March 2011 - Haskell-Cafe

Haskell and friends in Spanish
by Manuel Hernandez 22 Mar '11

22 Mar '11

Dear Haskellers, I have tried to compile from my own work and from several sources a university-level course in functional and logic programming, and the result is a prospect of a book (*right now only in Spanish*), which I have entitled "Programación declarativa". Perhaps someone could be interested in giving it a look. The book is big (more than 400pp and 2Mb) and eclectic wrt declarative topics (ranging from Scheme to Prolog), but Haskell has an ample treatment. Some parts of the book are even unfinished, but I think the book may be valuable for some Spanish-speaking people wishing to learn declarative programming through functional programming and Prolog. The book can be downloaded from: http://sites.google.com/site/writingsmhg/haskell/SpanishDeclarativeProgramm… Corrections and suggestions are welcome. Bests, -- Dr Manuel Hernández Gutiérrez Profesor-Investigador, Oficina num. 14 Coordinador del Doctorado en Tecnologías de Cómputo Aplicado. Área de Estudios de Posgrado. Instituto de Computación Universidad Tecnológica de la Mixteca Km 2.5 de la Carretera a Acatlima Huajuapan de León, Oaxaca, México 69000 Tel. +52 (953) 53 2 03 99 extensión 200 Fax +52 (953) 53 2 02 14 www.utm.mx

1 0

Bamse package virus false positive
by Isaac Potoczny-Jones 22 Mar '11

22 Mar '11

We have realized that a file in the Bamse distribution in Hackage is being flagged by some virus scanners as malicious, including Symantec and McAfee. We have worked with Microsoft, Symantec, and McAfee to analyze the data. All parties have concluded that this file is not malicious, but we wanted to let the community know in case you run across a similar issue. We have notified the maintainer. Symantec and McAfee have updated their rules accordingly and will no longer flag this file, but other virus scanners might continue to flag it. Note that if you use Bamse to build MSI packages, inclusion of this file could cause some scanners to flag the package. The details of this file are as follows: File name: Folder.exe File Size: 83249 byte MD5: 9f939a06344af9b242a7b43849defb40 SHA1: a9faf0915d1ea142b161bec57792ef2ca379ce5b URL: http://hackage.haskell.org/package/bamse From the package description: "Bamse is a framework for building Windows Installers for your Windows applications, giving you a comprehensive set of features to put together MSIs using Haskell." peace, isaac

1 0

wrong backquote in haskell 2010 report Prelude
by Albert Y. C. Lai 22 Mar '11

22 Mar '11

Haskell 2010 report chapter 9 Standard Prelude uses a wrong backquote, e.g., infixl 7 ⋆, /, ‘quot‘, ‘rem‘, ‘div‘, ‘mod‘ Those are U+2018. The grammar (Chapter 3) requires U+0060 and accepts no substitutes, e.g., varop → varsym | ` varid `

2 1

Uncertainty analysis library?
by Edward Amsden 22 Mar '11

22 Mar '11

Hi cafe, I'm looking for a library that provides an instance of Num, Fractional, Floating, etc, but carries uncertainty values through calculations. A scan of hackage didn't turn anything up. Does anyone know of a library like this? Thanks! -- Edward Amsden Student Computer Science Rochester Institute of Technology www.edwardamsden.com

