May 2012 - Beginners - Haskell.org

Choosing a function randomly...
by Stuart Hungerford 21 May '12

21 May '12

Hi, This is kind-of related to my earlier question on looking up functions by name. Suppose I have a module with a number of functions with the same signature: scale :: Int -> Int -> Int scale s x = s * x add :: Int -> Int -> Int add a x = a + x ... I'd like to choose and run one of these functions randomly at run time. I can see I could use some kind of case expression: op :: Int -> Int -> Int op p x = case random(1,2) of 1 -> scale p x 2 -> add p x Or some kind of pattern guards: op p x | random(1,2) == 1 = scale p x | otherwise = add p x Although that method won't work as is for more than two choices. Are these methods the most idiomatic way of randomly choosing a function? How hard would it be to use the machinery of the QuickCheck library for this, given it must be doing something similar in test suites? Thanks, Stu

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Choosing a function by (string) name...
by Stuart Hungerford 20 May '12

20 May '12

Hi, Suppose I have a module of functions with identical signatures: module Functions where a :: Int -> [Int] a x = [x + 1, x + 2, x + 3] b :: Int -> [Int] b x = [x + 4, x + 5, x + 6] and in my main function I receive a command line arguments (a string) naming one of the functions in MyModule. I'd like to lookup and use of the functions by name: lookup :: String -> (Int -> [Int]) lookup "a" = MyModule.a lookup "b" = MyModule.b ... Is there a more idiomatic Haskell way of going about this? I can see there's a fundamental tension between a runtime choice of function name expressed as a string and the need for compile-time known function choice. Thanks, Stu

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change to wikibook?
by Ashley Smith 13 May '12

13 May '12

I was wondering if someone could explain this error? I typed in the sample code from the Haskell Beginning Wikibook to define a function for absolute value. I have GHC 7.0.4 for OSX. The error went away when I put parentheses around the negative value, so did the Haskell interpreter think I was entering - and a second argument, namely 10, when the parentheses weren't there? Thank you! *Main> let abs x = if x < 0 then -x else x *Main> abs 5 5 *Main> abs -10 And got the following error <interactive>:1:6: No instance for (Num (a0 -> a0)) arising from the literal `10' Possible fix: add an instance declaration for (Num (a0 -> a0)) In the second argument of `(-)', namely `10' In the expression: abs - 10 In an equation for `it': it = abs - 10

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ANNOUNCE: Haskell Communities and Activities Report (22nd ed., May 2012)
by Janis Voigtländer 12 May '12

12 May '12

On behalf of all the contributors, I am pleased to announce that the Haskell Communities and Activities Report (22nd edition, May 2012) is now available in PDF and HTML formats: http://haskell.org/communities/05-2012/report.pdf http://haskell.org/communities/05-2012/html/report.html Many thanks go to all the people that contributed to this report, both directly, by sending in descriptions, and indirectly, by doing all the interesting things that are reported. I hope you will find it as interesting a read as I did. If you have not encountered the Haskell Communities and Activities Reports before, you may like to know that the first of these reports was published in November 2001. Their goal is to improve the communication between the increasingly diverse groups, projects, and individuals working on, with, or inspired by Haskell. The idea behind these reports is simple: Every six months, a call goes out to all of you enjoying Haskell to contribute brief summaries of your own area of work. Many of you respond (eagerly, unprompted, and sometimes in time for the actual deadline to the call. The editor collects all the contributions into a single report and feeds that back to the community. When I try for the next update, six months from now, you might want to report on your own work, project, research area or group as well. So, please put the following into your diaries now: ========================================= End of October 2012: target deadline for contributions to the November 2012 edition of the HC&A Report ========================================= Unfortunately, many Haskellers working on interesting projects are so busy with their work that they seem to have lost the time to follow the Haskell related mailing lists and newsgroups, and have trouble even finding time to report on their work. If you are a member, user or friend of a project so burdened, please find someone willing to make time to report and ask them to "register" with the editor for a simple e-mail reminder in October (you could point me to them as well, and I can then politely ask if they want to contribute, but it might work better if you do the initial asking). Of course, they will still have to find the ten to fifteen minutes to draw up their report, but maybe we can increase our coverage of all that is going on in the community. Feel free to circulate this announcement further in order to reach people who might otherwise not see it. Enjoy! Janis Voigtlaender <hcar at haskell.org>

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Issue installing reactive-banana-5.0.0.1
by Miguel Negrao 09 May '12

