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Re: [Haskell-cafe] chr/ord?
by michael rice 01 May '09

01 May '09

Thank you. I assumed when I saw this: "we could use Maybe's mplus to define a function, parent s = (mother s) `mplus` (father s)" that mplus was *already* present in Maybe. Michael --- On Fri, 5/1/09, Neil Mitchell <ndmitchell(a)gmail.com> wrote: From: Neil Mitchell <ndmitchell(a)gmail.com> Subject: Re: [Haskell-cafe] chr/ord? To: "michael rice" <nowgate(a)yahoo.com> Cc: "haskell-cafe(a)haskell.org" <haskell-cafe(a)haskell.org>, "Anton van Straaten" <anton(a)appsolutions.com> Date: Friday, May 1, 2009, 6:32 AM Hi > But I get this when I try to use it: > > sheep.hs:30:22: Not in scope: `mplus' > [michael@localhost ~]$ You need to import Control.Monad. You can find answers to these sorts of questions with Hoogle: http://haskell.org/hoogle/?hoogle=mplus Thanks Neil

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Re: [Haskell-cafe] chr/ord?
by michael rice 01 May '09

01 May '09

Understood. Thanks. A little further on in the tutorial they define a "parent" function. The mplus operator is used to combine monadic values from separate computations into a single monadic value. Within the context of our sheep-cloning example, we could use Maybe's mplus to define a function, parent s = (mother s) `mplus` (father s), which would return a parent if there is one, and Nothing is the sheep has no parents at all. For a sheep with both parents, the function would return one or the other, depending on the exact definition of mplus in the Maybe monad. But I get this when I try to use it: sheep.hs:30:22: Not in scope: `mplus' [michael@localhost ~]$ And here's the sheep.hs file, attempting to use "parent" near the bottom ========= {- Author: Jeff Newbern Maintainer: Jeff Newbern <jnewbern(a)nomaware.com> Time-stamp: <Mon Nov 10 11:59:14 2003> License: GPL -} {- DESCRIPTION Example 1 - Our first monad Usage: Compile the code and execute the resulting program. It will print Dolly's maternal grandfather. -} -- everything you need to know about sheep data Sheep = Sheep {name::String, mother::Maybe Sheep, father::Maybe Sheep} -- we show sheep by name instance Show Sheep where show s = show (name s) -- comb is a combinator for sequencing operations that return Maybe comb :: Maybe a -> (a -> Maybe b) -> Maybe b comb Nothing _ = Nothing comb (Just x) f = f x parent s = (mother s) `mplus` (father s) -- now we can use `comb` to build complicated sequences maternalGrandfather :: Sheep -> Maybe Sheep maternalGrandfather s = (Just s) `comb` mother `comb` father fathersMaternalGrandmother :: Sheep -> Maybe Sheep fathersMaternalGrandmother s = (Just s) `comb` father `comb` mother `comb` mother mothersPaternalGrandfather :: Sheep -> Maybe Sheep mothersPaternalGrandfather s = (Just s) `comb` mother `comb` father `comb` father -- this builds our sheep family tree breedSheep :: Sheep breedSheep = let adam = Sheep "Adam" Nothing Nothing eve = Sheep "Eve" Nothing Nothing uranus = Sheep "Uranus" Nothing Nothing gaea = Sheep "Gaea" Nothing Nothing kronos = Sheep "Kronos" (Just gaea) (Just uranus) holly = Sheep "Holly" (Just eve) (Just adam) roger = Sheep "Roger" (Just eve) (Just kronos) molly = Sheep "Molly" (Just holly) (Just roger) in Sheep "Dolly" (Just molly) Nothing -- print Dolly's maternal grandfather main :: IO () main = let dolly = breedSheep in do print (parent dolly) -- END OF FILE Michael --- On Wed, 4/29/09, Anton van Straaten <anton(a)appsolutions.com> wrote: From: Anton van Straaten <anton(a)appsolutions.com> Subject: Re: [Haskell-cafe] chr/ord? To: "haskell-cafe(a)haskell.org" <haskell-cafe(a)haskell.org> Date: Wednesday, April 29, 2009, 12:33 PM michael rice wrote: > Since I'm trying to learn Monads, let's look at this as a teaching moment. The example code (see below), which I pulled off YAMT (Yet Another Monad Tutorial ;-)), is the source of my 'comb' function. > > I understand the code as it now stands, and I understand that the Prelude (>>=) would replace the 'comb'. Adding whatever statements are needed, how would you "specialize" the (>>=) to Maybe and solve this particular problem. Saying that "comb is just (>>=) specialized to Maybe" just means that you can define comb like this: comb :: Maybe a -> (a -> Maybe b) -> Maybe b comb = (>>=) Which also of course means that you can typically use (>>=) instead of comb. Although in some cases, being more specific about the type can be useful. You can do this sort of specialization for any polymorphic function, e.g.: -- id is predefined in Haskell, definition given as example id :: a -> a id x = x intID :: Int -> Int intId = id In that case, the compiler basically specializes the function for you, providing a version of it that's specific to Ints. However, (>>=) is defined by the Monad type class, and as it happens there's also already a definition for it that's specific to the Maybe type. You can see GHC's source for it here: http://haskell.org/ghc/docs/latest/html/libraries/base/src/Data-Maybe.html#… Not surprisingly, that definition is essentially identical to the definition of comb: (Just x) >>= k = k x Nothing >>= _ = Nothing So defining "comb = (>>=)" just uses that definition. Anton _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

