November 2013 - Haskell-Cafe

Haskell talks at Code Mesh London 3-5 Dec - Early Bird ends in 3 days
by Andra Dinu 11 Nov '13

11 Nov '13

Hi, Between the 4th and 5th of December, inventors and users of the most discussed technologies of the moment will gather in London for the second edition of Code Mesh - The Alternative Programming Conference. Haskell talks include: * Haskell at Barclays: Exotic Tools for Exotic Trades - http://codemesh.io/index.html#tim-williams * The Haxl Project at Facebook - http://codemesh.io/index.html#simon-marlow * Haskell in the Newsroom - http://codemesh.io/index.html#erik-hinton Other languages discussed: Clojure, Erlang, Elixir, Rust, Julia and more! Among the 50 speakers you will also find Ruby guru and author of 'The Pragmatic Programmer' Dave Thomas, Erlang co-inventor Joe Armstrong, Elixir creator José Valim, Akka inventor Jonas Bonér, creator of the Parallela board Andreas Olofsson, creator of Catnip IDE Bodil Stokke, co-inventor of Julia Stefan Karpinski, Clojure experts Carin Meijer and Timothy Baldridge, creator of ZeroMQ Pieter Hintjens, as well as O'Reilly authors such as Francesco Cesarini and Dean Wampler. Check out the entire programme on the conference website: http://codemesh.io/ Thanks, Andra -- Andra Dinu Community & Social ERLANG SOLUTIONS LTD New Loom House 101 Back Church Lane London, E1 1LU United Kingdom Tel +44(0)2076550344 Mob +44(0)7983484387 www.erlang-solutions.com

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Installing pandoc without root
by Theo Moore 10 Nov '13

10 Nov '13

I am trying to install the Haskell libraries for pandoc on a local home folder and without root access, using the command: cabal install pandoc However, I run into the error: cabal: Missing dependency on a foreign library: * Missing (or bad) header file: zlib.h * Missing C library: z >From what I understand, I need to install zlib1g-dev. GHC and cabal install are available, but without root access I'm having trouble figuring out how to provide the necessary zlib files. I've tried to download the source of zlib1g-dev, but in order to compile it, I have to use "make" which I do not have the priviledge to invoke. Can someone guide me through getting pandoc up and running without root access? The system is Debian 3.2.41-2 x86_64 GNU/Linux.

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Expressing "self-composable" functions at the type level
by Ignas Vyšniauskas 10 Nov '13

10 Nov '13

Hi, I'm wondering if there's some way to express that a function can be composed with itself at the type level. Due to polymorphism this is not quite the same as saying that the domain and co-domain match. Example: swap :: (a, b) -> (b, a) swap . swap :: (a, b) -> (a, b) I'd like to write a function such as (in pseudocode): involute :: ((a -> b) &&& (b -> a)) -> (a -> a) involute f = f . f But I obviously can't write down the type for (a -> b) &&& (b -> a) Perhaps this can be done with some typeclass hackery? -- Ignas

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Re: [Haskell-cafe] IDE
by Schell Scivally 09 Nov '13

09 Nov '13

I use vim for all dev work except iOS. Specifically I use MacVim on OS X and GVim on Linux. Once I switched to vim I never looked back. Here are some plugins that I use: http://github.com/gmarik/vundle -- Package manager http://github.com/Shougo/vimproc -- Required for ghcmod-vim http://github.com/Shougo/neocomplcache -- Completion http://github.com/Shougo/neosnippet -- Snippets http://github.com/schell/vim-snippets -- Snippets http://github.com/scrooloose/syntastic -- Syntax checking http://github.com/scrooloose/nerdtree -- File drawer http://github.com/altercation/vim-colors-solarized -- Color theme http://github.com/ujihisa/neco-ghc -- Haskell completion for neocomplcache http://github.com/tpope/vim-fugitive.git-- Git integration http://github.com/kien/ctrlp.vim -- Sublime text's anything command (quick goto) http://github.com/Lokaltog/vim-powerline-- Status line http://github.com/majutsushi/tagbar -- Symbol goto based on ctags http://github.com/kana/vim-fakeclip -- OS X clipboard support http://github.com/flazz/vim-colorschemes-- More colorschemes http://github.com/eagletmt/ghcmod-vim -- Syntax/compilation checking, quickfix integration http://github.com/travitch/hasksyn -- Better haskell syntax coloring http://github.com/dag/vim2hs -- More haskell syntax coloring I have a lot of Haskell syntax plugins and some of those may be conflicting, IDK, but it works well IMO. Using all that I get a pretty good IDE feeling when developing in Haskell. I also use a tool called steeloverseer that I wrote to continuously compile my cabal projects. If you're interested it's on hackage and you can read the readme at https://github.com/schell/steeloverseer/. I hope that helps!