09 May '12

Hi It appears I can’t install reactive-banana-5.0.0.1: cabal install reactive-banana Resolving dependencies... cabal: cannot configure mtl-2.1. It requires transformers ==0.3.* For the dependency on transformers ==0.3.* there are these packages: transformers-0.3.0.0. However none of them are available. transformers-0.3.0.0 was excluded because reactive-banana-0.5.0.1 requires transformers ==0.2.* How can I solve this ? thank you, Miguel

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Haskell as a useful practical 'tool' for intelligent non-programmers
by Nicholas Kormanik 07 May '12

07 May '12

I am not a programmer, and have no intention of becoming one. I'm a stock and options trader. MetaStock is one of the primary programs I use. Other statistical and mathematical programs as well. Very often when some small need arises, I Google-search for a solution. There seems to be any number of freeware utilities out there in cyberland -- and more all the time -- that do pretty much whatever is needed. Additionally, Mathematica (as one example) has a powerful programming language built in. So, my question is: Does it make practical sense to spend time learning Haskell for the purpose of adding it to my assortment of 'tools' -- to quickly do this or that, as the need arises? Is there any better general practical 'tool' (or, if you want, 'programming language') to add to my arsenal. Thanks for your comments and suggestions. Nicholas Kormanik

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A wikipedia episode guide parser. [code review request]
by Thomas Nickson 07 May '12

07 May '12

I have written my first piece of haskell code which I'm, to be honest, not too proud of. I have used the tag soup library to parse and manipulate the wikipedia episode list page such as: http://en.wikipedia.org/wiki/List_of_Weeds_episodes to scrape some of the episode information and place it in to a series of data structures. Nothing too complicated. I know there are probably better libraries out there, I know of HXT and others but it uses arrows and I want to start getting to know the basics before moving on to that. The main point here being that I'm not to interested in the library I used but the code I produced with it. My main concern is in the layout of the code. It needs to keep state and I'm aware of the state monad but is it a good idea to push all of the code in to the state monad and restructure it that way or is that too complex for what I do. I'm not really sure how else I could refactor it but any suggestions on how to improve would be great. The code is here: http://pastebin.com/409SLPre Thanks.

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Signals and external bindings...
by Mike Meyer 05 May '12

05 May '12

I've just finished a QAD project in Python, and expect them to ask me to build the production version of the same project. I'd like to switch to Haskell, but (again, for those who noticed) have some questions I'd like answered. One problem I ran into is that I use Unix signals to control the various processes. In CPython, this causes a problem with extension code because the python signal handler at the C level just note the signal, then wait to run the Python "signal handler" the next time the interpreter. Since my extensions are doing the heavy lifting, they often run for long periods (by which I mean 10s of minutes). Meaning the signal is ignored for long periods. Since I expect to such extensions (the wrappers are available) and want to leave the control mechanisms in place, I'd like to know if I'm going to have similar problems in Haskell. Thanks, <mike -- Mike Meyer <mwm(a)mired.org> http://www.mired.org/ Independent Software developer/SCM consultant, email for more information. O< ascii ribbon campaign - stop html mail - www.asciiribbon.org

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Understanding "Signals, Not Generators"
by Nathan Hüsken 05 May '12

05 May '12

Hey I am currently reading Signals, Not Generators". I am on page 10 where the phantom type era is introduced. I have trouble understanding because I do not understand how the system is to be used. Let us assume I have a signal and an action to print the output: signal :: DSignal era Int action :: Int -> Io () action = putStrLn . show Now, I consume the signal: reactive = consume signal action => reactive :: Reactive era (IO ()) And make an io action: toIO reactive :: IO () My question is: what does this io action do? Thanks!

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Compilation error parsing arithmetic expressions with parsec-3.1.2
by j.romildo＠gmail.com 02 May '12

02 May '12

Hello. I want a parser to parse arithmetic expressions using Parsec-3.1.2. Therefore I wrote the attached Haskell module. But its compilation with ghc-7.4.1 fails with the message: Expr1.hs:22:22: No instance for (Stream s0 m0 Char) arising from a use of `pLexeme' Possible fix: add an instance declaration for (Stream s0 m0 Char) In the second argument of `(<$>)', namely `pLexeme (many1 (satisfy isDigit))' In the expression: read <$> pLexeme (many1 (satisfy isDigit)) In an equation for `pNum': pNum = read <$> pLexeme (many1 (satisfy isDigit)) Any clues on how to fix this? Romildo

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