- Haskell-Cafe - Haskell.org

threadDelay delays less time than expected (Windows)
by Lian Hung Hon 05 Jan '17

05 Jan '17

Dear Haskellers, I am baffled by a strange bug (?) with threadDelay on Windows. It delays for less time than expected (500 seconds vs expected 24 hours), and is consistently reproducible on my system. Can someone have a look (need some patience, ~8 minutes waiting), just to be sure? I glanced around trac but didn't find any related issues. How to reproduce: module Main where import Data.Time import Control.Concurrent import Control.Monad main :: IO () main = do _ <- forkIO thread getLine >>= print -- Just to keep the main thread running thread :: IO () thread = forever $ do now <- getCurrentTime print now threadDelay 86400000000 print "done waiting" Expected result: 2016-12-28 13:07:49.5113098 UTC ... (next timing should be the next day 2016-12-29) Actual result: 2016-12-28 13:07:49.5113098 UTC "done waiting" 2016-12-28 13:16:10.2159485 UTC "done waiting" 2016-12-28 13:24:30.8735845 UTC "done waiting" 2016-12-28 13:32:51.5292203 UTC ... System: Windows 7 Enterprise Service Pack 1, ghc 7.10.3 This seems specific to Windows, it doesn't appear on Debian 8. Help is appreciated! Regards, Hon Rant: It has cost me significant debugging time! Please tell me I made a rookie mistake or misread the docs somewhere!

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Deploying Scotty based web app to Amazon Elastic Bean - One approach
by Guru Devanla 03 Jan '17

03 Jan '17

Hello All, Recently, I have been working on a web app in Haskell using Scotty as the web framework. I had to go through different sources and compile together an approach I took to deploy a dockerized version of web app to Amazon Elastic Beanstalk. There may be other approaches available and I am happy to hear about it. Meanwhile, I documented my experience with a simple Hello World app and documented the steps I had to take to get my app up and running on Elastic Beanstalk. I look forward to suggestions on a better way to do this. https://github.com/gdevanla/helloscotty/blob/master/README.md Hopefully, this documentation is helpful, and does not proposed an anti-pattern! Thanks

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Code review - wrapping servant client to hide required parameters
by Nigel Rantor 03 Jan '17

03 Jan '17

Hello, So, I am still learning and would like someone to cast an eye over some code to see if I am missing something in terms of making it cleaner/more concise. I have begun wrapping the They Work For You API using the Servant Client library. All code is already online for you to grab at; https://github.com/wiggly/twfy-api-client https://hackage.haskell.org/package/twfy-api-client The API is not under my control so I can't change how it works or how it accepts parameters. Every API call requires a Key to be passed as a query string parameter. For a particular session/user this will always be the same, and there is no point in allowing the user to not provide it. Since it is a query parameter Servant forces the type to be within Maybe. For example; type TwfyAPI = "getConstituency" :> QueryParam "key" ApiKey :> QueryParam "name" T.Text :> QueryParam "postcode" T.Text :> Get '[JsonIso8859] Constituency Gives the following function signature; getConstituency :: Maybe ApiKey -- ^ API key -> Maybe T.Text -- ^ Name -> Maybe T.Text -- ^ Post code -> ClientM Constituency To make the API easier to use I have created a Client module that wraps these so that I can hide the passing of the API key for every call, and also allow the user to not have to deal with http client managers or URL parsing if they don't want to. The upshot is that I get a signature that now accepts a data type I have created that contains all the information required to run the client action and return a result, like so (where A.getConstituency is the function above taking a Maybe ApiKey as first argument); getConstituency :: Client -> Maybe T.Text -- ^ Name -> Maybe T.Text -- ^ Post code -> IO (Either ServantError Constituency) getConstituency c = do let key = Just $ clientApiKey c env = clientEnv c in (\name postCode -> (flip runClientM) env (A.getConstituency key name postCode)) So, this wraps up all of the calling of the servant client with an appropriate environment and arguments. I have two questions; 1) What do people think of this style of wrapping the servant client calls? 2) I am now returning a lambda that accepts the remaining arguments to the call, is there any way I can make this cleaner? i.e. is it possible for me to not have to specifically re-write all of the function parameters using something like uncurry or other machinery? I have played with trying to use uncurry but got nowhere fast. Thanks in advance, Nigel

