March 2008 - Haskell-Cafe

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 30 Mar '08

30 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 30 Mar '08

30 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 30 Mar '08

30 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 30 Mar '08

30 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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Monad instance for Data.Set
by oleg＠okmij.org 30 Mar '08

30 Mar '08

The following code solves exactly the problem of implementing (restricted) MonadPlus in terms of Data.Set: http://okmij.org/ftp/Haskell/DoRestrictedM.hs The code is written to demonstrate the do-notation. We write the monadic code as usual: > test1s_do () = do > x <- return "a" > return $ "b" ++ x and then instantiate it for Maybe String: > test1sr_do :: Maybe String > test1sr_do = unRM $ test1s_do () > -- Just "ba" or for Data.Set: > test2sr_do :: Set.Set String > test2sr_do = unSM $ test1s_do () > -- fromList ["ba"] It seems GHC 6.10 will support the do-notation even for the generalized monads.

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compilation succeeds -- execution fails
by Jason Dusek 30 Mar '08

30 Mar '08

I have a program here: https://svn.j-s-n.org/public/haskell/cedict currently at revision 302, which compiles okay but I can't get it to work. I'm using the FFI to take a (currently small) array and translate it into a Map. It compiles fine and loads fine -- but it doesn't run fine: GHCi, version 6.8.2: http://www.haskell.org/ghc/ :? for help Loading package base ... linking ... done. Prelude> import Data.Char.CEDICT Prelude Data.Char.CEDICT> traditional 'a' Loading package array-0.1.0.0 ... linking ... done. Loading package containers-0.1.0.1 ... linking ... done. Loading package parsec-2.1.0.0 ... linking ... done. Loading package utf8-string-0.2 ... linking ... done. Loading package cedict-0.1.1 ... linking ... <interactive>: unknown symbol `___stginit_cedictzm0zi1zi1_DataziCharziCEDICTziMatter_' ghc-6.8.2: unable to load package `cedict-0.1.1' Prelude Data.Char.CEDICT> I'm on a Mac -- Leopard. A whole bunch of things *could* be wrong -- I'd appreciate some help in narrowing the list. -- _jsn

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Re: [Haskell-cafe] Usage of Read Class
by Ariel J. Birnbaum 30 Mar '08

30 Mar '08

> > <interactive>:1:1: > > Ambiguous type variable `a' in the constraint: > > `Read a' arising from a use of `read' at <interactive>:1:1-4 > > Probable fix: add a type signature that fixes these type variable(s) > > What the error message says is, it doesn't know which type to read to. > read has the type Read a => String -> a, and unless you tell it what > 'a' is going to be, how would it know? If you say '(read.show) x :: > Color' it should work just fine. Just to sharpen the point: read cannot know from the string it processes which type to deliver. For example, try the following at the ghci prompt: read "3" -> Yields the same error as above read "3" + 3 -> 6 read "3" + 3.3 -> 6.3 read "3.3" + 3.3 -> 6.6 read "3.3" + 3 -> Error: no parse What's happening is that the type of the expression is determined by the literal on the _right_side_ of the + sign, and not by the contents of the string on the left side. The last example is a bit tricky. ghci sees 3 on the rhs, which can be of any type in the Num class (as opposed to 3.3, which has to be of a Fractional type). If unresolved, it will default to Integer (or Int?), and try to parse "3.3" as such -- which will of course fail. -- Ariel J. Birnbaum

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Usage of Read Class
by Simeon Mattes 30 Mar '08

30 Mar '08

Hello, I would like to ask something that results in when I have the following commands data Color = Red | Green | Blue | Indigo | Violet deriving (Enum,Show,Read) (read.show) x <interactive>:1:1: Ambiguous type variable `a' in the constraint: `Read a' arising from a use of `read' at <interactive>:1:1-4 Probable fix: add a type signature that fixes these type variable(s) I also receive the same message with the following: *Test > let x = Branch (Branch (Branch (Leaf 'a') (Leaf 'b')) (Leaf 'c') ) (Leaf 'd') *Test > x <<<'a'|'b'>|'c'>|'d'> *Test > (read.show) x <interactive>:1:1: Ambiguous type variable `a' in the constraint: `Read a' arising from a use of `read' at <interactive>:1:1-4 Probable fix: add a type signature that fixes these type variable(s) data Tree a = Leaf a | Branch (Tree a) (Tree a) --type ShowS = String -> String showsTree :: (Show a) => Tree a -> ShowS showsTree (Leaf x) = shows x showsTree (Branch l r) = ('<':).showsTree l . ('|':) . showsTree r . ('>':) --type ReadS a = String -> [(a,String)] readsTree :: (Read a) => ReadS (Tree a) readsTree s = [(Branch l r, x) | ("<",t) <- lex s, (l,u) <- readsTree t, ("|",v) <- lex u, (r,w) <- readsTree v, (">",x) <-lex w] ++ [(Leaf x, t) | (x,t) <- read s] instance Show a => Show (Tree a) where showsPrec _ x = showsTree x instance Read a => Read (Tree a) where readsPrec _ s = readsTree s Why is this happen? Maybe an example with a simple application of the Class Read would be helpful. Thanks -- View this message in context: http://www.nabble.com/Usage-of-Read-Class-tp16381441p16381441.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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