May 2009 - Beginners - Haskell.org

Empty list Exception
by Chicão Fernandes Junior 24 May '09

24 May '09

Hello! I'm starting programming in haskell and I got some problems running in GHCi (version 6.8.2) a module I've wrote. The following ... --The first string is the name and the second is the user ID, just for test > type User = (String, String, Integer) > > users = [("Marcelo Castro", "1234", 2), > ("Joao Vicente Claudino", "1235", 2), > ("Wilson Brandao", "1236", 2), > ("Humberto Teixeira", "1237", 2), > ("Luiz Gonzaga", "1238", 2), > ("Severino Dias", "1239", 2), > ("Hermeto Pascoal", "1234", 2)] > > checkUser :: User -> [User] -> Bool > checkUser user userList > = if (head (filter (\userParse -> userParse == user) userList)) == > user > then True > else False > > > userRegistration :: User -> [User] -> [User] > userRegistration user list > | checkUser user list = list > | not(checkUser user list) = (user : list) > | otherwise = [] > -- ____________________________________________________________________________ Francisco Borges "Chicão" Junior .......................................................................................... http://pedepinico.blogspot.com .......................................................................................... "Quem de boa vontade carrega o difícil, também carrega o menos difícil..." Lao Tsé - Tao Te Ching

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QuickCheck
by Thomas Friedrich 24 May '09

24 May '09

Hallo everyone, I am currently playing around with the QuickCheck library. I came across the Test.QuickCheck.Batch module a couple of times. I don't seem to have this module installed? Where can I get it and how do I install it? I also would like to invoke the following in ghci. generate 10 (System.Random.mkStdGen 1) arbitrary :: [Int] However, the function `generate` doesn't get imported when importing QuickCheck, and I just cannot find out which module this one would be in. I came to the conclusion that `generate` is just not the best word for a google search :( Cheers, Thomas

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Catching Network Connection Error
by aditya siram 23 May '09

23 May '09

Hi all, I am trying to Network.HTTP to connect to a website and catch the error if the network is down. Here's what I have so far: data Errors = DocumentParseError | WebsiteUnreachableError | ISBNNotFoundError respHTML' :: ISBN -> IO (Either Errors (IO String)) respHTML' isbn = do rsp <- simpleHTTP (getRequest $ " http://isbndb.com/api/books.xml?results=details&access_key=" ++ key ++ "&index1=isbn&value1=" ++ isbn :: Request_String) return $ Right $ getResponseBody rsp `E.catch` (\(e :: E.SomeException ) -> return $ Left $ WebsiteUnreachableError) I get the following error when I load it into GHCI: Couldn't match expected type `[Char]' against inferred type `Either Errors b' Expected type: IO String Inferred type: IO (Either Errors b) In the expression: return $ Left $ WebsiteUnreachableError In the second argument of `E.catch', namely `(\ (e :: E.SomeException) -> return $ Left $ WebsiteUnreachableError)' Failed, modules loaded: none. Control.Exception.catch has the following signature : catch :: forall a. IO a -> (IOError -> IO a) -> IO a I would expect that the handler function needs to return an IO (Either Errors (IO String)). Why is it trying to return an IO String?

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Searching Maybe lists
by aditya siram 23 May '09

23 May '09

Hi all, I would like to define a function that takes a list and a function that evaluates each member of the list to a Maybe value and output the first element in the list that evaluates to 'Just y', or 'Nothing' once the list has been completely processed. So something like: findMaybe :: [a] -> (a -> Maybe b) -> Maybe b The problem is that I don't want it to go through the entire list, but short-circuit when it hits a 'Just ...'. So far I have: orMaybe :: Maybe a -> Maybe a -> Maybe a orMaybe m1 m2 = case (m1,m2) of (_, Just a) -> Just a (Just a, _) -> Just a _ -> Nothing findMaybe :: [a] -> (a -> Maybe b) -> Maybe b findMaybe as f = foldr (\a sofar -> sofar `orMaybe` (f a)) Nothing as 'findMaybe', as far as I can tell, traverses the entire input list which is undesirable for long lists. How can I fix it? Curiously, the regular 'Data.List.find' function that applies a Boolean predicate to each member of the list also seems to first traverse the entire list using 'filter' and then grabs the head of the result. Thanks ... -deech

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HaL4: Haskell-Treffen in Halle/Saale, 12. Juni 2009 - Programm und Anmeldung
by unknown＠example.com 20 May '09