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ANN: hledger 1.1
by Simon Michael 01 Jan '17

01 Jan '17

Two months since last release, I'm pleased to announce hledger 1.1 ! Thank you to contributors Johannes Gerer, Nikolay Orlyuk, and Shubham Lagwankar, along with all others who contributed in other ways. hledger (http://hledger.org) is a dependable, precise, cross-platform program for tracking money, time, or any other commodity, using double-entry accounting and a simple plain text file format. Inspired by the Ledger project, hledger provides command-line, curses and web interfaces, and aims to be a robust, practical tool for daily use. Notable user-visible changes in this release: - hledger-ui --watch live-updates as files or the current date change, useful eg when reconciling or for a time log dashboard - Ledger-style balance assignments are now supported in journal files Full release notes: http://hledger.org/release-notes#hledger-1.1 To install this release: Get stack, eg from http://haskell-lang.org/get-started $ stack install --resolver=nightly hledger [hledger-ui] [hledger-web] [hledger-api] $ ~/.local/bin/hledger or see http://hledger.org/download for more installation options. Our IRC channel is #hledger on Freenode, and you can find out more at http://hledger.org. I hope you enjoy these tools and that they help you achieve your goals. If you like hledger, your support and participation is appreciated! Fund us on Gratipay or Bountysource, give feedback, report bugs, send pull requests, write about it, etc. Happy 2017! Best, -Simon

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How to call python from haskell
by jun zhang 01 Jan '17

01 Jan '17

Dear cafe I had try to use the http://hackage.haskell.org/package/cpython <http://hackage.haskell.org/package/cpython> package. I can call python function once by the package, but when I call function second time it will occur a "Segmentation fault”. Does there any stable package support call python from haskell? Thanks. Julian

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Re: [Haskell-cafe] help with type variable escaping scope/passing lens as function argument
by Li-yao Xia 01 Jan '17

