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ANN: contstuff: CPS-based monad transformers
by Ertugrul Soeylemez 01 Oct '10

01 Oct '10

Hello fellow Haskellers, (after failing (again) to send to the haskell list, I'm posting this here. I've contacted the mailing list owner about this.) I'm announcing the 0.4.0 release of the 'contstuff' monad transformer library, an alternative to libraries like 'mtl', 'transformers' and 'monadLib': http://hackage.haskell.org/package/contstuff The difference is that contstuff makes heavy use of continuation passing style both to increase performance and to make certain control flow patterns more accessible, including abortion, accumulation of multiple results, etc. On the wiki there is a quickstart introduction to those of you, who are familiar with monad transformers in general: http://haskell.org/haskellwiki/Contstuff Feedback, positive as well as negative, is highly appreciated. Thank you. Greets, Ertugrul -- nightmare = unsafePerformIO (getWrongWife >>= sex) http://ertes.de/

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ANNOUNCE: atomo 0.1, a simple, adventurous programming language
by Alex Suraci 01 Oct '10

01 Oct '10

Over the past few months I've been designing and implementing a new language called Atomo. The idea is to build up a very expressive language from as few parts as possible, akin to Io or Scheme. But first, the fundamentals: prototype-based object-orientation with multiple dispatch and pattern-matching. Still here? Great! Being object-oriented and dynamic, I expect a few boos and hisses, but if you want to dive in, it's a lot of fun. :) An immense amount of progress has been made in a short amount of time. It already sports a powerful documentation system (similar to Racket's Scribble), and a package manager is in the works (it's functional, but it's local-only). For a while I had atomo-lang.org serving up a site powered by Atomo itself, with Snap underneath via Atomo's Haskell interface. It's down at the moment (it was wrestling with darcsden over my VPS's resources), but the docs are always available: Documentation: http://atomo-lang.org/docs/ (work-in-progress) Repository: http://darcsden.com/alex/atomo Examples: http://darcsden.com/alex/atomo/browse/examples I recommend the generators example if you want some mind-bending, or html.atomo for a nice EDSL, or web.(atomo|hs) for an example of Haskell interop. On Hackage: http://hackage.haskell.org/package/atomo Installation: `cabal install atomo` should "just work", though I haven't worked out the appropriate version numbers for all of its dependencies yet. Hackage shows a build failure, but that seems to be caused by Haddock; I still haven't figured that out. If you want to get involved, you can find me on freenode in #atomo (and #haskell); my nick is alexsuraci. Or, you can fork the repository on darcsden. I'd just like to put this out there to see if anyone else is interested in playing with this. -- Alex

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Monad instance for partially applied type constructor?
by DavidA 30 Sep '10

30 Sep '10

Hi, I have the following code: {-# LANGUAGE TypeSynonymInstances #-} data Vect k b = V [(k,b)] -- vector space over field k with basis b -- for example, V [(5, E 1), (7, E 2)] would represent the vector 5 e1 + 7 e2 data Monomial v = M [(v,Int)] -- monomials over variables v -- for example, M [(A,3), (B,5)] would represent the monomial a^3 b^5 type Poly k v = Vect k (Monomial v) -- multivariate polynomials over field k and variables v instance Monad (Poly k) where return v = V [(1, M [(v,1)])] p >>= f = ... -- variable substitution So my thinking is: 1. The Monad type class is for one parameter type constructors (eg [], IO) 2. Poly is a two-parameter type constructor 3. So Poly k is a one-parameter type constructor 4. What I'm trying to express, that polynomials over field k are a monad, is true. However, GHC 6.12.2 complains: Type synonym `Poly' should have 2 arguments, but has been given 1 In the instance declaration for `Monad (Poly k)' What is going wrong?

