September 2009 - Beginners

How to import the Data.Char library in Hugs?
by Benjamin L.Russell 18 Sep '09

18 Sep '09

My apologies if this is an extremely elementary question, but I am having difficulties in importing the Data.Char library in Hugs. In GHCi, the command "import Data.Char" works correctly, as follows: --8<---------------cut here---------------start------------->8--- GHCi, version 6.10.3: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer ... linking ... done. Loading package base ... linking ... done. ___ ___ _ / _ \ /\ /\/ __(_) / /_\// /_/ / / | | GHC Interactive, for Haskell 98. / /_\\/ __ / /___| | http://www.haskell.org/ghc/ \____/\/ /_/\____/|_| Type :? for help. Prelude> import Data.Char Prelude Data.Char> --8<---------------cut here---------------end--------------->8--- However, in Hugs, the same command fails with an error, as follows: --8<---------------cut here---------------start------------->8--- __ __ __ __ ____ ___ _________________________________________ || || || || || || ||__ Hugs 98: Based on the Haskell 98 standard ||___|| ||__|| ||__|| __|| Copyright (c) 1994-2005 ||---|| ___|| World Wide Web: http://haskell.org/hugs || || Bugs: http://hackage.haskell.org/trac/hugs || || Version: 20051031 _________________________________________ Haskell 98 mode: Restart with command line option -98 to enable extensions Type :? for help Hugs> import Data.Char ERROR - Syntax error in expression (unexpected keyword "import") Hugs> --8<---------------cut here---------------end--------------->8--- Does anybody know how to import the Data.Char library in Hugs? -- Benjamin L. Russell -- Benjamin L. Russell / DekuDekuplex at Yahoo dot com http://dekudekuplex.wordpress.com/ Translator/Interpreter / Mobile: +011 81 80-3603-6725 "Furuike ya, kawazu tobikomu mizu no oto." -- Matsuo Basho^

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Using state inside quickcheck generators
by Alexander MacDonald 17 Sep '09

17 Sep '09

Hi guys, I¹ve been using quickcheck for a while and I¹m having no problem making generators for my data types, however recently I stumbled upon a situation where I need to maintain State inside the quickcheck generator functions. Here's a simple example, imagine I've got a basic language which only has two features, variable definitions and assignments: type Script = [Statement] data Name = Name String deriving (Show,Eq) data Statement = Def Name Int | Assign Name Name deriving Show Generating random scripts is easy enough, just make each type an instance of Arbitrary with a suitable generator function, however I want to make sure that the generated scripts are syntactically valid. In this case that means no "Name" can be used in an "Assign" statement unless it has previously been defined by a "Def" statement. My knowledge of using monads is somewhat basic so I'm struggling to figure out how to use the State monad with QuickCheck. I've pasted my current effort on the web: http://hpaste.org/fastcgi/hpaste.fcgi/view?id=9412 If anyone could spare a moment to glance at my code and give me a push in the right direction that would be much appreciated! Thanks, Alex Mac

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Re: [Haskell-beginners] Maybe, Either
by Michael P Mossey 14 Sep '09

14 Sep '09

Michael Snoyman wrote: > Can you give me a better idea of what you mean by catching errors? That > could mean a lot of things. > > Michael Hi Michael, I mean that I need my software not to exit the program on an error condition, but have a higher level "catch" of that condition which handles it gracefully. I'm writing a musical score editor for personal use in my spare hobby time. Because it's just a bit of spare time, I can't make a promise my software won't have bugs. When I'm working on a score, I don't want to lose my work when an error occurs, such as internal errors that violate the invariants I've established in my data structures. So any action that results in an error should trigger a message box that says "such-and-such error" and otherwise leave the data unchanged. Then I can save my work and attempt to debug the problem. Thanks, Mike

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parse error on input
by John Moore 13 Sep '09

13 Sep '09

Hi, Can anyone explain what I'm doing wrong. I'm learning Haskell using the book real world haskell. It tells me to load programs and update cabal using '$' symbol but everytime I enter it tells me parse error on input '$'. The programs are taken from the book so there nothing wrong with them.What amI misssing? Regards John