01 Jan '17

Hi Cody, I forgot to keep Haskell-cafe in copy. > Are you saying that my forall applies to both a and b, but not only a? And > that a is what (Lens (Record rs) Chicago -> (...)) expands to? That sounds about right. Try this: dateBetween :: (forall f. Functor f => (Chicago -> f Chicago) -> Record rs -> f (Record rs)) -> Day -> Day -> Pipe (Record rs) (Record rs) IO m Or, a bit refactored: type Lens s a = forall f. Functor f => (a -> f a) -> (s -> f s) dateBetween :: Lens (Record rs) Chicago -> Day -> Day -> Pipe (Record rs) (Record rs) IO m > Where does f1 come from? Does GHC just come up with it because my type > variable doesn't satisfy it? When you write this dateBetween :: forall (rs :: [*]) (m :: *) (f :: * -> *). (Functor f) => ((Chicago -> f Chicago) -> Record rs -> f (Record rs)) -> Day -> Day -> Pipe (Record rs) (Record rs) IO m dateBetween target start end = ... The parameter target is a monomorphic function, with f instantiated at the call site of dateBetween. This means that dateBetween can not assume anything about f apart from the fact that it is an instance of Functor, i.e., f must be abstract. Then we have: let targetDate = rget target r but this is the type of rget: rget :: (forall f . Functor f => (a -> f a) -> Record rs -> f (Record rs)) -> Record rs -> a rget expects a polymorphic function, while target is monomorphic. The compiler tries to unify "f" from the type signature of dateBetween, and "f" from the type signature of rget. To avoid confusion, the compiler does renaming, so that different variables (identified by where they are bound) have different names, and that's where "f1" comes from: rget :: forall a1 rs1. (forall f1 . Functor f1 => (a1 -> f1 a1) -> Record rs1 -> f1 (Record rs1)) -> Record rs1 -> a1 Applying rget to (target :: (Chicago -> f Chicago) -> Record rs -> f (Record rs)), the compiler tries to unify f and f1, and fails because they are "rigid", meaning, they can't be instantiated. f is rigid because it is abstract in the body of dateBetween, f1 is rigid because it is the type parameter of the polymorphic function rget expects. Li-yao On 12/31/2016 09:07 AM, Cody Goodman wrote: > Hi Li-yao, > > Are you saying that my forall applies to both a and b, but not only a? And > that a is what (Lens (Record rs) Chicago -> (...)) expands to? > > In either case, I don't understand what action I can take based on what was > you said and what I understand from it :/ > > Where does f1 come from? Does GHC just come up with it because my type > variable doesn't satisfy it? > > Thanks, > > Cody > > On Sat, Dec 31, 2016 at 2:54 AM, Li-yao Xia <li-yao.xia(a)ens.fr> wrote: > >> Hi Cody, >> >> You wrote something of the shape >> >> forall f. Functor f => a -> b >> >> which means >> >> forall f. Functor f => (a -> b) >> >> but not >> >> (forall f. Functor f => a) -> b >> >> which would be what (Lens (Record rs) Chicago -> (...)) expands to. >> >> Regards, >> Li-yao >> >> >> On 12/31/2016 08:45 AM, Cody Goodman wrote: >> >>> Thanks for the help Li-yao, but I still have a type error after using Rank >>> N Types. I have updated my code on github >>> https://github.com/codygman/frames-credit-card-trans-demo/bl >>> ob/master/src/Main.hs#L45() >>> and pasted below: >>> >>> dateBetween :: forall (rs :: [*]) (m :: *) (f :: * -> *). >>> (Functor f) => >>> ((Chicago -> f Chicago) -> Record rs -> f (Record rs)) >>> -> Day >>> -> Day >>> -> Pipe (Record rs) (Record rs) IO m >>> dateBetween target start end = P.filter go >>> where go :: Record rs -> _ >>> go r = let targetDate = (rget target r) :: Chicago >>> targetDate' = chicagoToZoned targetDate :: ZonedTime >>> targetDay = localDay (zonedTimeToLocalTime targetDate') >>> :: Day >>> in >>> targetDay >= start && targetDay < end >>> -- type error >>> -- src/Main.hs:53:39: error: … >>> -- • Couldn't match type ‘f’ with ‘f1’ >>> -- ‘f’ is a rigid type variable bound by >>> -- the type signature for: >>> -- dateBetween :: forall (rs :: [*]) m (f :: * -> *). >>> -- Functor f => >>> -- ((Chicago -> f Chicago) -> Record rs -> f >>> (Record rs)) >>> -- -> Day -> Day -> Pipe (Record rs) (Record rs) >>> IO m >>> -- at >>> /home/cody/source/frames-credit-card-trans-demo/src/Main.hs:45:45 >>> -- ‘f1’ is a rigid type variable bound by >>> -- a type expected by the context: >>> -- forall (f1 :: * -> *). >>> -- Functor f1 => >>> -- (Chicago -> f1 Chicago) -> Record rs -> f1 (Record rs) >>> -- at >>> /home/cody/source/frames-credit-card-trans-demo/src/Main.hs:53:34 >>> -- Expected type: (Chicago -> f1 Chicago) >>> -- -> Record rs -> f1 (Record rs) >>> -- Actual type: (Chicago -> f Chicago) -> Record rs -> f (Record >>> rs) >>> -- • In the first argument of ‘rget’, namely ‘target’ >>> -- In the expression: (rget target r) :: Chicago >>> -- In an equation for ‘targetDate’: >>> -- targetDate = (rget target r) :: Chicago >>> -- • Relevant bindings include >>> -- target :: (Chicago -> f Chicago) -> Record rs -> f (Record rs) >>> -- (bound at >>> /home/cody/source/frames-credit-card-trans-demo/src/Main.hs:51:13) >>> -- dateBetween :: ((Chicago -> f Chicago) >>> -- -> Record rs -> f (Record rs)) >>> -- -> Day -> Day -> Pipe (Record rs) (Record rs) >>> IO m >>> -- (bound at >>> /home/cody/source/frames-credit-card-trans-demo/src/Main.hs:51:1) >>> -- Compilation failed. >>> >>> On Sat, Dec 31, 2016 at 2:21 AM, Li-yao Xia <li-yao.xia(a)ens.fr> wrote: >>> >>> Hello Cody, >>>> >>>> Complete the type annotation, using RankNTypes >>>> >>>> dateBetween :: Lens Record Chicago -> ... >>>> >>>> GHC does not infer higher-rank types, because it is an undecidable >>>> problem >>>> in general, so you must annotate polymorphic parameters explicitly. >>>> >>>> Regards, >>>> Li-yao >>>> >>>> >>>> On 12/31/2016 07:56 AM, Cody Goodman wrote: >>>> >>>> I have the following code and error(also seen at >>>>> https://github.com/codygman/frames-credit-card-trans-demo/bl >>>>> ob/master/src/Main.hs#L28 >>>>> ): >>>>> >>>>> >>>>> -- between meaning on or after start but before end >>>>> dateBetween :: _ >>>>> -> Day >>>>> -> Day >>>>> -> Pipe (Record rs) (Record rs) IO r >>>>> dateBetween target start end = P.filter go >>>>> where go :: Record rs -> _ >>>>> go r = let targetDate = rget target r :: Chicago >>>>> targetDate' = chicagoToZoned targetDate :: ZonedTime >>>>> targetDay = localDay (zonedTimeToLocalTime >>>>> targetDate') >>>>> :: Day >>>>> in >>>>> targetDay >= start && targetDay < end >>>>> -- type error >>>>> -- src/Main.hs:48:38: error: … >>>>> -- • Couldn't match expected type ‘(Chicago -> f Chicago) >>>>> -- -> Record rs1 -> f (Record rs1)’ >>>>> -- with actual type ‘t’ >>>>> -- because type variable ‘f’ would escape its scope >>>>> -- This (rigid, skolem) type variable is bound by >>>>> -- a type expected by the context: >>>>> -- Functor f => (Chicago -> f Chicago) -> Record rs1 -> f >>>>> (Record >>>>> rs1) >>>>> -- at >>>>> /home/cody/source/frames-credit-card-trans-demo/src/Main.hs:48:33-45 >>>>> -- • In the first argument of ‘rget’, namely ‘target’ >>>>> -- In the expression: rget target r :: Chicago >>>>> -- In an equation for ‘targetDate’: >>>>> -- targetDate = rget target r :: Chicago >>>>> -- • Relevant bindings include >>>>> -- r :: Record rs1 >>>>> -- (bound at >>>>> /home/cody/source/frames-credit-card-trans-demo/src/Main.hs:48:12) >>>>> -- go :: Record rs1 -> Bool >>>>> -- (bound at >>>>> /home/cody/source/frames-credit-card-trans-demo/src/Main.hs:48:9) >>>>> -- target :: t >>>>> -- (bound at >>>>> /home/cody/source/frames-credit-card-trans-demo/src/Main.hs:46:13) >>>>> -- dateBetween :: t -> Day -> Day -> Pipe (Record rs) (Record >>>>> rs) >>>>> IO r >>>>> -- (bound at >>>>> /home/cody/source/frames-credit-card-trans-demo/src/Main.hs:46:1) >>>>> -- Compilation failed. >>>>> >>>>> Perhaps it's late, but I'm fairly sure I've done something like this >>>>> before. >>>>> >>>>> Thanks, >>>>> >>>>> Cody >>>>> >>>>> >>>>> >>>>> _______________________________________________ >>>>> Haskell-Cafe mailing list >>>>> To (un)subscribe, modify options or view archives go to: >>>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe >>>>> Only members subscribed via the mailman list are allowed to post. >>>>> >>>>> >>>>> >>> >