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Spell-checking lhs files
by Petr Pudlak 30 Sep '10

30 Sep '10

Hi, recently I was spell-checking a literate Haskell source file (within the vim editor). The spell checker correctly ignored LaTeX commands, but I had to manually skip over code blocks inside |...|. This was really annoying and I couldn't figure out how to configure the spell checker to skip over code. Does anybody have a solution for that? Thanks a lot, Petr

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Fourth Ghent Functional Programming Group Meeting on Thursday, Oct 7th: Program, Final Details and BelHac Commercial
by Jeroen Janssen 30 Sep '10

30 Sep '10

(apologies if you receive multiple copies) Dear All, We are pleased to announce that the fourth Ghent Functional Programming Group (GhentFPG) meeting will take place on Thursday, October 7th, at 19h in the Technicum building of Ghent University (Sint-Pietersnieuwstraat 41, 9000 Gent). As before, you should enter through the electronic sliding doors on the left. The doors will be locked, but there will be a sign with a number that you can call to get in. The final program for our fourth meeting is as follows: 1. Stijn Timbermont - Mapping Interpreters onto Runtime Support When constructing an interpreter, it is hard to achieve a concise and abstract definition of the language semantics and at the same time have sufficient control over how the underlying runtime support is implemented. We illustrate with a number of examples that gaining more control over runtime support implementation forces us to adopt a more machine-like interpreter structure. This is problematic for reuse and evolution of interpreters. We present a solution to this problem that automates this restructuring of interpreters. The solution is based on three cornerstones: the theory of defunctionalized interpreters, effect-driven transformations and generic programming. It allows us to start from a concise and abstract definition of the language features and automatically transform it into a more machine-like form that allows for a customized runtime support implementation to be integrated. 2. Tom Schrijvers - Dictionaries: Eager or Lazy Type Class Witnesses? Type classes are Haskell’s acclaimed solution to ad-hoc overloading. This talk gives an introductory overview of type classes and their runtime witnesses, dictionaries. It asks the questions whether dictionaries should abide by Haskell’s default lazy evaluation strategy. Conceptually, a type class is a type-level predicate: a type is an instance of a type class iff it provides an implementation for overloaded functions. For instance, `Eq a’ declares that type `a’ implements a function `(==) :: a → a → Bool’ for checking equality. Type classes are used as constraints on type variables, in so-called constrained polymorphic functions. E.g. `sort :: Ord a => [a] → [a]’ sorts a list with any type of elements `a’ that are an instance of the Ord type class, i.e. provide implementations for comparison. Witnesses for type class constraints are necessary to select the appropriate implementation for the overloaded functions at runtime. For instance, if `sort’ is called with Int elements, the Int comparison must be used, versus say Float comparison for Float elements. Two forms of witnesses have been considered in the literature, runtime type representations and so-called dictionaries, of which the latter are the most most commonly implementation, e.g., in GHC . Haskell implementations treat dictionaries just like all other data, as lazy values that may potentially consists of non-terminating computations. This way part of the type checker’s work, who has made sure that the dictionaries do exist, is simply forgotten. Is this really necessary? 3. Dominique Devriese - Grammar Combinators - A new model for shallow parser DSL's Parser combinator libraries are a well-known alternative to parser generators in the functional programming community. However, their power is currently fundamentally limited by their representation of recursion in context-free grammars. We present the grammar-combinators Haskell library which removes these limitations, and brings some other advantages (semantic value family polymorphism, natural concepts). In this talk, I will briefly explain the motivation for the library, and then present our approach and some of its most important aspects in a live demonstration and finish with a discussion. Last, note that GhentFPG is organizing the first Belgian Haskell Hackathon, called BelHac, on 5-7 November 2010. The fifth GhentFPG meeting will take place at BelHac and will focus on talks concerning the use of functional programming in industry. More info will be provided soon. As usual, if you want get all the latest info on GhentFPG, you can subscribe to our google group (http://groups.google.com/group/ghent-fpg) or follow us on twitter (@ghentfpg). Hope to see you at both of these events! The GhentFPG Organizing committee.

