November 2014 - Haskell-Cafe

xs not in scope
by Roelof Wobben 13 Nov '14

13 Nov '14

Hello, Im trying to find several answers to problem1 of the 99 haskell problems. Find the last item in a list. Now im trying to find a guarded solution. So far I have this: last3::[a]-> Maybe a; last3 a | [] = Nothing | ([a]) = Just a | (_:xs) = last3 xs But I see this error messages : src/Main.hs@10:8-10:10Not in scope: xs How to solve this ? Roelof

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Using multiple versions of the Haskell Platform on Windows
by Yitzchak Gale 13 Nov '14

13 Nov '14

The win-hp-path project provides the use-hp command, which makes it easy to switch between different versions of Haskell Platform on Windows. https://github.com/ygale/win-hp-path We are using it for running cabal and GHC in command prompt windows. In particular, we can also use it in build scripts used by the build management team who are not Haskell programmers. Please let me know if you can use this in more complex dev environments, or if you have suggestions about how it could be enhanced to do that. Pull requests are welcome. Thanks, Yitz

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How is maintaining the iCalendar package?
by Alfredo Di Napoli 13 Nov '14

13 Nov '14

Hello fellow Haskellers, anybody knows who is maintaining https://github.com/tingtun/iCalendar ? I have tried to email the maintainer listed on Github, but his activity seems quite low. Posting it here hoping to get a bit more resonance. Alfredo

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open-source project looking for novice help?
by Richard Eisenberg 13 Nov '14

13 Nov '14

I'm teaching an introductory Haskell course this semester (http://cis.upenn.edu/~cis194/fall14/) and am about to assign students their final project. Is there anyone out there who would welcome getting some novice help on an open-source project? You certainly don't need to commit to accept their patch(es), but I know it would be a great experience for some of the students to contribute to the "real" world of Haskell, instead of just doing exercises. You can get an accurate summary of what we've covered by looking at the lecture titles at http://cis.upenn.edu/~cis194/fall14/lectures.html Students will be choosing projects starting next week, and will need to have them completed by Dec. 15 or so. A good project is about 20 hours of work. Thanks! Richard

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Representing record subtypes, sort of.
by Kannan Goundan 13 Nov '14

13 Nov '14

I have an API that, in a language with subtyping, would look like: class FsEntry id: String class FsFile extends FsEntry modified: Date size: Int class FsFolder extends FsEntry owner: String listFolder :: Path -> [FsEntry] createFile :: Path -> FsFile createFolder :: Path -> FsFolder (I'm assuming some way of specifying that FsEntry will only ever have those two subtypes.) How would you represent this in Haskell? My first thought was: newtype FsEntry = FsEntry FsCommon FsExtra data FsCommon = FsCommon { id: String } data FsExtra = FsFile { modified: Date, size: Int } | FsFolder { owner: String } But then I couldn't have precise return types for `writeFile` and `createFolder`. My next attempt was to use a type-parameterized top-level class: data FsCommon = FsCommon { id: String } data FsFileExtra = FsFileExtra { modified: Data, size: Int } data FsFolderExtra = FsFolderExtra { owner: String } data FsEither = FsFile FsFileExta | FsFolder FsFolderExtra newtype FsEntryBase extra = FsEntryBase FsCommon extra type FsEntry = FsEntryBase FsEither type FsFile = FsEntryBase FsFileExtra type FsFolder = FsEntryBase FsFolderExtra 1. This seems complicated. 2. I can't pass an `FsFolder` to a function expecting an `FsEntry`, but maybe that's just the nature of having subtyping and I have to give up on that (which I'm ok with). Any suggestions on how to do this? Thanks!

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IO monads, stream handles, and type inference
by Jeffrey Brown 12 Nov '14