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help with type variable escaping scope/passing lens as function argument
by Cody Goodman 31 Dec '16

31 Dec '16

I have the following code and error(also seen at https://github.com/codygman/frames-credit-card-trans-demo/blob/master/src/M… ): -- between meaning on or after start but before end dateBetween :: _ -> Day -> Day -> Pipe (Record rs) (Record rs) IO r dateBetween target start end = P.filter go where go :: Record rs -> _ go r = let targetDate = rget target r :: Chicago targetDate' = chicagoToZoned targetDate :: ZonedTime targetDay = localDay (zonedTimeToLocalTime targetDate') :: Day in targetDay >= start && targetDay < end -- type error -- src/Main.hs:48:38: error: … -- • Couldn't match expected type ‘(Chicago -> f Chicago) -- -> Record rs1 -> f (Record rs1)’ -- with actual type ‘t’ -- because type variable ‘f’ would escape its scope -- This (rigid, skolem) type variable is bound by -- a type expected by the context: -- Functor f => (Chicago -> f Chicago) -> Record rs1 -> f (Record rs1) -- at /home/cody/source/frames-credit-card-trans-demo/src/Main.hs:48:33-45 -- • In the first argument of ‘rget’, namely ‘target’ -- In the expression: rget target r :: Chicago -- In an equation for ‘targetDate’: -- targetDate = rget target r :: Chicago -- • Relevant bindings include -- r :: Record rs1 -- (bound at /home/cody/source/frames-credit-card-trans-demo/src/Main.hs:48:12) -- go :: Record rs1 -> Bool -- (bound at /home/cody/source/frames-credit-card-trans-demo/src/Main.hs:48:9) -- target :: t -- (bound at /home/cody/source/frames-credit-card-trans-demo/src/Main.hs:46:13) -- dateBetween :: t -> Day -> Day -> Pipe (Record rs) (Record rs) IO r -- (bound at /home/cody/source/frames-credit-card-trans-demo/src/Main.hs:46:1) -- Compilation failed. Perhaps it's late, but I'm fairly sure I've done something like this before. Thanks, Cody

