- Beginners - Haskell.org

Get resource from number of possible sources
by Libor Wagner 29 Jan '13

29 Jan '13

Hi, say I have a number of functions tryGetA, tryGetA', tryGetA'' … with the type tryGetA :: IO (Maybe A), what I want is to get A, trying these function one by one until a get Just a. The first shot code I came up with is using if: getA :: IO A getA = do a <- tryGetA if isJust a then return $ fromJust a else do a' <- tryGetA' if isJust a then return $ fromJust a' … there would be some default value at the end, this looks really ugly so I have tried another shot: firstJust :: a -> [Maybe a] -> a firstJust a [] = a firstJust a (x:xs) = if isJust x then fromJust x else firstJust a xs getA :: IO A getA = do r <- sequence [tryGetA, tryGetA', tryGetA'', …] return $ firstJust "OK" r which is a lot better, but I will appreciate any comment or suggestion. Thanks, Libor

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reorganizing lists
by Bryce Verdier 29 Jan '13

29 Jan '13

Hi All, At the moment I have a list of lists. The inner list is a coordinate, like (x,y) but is [x,y] instead. What I would like to do is group all the x's into one list, and the y's into another. I know I can do this with calling 2 maps on the container, but I would also like to do this in one iteration. Does anyone have any ideas? Thanks in advance, Bryce

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Re: missing ghci, need to install cca
by Franco 28 Jan '13

28 Jan '13

> Hi Franco, > > I've seen this problem when you do not have ~/.cabal/bin in your path. Can > you re-try cabal install Euterpea after adding ~/.cabal/bin in your path. > Also, just check if ~/.cabal/bin has cca executable in there or not. > Regards, > Kashyap I have added the directory to PATH, unfortunately compile fails with the same error: src/Language/Haskell/TH/Instances.hs:175:18: Template Haskell splice illegal in a stage-1 compiler return (LitE (DoublePrimL (toRational d))) cabal: Error: some packages failed to install: CCA-0.1.3 failed during the building phase. The exception was: ExitFailure 1 demo@efikamx:~/Desktop/media/git/Euterpea$ $ echo $PATH /usr/local/bin:/usr/bin:/bin:/usr/local/games:/usr/games:/home/demo/.cabal/bin there is no cca executable in ~/.cabal/bin I will try to see if other packages will do. Thanks! -F

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Re: [Haskell-beginners] missing ghci, need to install cca
by Franco 28 Jan '13

28 Jan '13

> What version of ghc do you have? I thought the 7.6.1 version > of ghci for armhf did support a working ghci. 7.4.1 (I am running wheezy/sid). 7.6.1 is being tested in experimental [1], but debian experimental is quite a nightmare for me, I plan to stay on stable as soon as wheezy is released. [1] http://packages.debian.org/experimental/ghc

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Why Functional Programming Matters <-- this will make you smarter
by Costello, Roger L. 27 Jan '13

27 Jan '13

Hi Folks, I have written a summary of the outstanding paper by John Hughes titled, Why Functional Programming Matters. Hughes says that the purpose of his paper is to demonstrate to the "real world" that functional programming is vitally important. I say that the purpose of his paper is to make you smarter. The paper presents a series of problems. As I proceeded through the problems, I realized that I was on a beautiful journey to the promised land of solving problems by simply combining existing parts. It was a thrilling, mind-expanding journey. Hughes implemented the solutions to the problems using Miranda. I converted them to Haskell. http://www.xfront.com/Haskell/Why-Functional-Programming-Matters.pdf /Roger

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Laziness problem, some code never being executed, when using Control.Monad.State.Strict
by Olivier Boudry 27 Jan '13

27 Jan '13

Hi folks, I'm using the state monad (Control.Monad.State.Strict) to keep track of the state of a monopoly game (Project Euler problem #84). I'm really close to get something that works, but I just have some code with side effects only that does not execute and I struggle force it to execute. I used hpc to discover which part of the code was never run and found lines 93 to 95. In those lines I'm trying to update a visits Map that holds the number of times I've been on a particular cell of the board. What I don't understand is that the code in lines 66 to 68 is almost identical and this one gets executed. The visit Map is used at the very end to retrieve results. 57 execAction :: State Monopoly () 58 execAction = do 59 m <- get 60 let b = board m 61 let s = CL.value b 62 case s of 63 (CC _) -> execCommunityChest 64 (CH _) -> execChance 65 G2J -> execMove gotoJail 66 _ -> do 67 let v = insertWith (+) s 1 (visit m) 68 put m {visit = v}... 80 let c = cc m 81 c' <- execCard c 82 put m {cc = c'} 83 84 execCard :: CardsDeck -> State Monopoly CardsDeck 85 execCard cd = do 86 let cd' = CL.next cd 87 case CL.value cd of 88 Just mv -> do 89 execMove mv 90 return cd' 91 Nothing -> do 92 m <- get 93 let s = CL.value $ board m 94 let v = insertWith (+) s 1 (visit m) 95 put m {visit = v} 96 return cd' I tried to play with some $! to force execution but it never goes deep enough and I don't really know where to place these. I did not copy the whole solution as requested on the Project Euler site. Any help on this would be greatly appreciated, Thanks, Olivier.

