February 2005 - Haskell-Cafe

how to name third party libraries?
by S. Alexander Jacobson 07 Feb '05

07 Feb '05

Is there documentation somewhere about how to name third party libraries? For example, if I have HTTP and SMTP libraries, what should I call them? -alex- ______________________________________________________________ S. Alexander Jacobson tel:917-770-6565 http://alexjacobson.com

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Re: [Haskell] help with some basic code that doesn't work
by Cale Gibbard 07 Feb '05

07 Feb '05

Delete data Maybe Dir = Just Dir | Nothing as it is unnecessary "Maybe a" is defined already for any type "a" (and doesn't make sense, Dir occuring before the = sign would indicate that it is a type variable, but it is uppercase), and also change data Dir = Left | Right | Up | Down data Piece = Vertical | Horizontal | CodeA | CodeB to data Dir = Left | Right | Up | Down deriving Eq data Piece = Vertical | Horizontal | CodeA | CodeB deriving Eq Equality isn't defined by default on user-defined datatypes, so that you can add your own definition. In this case your type is pretty simple, so you can use the derived instance of Eq, so that (==) and (/=) get defined for Dir and Piece. You should also add the line import Prelude hiding (Left, Right) as these are the names of the data constructors for the Either type, and you will get ambiguity errors if you don't rename them or hide the ones in the Prelude. Lastly, you should probably rename your 'fst' to 'frst' or some such, so that it doesn't overlap with the Prelude function of the same name. hope this helps, - Cale On Mon, 7 Feb 2005 20:36:55 +0000, pablo daniel rey <listas(a)pablodanielrey.com.ar> wrote: > hello > i'm new to haskell so i'm sorry if this is a stupid question, but i'm having problems with some basic code. > the code : > > data Maybe Dir = Just Dir | Nothing > data Dir = Left | Right | Up | Down > data Piece = Vertical | Horizontal | CodeA | CodeB > > flow = [(Horizontal, Left, Left), > (Horizontal, Right, Right), > (Vertical, Down, Down), > (Vertical, Up, Up), ................ etc ] > > fst :: (a,b,c) -> a > fst (x,y,z) = x > > scnd :: (a,b,c) -> b > scnd (x,y,z) = y > > third :: (a,b,c) -> c > third (x,y,z) = z > > element :: [(Piece, Dir, Dir)] -> Maybe Dir > element [] = Nothing > element xs = Just (third (head xs)) > > chgDir :: Piece -> Dir -> Maybe Dir > chgDir p d = element (filter (\x -> p == (fst x)) (filter (\x -> d == (scnd x)) flow)) > > the error i get : > > Instances of (Eq Dir, Eq Piece) required for definition of chgDir > > i don't know what's happening. > help!!! > thanks in advance > _______________________________________________ > Haskell mailing list > Haskell(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell >

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"Parsing" a string
by Dmitri Pissarenko 07 Feb '05

07 Feb '05

Hello! I have to read a PGM image and transform it into a list of Int values. I read the image (this is an ASCII PGM format) using the readFile function and get a string with the contents of the file. This string contains height and widht of the image at the beginning, and then the pixel values follow. I need to read the height and width, then "cut" them from the string, create an array (or finite map) of Int's (for this I need to know the height and width), and then recursively process the pixel values (i. e. put them into the array). The string is structured as follows: <string> P2 # comment # comment 320<WHITESPACE>243<WHITESPACE>255 130 130 130 130 130 130 130 130 130 130 </string> P2 is the magic number. All lines starting with # are comments and must be ignored. 320 and 243 are width and height. <WHITESPACE> is either a space, or tab, or newline. At the moment, I don't have an idea about how to read width and height. One possible approach would be to convert the string into a list of strings A, using newline as separator. Then, I could create list A' with comments removed. Then, A' can be transformed into a string A'' again. From A'' I know that width is contained between the end of P2 and the first occurrence of whitespace (Char.isSpace). I also know that height is contained between first and second occurrence of white space. I could use some sort of regular expression analogons to access width and height. I have several questions concerning this approach: 1) How can I transform a string into a list of strings, separated by some character (in Java one uses StringTokenizer for this) ? 2) How do the aforementioned "regular expressions' analogons" work in Haskell? Thanks in advance Dmitri Pissarenko -- Dmitri Pissarenko Software Engineer http://dapissarenko.com

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Haskell binding to Berkeley DB?
by Dimitry Golubovsky 05 Feb '05

05 Feb '05

Hi, Has anyone tried/been successful on developing such a binding? Google returns reference to a message written 2 years ago (on the gtkhs mailing list) that someone developed it partially: http://www.haskell.org/pipermail/gtkhs/2003-January/000317.html and a description of a student's assignment: http://www.cse.unsw.edu.au/db/thesis/topicinfo/MMTC13.html The rest of links that Google returns are of little relevance. Any ideas? Dimitry Golubovsky Middletown, CT

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Haskell binding to Berkeley DB?
by Alain Cremieux 04 Feb '05

04 Feb '05

> > >Hi, > >Has anyone tried/been successful on developing such a binding? > >Google returns reference to a message written 2 years ago (on the gtkhs >mailing list) that someone developed it partially: > >http://www.haskell.org/pipermail/gtkhs/2003-January/000317.html > > > Hi Dimitry, I have made some progress since this posting, but it is not finished. My binding is written using the c-code from the lisp binding available here : http://users.actrix.gen.nz/mycroft/cl.html I have a low-level binding, and a medium-level binding, and both work. But I'd like to make a high-level binding, maybe based on the ideas from Oleg described here : From enumerators to cursors: turning the left fold inside out <http://okmij.org/ftp/Haskell/index.html#fold-stream> and here : Takusen: a DBMS access library built around a left-fold enumerator <http://okmij.org/ftp/Haskell/index.html#takusen> Anyhow, I can send you my code, no documentation available but with some examples, Alain