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Netwire woes: creating a wire with mkGen_
by Francesco Ariis 31 Dec '16

31 Dec '16

Hello -cafe, a few days ago I posted this [1] in beginners, I'll pester you too before giving up. tl;dr: I want to create a wire which outputs a stream of 'q's when the letter Q is pressed and a stream of 'a' when the letter A is pressed. What I wrote is not working and I suspect I am not using mkGen_ correctly. Any pointers appreciated! [1] https://mail.haskell.org/pipermail/beginners/2016-December/017276.html

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ANN: rapid, rapid-term: Rapid prototyping for Haskell
by esz＠posteo.de 31 Dec '16

31 Dec '16

Hi everybody, I'd like to officially announce my rapid prototyping suite unimaginatively called [Rapid] along with a helper library called [rapid-term]. The traditional approach to rapid prototyping with Haskell is to have a process running in the background that listens for source code changes and then recompiles/reloads and restarts the application. I'll call this process the *watcher*. This approach has a few drawbacks: * The watcher is separate from the main GHCi instance used for development. This means that the only access to the application state you have is through the interfaces you have explicitly defined in your code. * Restarting the application entails losing all ephemeral state, all network connections, all computed in-memory data, etc. * The watcher typically doesn't understand individual application components, so it's impossible to restart them individually. Rapid uses Chris Done's [foreign-store] library to provide a rapid prototyping framework for GHCi that lets you reload just the relevant parts of your application while keeping everything else running untouched, including all state, database/network connections, etc. It runs directly in the GHCi instance used for development, so you can talk to Rapid as well as your application from the REPL as it runs; no need to define debugging interfaces. The goal is to shorten the development cycle as much as possible. Another example use case is the development of interactive GUI applications. You can keep the GUI window open and preserve all of its run-time state (text you have entered, list items you have created, etc.), while reloading your application logic. It can also be used by batch applications that don't run for extended periods of time, for example to keep expensive resources in memory. For example load and parse your giant data file once and just reuse the resulting value across reloads. There is a detailed [Rapid tutorial] in the documentation of the `Rapid` module. It also walks you through Emacs haskell-mode integration. Rapid-term is a helper library for the development of applications that require an actual terminal. It spawns a separate terminal and keeps it open for the duration of the GHCi session. The application can then access this terminal and reuse it across runs. There is a [rapid-term tutorial] in the documentation of the `Rapid.Term` module, which also explains how to use rapid-term for rapid prototyping of Vty applications (same story as for GUI applications, but for Vty: keep your UI running, preserve your state, reload your application logic). Feedback, bug reports and code contributions are highly welcome! Greets, ertes [foreign-store]: https://hackage.haskell.org/package/foreign-store [rapid tutorial]: https://hackage.haskell.org/package/rapid-0.1.3/docs/Rapid.html [rapid-term tutorial]: https://hackage.haskell.org/package/rapid-term-0.1.0/docs/Rapid-Term.html [rapid-term]: https://hackage.haskell.org/package/rapid-term [rapid]: https://hackage.haskell.org/package/rapid

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Doing the course "Functional Systems in Haskell"
by Damian Nadales 31 Dec '16

31 Dec '16

Hi, I want to follow the material in this course: http://www.scs.stanford.edu/16wi-cs240h/ Is anybody interested in studying the course material and completing the exercises? Has anybody tried to complete earlier versions of the course? Cheers, Damian.

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