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hide Perl code inside Haskell program
by Hong Yang 12 Sep '09

12 Sep '09

I was trying to hide Perl code inside Haskell program like the following: > module Main where > import System.Environment (getArgs) > import System.Cmd > main = do > args <- getArgs > rawSystem pl args > where pl = "#!/usr/bin/perl\n" ++ > "print \"testing ...\n\";" The program can run, but generates no output at all. Can someone tell me any other tricks which might be missing? (Do we need to use pipe here?) Thanks, Hong

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Parsec 2 and Parsec 3
by John Velman 11 Sep '09

11 Sep '09

I've just started picking up Haskell where I left off a couple years ago when I switched from Linux to OS X (and some other things interfered with continuing my Haskell projects). I was using Parsec at the time -- must have been Parsec 2. When I loaded HOC it seemed to depend on Parsec 3, so I installed Parsec 3. Now -- suppose I want to use Parsec 2 for my own work. Is there a reasonably easy way to install Parsec 2 as, say Parsec2 and have both? Thanks, John Velman

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problem running yi on OS X
by Richard Talley 09 Sep '09

09 Sep '09

I'm running OS X 10.5.8 on Intel. I installed GHC 6.10.4 and the Haskell Platform 2009.2.0.2 with no problem (thanks to everybody who made this so easy on OS X). The yi editor seems very interesting, so I installed it with 'cabal install yi' (after running 'cabal update'). That went OK (with some warnings about deprecated GHC features) and I was able to run yi which I found in ~/.cabal/bin Pressing h for help gave me a sample configuration file for yi which I edited to use cua keybindings and the (experimental) cocoa gui. Now when I tried to run yi again, I got this: Recompiling custom yi Launching custom yi: "/Users/rbtalley/.yi/yi-i386-darwin" yi: /Users/rbtalley/.yi/yi-i386-darwin: executeFile: does not exist (No such file or directory) Any suggestions? -- Rich -- Dealing with failure is easy: Work hard to improve. Success is also easy to handle: You've solved the wrong problem. Work hard to improve. -- Alan Perlis

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Problems defining a type with a multiplication function
by Amy de Buitléir 09 Sep '09

09 Sep '09

I'm trying to define a Quaternion, which is sort of a four-element vector with special rules for multiplication. ----- Quaternion.hs ----- data Quaternion = Quaternion Double Double Double Double deriving (Show) (*) :: (Quaternion a) => a -> a -> a (*) (Quaternion w1 x1 y1 z1) (Quaternion w2 x2 y2 z2) = Quaternion w x y z where w = w1*w2 - x1*x2 - y1*y2 - z1*z2 x = x1*w2 + w1*x2 + y1*z2 - z1*y2 y = w1*y2 - x1*z2 + y1*w2 + z1*x2 z = w1*z2 + x1*y2 - y1*x2 + z1*w2 ----- end code ----- When I try to load this into ghci, I get: Quaternion.hs:6:13: Ambiguous occurrence `*' It could refer to either `Main.*', defined at Quaternion.hs:5:0 or `Prelude.*', imported from Prelude ... and lots more messages like that. I understand roughly what the message means, but I don't know how to tell it that when I use "*" within the definition, I just want ordinary multiplication. Thanks in advance for any help! Amy

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number formatting with locale
by Dmitry Simonchik 08 Sep '09

08 Sep '09

Hi all haskell lovers I'm trying to print out Double in russian locale (we use "," as decimal separator insetad of "."). I'm trying to set locale with System.Locale.SetLocale like setLocale LC_ALL $ Just "ru_RU.UTF-8" this returns Just "ru_RU.UTF-8", so it seems that function call succeeded, but when I call show 20.2 it just prints 20.2 and not desired 20,2 Can anyone please help?

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Urgent: Defining Momory data Types in Haskell
by Akshay Dave 08 Sep '09

08 Sep '09

Hi All, I am stuck in converting the transition semantics in Haskell. Please let me know how to define memory data types in Haskell( like we define pointer in C). Prompt help would be greatly appreciated. Sincerely, Akshay _________________________________________________________________ With Windows Live, you can organize, edit, and share your photos. http://www.windowslive.com/Desktop/PhotoGallery

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