January 2011 - Haskell-Cafe

Hexpat: Lazy I/O problem with huge input files
by Aleksandar Dimitrov 10 Dec '11

10 Dec '11

Hello Haskell Cafe, I really hope this is the right list for this sort of question. I've bugged the folks in #haskell, they say go here, so I'm turning to you. I want to use Hexpat to read in some humongous XML files (linguistic corpora,) since it's the only Haskell XML library (I could find) that takes ByteStrings as input. I stumbled on a problem when using one of the examples from the docs of Text.XML.Expat.Tree. The "cookbook recipe" there suggests *first* processing the data, and only then looking into the parser error to see if there has been an error. I understand this should prevent the parse tree from being fully evaluated before use. Unfortunately, that is not what happens on my system (ghc 6.12.1, if that's of importance.) This is the code from the docs, that I modified to read files: > import Text.XML.Expat.Tree > import System.Environment (getArgs) > import Control.Monad (liftM) > import qualified Data.ByteString.Lazy as C >· > -- This is the recommended way to handle errors in lazy parses > main = do > f <- liftM head getArgs >>= C.readFile > let (tree, mError) = parse defaultParseOptions f > print (tree :: UNode String) >· > -- Note: We check the error _after_ we have finished our processing > -- on the tree. > case mError of > Just err -> putStrLn $ "It failed : "++show err > Nothing -> putStrLn "Success!" Given a 42M test file, an invocation like this: % ghc --make -O2 Hexpat.hs % ./Hexpat input.xml > dump.xml will gobble up some 2Gigs of RAM (at least. I usually kill it before it starts thrashing the swap space, since that almost crashes my entire machine.) If I remove the last 3 lines: > import Text.XML.Expat.Tree > import System.Environment (getArgs) > import Control.Monad (liftM) > import qualified Data.ByteString.Lazy as C > > main = do > f <- liftM head getArgs >>= C.readFile > let (tree, mError) = parse defaultParseOptions f > print (tree :: UNode String) the same invocation and input file barely uses a megabyte or two of RAM and finishes really quickly. Why is that? Is this a mistake in the Hexpat docs, or am I doing something wrong? Lazy IO has always been a little bit of a mystery to me, and just when I thought I had it... Thanks for any help on the matter! Aleks

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Re: [Haskell-cafe] Re: Functional dependencies and Peano numbers (and hoogle-bug?)
by Oscar Finnsson 03 May '11

03 May '11

Thanks for the great feedback. The bijective example was especially interesting. While reading "Fun with Type Functions" I notices GNum as an interesting alternative to the Num type class but I couldn't find any such package on hackagedb. Do anyone know if there is anything like GNum on hackagedb? On an unrelated note: I hoogled "to" (i.e. http://haskell.org/hoogle/?hoogle=to) and just got a blank page. Nothing. Nil (not even <html>...</html>). Is this a bug or a "feature"? :) The reason I hoogled it was because I'm searching for something like explicit casting found in many other languages, something similar to > class To f t where > to :: f -> t > > instance To a a where > to x = x > > instance (Real a, Fractional b) => To a b where > to = realToFrac > > instance (Read a) => To String (Maybe a) where > to = maybeRead -- not from cgi :) so I can write something like > ("23.2" `to`) :: Maybe Double or > ((42 :: Integer) `to`) :: Float Anyone made a module/package that solves this problem already? I cannot be the first that needs generic type safe conversion... . -- Oscar

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Trouble installing FileManipCompat
by Tony Miller 28 Apr '11

28 Apr '11

Hi, I'm trying to install FileManipCompat(well, really HStringTemplateHelpers, but the error is from FileManipCompat) and I'm getting this error: [1 of 1] Compiling System.FilePath.FindCompat ( System/FilePath/FindCompat.hs, dist/build/System/FilePath/FindCompat.o ) System/FilePath/FindCompat.hs:175:20: Not in scope: data constructor `State' I've attached the complete output of `cabal --verbose install HStringTemplateHelpers`.

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ANNOUNCE: Multiplate 0.0.1
by roconnor＠theorem.ca 25 Mar '11

25 Mar '11

Multiplate is a lightweight generic library for mutually recursive data types that won't make Conor lose his lunch. Multiplate is an alternative extension of the Uniplate/Compos core design to support mutually recursive datatypes in a way that is as powerful as Compos, almost as easy to use as Uniplate, and more portable than both of them. Multiplate does not require you to rewrite your data type, does not require run-time reflection, does not require GADTs, and does not even require multi-parameter type classes. It only requires rank 3 polymorphism. <http://hackage.haskell.org/package/multiplate-0.0.1> A more detailed paper is forthcoming, but the library is available to try right now. -- Russell O'Connor <http://r6.ca/> ``All talk about `theft,''' the general counsel of the American Graphophone Company wrote, ``is the merest claptrap, for there exists no property in ideas musical, literary or artistic, except as defined by statute.''