20 May '09

Kindly excuse the German noise, please ... Liebe Haskell-Freunde! Das Programm fuer unser Haskell-Treffen steht jetzt so gut wie fest und man kann sich online anmelden. --------------------------------------------- HaL4 : Haskell - Tutorial + Workshop + Party am Freitag, dem 12. Juni 2009, in Halle/Saale --------------------------------------------- Das traditionsreiche HaL-Treffen bietet eine gute Mischung von Haskell-bezogenen Themen aus Forschung, Anwendung und Lehre mit vielen Möglichkeiten zu Diskussion und Unterhaltung bei der anschließenden Party. Der Workshop wird in diesem Jahr ergänzt durch Tutorien für Haskell-Ein- und Umsteiger. Diesmal findet das Treffen in Halle/Saale im Institut für Informatik der Martin-Luther-Universität Halle-Wittenberg statt. Wir bieten: 10 Uhr: Entwurfsmuster vom Kopf auf die Fuesse gestellt: data, fold und laziness statt Kompositum, Visitor, Iterator. Johannes Waldmann, HTWK Leipzig. 11 Uhr: CAL/openquark: (openquark.org) Haskell + Java => CAL, eine praktische Einfuehrung (Eclipse als CAL-IDE, Excel lesen, typsicheres SQL = LINQ fuer Java?) Alf Richter, iba Consulting, Leipzig 12 Uhr: Wie man das Semikolon ueberlaedt: Code-Strukturierung und -Wiederverwendung durch Monaden 15 Uhr: Hayoo! Haskell API Search Timo Huebel, FH Wedel 16 Uhr: Haskell als reine Spezifikationssprache Baltasar Trancon y Widemann, Universit Bayreuth 17 Uhr: Funktional-logische Programmierung mit Curry Jan Christiansen, Universitaet Kiel 19 - 22 Uhr: Grillparty Wir freuen uns auf rege Teilnahme sowie spannende Vorträge mit heißen Diskussionen und bitten um Anmeldung bis zum 31. Mai. Weitere Informationen auf http://www.iba-cg.de/hal4.html Beste Gruesse Henning Thielemann

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"computation", "action"
by Michael P Mossey 19 May '09

19 May '09

I've seen the following language used: - "computations" - "actions" - "effects" - "context" (For example, a functor is a context, and fmap applies a function to an object in that context without changing the context.) One words that seem to come up in Haskell are "algorithm" and "procedure." For example, a monad is sometimes called a "computation" or an "effect", but I've not seen it called an "algorithm." Can someone elaborate on these terms and why they are used? What is the difference between a computation and an algorithm? The word "effect" has a fairly obvious meaning (like an IO side-effect) but I suspect there's more to it. Why is the word "context" important, and when it is more appropriate than "container"? thanks, Mike

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Intervals?
by Nathan Holden 19 May '09

19 May '09

I have been trying to optimize some research I was doing in my spare time. To spare complexity, it involves music-- I have a class, defining each note in an octave, and I want to write a function that will convert between two notes and an interval-- so, for instance: Foo As {- A sharp-- can't use # -} B = 1 because in the standard scale, A# and B are one semitone apart. My question, however, boils primarily down to this: Is there a way to make Foo :: Note -> Note -> Int Foo a b = a-b work?

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Re: [Haskell-beginners] Intervals?
by Nathan Holden 19 May '09

19 May '09

Yeah, it would. But I always write code like that, it feels ungainly... writing super-long, case-by-case instance declarations (So that I can add in one little caveat at the end, or something), and stuff. Makes me feel like I must be dumb! On Tue, May 19, 2009 at 2:54 PM, Derek Gladding <derek(a)ebollocks.net> wrote: > Nathan Holden wrote: > >> [snip] >> My question, however, boils primarily down to this: Is there a way to make >> >> Foo :: Note -> Note -> Int >> Foo a b = a-b >> >> work? >> > > Would a function > > Bar :: Note -> Int > > make writing Foo easier? > > - Derek >

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Haskell vs Clean (speed)
by Daniel Carrera 19 May '09

19 May '09

Hi, I can't subscribe to Haskell-Cafe, so I'll post here. I found something interesting. "General wisdom" is that Clean (or OCaml) is faster than Haskell. The claim is often followed by a link to the Debian shootout. But on closer inspection, I question this conclusion. The Debian shoot out actually has four sets of benchmarks: 1) Intel 32-bit one core. 2) Intel 32-bit quad-core. 3) Intel 64-bit one core. 4) Intel 64-bit quad-core. It turns out that Clean is only faster for (1). For the others, Haskell is faster. Here I compare Haskell, Clean, OCaml, Lisp SBCL, C# Mono and Fortran because they are all in the same ball mark: 32-bit sing core [1]: Lisp, Fortran, Clean, Haskell, C# Mono. 32-bit quad-core [2]: Haskell, C# Mono, Lisp, Clean, Fortran. 64-bit sing core [3]: Fortran, OCaml, Haskell, Clean, C# Mono, Lisp. 64-bit quad-core [4]: Haskell, OCaml, Lisp, C# Mono, Fortran, Clean. Notes: * The order is "fast language first". * There are no results for OCaml for 32-bit. * "Lisp" is "List SCBL" whatever that is. Tentative conclusions: 1) Haskell makes very good use of multiple cores. It smokes Clean. 2) For single core, they are neck and neck. Whether Clean is faster depends non the architecture. What do you think? Daniel. P.S. [1]http://shootout.alioth.debian.org/u32/benchmark.php?test=all&lang=all&d=d… [2]http://shootout.alioth.debian.org/u32q/benchmark.php?test=all&lang=all&d=… [3]http://shootout.alioth.debian.org/u64/benchmark.php?test=all&lang=all&d=d… [4]http://shootout.alioth.debian.org/u64q/benchmark.php?test=all&lang=all&d=…

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Unable to Cabal Upgrade
by aditya siram 15 May '09

15 May '09

Hi all, I'm unable to cabal upgrade with the following error: Resolving dependencies... cabal: dependencies conflict: ghc-6.10.1 requires process ==1.0.1.1 however process-1.0.1.1 was excluded because ghc-6.10.1 requires process ==1.0.1.0 I am currently running ghc 6.10.1 and process 1.0.1.1. thanks ... deech

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