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Distribution needs
by Joachim Breitner 30 Sep '10

30 Sep '10

Hi, on planet.debian.org, there is some ill-tempered discussion about the seemingly bad relationship between the Ruby community and Debian maintainers. The following blog post summarizes the issues quite well and calmly: http://gwolf.org/blog/ruby-dissonance-debian-again With the Haskell community, luckily the relationship is much better. But since the technical situation is similar, i.e. a custom distribution channel via hackage/cabal-install, the possibility to install multiple versions of a package at once, I thought this might still be an interesting read. I hope this improves the understanding of a distribution’s needs and why we don’t like multiple versions of one package or a splitting in too many single small packages. But let me stress that I’m very happy with the interaction of the communities, Greetings, Joachim -- Joachim "nomeata" Breitner mail: mail(a)joachim-breitner.de | ICQ# 74513189 | GPG-Key: 4743206C JID: nomeata(a)joachim-breitner.de | http://www.joachim-breitner.de/ Debian Developer: nomeata(a)debian.org

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Problem with a sample from RWH
by C K Kashyap 30 Sep '10

30 Sep '10

Hi, I was going over the Error Handling chapter in RWH and tried out this sample - Prelude> :m Control.Exception Prelude Control.Exception> let x=5 `div` 0 Prelude Control.Exception> let y=5 `div` 1 Prelude Control.Exception> handle (\_ -> putStrLn "Text") (print x) <interactive>:1:0: Ambiguous type variable `e' in the constraint: `Exception e' arising from a use of `handle' at <interactive>:1:0-39 Probable fix: add a type signature that fixes these type variable(s) Prelude Control.Exception> Could someone please tell me what the problem is and how to resolve it? -- Regards, Kashyap

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Parsec question (new user): unexpected end of input
by Peter Schmitz 30 Sep '10

30 Sep '10

I am a new Parsec user, and having some trouble with a relatively simple parser. The grammar I want to parse contains tags (not html) marked by angle brackets (e.g., "<some tag>"), with arbitrary text (no angle brackets allowed) optionally in between tags. Tags may not nest, but the input must begin and end with a tag. Whitespace may occur anywhere (beginning/end of input, inside/between tags, etc.), and is optional. I think my problem may be a lack of using "try", but I'm not sure where. At runtime I get: Error parsing file: "...\sampleTaggedContent.txt" (line 4, column 1): unexpected end of input expecting "<" The input was: <tag1>stuff<tag 2> more stuff < tag 3 > even more <lastTag> The code is below. (I'm using Parsec-2.1.0.1.) I don't really want to return anything meaningful yet; just parse okay. Any advice about the error (or how to simplify or improve the code) would be appreciated. Thanks much, -- Peter > -- Parsers: > taggedContent = do > optionalWhiteSpace > aTag > many tagOrContent > aTag > eof > return "Parse complete." > > tagOrContent = aTag <|> someContent <?> "tagOrContent" > > aTag = do > tagBegin > xs <- many (noneOf [tagEndChar]) > tagEnd > optionalWhiteSpace > return () > > someContent = do > manyTill anyChar tagBegin > return () > > optionalWhiteSpace = spaces -- i.e., any of " \v\f\t\r\n" > tagBegin = char tagBeginChar > tagEnd = char tagEndChar > > -- Etc: > tagBeginChar = '<' > tagEndChar = '>' --------

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When I change the cabal file to say preference: base >= 4
by caseyh＠istar.ca 29 Sep '10

29 Sep '10

When I change the cabal file to say preference: base >= 4 I still get, "you are using base 3.0 which is deprecated." When I change the overall cabal profile, the error message still comes up. It seems like some other part of the install process is controlling the base version, besides the *.cabal and cabal profile file.

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ICFP2010 contest results?
by Ryan Ingram 29 Sep '10

29 Sep '10

I saw the winner was announced. Is there a highscore table? We were in the top 5, I want to see how well we did. -- ryan

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