July 2014 - Haskell-Cafe

Webdriver: Couldn't match type `Element' with `WD Element'
by Cody Goodman 09 Jul '14

09 Jul '14

I received some help/guidance in #haskell, but I still can't get this to work. I'm basically trying to find all the text elemnts in a webpage with the webdriver package. Here is my code and errors: {-# LANGUAGE OverloadedStrings #-} import Control.Monad import Control.Monad.IO.Class import Test.WebDriver import Test.WebDriver.Classes (WebDriver (..)) import Test.WebDriver.Commands import Test.WebDriver.Commands.Wait main = do runSession defaultSession capabilities $ do openPage "http://www.appnitro.com/demo/view.php?id=1" inputs <- findElems $ ByTag "input" textElems <- filterM (liftM $ ((==) "text" . (`attr` "type"))) inputs -- wait 20 seconds waitUntil 20 (getText <=< findElem $ ByCSS ".doesnotexist") `onTimeout` return "" liftIO $ putStrLn "done" where capabilities = allCaps { browser=firefox } -- [1 of 1] Compiling Main ( src/Main.hs, interpreted ) -- src/Main.hs:168:70: -- Couldn't match type `Element' with `WD Element' -- Expected type: [WD Element] -- Actual type: [Element] -- In the second argument of `filterM', namely `inputs' -- In a stmt of a 'do' block: -- textElems <- filterM -- (liftM $ ((==) "text" . (`attr` "type"))) inputs -- In the second argument of `($)', namely -- `do { openPage "http://www.appnitro.com/demo/view.php?id=1"; -- inputs <- findElems $ ByTag "input"; -- textElems <- filterM -- (liftM $ ((==) "text" . (`attr` "type"))) inputs; -- waitUntil 20 (getText <=< findElem $ ByCSS ".doesnotexist") -- `onTimeout` return "" }' -- Failed, modules loaded: none.

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Designing widgets in Reactive style
by JP Moresmau 09 Jul '14

09 Jul '14

Hello, I'm a bit confused on how to design graphical widgets to take advantage of FRP design. I'm trying to do things in threepenny-gui but I suppose the same question applies to any framework. Suppose I want to create a "file selection widget". Currently I have a function that takes a (FilePath -> UI()) function as a parameter: what to do when a file is selected; my function returns the "widget", so that the calling code can incorporate it in its UI. This works fine, but is not FRP, right? Should my function take no arguments and return a (Widget,Event FilePath) tuple, and let the calling code decide what to do with that event? Should it be a Behavior (the currently selected file of the widget)? I *think* I understand the principles of FRP, but I'm not getting what it means in terms of design. What are the advantages of returning an Event and letting the calling code do "on myEvent myWidget $ \file -> ..." instead of just passing the handler function as a parameter? I don't know if I make much sense. In any case, any pointer to non trivial FRP GUI code would be good! Thanks a million all, -- JP Moresmau http://jpmoresmau.blogspot.com/

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Turning a message parser into a protocol handler
by Magnus Therning 09 Jul '14

09 Jul '14

The last few days I've been playing around with the combination of attoparsec/blaze/conduit to build a parser and a generator of messages in a small proprietary protocol used at work. The goal is to easily build small clients of the protocol, and possibly, if time permits, build a server in Haskell which mimics the C implementation of the server. I've come so far that the message parsing and generating is working. I've also put together a couple of tiny clients to verify that my Haskell code can communicate with the production implementation of the server. I've done this by manually tying the toy example behaviour into my conduit process. However, I recognise that as the required behaviour becomes more and more complex my thinking about the behaviour more and more resembles a finite automaton (or FSM). This led me to look around for an FSM generator tool or library for Haskell, but found none. Did I miss something in my search? Or maybe Haskell already IS the FSM generator tool I need. /M -- Magnus Therning OpenPGP: 0xAB4DFBA4 email: magnus(a)therning.org jabber: magnus(a)therning.org twitter: magthe http://therning.org/magnus I invented the term Object-Oriented, and I can tell you I did not have C++ in mind. -- Alan Kay

