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Storable types
by Clerton Filho 20 Dec '07

20 Dec '07

Hi, I'm newbie in Haskell, and I have some doubts... In this programming language, do we have storable values? Case affirmative, what are the storable types in Haskell, and how can I implement then... thanks! -- Clerton Ribeiro de Araujo Filho Graduando em Ciência da Computação Integrante do PET - Programa de Educação Tutorial Integrante do LIA - Laboratório de Inteligência Artificial Universidade Federal de Campina Grande - UFCG

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Re: [Haskell-cafe] [HXT] Simple question
by Miguel Mitrofanov 20 Dec '07

20 Dec '07

Seems rather strange for me, I've just installed HXT and got this: Prelude Text.XML.HXT.Arrow> runX $ readString [(a_validate,"0")] "<this> </this>" >>> writeDocumentToString [] ["<?xml version=\"1.0\" encoding=\"UTF-8\"?>\n<this> </this>"] > Everything works fine except for the fact that all the nodes <this> > </this> > (that is, a space (an XML text node whose contents are a single space > character) > within a <this> element node) get transformed to a <this/> element node,

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[HXT] Simple question
by Fernand 20 Dec '07

20 Dec '07

Hi, I'm using HXT (7.4) with GHC to make some XML transformations (BTW, congratulations to the maintainers of that package for their impressive work). Everything works fine except for the fact that all the nodes « <this> </this> » (that is, a space (an XML text node whose contents are a single space character) within a <this> element node) get transformed to a « <this/> » element node, which I do not want (I want to keep those nodes as they are). Note that my transformations do not change the <this> nodes or their children (my transformations works on others parts of the document). This seems to be related to the XML parsing or writing of the XML file, and I have tried several read/write options (using a_remove_whitespace and a_canonicalize with v_0, for both writing and reading) to prevent this behaviour, without success. I have even tried to add text childs nodes to all the empty <this> nodes, but the issue seems to lie in the XML output options. Has anybody an idea ? Excuse me if this is a trivial issue, Sincerely yours Fernand -- Excuse my french, it's my mother tongue.

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Re: type trickery
by oleg＠okmij.org 20 Dec '07

20 Dec '07

Adrian Neumann wrote: > I figured I'd need something like this > data GF = GF Integer Integer > so that each element of the finite field would remember p. However I > can't think of a way to use the typesystem to ensure that p is always > the same. You might like: Vectro: Haskell library for "statically typed linear algebra" http://ofb.net/~frederik/stla/ which marks each element of a vector space with its dimension. The type system makes sure that you can only add vectors of the same dimension (the type system does even more: it computes the dimension of a result of multiplying a vector by a non-square matrix, for example). > I think that would need an infinite number of different types, You think correctly. Haskell already has the infinite number of different types (e.g., the infinite number of function types).

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Class deriving in GHC 6.8
by Emil Axelsson 20 Dec '07

20 Dec '07

Hello all! How come in GHC 6.6 I could to write > {-# OPTIONS_GHC -fglasgow-exts -fallow-undecidable-instances #-} > data Foo = Foo deriving Show > data Bar c = Bar (c Foo) deriving Show but in GHC 6.8.2 I get the error > No instance for (Show (c Foo)) > arising from the 'deriving' clause of a data type declaration > at Ert.hs:3:0-37 > Possible fix: add an instance declaration for (Show (c Foo)) > When deriving the instance for (Show (Bar c)) Thanks, / Emil

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type trickery
by Adrian Neumann 20 Dec '07

20 Dec '07

Hello haskell-cafe! After making "data Number = Zero | Succ Number" an instance of Integral, I wondered how I could do the same with galois fields. So starting with Z mod p, I figured I'd need something like this data GF = GF Integer Integer so that each element of the finite field would remember p. However I can't think of a way to use the typesystem to ensure that p is always the same. I think that would need an infinite number of different types, but the type hackers here probably know something better. Adrian

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Re: [xmonad] Re: XMonad.Layout.NoBorders
by gwern0＠gmail.com 20 Dec '07