12 Nov '14

In the thread "Precise timing <https://groups.google.com/forum/#!topic/haskell-cafe/sf8W4KBTYqQ>", in response to something ugly I was doing, Rohan Drape provided the following code: import Control.Concurrent import Control.Monad import System.IO import Sound.OSC main = withMax $ mapM_ note (cycle [1,1,2]) withMax = withTransport (openUDP "127.0.0.1" 9000) sin0 param val = sendMessage (Message "sin0" [string param,float val]) pause = liftIO . pauseThread . (* 0.1) note n = do sin0 "frq" 300 sin0 "amp" 1 pause n sin0 "amp" 0 pause n For days I have monkeyed with it, and studied the libraries it imports, and I remain sorely confused. *How can the "a" in "IO a" be a handle?* Here are two type signatures: openUDP :: String -> Int -> IO UDP withTransport :: Transport t => IO t -> Connection t a -> IO a Rohan's code makes clear that openUDP creates a handle representing the UDP connection. openUDP's type signature indicates that its output is an "IO UDP". How can I reconcile those two facts? When I read about the IO type, all sources seem to indicate that "IO a" represents a value of type "a" wrapped in an IO context. For instance, when putting Strings to the screen, one passes around "IO String" values. Until this OSC library, I had never seen the "a" in "IO a" represent a pipe; it had always represented data to be passed *through* a pipe. *Why the long signature?* When I ask for it, GHC provides the following additional type signatures: > :t pause pause :: Double -> transformers-0.3.0.0:Control.Monad.Trans.Reader.ReaderT UDP IO () What's up with that? *What type is note? (and related questions)* GHCI goes on: > :t sin0 sin0 :: (SendOSC m, Real n) => String -> n -> m () > :t note note :: Double -> transformers-0.3.0.0:Control.Monad.Trans.Reader.ReaderT UDP IO () note calls both sin0 and pause. It appears that note's type signature takes pause, but not sin0, into account, but I must be wrong about that. sin0 returns a SendOSC. pause applies liftIO to pauseThread. The result must be a SendOSC too, because sin0 and pause are both called in the same do loop. SendOSC implements these three classes: (Monad (ReaderT t io), Transport t, MonadIO io) => SendOSC (ReaderT t io) Is the liftIO that pause applies to pauseThread, then, the "default" liftIO defined in the MonadIO library? *How to read the "instances" portion of Hackage documentation?* In the Hackage documentation <https://hackage.haskell.org/package/hosc-0.13/docs/Sound-OSC-Transport-Mona…> for the SendOSC type, how should I be reading this line? (Monad (ReaderT t io), Transport t, MonadIO io) => SendOSC (ReaderT t io) I understand the middle two clauses: that io should be of type MonadIO, and t should be of type Transport. The outer two clauses, though, I don't know how to interpret. (I looked at the code <https://hackage.haskell.org/package/hosc-0.13/docs/src/Sound-OSC-Transport-…> and saw nothing that clearly corresponded to that line in the documentation.)

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A Very Simple Type Class Question
by Larry Lee 11 Nov '14

11 Nov '14

Hi I have a very simple problem. I have a class and want to define a function in that class that returns a different instance of the same class. I tried accomplishing this as follows: class A a where f :: A b => a -> b This fails however when I try to instantiate it. For example: instance A String where f x = x I get an error message that makes absolutely no sense to me: src/CSVTree.hs:12:9: Could not deduce (b ~ [Char]) from the context (A b) bound by the type signature for f :: A b => String -> b at src/CSVTree.hs:12:3-9 `b' is a rigid type variable bound by the type signature for f :: A b => String -> b at src/CSVTree.hs:12:3 In the expression: x In an equation for `f': f x = x In the instance declaration for `A String' make: *** [compile] Error 1 Can someone please explain: how I can achieve my goal; and why my code is failing; simply and in plain English. Thanks, Larry

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Hackage’s account creation down?
by Pablo Couto 10 Nov '14

10 Nov '14

Is Hackage’s account creation (at http://hackage.haskell.org/users/register-request) down? I tried it a few times since last night, without luck. This is the message I get: Server error: fd:57: hClose: resource vanished (Broken pipe) Best regards, Pablo

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New Functional Programming Job Opportunities
by Functional Jobs 10 Nov '14

10 Nov '14

Here are some functional programming job opportunities that were posted recently: Senior Software Engineer at McGraw-Hill Education http://functionaljobs.com/jobs/8760-senior-software-engineer-at-mcgraw-hill… Cheers, Sean Murphy FunctionalJobs.com

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list comprehension with multiple generator|targets
by Donn Cave 10 Nov '14

10 Nov '14

I'm guessing this isn't supported, but might be worth asking - can I extend a list comprehension like ['A' | A <- s] to multiple values? Like, data V = A | B | C pv :: [V] -> [Char] pv [] = [] pv (A:x) = 'A':(pv x) pv (B:x) = 'B':(pv x) pv (_:x) = pv x -- can that be a list comprehension, like pv s = [ 'A' | A <- s -- ?? ] thanks, Donn

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