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attoparsec and vectors
by Eric Rasmussen 29 Jun '11

29 Jun '11

Hi everyone, I have a task that involves parsing a flat-file into a vector from the Data.Vector package. In the interest of keeping things simple, I first used Attoparsec.Text's version of "many" and then converted the resulting list to a vector: import qualified Data.Vector as V import Data.Attoparsec.Text as A import Control.Applicative getData = do results <- A.many someParser return (V.fromList results) It runs quickly, but I naively thought I could outperform it by reworking "many" to build a vector directly, instead of having to build a list first and then convert it to a vector: manyVec :: Alternative f => f a -> f (V.Vector a) manyVec v = many_v where many_v = some_v <|> pure V.empty some_v = V.cons <$> v <*> many_v Unfortunately, manyVec either quickly leads to a stack space overflow, or it takes an enormous amount of time to run. Does anyone know of a better way to build up a vector from parsing results? Thanks for any tips or insight, Eric

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Possible bug in GHC 7.0.3
by Alberto G. Corona 29 Jun '11

29 Jun '11

I have an "'impossible' happened" error. The code may look a little bit convoluted but it is part of my real code.: --------- begin of code------ {-# LANGUAGE FlexibleInstances, UndecidableInstances , MultiParamTypeClasses #-} class Serializable a b class IResource a --The rest of the instance definitions does not matter for the error instance Serializable a b => IResource a data DBRef a= DBRef String a instance (IResource a) => Read (DBRef a) data Votation a= Votation{ content :: DBRef a } deriving (Read) ------------------------------- end of code --- gives the following error at compilation time: tests>runghc impossiblelloop.hs ghc: panic! (the 'impossible' happened) (GHC version 7.0.3 for i386-unknown-mingw32): solveDerivEqns: probable loop (impossiblelloop.hs:20:13-16 main:Main.$fReadVotation{v rhI} [a{tv abB} [tv] ] base:GHC.Read.Read{tc 2d} [main:Main.Votation{tc rbo} a{tv abB} [tv]] = [base:GHC.Read.Read{tc 2d} (main:Main.DBRef{tc rbu} a{tv abB} [tv])]) [[main:Main.Serializable{tc rbA} a{tv abB} [tv] b{tv ajE} [tcs]]] Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug

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Re: [Haskell-cafe] Possible bug in GHC 7.0.3
by Alberto G. Corona 29 Jun '11

29 Jun '11

Ryan' P robably the read instance can not be infered without the functional dependency but the bug is also triggered with it. I simplified ithe coder as much as possible El 29/06/2011 02:26, "Ryan Ingram" <ryani.spam(a)gmail.com> escribió:

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IterIO type restricted functions
by John Ky 29 Jun '11

29 Jun '11

Hi all, >From the IterIO tutorial: enumFile' is like enumFile above, but type restricted to data in the lazy ByteString format, which is more efficient than plain Strings. (enumFile supports multiple types, but in this example there is not enough information for Haskell to choose one of them, so we must use enum*f*ile' or use :: to specify a type explicitly. Which is fine, but shouldn't there also be iterHandle' and iterStream'? Also I'm guessing the *f* is a typo that should be *F*. Cheers, -John

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Problem building with Cabal + TH + Library with FFI to C++ via C Wrapper
by Ozgun Ataman 28 Jun '11

28 Jun '11

Dear Cafe, I have recently run into a very annoying issue that I was not able to get around. I have a local package that makes use of the snappy compression library, which boasts bindings to C++ through a C wrapper. Everything compiles fine with ghc --make, but cabal install hits a wall when TemplateHaskell kicks in and decides to load the entire dependency chain. Here is the important bit of the error message I get after running "cabal install" Loading package snappy-0.2.0.0 ... linking ... ghc: /home/ubuntu/.cabal/lib/snappy-0.2.0.0/ghc-7.0.3/HSsnappy-0.2.0.0.o: unknown symbol `_ZTV8BSSource' ghc: unable to load package `snappy-0.2.0.0' I'm quite certain all the libs are in PATH. "cabal install snappy" installs fine when typed separately by itself and everything works fine (the program works, including compression via snappy) when I compile my local package with ghc --make. The problem only occurs on a Ubuntu 64-bit server, and I wasn't able to replicate the issue on my local OS X box. On my local, cabal install works just fine. I did find a previous note on Cafe with the exact same problem, but no solution was posted at the time: http://www.haskell.org/pipermail/haskell-cafe/2010-December/087748.html Any help is much appreciated. Cheers, Ozgun

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Installation Failed: Haskell Platform 2011.2.0.1-i386 for OS X
by John Velman 28 Jun '11

28 Jun '11

I'm running OS X 10.6.7, XCode 3.2.5. When I try to install "The Haskell Platform 2011.2.0.1 for Mac OS X 10.6 (Snow Leopard)" it goes all the way through to "running package scripts", then says "installation failed" I did two separate downloads, and tried the first installation two time, the second download one time. Same result all three times. Is this a problem with the package, or ? Any suggestions? Thanks, John Velman

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Enumerators, Enumeratees and Iterators
by Sævar Berg 28 Jun '11