20 Dec '07

On 2007.12.19 10:57:33 -0500, David Roundy <daveroundy(a)gmail.com> scribbled 0.3K characters: > No, I don't have time. And I'm not sure why one would want no borders > on floating windows... > > David Perhaps I don't fully understand the issues, but wouldn't such a thing be nice for things like MPlayer? The other day, I happened to play a video through MPlayer, and I noticed (using smartBorders/NoBorders) that there was an ugly border on the left and upper edges, and that it detracted from the cinematic experience. It was otherwise fullscreen and nice, but not as good as it could've been. -- gwern picking FID DES Mole Cap-Stun timers Z7 NAAP 2.6.2. ARC

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Re: [Haskell-cafe] New to Haskell
by Miguel Mitrofanov 20 Dec '07

20 Dec '07

> >> As a user, I just want to be able to spot the basic components of a > >> source > >> file without thinking about transformation rules. > > Well, most users are. > Are what ? Sorry if I've confused you. English isn't my native language. "Are able", of course. > Have you asked them all ? If you're unsure, we can vote here or somewhere else. > >> Is this legal (`plus`) x y ? > > > > Never tried to write this myself, it looks stupid. > What else haven't you tried to write by know ? Well, I hope, I haven't tried to write the most of stupid-looking things. > > What kind of guarantees do you want? > Written in blood. Write it yourself, I don't have too much blood. > >> I just want the sistem to be able to print one of these expressions ! > >> Its this too much to ask ? > > Yes, 'cause it means you want to embed almost all source code into the > > compiled program. > So ? So, I don't know any compiler of any language which does it.

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ANN: SearchPath v0.9
by Alex Jacobson 19 Dec '07

19 Dec '07

SearchPath v0.9 does recursive module chasing accross the internet using a combination of mapfiles you provide and the default map file, caching the downloaded modules in a local directory. Searchpath can handle modules in module hierarchies based at a URLs, in tgz archives accessible via URL, and in accessible darcs/svn/cvs etc repos at particular tags. New in v0.9 * handling tagged darcs/svn repos * handling .tgz archives * massive code cleanup * substantially faster * easier/better command line options * better usage documentation * handling non-local haskell files Check it out at http://searchpath.org. -Alex-

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Is StateT what I need?
by Andre Nathan 19 Dec '07

19 Dec '07

Hello (Newbie question ahead :) I'm trying to write a program which will build a tree (here represented as a Map) of unix processes. The tree should be built by reading the process information stored in /proc/PID/status. There is also another Map which will be used for faster insertions on the process tree, which I'd like to handle as my program state. So far I have the functions to get a list of entries from /proc, filter the ones that represent processes and get the information from their status file. Now I need an "insertProc" function, which should get the information for a given process ID, update the state and return that information. This is where I think I need StateT, but I couldn't find out how to use it (never used StateT before...). This is what I have so far: > type Pid = FilePath > type Uid = String > > type PsData = Map String Uid > type PsChildren = Map Pid PsInfo > > data PsInfo = PsInfo PsData PsChildren > type PsMap = Map Pid PsInfo > type PsTree = Map Pid PsInfo > > parent :: PsData -> Pid > parent psData = fromJust $ Map.lookup "PPid" psData > > getProcInfo :: PsData -> String -> IO PsData > getProcInfo psData line = do > case matchRegex (mkRegex "^([a-z]+):[[:space:]]+(.*)$") line of > Nothing -> return (psData) > Just [key, value] -> return (Map.insert key value psData) > > procInfo :: Pid -> IO PsInfo > procInfo pid = do > procData <- readFile $ "/proc/" ++ pid ++ "/status" > psData <- foldM getProcInfo Map.empty (lines procData) > let [rUid, eUid, _] = words $ fromJust (Map.lookup "Uid" psData) > let [rGid, eGid, _] = words $ fromJust (Map.lookup "Gid" psData) > let uids = Map.fromList [("RUid", rUid), ("EUid", eUid), > ("RGid", rGid), ("EGid", eGid)] > let psData' = Map.union psData uids > return (PsInfo psData' Map.empty) I tried this for insertProc, but it obviously doesn't work... what would be the correct way to do this? > insertProc :: Pid -> StateT PsMap IO PsInfo > insertProc pid = do > proc <- procInfo pid -- XXX this is obviously wrong... > psMap <- get > put (Map.insert pid proc psMap) > return (proc) A second question: is it possible to make getProcInfo's type to be PsData -> String -> PsData and use some operation similar to lift so that it can be used with foldM, instead of making its return type to be IO PsData explicitely? Thanks in advance, Andre

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