.o: In function `sGp_info': (.text+0x869): undefined reference to `mtlzm1zi1zi0zi2_ControlziMonadziErrorziClass_catchError_info' ex14.o: In function `rF8_srt': (.data+0x0): undefined reference to `mtlzm1zi1zi0zi2_ControlziMonadziError_zdf17_closure' ex14.o: In function `Main_main_srt': (.data+0x6c): undefined reference to `mtlzm1zi1zi0zi2_ControlziMonadziError_zdf17_closure' collect2: ld returned 1 exit status [michael@localhost ~]$

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