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ANNOUNCE: sai-shape-syb Generic mapping to homogeneous types (etc.)
by Andrew Seniuk 09 Jul '14

09 Jul '14

This package provides some support for dealing with polytypic data. It lets you escape from the generics world and work with a homogeneous rose tree, which can sometimes be convenient. I realise a more experienced programmer would probably do this differently, but anyhow it was useful to me and I've uploaded it. http://hackage.haskell.org/package/sai-shape-syb There are some examples at http://fremissant.net/shape-syb In particular, it supports GHC staged traversals, so for those trying to work with the GHC AST this might be a helpful tool for filtering, debugging, and suchlike. Kind Regards, Andrew Seniuk rasfar on #haskell

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Travis CI
by Kazu Yamamoto 09 Jul '14

09 Jul '14

Hi, This is just FYI. As you may know, Travis CI itself provides multiple versions of GHC now: http://docs.travis-ci.com/user/languages/haskell/ But I was suffering from test failures indicated by: ghc_find: command not found I heard that this problem is fixed today. https://github.com/travis-ci/travis-ci/issues/2364 --Kazu

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How to have a managed window unfocusable?
by Magicloud Magiclouds 08 Jul '14

08 Jul '14

Hi, I'd like to have a transparent clock floating on my desktop. If I made it ignored (unmanaged), it would not be above and would be blocked by other windows. If I made it managed and float, when I floated up and down, it would be focused and I needed to press hotkey again to move focus to correct window. Anyway I could manage it, but not focusing on it? -- 竹密岂妨流水过山高哪阻野云飞 And for G+, please use magiclouds#gmail.com.

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Nottingham Research Fellowships
by Graham Hutton 08 Jul '14

08 Jul '14

Dear all, The University of Nottingham is seeking applications for a number of Nottingham Research Fellowships: http://tinyurl.com/notts-fellows These are highly prestigious three-year fellowships, which are normally expected to lead into permenant academic positions. Candidates should have no more than 8 years postdoctoral experience, and fellowships normally start in October 2015. The deadline for submission of initial Expressions of Interest is ** 20th October 2014 **. Applicants in the area of the Functional Programming (FP) lab would be most welcome. The FP lab is keen to receive applications from outstanding candidates with an excellent publication record, experience in combining theory with practice, and the ability to secure external funding to support their research. Further information about the FP lab is available from http://fp.cs.nott.ac.uk As an approximate guideline, candidates in the area of the FP lab would normally be expected to have at least 3 years postdoc experience, and a number of strong publications in leading international venues such as ICFP, POPL, LICS, JFP, Haskell Symposium, etc. Best wishes, Graham Hutton -- Prof Graham Hutton Functional Programming Lab School of Computer Science University of Nottingham, UK http://www.cs.nott.ac.uk/~gmh This message and any attachment are intended solely for the addressee and may contain confidential information. If you have received this message in error, please send it back to me, and immediately delete it. Please do not use, copy or disclose the information contained in this message or in any attachment. Any views or opinions expressed by the author of this email do not necessarily reflect the views of the University of Nottingham. This message has been checked for viruses but the contents of an attachment may still contain software viruses which could damage your computer system, you are advised to perform your own checks. Email communications with the University of Nottingham may be monitored as permitted by UK legislation.

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Job opening: further Haskell Developers at Anchor
by Andrew Cowie 08 Jul '14