28 Jun '11

Hey Café. I've been playing with Enumerators, Iteratees and Enumeratees today after having spent a few days writing a server application using lazy IO, then reading slides from Oleg's DEFUN8 talk notes, and I quote: "Lazy IO in serious, server-side programming is unprofessional", "I can talk a lot how disturbingly, distressingly wrong lazy IO is theoretically, how it breaks all equational reasoning" after which my OCD self has been eating me up to take a proper look at this, so here I am. I'm already beginning to see how Iteratees, Enumerators and Enumeratees can be nifty, but I have a couple of questions about a couple of ideas and whether, and if so how they can be implemented. The first question is, I think, to be solved with enumeratees but I can't really grok how. Let's say I have an iteratee that consumes all input. Is it possible to implement an enumeratee or something else to stick between the enumerator and the iteratee to basically modify the input to the iteratee to only be a part of the input? Something like this: enumFile "someFile" && printFileLines -- prints file with prepended line numbers enumFile "someFile" ?? onlyGet10Lines && printFileLines -- print only 10 lines The second question could actually have an application in the server I'm writing. I was wondering if it was possible to write iteratees/enumerators that would only generate/consume when a certain something was the next chunk to be processed? It's a bit hard to explain, but let me try to give you an example. Let's say my application takes input from the network which contains commands that the server reacts to. Let's say I have a list of command handlers that I want to run this command through, but the handlers will only execute when they are passed the command that they are responsible for handling. Can this list of command handlers be enumerators/iteratees/enumeratees? E.g. is it possible to 'concat' them, making them each peek at the next chunk to see if it's theirs to handle and put it back if it isn't? I know this is possible to do differently and I have an idea for how I would do this, but I'm wondering if this is possible. If I can get answers to either or both of the questions it would help me a bit on my way to completely realize the roles that enumerators vs. iteratees vs. enumeratees have, as the lines seem a bit blurred -- sometimes, anyway. Thanks in advance. :)

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Re: [Haskell-cafe] Possible bug in GHC 7.0.3
by Alberto G. Corona 28 Jun '11

28 Jun '11

Reported: http://hackage.haskell.org/trac/ghc/ticket/5287 2011/6/28 Alberto G. Corona <agocorona(a)gmail.com> > > 2011/6/28 Jason Dagit <dagitj(a)gmail.com> > >> On Tue, Jun 28, 2011 at 3:43 AM, Alberto G. Corona <agocorona(a)gmail.com> >> wrote: >> > I have an "'impossible' happened" error. >> > The code may look a little bit convoluted but it is part of my real >> code.: >> >> I don't know why it's crashing, but did you already report it as a >> bug? If not, you definitely should. >> >> > Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug >> >> That URL should have the info you need to report it. >> >> Jason >> > > Not published yet. I wanted to publish it in the list before reporting the > bug. Just in case anyone tell something interesting about it. A sorter > expression that produces the same bug or something similar. >

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What is a "simple pattern binding"?
by dm-list-haskell-cafe＠scs.stanford.edu 28 Jun '11

28 Jun '11

Section 4.4.3.2 of the 2010 Haskell report says: A simple pattern binding has form p = e. The pattern p is matched “lazily” as an irrefutable pattern, as if there were an implicit ~ in front of it. This makes it sound as though p is a pattern, which I assume means what section 3.17 defines as the non-terminal "pat". pat -> lpat qconop pat | lpat This is further suggested by the explicit mention of ~, which would be redundant if p had to be a var, since variables always match (according to section 3.17.2 rule 1). So my reading of section 4.4.3.2 is that the following is considered a simple pattern binding (because it has no guards): (f, g) = (\x -> x, f) However, section 4.5.5 seems to contradict this. It reads: Recall that a variable is bound by either a function binding or a pattern binding, and that a simple pattern binding is a pattern binding in which the pattern consists of only a single variable (Section 4.4.3). Moreover, it goes on to give an example and explanation: [(n,s)] = reads t ... Hence, when non-simple pattern bindings are used This text makes it sound as though a "simple pattern binding" can have only a single variable to the left of the = sign, meaning: f = \x -> x Further confusing things, GHC accepts the following: g1 x y z = if x>y then show x ++ show z else g2 y x g2 :: (Show a, Ord a) => a -> a -> String g2 | False = \p q -> g1 q p () | otherwise = \p q -> g1 q p 'a' where x = True and infers type: g1 :: (Show a, Show a1, Ord a1) => a1 -> a1 -> a -> [Char] According to 4.4.3.2, g2 definitely does not have a simple pattern binding, as its binding is not of the form p = e where p is a pattern. Yet by section 4.5.5, if g2 were not considered a simple pattern binding, the constrained type variables in the binding group containing g1 and g2 (in particular the inferred type (Show a => a) of z in g1) would not be allowed to be generalized. So is section 4.4.3.2 of the Haskell 2010 report just wrong? Or is GHC allowing code prohibited by the standard? Or am I somehow misreading the standard? Anyway, if someone can provide a less ambiguous definition of the term "simple pattern binding", I would appreciated it, particularly if you can point to support for your definition in the Haskell 2010 report... Thanks, David

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Cleaner way to write code and handle errors?
by John Ky 28 Jun '11

28 Jun '11

Hi all, I'm practising my Haskell by writing a simple TCP echo server and finding that getting my program control to be succinct is rather tricky. In particular, I have return () everywhere, my error handling is verbose and I'm not entirely sure my recursion is the cleanest way syntactically to get my loops going and terminating. I must be doing something obviously un-Haskell-like. Any suggestions on how I can improve my code? Code below. Cheers, -John import Control.Concurrent import Control.Exception import Control.Monad import Network import System.IO import System.IO.Error (isEOFError) main = withSocketsDo $ do sListen <- listenOn (PortNumber 8000) putStrLn "Listening on Port 8000" forkIO $ forever $ do (sSession, hostname, port) <- accept sListen putStrLn ("Connected to " ++ hostname ++ ":" ++ (show port)) let processLine = forkIO $ do lineResult <- try (hGetLine sSession) case lineResult of Right line -> do putStrLn line processLine return () Left e -> if isEOFError e then putStrLn (show e) else do ioError e return () return () processLine return() line <- getLine return ()

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