February 2007 - Haskell-Cafe

process
by h. 23 Feb '07

23 Feb '07

Hello, I need to interact with some other program, and wrote the following code: module Main where import System.Process import System.IO main :: IO () main = do putStrLn "Running proc9..." (inp,out,err,pid) <- runInteractiveProcess "prog1" [] Nothing Nothing hSetBuffering inp LineBuffering hSetBuffering out LineBuffering hSetBuffering err LineBuffering hPutStrLn inp "1" a <- hGetLine out hPutStrLn inp a a <- hGetLine out waitForProcess pid putStrLn a But it does not work as I expected. As long as there is no need to put some input after having received some output it is no problem, but real interaction seems not possible. Is there a solution to work with the Handles as with stdin,... ? using ghc 6.6 on Windows Thanks in advance for your help.

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exists . a psuedo-standard non-empty list module
by oleg＠pobox.com 23 Feb '07

23 Feb '07

Nicolas Frisby wrote: > Some of the code from the previous wiki link, type-level decimal > numbers I would rather advice against type-level decimal numbers, if we are looking at a lightweight solution. The [complete] code at http://www.haskell.org/haskellwiki/Non-empty_list is Haskell98! It also provides for a head and tail functions -- which are total and so raise no errors. Incidentally, inserting NList into the existing Safe.List does not seem like a good match as NList critically relies on being in a separate module with a limited export. NList constitutes a security kernel, whose invariants are to be described explicitly (and hopefully, automatically proven at some point). For that reason, it is beneficial to keep module NList separate and compact. It could of course be placed within Safe.* hierarchy, etc -- as a separate module. Neil Mitchell wrote: > Fortunately the problem of pattern match errors is being tackled by at > least two projects: > > * Catch: http://www-users.cs.york.ac.uk/~ndm/projects/catch.php > * ESC-Haskell: http://www.cl.cam.ac.uk/~nx200/research/escH-hw.ps I would submit the approach of the lightweight static capabilities (cf above Wiki link) to be counted as the third project in that area. The latter has an advantage that it is available in Haskell right now and requires no extra tools.

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exists . a psuedo-standard non-empty list module
by Nicolas Frisby 23 Feb '07

23 Feb '07

I don't particularly like using fromJust or head, and there's been plenty of discussion on those issues. For the cases where it makes good sense to do so, I'm about to write a module for non-empty lists that looks to strike a balance between usability and static checks. But before I re-invent the wheel, I was wondering if there's already a popular module along those lines. I would also appreciate references to your favorite discussion about uses of head and other unsafe functions or various Oleg posts showing how they're all unnecessary. I'll find these on my own, but I would also like to know which ones strike a chord with the community. Thanks for your time. -Nick

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Map list of functions over a single argument
by Paul Moore 23 Feb '07

23 Feb '07

I'm after a function, sort of equivalent to map, but rather than mapping a function over a list of arguments, I want to map a list of functions over the same argument. The signature would be [a -> b] -> a -> [b], but hoogle didn't come up with anything. It seems like an obvious analogue of map, so I'm pretty sure I'm missing something (otherwise, I'd just write it myself, and not bother :-)) Can anyone point me in the right direction? Thanks, Paul.

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Re: [Haskell-cafe] TFP 2007: Registration and Program
by TFP 2007 23 Feb '07

23 Feb '07

>> >> Dear Colleagues, >> >> You may now resgister for TFP 2007! TFP 2007 will be held April 2-4, >> 2007 in New York City, USA. > April 2 is the first night of Passover. This is not one of those Your point is well taken. It is very unfortunate that the overlap you have pointed out exists. The organizing committee had to consider _many_ variables including, among others, the availability of a venue and of hotels for participants in NYC. Sometimes the logistics of a situation force your hand. ************************************************************************************ Dr. Marco T. Morazan TFP 2007 Program Committee Chair http://cs.shu.edu/tfp2007/

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Prime finding
by Ruben Zilibowitz 23 Feb '07