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Idiomatic way of working with Either and lists
by Emmanuel Surleau 26 Jan '13

26 Jan '13

Hi, I'm trying to figure out the best way to work with Either and lists. I have a function f which goes: f :: a -> Either SomeError b Now, I'd like to do: applyF :: [a] -> (a -> Either SomeError b) -> Either SomeError [b] That is, map f over a list and return either a list or the first error found. The function should stop at the first error. I have implemented applyF with a fold, but I'm sure there must be something like this already, being a fairly generic pattern. Thanks, Emm

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missing ghci, need to install cca
by Franco 26 Jan '13

26 Jan '13

Hello haskellers, I am trying to install Euterpea [1], a DSL computer music development written in Haskell. Its git repository lies here [2]. Once finished cloning it, i tried running 'cabal install'. Packet CCA-1.0.3 fails to install with errors like > src/Language/Haskell/TH/Instances.hs:175:12: > Template Haskell bracket illegal in a stage-1 compiler > [| D# ($(return (LitE (DoublePrimL (toRational d))))) |] > > src/Language/Haskell/TH/Instances.hs:175:18: > Template Haskell splice illegal in a stage-1 compiler > return (LitE (DoublePrimL (toRational d))) > > cabal: Error: some packages failed to install: > CCA-0.1.3 failed during the building phase. The exception was: > ExitFailure 1 > Euterpea-1.0.0 depends on CCA-0.1.3 which failed to install. I have been warned by the #haskell irc channel on freenode that those errors essentially mean "you don't have ghci installed, so no template haskell for you". Indeed my platform (ARM, running Debian ARMHF) does not have ghci. Question is: do you think there is a workaround to install package Euterpa (or CCA?). Or should I just give up? [1] http://haskell.cs.yale.edu/euterpea-2/ [2] https://github.com/dwincort/Euterpea

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Reply-To fixed for this mailing list
by Kim-Ee Yeoh 25 Jan '13

25 Jan '13

Hey Miguel, Just got word from Benjamin (list owner) that he's fixed it. -- Kim-Ee On Mon, Jan 21, 2013 at 3:26 AM, Kim-Ee Yeoh <ky3(a)atamo.com> wrote: > On Mon, Jan 21, 2013 at 3:07 AM, Miguel Negrao < > miguel.negrao-lists(a)friendlyvirus.org> wrote: > >> A 20/01/2013, às 05:50, Kim-Ee Yeoh escreveu: >> > p.s. Everyone, please "Reply to All" to make sure your email gets to >> the list reflector at haskell.org. Otherwise your responses are private >> to Martin and you lose out on the aspect of community. >> >> I don’t understand: why can’t this mailing automatically set the >> “reply-to” field to beginners(a)haskell.org ? A lot of the other mailing >> lists that I’m subscribed to do that... Is it a conscious decision or a >> technical limitation ? >> > > Excellent question! I believe we're on mailman software and the list admin > is Benjamin Russell: DekuDekuplex(a)Yahoo.com or beginners-owner(a)haskell.org > > I've cc'ed both to see if we can get a response from him. > > It does look as if it's merely a config option: > > http://ccit.mines.edu/Mailman-FAQ#25 > > -- Kim-Ee >

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Can I lift this interestingly typed function into a Maybe?
by Simon Peter Nicholls 24 Jan '13

24 Jan '13

Hi all, My issue is generic Haskell use, but I should provide the specific background. I have some Snap code: eres <- eitherWithDB $ DB.findOne (DB.select [] "someCollection") let maybeVal = either (error "Mongoed") (maybe Nothing fromBson) eres and this works so that "maybeVal" will be of type Maybe MyType. "eres" is either a MongoDB Failure, or a Maybe Document (I find one document, or not). I'd like to convert this Maybe Document to a Maybe MyType without unpacking and repacking the Maybe. Is this possible? The fromBson function, that converts from a Document to an end user type, confuses me due to it's type: fromBson :: Monad<http://hackage.haskell.org/packages/archive/base/4.5.0.0/doc/html/Control-M…> m => Document<http://hackage.haskell.org/packages/archive/bson/0.1.7/doc/html/Data-Bson.h…> -> m a Thanks, Si

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