5 13

ANNOUNCE: incremental-parser library package
by Mario Blažević 22 Mar '11

22 Mar '11

The first version of incremental-parser has been released on Hackage [1]. It's yet another parser combinator library, providing the usual set of Applicative and Monad combinators. Apart from this, it has three twists that make it unique. First, the parser is incremental. That means it can be fed its input in chunks, and in proper circumstances it can also provide the parsed output in chunks. For this to be possible the result type must be a Monoid. The complete parsing result is then a concatenation of the partial results. In order to make the incremental parsing easier, the combinator set is optimized for monoidal results. The usual combinator many1, for example, assumes the result type is a monoid and concatenates its components instead of constructing a list. In Parsec: > many1 :: Stream s m t => ParsecT s u m a -> ParsecT s u m [a] In incremental-parser: > many1 :: (Monoid s, Monoid r) => Parser s r -> Parser s r The second weirdness is that the the parser is generic in its input stream type, but this type is parameterized in a holistic way. There is no separate token type. Primitive parsers that need to peek into the input require its type to be an instance of a monoid subclass. In Parsec: > string :: Stream s m Char => String -> ParsecT s u m String > char :: Stream s m Char => Char -> ParsecT s u m Char > anyToken :: (Stream s m t, Show t) => ParsecT s u m t In Attoparsec: > string :: ByteString -> Parser ByteString > word8 :: Word8 -> Parser Word8 > anyWord8 :: Parser Word8 In incremental-parser: > string :: (LeftCancellativeMonoid s, MonoidNull s) => s -> Parser s s > token :: (Eq s, FactorialMonoid s) => s -> Parser s s > anyToken :: FactorialMonoid s => Parser s s The monoid subclasses referenced above provide methods for analyzing and subdividing the input stream. The classes are not particularly demanding, and any reasonable input stream should be able to accommodate them easily. The library comes with instances for lists, ByteString, and Text. > class Monoid m => MonoidNull m where > mnull :: m -> Bool > class Monoid m => LeftCancellativeMonoid m where > mstripPrefix :: m -> m -> Maybe m > class Monoid m => FactorialMonoid m where > factors :: m -> [m] > primePrefix :: m -> m > ... Finally, the library being implemented on the basis of Brzozowski derivatives, it can provide both the symmetric and the left-biased choice, <|> and <<|>. This is the same design choice made by Text.ParserCombinators.ReadP and uu-parsinglib. Parsec and its progeny on the other hand provide only the faster left-biased choice, at some cost to the expressiveness of the combinator language. [1] http://hackage.haskell.org/package/incremental-parser-0.1

4 4

Re: [Haskell-cafe] [Haskell] ANNOUNCE: incremental-parser library package
by Mario Blažević 22 Mar '11

22 Mar '11

2011/3/22 Philippa Cowderoy <postmaster(a)flippac.org> > This is what newtypes are for, no? > I did not think of that approach. I'm not sure how well it would work out, but it would solve another problem I have, which is the duplication of combinators many, some, and optional. Each of these could exist in two forms, the lazy one and the greedy one, and the only difference is the underlying choice combinator, (<|>) vs. (<<|>). I'm not aware of any other parsing library taking this road, though, and there must be a good reason. I'll try and see. > 2011/3/22 Mario Blažević <mblazevic(a)stilo.com> > >> >> This seems very interesting. One question: >>> >>> > The MonadPlus and the Alternative instance differ: the former's mplus >>> > combinator equals the asymmetric <<|> choice. >>> >>> Why? >>> >> >> >> Good question. Basically, I see MonadPlus as a union of Monad and >> Alternative. The class should not exist at all. But as long as it does, I >> figured I should provide an instance, and I made it different from the >> Monoid+Alternative combination because otherwise it would be useless. My >> second choice would be to remove the instance completely. >> > > I have to admit I really do not like having Applicative and MonadPlus with > different behavior. Yes, one is redundant, but that is more an artifact of > language evolution, than an intentional opportunity for diverging behavior. > > Every library I am aware of to date, save of course this one, has > maintained their compatibility. > > If the instance for Alternative satisfies the underspecified MonadPlus > laws, I'd just as soon have the 'useless redundant' instance. The power of > MonadPlus is in the combinators that are built on top of it. Not in the > primitives themselves. > > If the Alternative instance would not be a legal MonadPlus instance, then > I'd feel much less queasy with your second scenario, and it simply removed. > > -Edward >

1 0

Berlin Haskell Meeting
by Sönke Hahn 22 Mar '11

22 Mar '11

Hi all! It's been some time since the last Berlin Haskell Meeting, but we're doing it again: Date: Wednesday, March 30th Time: from 20:00 Location: c-base, Rungestrasse 20, 10179 Berlin If you're in Berlin and interested in Haskell, save the date! Cheers, Sönke

1 0

Loops and ray casting with GPipe?
by Jarkko Vilhunen 22 Mar '11

22 Mar '11

Greetings, With OpenGL shaders it is possible to write code utilizing loops with unknown number of iterations, I'm wondering if the same can be done with GPipe? Fractals would be one use case and implicit surfaces another. Fragment Floats had to be compared with IfB from Data.Boolean, so I tried something recursive like foo x = boolean x (foo (x * 0.9)) (x <* 1) which compiles fine but seems to concentrate on eating stack space while running :)

3 2

Mime / Mail library
by Pieter Laeremans 22 Mar '11

22 Mar '11

Hi all, The MIME package that can be found on hackage, uses String as input. Would i be considered better if there would be a version based on Text, or ByteString ? kind regards, Pieter -- Pieter Laeremans <pieter(a)laeremans.org> "The future is here. It's just not evenly distributed yet." W. Gibson

4 3

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

12 15