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Few Alex questions
by Dimitry Golubovsky 01 May '09

01 May '09

Hi, Q1: I am trying to create an Alex equivalent of the following flex file: http://code.haskell.org/yc2js/es-idl/lexer.ll (lexer for Web IDL which is a dialect of OMG IDL) I haven't been able to find Alex equivalent for the following line: PoundSign ^{WhiteSpace}*# that is any amount of whitespace at the beginning of line followed by '#' I defined a @WhiteSpace macro some place earlier, but Alex gives me parser error on the line I created: @PoundSign = ^@WhiteSpace*# in the position 28 that is position of '@', next to caret, that is, caret is not recognized by Alex. Can beginning of line (caret) be recognized by Alex? Q2: The original line says: MultiLineComment \/\*(([^*])|(\*[^/]))*\*\/ basically same as comments in C or Java. I had to prefix star and slash even within square brackets with backslashes to make it compile. So I got: @MultiLineComment = \/\*(([^\*])|(\*[^\/]))*\*\/ Is this correct understanding that if we want to match any character except for an asterisk, then Alex would like to see [^\*] rather than [^*]? And [^\/] rather than [^/]? Or would it be better to use a hex code for the asterisk and slash? Thanks. -- Dimitry Golubovsky Anywhere on the Web

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ANNOUNCE: graphviz-2009.5.1
by Ivan Lazar Miljenovic 01 May '09

01 May '09

I'd like to announce version 2009.5.1[1] of the graphviz[2] library, less than a week since Matthew Sackman passed maintainership onto me[3]. The graphviz library provides a Haskell interface to the GraphViz[4] program. [1] http://hackage.haskell.org/packages/archive/graphviz/2009.5.1/graphviz-2009… [2] http://hackage.haskell.org/cgi-bin/hackage-scripts/package/graphviz [3] http://www.mail-archive.com/haskell@haskell.org/msg21994.html [4] http://graphviz.org Major changes with this release are: * Supports polyparse >= 1.1 (the previous version didn't work with 1.3). * Requires base == 4.* (i.e. GHC 6.10.*) due to the new exception handling. * Includes functions from my Graphalyze [5] library for actually running the various GraphViz commands, etc. * Slight API breakages: - dirCommand, undirCommand and commandFor no longer return String, but GraphvizCommand - Data.GraphViz.ParserCombinators is no longer exported by the Cabal file, since I see no need for anyone to require acccess to this. If this is a problem for you, please let me know and I'll re-export it. * A lot more Haddock documentation. * A darcs repository is now available [6]. Patches are welcome! [5] http://hackage.haskell.org/cgi-bin/hackage-scripts/package/Graphalyze [6] http://code.haskell.org/graphviz Plans for the future: * Add support for Data.Graph-style graphs (maybe even making it generic such that custom graph types can be used). * Allow the user to choose whether the graph is directed or undirected. * Include all attributes offered by the Dot language. * Remove (or at least minimise) usage of extensions to increase portability. Oh, and to pre-empt Don, this is already available in the Gentoo-Haskell overlay ;-) -- Ivan Lazar Miljenovic Ivan.Miljenovic(a)gmail.com IvanMiljenovic.wordpress.com

