- Haskell-Cafe - Haskell.org

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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Test Framework, HUnit
by spopejoy 10 Nov '14

10 Nov '14

Hi, I started using Test Framework following the example of the lens library, specifically using Test.Framework.TH. It looks really nice, however I noticed that the TH project hasn't had any activity in 2 years ... plus the whole test framework project is very light on docs. I've been happy with straight HUnit, but I'm wanting something more structured to bring others into the project I'm working on. 1) Are there more resources on TF? 2) Is TF-TH actively maintained? Cheers, Stuart

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Can I check the length in a guarded expresion
by Roelof Wobben 10 Nov '14

10 Nov '14

Hello, I try to make a guarded solution to find the last item in a list. I thought I could do something like this | length a == 1 = a But then I see a lot of error messages. Is there another way I can check the length of a list with guards ? Roelof

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IO and exceptions
by Ömer Sinan Ağacan 10 Nov '14

10 Nov '14

Hi all, I'm having some trouble with IO functions and exceptions. When I call an IO function in Haskell, I feel very uncomfortable, because you never know what kind of exceptions your function may throw. There are a lot of cases where thrown exceptions are not specified in the documentation. What's worse, there are cases where it's impossible to catch thrown exceptions in a safe way. Example to not knowing what exceptions may be thrown: http://hackage.haskell.org/package/base-4.6.0.1/docs/Control-Concurrent-Cha… `readChan` blocks the thread until it reads something. But what happens when channel is closed while one thread is blocked to read? It's not mentioned in the documentation and it's not specified in terms of types. (e.g. by using Either, Maybe or similar) Example to not being able to catch the exception: http://hackage.haskell.org/package/network-2.6.0.2/docs/Network-Socket.html `recvFrom` blocks the thread until it reads something. However, what happens if socket is closed while `recvFrom` is blocking to read? Similar to `readChan`, we can't know this. What's worse is this: sockets have a `SO_RCVTIMEO` option, a timeout value for `recv_from` calls. What happens when we set that using `setSocketOption` in the same module and call `recvFrom` is unknown. This is really horrible, because a timeout exception is really something that you'd like to catch. Example: I have a thread that listens from a socket and handles some other events. I want to stop listening the socket after every second and handle events, then return listening the socket. But I can't know what it throws when timeout happens, so I have to catch all exceptions, which is horrible for a lot of reasons. I was wondering if I'm doing something wrong. I'm sure we can find a lot of cases like these and I feel like libraries are not designed very good for errors/exceptions. So I'd like to get some ideas/suggestions about this. Am I doing something wrong or did I misunderstand something? Should we improve the libraries for better types/error handling? Am I only one here that has problems with those functions? Thanks.

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Extensionality principles for kinds?
by Rob 09 Nov '14

09 Nov '14

Haskellers, I've waded into some pretty deep waters, but I'm hoping there's a GHC type system wizard in here who can help me. The following code fragment is extracted from something I've been working on involving some pretty fancy type-level programming. I'm using GHC 7.8.3. I've run into a problem where the typechecker refuses to believe that two types are equal; it seems to lack the necessary extensionality principles for lifted data kinds. Basically (see below for definitions) I get into a contex where I know that (TwiceLeft a ~ TwiceLeft b) and (TwiceRight a ~ TwiceRight b), but the typechecker will not believe that (a ~ b). Are there any tricks I can employ to make this code typecheck (using only safe operations)? Bonus if it doesn't add any runtime cost. Thanks, Rob Dockins ====== code follows ====== {-# LANGUAGE DataKinds #-} {-# LANGUAGE PolyKinds #-} {-# LANGUAGE RankNTypes #-} {-# LANGUAGE TypeOperators #-} {-# LANGUAGE TypeFamilies #-} import Data.Type.Equality -- lifted kind (Twice k) consists of a pair of things of kind k data Twice k = TwiceVal k k -- type family accessor functions for the Twice kind type family TwiceLeft (x::Twice k) :: k where TwiceLeft (TwiceVal a b) = a type family TwiceRight (x::Twice k) :: k where TwiceRight (TwiceVal a b) = b -- Double is already taken, use Dbl instead... data Dbl (f :: k -> *) (tv :: Twice k) = Dbl (f (TwiceLeft tv)) (f (TwiceRight tv)) -- This function doesn't typecheck... doubleEq :: forall (f:: k -> *) . (forall (a::k) (b::k). f a -> f b -> Maybe (a :~: b)) -> (forall (a::Twice k) (b::Twice k). Dbl f a -> Dbl f b -> Maybe (a :~: b)) doubleEq teq (Dbl x y) (Dbl z w) = case teq x z of Nothing -> Nothing Just Refl -> case teq y w of Nothing -> Nothing Just Refl -> Just Refl

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