- Haskell-Cafe - Haskell.org

Seeking Daan Leijen
by John Goerzen 31 Mar '08

31 Mar '08

I tried to email this to Daan, but his mail is bouncing... Subject: http://legacy.cs.uu.nl/daan/parsec.html Hi Daan, I noticed Parsec 3.0.0 on Hackage, and went to your homepage to read about the new package. But it looks like your homepage still has the old version put out in 2003. This might cause confusion for some that are looking for new releases at your website. Thanks, -- John PS... The HaXml website has the same problem, and has an incompatible API with the version in Hackage... Some Linux distros are using one, some the other.

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HTTP client libraries
by John Goerzen 31 Mar '08

31 Mar '08

Hi folks, I was recently looking for HTTP client libraries for Haskell. I investigated several, and it seems that there aren't any really ready for primetime. Am I missing anything? Here's what I found: * Bjorn's String-based HTTP It eats RAM. Does not appear to read data lazily, returns a String, and may have a memory leak as well. Does not appear to be suited for anything except very small file downloads. * nominolo's ByteString port of Bjorn's HTTP Appears to exist only in a darcs repo linked from his personal blog. Makes me hesitant to use it in production code, for fear that it won't be around long-term. * tagsoup Broken in many ways. Does not support chunked encoding that is mandated by HTTP RFC. Will be incompatible with numerous HTTP servers, and the manual states as much. Port 80 hardcoded. * network-minihttp Doesn't appear to actually be very useful as a client. Also, as far as I have been able to deduce, none of these have built-in support for https (SSL) URLs. There is also a Curl binding (or two?) floating around, which I haven't investigated. It would be nicer to have a native Haskell solution. Or should I just go write one? -- John

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lexicographic order
by Simeon Mattes 31 Mar '08

31 Mar '08

Hello everyone, I would like to ask something that I found in the ebook "a Gentle Introduction to Haskell". http://haskell.org/tutorial/stdclasses.html#sect8.4 data Tree a = Leaf a | Branch (Tree a) (Tree a) instance (Ord a) => Ord (Tree a) where (Leaf _) <= (Branch _) = True (Leaf x) <= (Leaf y) = x <= y (Branch _) <= (Leaf _) = False (Branch l r) <= (Branch l' r') = l == l' && r <= r' || l <= l' The latter specifies a lexicographic order: Constructors are ordered by the order of their appearance the data declaration, and the arguments of a constructor are compared from left to right. Although I have tried to make sense what lexicographic order means I haven't figured out. Maybe an example with a simple application of this would be helpful. To be honest I can't understand what the symbol <= really means. Thanks -- View this message in context: http://www.nabble.com/lexicographic-order-tp16381434p16381434.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Possible to automatically determine typeclass membership?
by Ryan Ingram 31 Mar '08

31 Mar '08

Is it possible in Haskell + GHC extensions to use reflection techniques to determine typeclass membership? I'm thinking of things like the following: Idea 1: > data MaybeEq a = NoEq a | Eq a => HasEq a > checkEq :: (some typeclass constraint) => a -> MaybeEq a (such that checkEq x returns HasEq x if and only if x has an Eq instance.) Idea 2: > data HTrue > data HFalse > class MaybeEq a b | a -> b > instance Eq a => MaybeEq a HTrue > instance (otherwise) => MaybeEq a HFalse -- ryan

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Wumpus World
by iliali16 31 Mar '08

31 Mar '08

Hi guys I have to build the wumpus world problem. I didn't start yet since this is the first time in my life I have to do something like that and I feel not confident in starting it. So I have basic idea of what prolog and haskell can do and I know a bit of Java. I am wandering if you can tell me which one is best to use to build this problem.Thanks couse I am really confused -- View this message in context: http://www.nabble.com/Wumpus-World-tp16310198p16310198.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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SoC project: Python-Haskell bridge - request for feedback
by Michał Janeczek 31 Mar '08

31 Mar '08

Hi, I am a student interested in participating in this year's SoC. At http://tsk.ch.uj.edu.pl/~janeczek/socapp.html (and also below in this email) you can find a draft of my project proposal. I'd like to ask you to comment on it, especially the deliverables part. Are you interested in such a project, and if yes, what features would be most important to you? Is anything missing, or should something get more priority or attention? Regards, Michal Python-Haskell bridge ===================== Description ----------- This project will seek to provide a comprehensive, high level (and thus easy to use) binding between Haskell and Python programming languages. This will allow using libraries of either side from each language. Benefits for Python ------------------- * Robust, high assurance components It might be beneficial to implement safety-critical components in a strongly, statically typed language, using Python to keep them together. Cryptography or authentication modules can be an example. * Performance improvements for speed-critical code Haskell compiled to native code is typically an order of magnitude faster than Python. Aside from that, advanced language features (such as multicore parallel runtime, very lightweight threads and software transactional memory) further serve in improving the performance. Haskell could become a safe, high level alternative to commonly used C extensions. * Access to sophisticated libraries While its set of libraries is not as comprehensive as that of Python, Haskell can still offer some well tested, efficient libraries. Examples might be rich parser combinator libraries (like Parsec) and persistent, functional data structures. QuickCheck testing library could also be used to drive analysis of Python code. Benefits for Haskell -------------------- The project would benefit Haskell by providing it with access to an impressive suite of libraries. It also has a potential to help Haskell adoption, by mitigating risk of using Haskell in a project. Deliverables ------------ * A low level library to access Python objects from Haskell * A set of low level functions to convert built-in data types between Haskell and Python (strings, numbers, lists, dictionaries, functions, generators etc.) * A higher level library allowing easy (transparent) access to Python functions from Haskell, and wrapping Haskell functions for Python to access * A way to easily derive conversion functions for user-defined data types/objects. Functions derived in such a way should work well with both low level and high level access libraries * Documentation and a set of examples for all of above Optional goals -------------- These are of lower priority, and might require a fair amount of work. I would like to implement most of them, if technically feasible. If they don't fit into Summer of Code timeframe, I am planning to finish afterwards. * A Python module for accessing functions from Haskell modules without manual wrapping (such wrapping should be already easy thanks to the high level library). It'd be accomplished through GHC api - if it allows it. The Haskell side of the high level library will already support such mode of operation * Extend and refactor the code, to make it support other similar dynamic languages. This is a lot of work, and definitely out of the scope of Summer of Code project, but some design decisions may be influenced by this. Related projects ---------------- They (and quite possibly some others) will be referenced for ideas. * MissingPy Provides a one way, low level binding to Python. Some of the code can be possibly reused, especially data conversion functions. It doesn't seem to export all features, in particular function callbacks are not supported * HaXR XML-RPC binding for Haskell. It could provide inspiration for reconciling Haskell and Python type systems, resulting in a friendly interface * rocaml A binding between Ruby and OCaml