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How to install HOpenGL to Windows?
by Alexandr N. Zamaraev 30 Apr '09

30 Apr '09

I get darcs source HOpenGL Try runghc Setup.hs configure I see: Configuring OpenGL-2.3... Warning: The 'build-type' is 'Configure' but there is no 'configure' script. Setup.hs: configure script not found. runghc Setup.hs build wrote: Preprocessing library OpenGL-2.3... Building OpenGL-2.3... Graphics\Rendering\OpenGL\GL\BasicTypes.hs:24:0: HsOpenGLConfig.h: No such file or directory How to build HOpenGL? Os Windows Vista Home Ru + sp ghc 6.10.1

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Google SoC: Space profiling reloaded
by Patai Gergely 30 Apr '09

30 Apr '09

Hello all, I'm one of the lucky individuals accepted for this year's Summer of Code. Here's the abstract of my proposal: http://socghop.appspot.com/student_project/show/google/gsoc2009/haskell/t12… There's less than a month left before I'm supposed to jump into coding, and I'd love to hear about any little idea you think would make this project even better! I created a project page with a rough draft of what I intend to concentrate on, but the issue tracker is obviously begging for content, so feel encouraged to populate it. :) http://code.google.com/p/hp2any/ I also intend to expand the description later. Gergely -- http://www.fastmail.fm - The professional email service

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Arrow preprocessor and *** combinator
by Peter Verswyvelen 30 Apr '09

30 Apr '09

After doing some pragmatic tests, it seems that neither the arrow preprocessor nor GHC's builtin one generate / optimize to the (***) combinator. For example, p = proc (x,y) -> do x' <- f <- x y' <- g <- y return (x',y') is equivalent to p = f *** g But I have the impression this kind of rewrites is not being done? Is it possible to provide rewrite rules for arrows? Since *** is a method of the arrow type class, it might have a more optimal implementation. I haven't looked at the arrow preprocessor or GHC's code yet, but comments are welcome, I might see things wrongly. Cheers, Peter

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unsafeSTToIO and stToIO
by Xiao-Yong Jin 30 Apr '09

30 Apr '09

Hi, Between the following two functions stToIO :: ST RealWorld a -> IO a stToIO (ST m) = IO m unsafeSTToIO :: ST s a -> IO a unsafeSTToIO (ST m) = IO (unsafeCoerce# m) All I can see is that the safe one uses RealWorld instead of an arbitrary thread s used in the unsafe one. I really don't understand the difference between these two. Why is the one without RealWorld unsafe? I tried google, but couldn't find anything helpful. -- c/* __o/* <\ * (__ */\ <

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Help from C libraries experts
by Maurício 30 Apr '09

30 Apr '09

Hi, Do you understand very well a C library and would like Haskell to have a binding for it? I've been working on this package: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/bindings My goal is to have a place where one can find reliable and comprehensive low-level bindings to foreign libraries, so that writing higher level bindings becomes an easier task. If you are willing to help me with questions about your favorite C library, like compile options I should be carefull about or differences I may find between systems, I'll be glad to write a low level binding to it. Best, Maurício

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Getting started - help
by applebiz89 30 Apr '09

30 Apr '09

What I need to do is build a type of database. Firstly have a few questions, whether what I have done it right...and how to do something else. This is my algebraic type for the database (correct me if its wrong) data Film = Film String String Int String with this as the data. testDatabase :: [Film] testDatabase = [("Casino Royale", "Martin Campbell",2006, "Garry, Dave, Zoe")] the last set of strings are the fans to the film. I want to be able to add a new fan, but not sure how to specifically insert it to there this is what I have so far: becomeFan :: String -> String -> [Film] -> [Film] becomeFan = do putStr "Which film are you a fan of: " filmTitle <- getLine do putStr "What is your name: " fanName <- getLine (if filmTitle == filmName then fanName : [Film]) -- View this message in context: http://www.nabble.com/Getting-started---help-tp23314466p23314466.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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