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create a type for selecting functions already in the module
by Oterman-Fernandez, Ana-Maria 04 Feb '05

04 Feb '05

Hi! I have implemented a module with some functions and now I would like to create a type for selecting between this functions: For example: if f1, f2,...fn are the names of the functions in the module, I need to do something like this: data Consult = f1 | f2 |....| fn for applying them whenever the user wants. but by this way is not possible. Do you know how can I do this? Thank you! Ana Maria The data contained in, or attached to, this e-mail, may contain confidential information. If you have received it in error you should notify the sender immediately by reply e-mail, delete the message from your system and contact +44 (0) 1332 242424 (the Rolls-Royce IT Security Director) if you need assistance. Please do not copy it for any purpose, or disclose its contents to any other person. An e-mail response to this address may be subject to interception or monitoring for operational reasons or for lawful business practices. (c) 2005 Rolls-Royce plc Registered office: 65 Buckingham Gate, London SW1E 6AT Company number: 1003142. Registered in England.

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MonadPlus instance for IO
by David Roundy 02 Feb '05

02 Feb '05

I'm sure I'm doing something stupid, but somehow ghc isn't recognizing the existance of a MonadPlus instance for IO: DarcsIO.lhs:48: No instance for (MonadPlus IO) arising from use of `mplus' at DarcsIO.lhs:48 In the definition of `foo': foo = (fail "aaack") `mplus` (fail "foobar") Any idea what I'm doing wrong? I'm using $ ghc --version The Glorious Glasgow Haskell Compilation System, version 6.2.2 packaged for debian. -- David Roundy http://www.darcs.net

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RE: [Haskell-cafe] File path programme
by Simon Marlow 02 Feb '05

02 Feb '05

[ moving to libraries(a)haskell.org ] On 26 January 2005 12:22, Malcolm Wallace wrote: >> Could we just punt this library for this release. After all we can >> add libraries in a later point release (eg 6.4.1) you just can't >> change existing APIs. > > FWIW, I agree with Duncan, Ben, and Peter, that the new > System.FilePath interface is broken, and the implementation more so. > It would be better to redesign FilePaths as an algebraic datatype. Ok, I'll go with the concensus. System.FilePath in its current state will be removed from the base package. Isaac or Krasimir: can you make the required changes to Cabal? (the code is already duplicated in Distribution.Compat.FilePath, just remove the import for GHC). So let's aim for 6.6 to do it right, and design an abstract type for path names. We have to think about the migration path: Haskell 98 programs must continue to work, so at least IO.FilePath must still be a String, and IO.openFile must still take a String. We can therefore: (a) make System.IO.FilePath be the new type, which is different from, and incompatible with, IO.FilePath. Similarly for System.IO.openFile, System.Directory.removeFile, and so on. (b) or just define a new type, and force you to insert read/show to convert where necessary, eg. before calling openFile. (a) is kind of the right thing, but (b) is a lot less painful in the short term. Since we'll be migrating to a new IO library at some point, (b) is probably fine (the new IO library can use the new FilePaths exclusively), but we'll need to migrate System.Directory too. Would someone like to take up the reigns on the design for the new library? Cheers, Simon

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RE: [Haskell-cafe] MonadPlus instance for IO
by Simon Peyton-Jones 02 Feb '05

02 Feb '05

that instance seems to be only in Control.Monad.Error in the 'mtl' package. It's not in the standard base package. You may need to import an mtl module to tell GHC to look there. Simon | -----Original Message----- | From: haskell-cafe-bounces(a)haskell.org [mailto:haskell-cafe-bounces@haskell.org] On Behalf Of | David Roundy | Sent: 02 February 2005 13:18 | To: haskell-cafe(a)haskell.org | Subject: [Haskell-cafe] MonadPlus instance for IO | | I'm sure I'm doing something stupid, but somehow ghc isn't recognizing the | existance of a MonadPlus instance for IO: | | DarcsIO.lhs:48: | No instance for (MonadPlus IO) | arising from use of `mplus' at DarcsIO.lhs:48 | In the definition of `foo': | foo = (fail "aaack") `mplus` (fail "foobar") | | Any idea what I'm doing wrong? I'm using | | $ ghc --version | The Glorious Glasgow Haskell Compilation System, version 6.2.2 | | packaged for debian. | -- | David Roundy | http://www.darcs.net | _______________________________________________ | Haskell-Cafe mailing list | Haskell-Cafe(a)haskell.org | http://www.haskell.org/mailman/listinfo/haskell-cafe

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The Nature of Char and String
by John Goerzen 02 Feb '05

02 Feb '05

Char in Haskell represents a Unicode character. I don't know exactly what its size is, but it must be at least 16 bits and maybe more. String would then share those properties. However, usually I'm accustomed to dealing with data in 8-bit words. So I have some questions: * If I use hPutStr on a string, is it guaranteed that the number of 8-bit bytes written equals (length stringWritten)? + If no, what is the representation written? I'm assuming UTF-8. How could I find out how many bytes were actually written? + If yes, what happens to the upper 8 bits? Are they simply stripped off? * If I run hGetChar, is it possible that it would consume more than one byte of input? How can I determine whether or not this has happend? * Does Haskell treat the "this is a Unicode file" marker special in any way? * Same questions on withCString and related String<->CString conversions. -- John

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