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Haskell job opening
by Chris Smith 31 Mar '08

31 Mar '08

MindIQ Corporation is looking for a Haskell programmer. Details can be found at http://cdsmith.twu.net/jobposting.html. This isn't "officially" announced yet, but it will be in a few days. >From the announcement: Why You Want This Job * You can program in Haskell and get a salary for it. * Work from home. Choose your hours. Travel only if you want to. * Build a new software application from scratch. * Chance to work some (but not entirely) on open source stuff. * Small company, no bureaucracy, but quite stable (around since 1986). * You will be "Lead Software Developer" within a very short time. Hope this is of interest. -- Chris Smith Lead Software Developer, MindIQ Corporation

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[GSoC] Porting HaRe to use the GHC API
by Chaddaï Fouché 31 Mar '08

31 Mar '08

For those unfamiliar with it, HaRe[1] is a refactoring tool in Haskell for Haskell. It already supports a wide range of useful refactoring and have Emacs and Vim bindings. Unfortunately, HaRe currently uses the Programmatica front-end to do his parsing, and Programmatica only knows Haskell 98, which make the tool far less useful for those that use GHC extensions in their programs. Moreover, Haskell' is bound to arrive one day and will encompass some of those extensions. My proposal for the SoC is to port HaRe (its parsing and refactoring) to use the GHC API instead of Programmatica. The advantages are : * GHC API already supports (by definition) all the GHC extensions, HaRe could be used for any Haskell program. * GHC development is unlikely to stop in the foreseeable future, thus HaRe could evolve with the language rather than having to catch up from time to time. * A certain number of refactoring can be added that exploit the GHC extensions * The GHC API is becoming a mature and generally useful tool, having more libraries that use it can't hurt its growth There already has been some works done to port HaRe to the GHC API in 2005 that did not run to completion but the API has evolved for the better since then and using this report [2] and the current API, I hope to complete the port during the summer. I'm familiarizing myself with the GHC API, Programmatica, Strafunski and HaRe and I'll do an internship at the university of Kent for three months in the summer, which should place me in the best situation to do this work. Refactoring is an important feature of the modern IDE, often quoted by the newcomers who search for the best environment to develop in Haskell. HaRe addresses this point but isn't usable for most programs actually written, which is really a shame (consult the list of supported and future refactoring [3] to get a taste of what you're missing...). Comments, reactions ? You can also propose refactoring, if I complete the port early, I'll try to add some refactoring to HaRe. [1] http://www.cs.kent.ac.uk/projects/refactor-fp/hare.html [2] http://www.cs.kent.ac.uk/pubs/2005/2266/ [3] http://www.cs.kent.ac.uk/projects/refactor-fp/catalogue/ -- Chaddaï Fouché, student in M1, majoring in CS, Ecole Normale Supérieure in Lyon.

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Equality constraints in type families
by Hugo Pacheco 31 Mar '08

31 Mar '08

Hi all, I have encoded a type family such that: type family F a :: * -> * and a function (I know it is complicated, but I think the problem is self explanatory): hyloPara :: (Functor (F a), Functor (F d), F d a ~ F a (a,a), F d c ~ F a (c,a)) => d -> (F d c -> c) -> (a -> F d a) -> a -> c hyloPara d g h = g . fmap (hyloPara d g h) . h it all works fine. However, if I change the declaration to (changed F d c for the "supposedly equivalent" F a (c,a)): hyloPara :: (Functor (F a), Functor (F d), F d a ~ F a (a,a), F d c ~ F a (c,a)) => d -> (F a (c,a) -> c) -> (a -> F d a) -> a -> c and I get Occurs check: cannot construct the infinite type: c = (c, a) When generalising the type(s) for `hyloPara' This really messes up my notions on equality constraints, is it the expected behavior? It would be essential for me to provide this definition. Thanks, hugo

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Functional dependencies with Type Classes
by Henning Günther 31 Mar '08

31 Mar '08

Hi, suppose there are two (identical) classes: > class Res a b | a -> b where > getRes :: a -> b and > class Res2 t where > type Member t > getRes2 :: t -> Member t It is easy to automatically make every instance of Res2 an instance of res: > instance Res2 a => Res a (Member a) where > getRes x = getRes2 x However, declaring every instance of Res an instance of Res2 seems impossible, as the following doesn't compile > instance Res a b => Res2 a where > type Member a = b > getRes2 x = getRes x Question is: How to do this? The reason I need it is because I use a library which uses functional dependencies, but my classes shall be type families. Regards, Henning

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