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Anyone recommend a VPS?
by Michael Snoyman 21 Mar '11

21 Mar '11

Hi all, I'm working with a client right now on deploying an app. I was wondering if anyone had some recommendations for a Haskell-friendly host. I'm inclined to go with a VPS for this setup, but if there's reliable shared hosting, that would do as well. This client is fairly price-sensitive, so I don't think anything past the $20/month range to start off with. The only other requirement for this project is PostgreSQL. As far as wants: * Linux is preferable * A server that isn't on everyone's spam list would be better. Of course, I can route through an outside e-mail provider, but local would be nicer. * Have room to grow. For now, this is a fairly small app, but the hope of course is for it to expand. Thanks in advance. Michael

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Downloading Haskell repos from GitHub
by Gwern Branwen 20 Mar '11

20 Mar '11

Along the lines of http://blog.patch-tag.com/2010/03/13/mirroring-patch-tag/ for downloading all patch-tag.com repositories, I've begun to wonder how to download all Github repositories since more and more people seem to be using it. Nothing in http://develop.github.com/ seems especially useful for grabbing the git:// URLs of all repos by language - just by user. The only real list of repos by language seems to be gotten at via http://github.com/languages/Haskell/updated or http://github.com/languages/Haskell/created . (You might think http://github.com/languages/Haskell would be good, but no, it's just a few random repos by interest and not a full listing.) I looked at the HTML, and it looks possible to use tagsoup to get all 98 pages and then parse the entries to get the HTTP URLs of the repos, and then turn *that* into git:// URLs suitable for shelling out to 'git clone', but I can't help but wonder if maybe there's a better approach someone more familiar with Github would know. -- gwern

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tplot and splot - analyst's swiss army knifes for visualizing log files
by Eugene Kirpichov 18 Mar '11

18 Mar '11

Hi cafe, I've published a large presentation about two Haskell-based tools of mine - tplot and splot. Their motto is "visualize system behavior from logs with a shell one-liner". Based on my experience, they usually seem to live up to this motto. http://www.slideshare.net/jkff/two-visualization-tools [attention attractor: the presentation has *really a lot* of pictures] hackage: http://hackage.haskell.org/package/timeplot http://hackage.haskell.org/package/splot other docs: http://www.haskell.org/haskellwiki/Timeplot http://www.haskell.org/haskellwiki/Splot -- Eugene Kirpichov Senior Software Engineer, Grid Dynamics http://www.griddynamics.com/

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Cmdargs and common flags
by Magnus Therning 12 Mar '11

12 Mar '11

I'm looking for a good way of dealing with common flags in cmdargs. Currently what I do requires a bit of repetition that I'd really like to get rid of: 1. Data types data Modes = Cmd1 { foo :: String, ... } | Cmd2 { foo :: String, ... } 2. Mode specifications cmd1 = Cmd1 { foo = def &= help "..", ...} cmd2 = Cmd2 { foo = def &= help "..", ...} I have no idea how to deal with the repetition in the data types, so all suggestions are welcome. For the repetition in the mode specifications I attempted to factor the common bits into a "variable": flagFoo = "" &= help ".." but that resulted in only the first command getting the proper specification. I suppose this comes down to the impurity, but how do I work around that? (Especially, are there any good examples of using the pure part of the API?) /M -- Magnus Therning OpenPGP: 0xAB4DFBA4 email: magnus(a)therning.org jabber: magnus(a)therning.org twitter: magthe http://therning.org/magnus

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Infinite types should be optionally allowed
by Job Vranish 22 Feb '11

22 Feb '11

There are good reasons against allowing infinite types by default (mostly, that a lot of things type check that are normally not what we want). An old haskell cafe conversation on the topic is here: http://www.nabble.com/There%27s-nothing-wrong-with-infinite-types!-td771373… However, I think infinite types should be allowed, but only with an explicit type signature. In other words, don't allow infinite types to be inferred, but if they are specified, let them pass. I think it would be very hard to shoot yourself in the foot this way. Newtype is the standard solution to situations where you really need an infinite type, but in some cases this can be a big annoyance. Using newtype sacrifices data type abstraction and very useful type classes like Functor. You can use multiparameter type classes and functional dependencies to recover some of the lost abstraction, but then type checking becomes harder to reason about and the code gets way more ugly (If you doubt, let me know, I have some examples). Allowing infinite types would fix this. I'm imagining a syntax something like this: someFunctionThatCreatesInfiniteType :: a -> b | b = [(a, b)] Thoughts? Opinions? Am I missing anything obvious? - Job

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Haskell for children? Any experience?
by Chris Smith 19 Feb '11

19 Feb '11

So I find myself being asked to plan Haskell programming classes for one hour, once a week, from September through May this coming school year. The students will be ages 11 to 13. I'm wondering if anyone has experience in anything similar that they might share with me. I'm trying to decide if this is feasible, or it I should try to do something different. To be honest, as much as I love Haskell, I tried to push the idea of learning a different language; perhaps Python. So far, the kids will have none of it! This year, I've been teaching a once-a-week exploratory mathematics sort of thing, and we've made heavy use of GHCi... and they now insist on learning Haskell. (By the way, GHCi is truly amazing for exploratory mathematics. We really ought to promote the idea of Haskell for elementary / junior-high level math teachers! It's so easy to just try stuff; and there are so many patterns you can just discover and then say "Huh, why do you think that happens? Can you write it down precisely? ...") -- Chris Smith

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