23 Feb '07

I see that there has been some discussion on the list about prime finding algorithms recently. I just wanted to contribute my own humble algorithm: primes :: [Integer] primes = primesFilter 1 [2..] primesFilter :: Integer -> [Integer] -> [Integer] primesFilter primorial (n:ns) | (gcd primorial n == 1) = n : (primesFilter (primorial*n) ns) | otherwise = primesFilter primorial ns Comparing it to some of the algorithms in: http://www.haskell.org/pipermail/haskell-cafe/2007-February/022765.html It seems to perform reasonably well. It also has the advantage of being quite short. Ruben

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HList, hOccurs and MonadReader
by Marc Weber 23 Feb '07

23 Feb '07

Having the module given below I can't see why using printAndRerun l1 printAndRerun2 l2 but not printAndRerun l1 printAndRerun l2 ? They only differ in their name. Can you point me in the right direction? ------------------------------------------------ {-# OPTIONS -fglasgow-exts #-} {-# OPTIONS -fallow-undecidable-instances #-} {-# OPTIONS -fallow-incoherent-instances #-} {-# OPTIONS -fallow-overlapping-instances #-} module HListTest.MonadReader where import HList hiding (liftIO) import Monad import Control.Monad.Reader printAndRerun = do v <- liftM hOccursFst ask liftIO $ print (v :: Int ) v2 <- liftM hOccursFst ask return (v2 :: String) printAndRerun2 = do v <- liftM hOccursFst ask liftIO $ print (v :: Int ) v2 <- liftM hOccursFst ask return (v2 :: String) a = (2 :: Int) b = "str" l a b = HCons a (HCons b HNil) --- l1 :: ( HCons Int (HCons String HNil)) l1 = l a b l2 = l b a printBoth f l = runReaderT f l >>= print printBoth2 l = do print (hOccurs l :: Int) print (hOccurs l :: String) hlistMonadReaderTest = do print "hlistMonadReaderTest" printBoth printAndRerun l1 -- printBoth printAndRerun l2 -- < this doesn't work but the next line ? Where is the difference between printAndRerun printAndRerun2 ? printBoth printAndRerun2 l2 -- ^ printBoth2 l1 -- here is no trouble.. printBoth2 l2 main = hlistMonadReaderTest ------------------------------------------------ when not commenting the line above I'm getting this error: HListTest/MonadReader.hs|35| 26: Couldn't match expected type `Int' against inferred type `[Char]' Expected type: HCons Int (HCons [Char] HNil) Inferred type: HCons [Char] (HCons Int HNil) In the second argument of `printBoth', namely `l2' In the expression: printBoth printAndRerun l2 Thanks Marc

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functional database queries
by Henning Thielemann 22 Feb '07

22 Feb '07

At http://www.haskell.org/hawiki/HaskellDbTutorial it is described, how database queries can be modelled with a monad. However, I wonder if this is also possible without monads. Say, writing "DB.map col1 $ DB.filter (\row -> col2 row == 10+2) myTable" for "SELECT col1 FROM MyTable where col2 = 10+2"

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TFP 2007: Registration and Program
by TFP 2007 22 Feb '07