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float show instances
by Evan Laforge 08 Nov '13

08 Nov '13

Ok, here's a bit of shameless bike-sheddery, so feel free to move on if you don't have time for that kind of thing. It seems silly, but this is something that's been bothering me for something like 5 years now, namely that ghc's float show instance is very aggressive about resorting to scientific notation for numbers below 1. If you get to 0.01 it writes it as 1.0e-2. Python and ruby, by comparison, have the cutoff at 1e-5. But it's only for <1, if you go higher you have to get all the way up to 1.0e7 before it goes exponential (python and ruby are even more reluctant: 1e16 and 1e14 respectively). This little detail has been a constant low-grade hassle for me because I don't read scientific notation fluently, and it's hard for me to see relative magnitudes in a big chunk of floats. In the past I've resorted to copy pasting a list into a python prompt or a separate pretty printer program, and in the present I have my own replacement for Show that is less aggressive with scientific notation, in addition to other things. So I was wondering, was this a considered design choice, or just the preference of the person who happened to write Show Float and Show Double way back when? Are there people who read scientific notation just as easily as positional (and if so, would they prefer to see 100 as 1e3)? Is there hope that if I look at it enough maybe someday it'll be clear to me too? I'm not actually expecting anyone will say "why didn't we think of that, let's change it right away", but I'm curious about how it came about, and whether other people have resorted to as drastic measures as I have. As an aside, I've noticed that when I'm debugging, the formatting of the output is very important. Surprisingly so. It's worth it for me to spend extra time making test and debugging output minimal and clear, otherwise my brain is already overtaxed trying to understand the problem, and just can't handle extra load like skipping irrelevant data, figuring out scientific notation, and even trying to figure out where the breaks are in un-spaced output. Maybe a personal quirk.

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a tiny HTTP server.. that doesn't work
by Corey O'Connor 08 Nov '13

08 Nov '13

I attempted to write a tiny HTTP server using only Haskell Platform packages: * https://github.com/coreyoconnor/tiny-http-hp Which works... Unless the threaded runtime is used. When compiled using the threaded runtime and run with +RTS -N this server fails to reply correctly to ~3% of requests. The expectation is that the reply will be exactly the request body. However, 3% of the time the HTTP request fails to be parsed due to an "ErrorClosed" The server then executes: * https://github.com/coreyoconnor/tiny-http-hp/blob/master/TinyHttp.hs#L23 which responds to the client. The client receives the response correctly most of the time. My current hypothesis is that some aspect of lazy IO is not playing nice. The connection is being closed before the request can be completely parsed. Is this correct? What am I missing? I know there are other HTTP server packages that could be used. However, the exercise was to build a HTTP server using only Haskell Platform packages. Below is the main body of the code: main = withSocketsDo $ do http_socket <- listenOn $ PortNumber 9090 dispatch_on_accept http_socket $ either handle_failed_request handle_valid_request sClose http_socket handle_failed_request failure = return $ Response (4,0,0) "Bad Request" [mkHeader HdrConnection "close"] (encodeUtf8 $ pack $ show failure) handle_valid_request request = do let request_body = rqBody request return $ Response (2,0,0) "OK" [mkHeader HdrConnection "close"] (encodeUtf8 $ pack $ show request_body) dispatch_on_accept http_socket handler = forever $ accept http_socket >>= forkIO . httpHandler . fst where httpHandler client_socket = bracket (socketConnection "client" 0 client_socket) Network.HTTP.close client_interact client_interact :: HandleStream BS.ByteString -> IO () client_interact byte_stream = receiveHTTP byte_stream >>= handler >>= respondHTTP byte_stream -Corey O'Connor coreyoconnor(a)gmail.com http://corebotllc.com/