08 Jul '14

Openings for additional Haskell developers... About Anchor ------------ Anchor Systems, based in Sydney Australia, has successfully been providing managed hosting and infrastructure to clients around the world for almost 15 years. The critical difference in the service we offer our customers often boils down to providing operations expertise and infrastructure configuration balancing the short term needs to get a new installation running with the long term imperatives of security, scalability, and flexibility. The engineering department is building the analytics capabilities, internal systems, cloud infrastructure, deployment tooling, and operations practices that will be needed to take the company to the next level. We have every intention of changing the hosting industry to make developing applications — and operating them at scale — easier for people facing massive growth. Haskell is the working language for all internal development, and we're exploring it for web facing code too. The benefits of type safety, resilience when refactoring, and stability over time added to the power of functional programming has already paid dividends. We have a significant programme of work ahead, and we're looking to add further capability as we face these challenges. Interested in helping? Requirements ------------ You'll need to have demonstrable experience programming in Haskell, familiarity with open source development practices, and the ability to tolerate other people refactoring your code. Exposure to the harsh realities of IT operations and experience of the thrill of Linux systems administration will stand you in good stead. Knowledge of the history, progress, and problems in configuration management will be helpful. A history of contributing to libraries on Hackage would be relevant. More relevant would be a track record of working well in the open (regardless of language or project). Good documentation is as important as good code (a type signature is not sufficient), and command of written English is necessary both to collaborate with your peers and to describe and promote your results. A Bachelor's in science, engineering, or xenobiology will be well thought of. Your specific field of study certainly doesn't matter, but how you use computing to solve problems in your chosen field does. A degree in engineering will be a distinct asset. An undergraduate degree in computer science will not be held against you. Applying -------- If you're interested, send a PDF copy of your CV to <jobs(a)anchor.com.au>, along with the URL to your GitHub account. We also invite you to include a pointer to something that shows you in a good light: a mailing list thread where you argued a contentious position, a piece of particularly thorny code in which you found an elegant solution to a problem, or even an essay or article you've written. We don't do international relocations (sorry everyone who wants to work remotely from Hawaii; I want to work remotely from Hawaii too), but we readily sponsor work visas for new hires should they need one. So whether you've just moved to Australia or have been here for generations, give us a shout next time you're in Sydney. AfC Sydney -- Andrew Frederick Cowie Head of Engineering Anchor Systems 201 Elizabeth Street Level 11, Sydney NSW 2000, Australia http://www.anchor.com.au | afcowie(a)anchor.com.au | +61 2 8296 5133

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strange behavior of GHC 7.8.2
by Kazu Yamamoto 08 Jul '14

08 Jul '14

Hi cafe, I noticed that strange behavior of GHC 7.8.2. Consider the following example which requires the "ghc" package: ---- {-# LANGUAGE GeneralizedNewtypeDeriving #-} module A where import GHC import MonadUtils import Control.Monad.Trans.RWS.Lazy (RWST(..)) newtype M a = M (RWST () () () IO a) deriving (Functor,Applicative,Monad,MonadIO) ---- The "ghc" library depends on "transformers" 0.3.0.0. If "transformers" *0.4.1.0* is NOT installed, GHCi can handle the code above well: ---- % ghci -package ghc A.hs ... Loading package transformers-0.3.0.0 ... linking ... done. ... Ok, modules loaded: A. [*A] > ---- However, if "transformers" *0.4.1.0* is installed, an error happens: ---- % cabal install transformers % ghci -package ghc A.hs ... Loading package transformers-0.3.0.0 ... linking ... done. ... [1 of 1] Compiling A ( A.hs, interpreted ) A.hs:10:49: No instance for (MonadIO (RWST () () () IO)) arising from the 'deriving' clause of a data type declaration Possible fix: use a standalone 'deriving instance' declaration, so you can specify the instance context yourself When deriving the instance for (MonadIO M) Failed, modules loaded: none. [Prelude] > ---- A you can see, "transformers" 0.3.0.0 is certainly linked. How can I interpret this behavior? If this is a bug of GHC 7.8.2, I will file this to GHC's trac. P.S. We noticed this because "doctest" of ghc-mod fails only for GHC 7.8.2. --Kazu

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withFile variant which takes a ByteString?
by Bardur Arantsson 08 Jul '14

08 Jul '14

Hi all, Is there a variant of withFile which takes a ByteString? (I'm aware that I could use the lower-level System.Posix FD-based functions, but if someone already has a package out there, I'd rather not.) Regards,

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