22 Feb '07

Dear Colleagues, You may now resgister for TFP 2007! TFP 2007 will be held April 2-4, 2007 in New York City, USA. Our invited speaker is John McCarthy, Stanford University. Further details can be found at our homepage: http://cs.shu.edu/tfp2007/ . You may register at: http://cs.shu.edu/tfp2007/registration.html . The registration deadline is March 2, 2007 (11:59 p.m. EST). Accomodations information may be found at: http://cs.shu.edu/tfp2007/accomodations.html . We kindly remind you that the deadline to make a hotel reservation at the guaranteed rates offered to TFP 2007 participants is also quickly approaching. We are proud to announce our program of accepted talks: Unifying Hybrid Types and Contracts Jessica Gronski and Cormac Flanagan A Dual Semantics for the Data Description Calculus Yitzhak Mandelbaum, Kathleen Fisher, and David Walker A Metalanguage for Structural Operational Semantics Matthew Lakin and Andrew Pitts An Arrow Based Semantics for Interactive Applications Peter Achten, Marko van Eekelen, Maarten de Mol, and Rinus Plasmeijer Dependent Types: Easy as Pie Dimitrios Vytiniotis and Stephanie Weirich Constructing Correct Circuits -- Hardware Modelling with Dependent Types Edwin Brady, James McKinna, and Kevin Hammond Why Would Extensible Dependent Types Matter Pablo Nogueira and Bruno Oliveira Bytecode Verification for Haskell Robert Dockins and Samuel Z. Guyer UnreadTVar: Extending Haskell Software Transactional Memory for Performance Nehir Sonmez, Cristian Perfumo, Srdjan Stipic, Adrian Cristal, Osman S. Unsal, and Mateo Valero A New Functional Implementation of Grover's Fast Search Algorithm Justin Stallard and Murray Gross An Inference Algorithm for Guaranteeing Safe Destruction Manuel Montenegro, Ricardo Peña, and Clara Segura Hierarchical Master/Worker Skeletons Jost Berthold, Mischa Dieterle, Rita Loogen, and Steffen Priebe Property Directed Generation of First-Order Test Data Fredrik Lindblad Refactoring for Comprehension Gustavo Villavicencio Towards a Box Calculus for Hume Gudmund Grov and Greg Michaelson Scaled Regression: A Refinement of Primitive Recursion Daniel Leivant Equality-Based Uniqueness Typing Edsko de Vries, Rinus Plasmeijer, and David Abrahamson Lightweight Static Resources: Sexy Types for Embedded and Systems Programming Oleg Kiselyov and Chung-chieh Shan Space-Efficient Gradual Typing David Herman, Aaron Tomb, and Cormac Flanagan Use-Based Reference of Polymorphism Dave King and John Hannan Designing a Generic Graph Library Using ML Functors Sylvain Conchon, Jean-Christophe Filliatre, and Julien Signoles The SCIence Joint Research Activity Kevin Hammond, Dana Petcu, Phil Trinder, Abdallah Al Zain, Steve Linton, and Greg Michaelson Generic and Index Programming Jeremy Gibbons, Meng Wang, and Bruno C d. S. Oliveira The AHA Project Marko van Eekelen, Olha Shkaravska, Ron van Kesteren, Bart Jacobs, Sjaak Smetsers, and Erik Poll Studying Helium Program Bahaviour with the Neon Library Jurriaan Hage and Peter van Keeken Design and Implementation of JFP Hao Xu Hop Client-Side Compilation Florian Loitsch Adaptive High-Level Scheduling in a Generic Parallel Runtime Environment Jost Berthold, Abyd Al-Zain, and Hans-Wolfgang Loidl Bundles Pack Tighter than Lists Francisco Lopez-Fraguas, Juan Rodriguez-Hortala, and Jaime Sanchez-Hernandez Model-Based Testing of Thin-Client Web Applications and Navigation Input Pieter Koopman, Peter Achten, and Rinus Plasmeijer We look forward to seeing you at TFP 2007! Cheers, Marco ************************************************************************************ Dr. Marco T. Morazan TFP 2007 Program Committee Chair http://cs.shu.edu/tfp2007/

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Haskell vs Ruby as a scripting language
by Joel Reymont 22 Feb '07

22 Feb '07

Is anyone using Haskell as a scripting language in their app? I'm thinking of viable it would be to embed ghc in a Mac (Cocoa) app. TextMate [1] uses Ruby as the extension language and quite successfully at that. Everybody loves Ruby since it's simple. I need a trading systems language and I'm afraid the combination of ... everything that makes Haskell otherwise great will prove a drawback in my case. What do you think? Thanks, Joel [1] http://macromates.com/ -- http://wagerlabs.com/

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