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Help using Groundhog Template Haskell.
by Andrew Myers 08 Nov '13

08 Nov '13

Hi Cafe, I'm trying to experiment with groundhog because I want something that I can map between arbitrary ADTs and arbitrary Schema. Groundhog looks like exactly what I want but I'm hung up on an error coming from the mkPersist TH function. When I compile the code below with ghc --make groundhog.hs I get this error: groundhog.hs:23:1: Function binding for `Database.Groundhog.Core.entityFieldChain' has no equations When splicing a TH declaration: I'm pretty sure it has something to do with the TransactionType not being made up of primitive types but I'm not sure what to do about it. Can someone familiar with the library point me in the right direction? Thanks, Andrew Myers {-# LANGUAGE TemplateHaskell, QuasiQuotes #-} module Checking.Database.Groundhog where import qualified Database.Groundhog.TH as TH import qualified Data.Text as T import qualified Data.Time as D import qualified Control.Lens as L data TransactionType = Deposit | Debt | Informational deriving (Show, Eq, Read) data Transaction = Transaction { _transactionDate :: D.UTCTime , _transactionAmount :: Double , _transactionDescription :: T.Text , _transactionBalanced :: Bool , _transactionCheckNumber :: Maybe Int , _transactionTransactionType :: TransactionType } deriving (Show, Read, Eq) TH.mkPersist TH.defaultCodegenConfig [TH.groundhog| - entity: Transaction constructors: - name: Transaction fields: - name: _transactionDate dbName: date exprName: DateField - name: _transactionAmount dbName: amount exprName: AmountField - name: _transactionDescription dbName: description exprName: DescriptionField - name: _transactionBalanced dbName: balanced exprName: BalancedField - name: _transactionCheckNumber dbName: check_number exprName: CheckNumberField - name: _transactionTransactionType dbName: transaction_type exprName: TransactionTypeField - entity: TransactionType |]

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Job opportunities at Startup - London and Exeter
by Ben Moseley 08 Nov '13

08 Nov '13

If you are an experienced developer, have a keen interest in functional programming and would be interested in working at an ambitious, funded, consumer-focused UK startup, please get in touch with me. Our main server-side infra is Haskell-based but we also use Java and Objective-C for the mobile platforms we target. Beyond the obvious technical skills, successful candidates will need focus and determination to get the job done. In return we offer a hugely exciting and challenging working environment, competitive compensation, a great culture (we'll probably try and drag you away from your keyboard and onto a mountain bike) and an experienced management team with a proven track-record. We are looking for developers to be based in London and Exeter. -- Ben Moseley - Programmer ben(a)benmoseley.net

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QuickCheck
by Tony Morris 08 Nov '13

08 Nov '13

Hello, I have implemented a function for QuickCheck: untilJust :: Gen a -> (a -> Maybe b) -> Gen b I based it on the code for suchThat[1] and suchThatMaybe[2]. I am wondering if I have potentially re-implemented an existing function in part or full. In other words, is there an easier way of achieving this function without pulling the generator apart as much as I have in this implementation: http://lpaste.net/95317 [1] suchThat :: Gen a -> (a -> Bool) -> Gen a http://hackage.haskell.org/package/QuickCheck-2.6/docs/src/Test-QuickCheck-… [2] suchThatMaybe :: Gen a -> (a -> Bool) -> Gen (Maybe a) http://hackage.haskell.org/package/QuickCheck-2.6/docs/src/Test-QuickCheck-… -- Tony Morris http://tmorris.net/

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concatMap with state for Data.Vector?
by Evan Laforge 07 Nov '13

07 Nov '13

This is a Data.Vector question, hopefully here is a good place for those. Or would it be better to ask on stackoverflow? I want to generate a vector from another, where I interpolate between each point. So I need something like concatMap because the vector gets longer, but I need state because I need to remember the previous value (and I need to look at the next value to avoid overrunning, but that can be done with state and 'tails'). Stated imperatively, it would look like this: state = Nothing out = [] for (x, y) in signal: case state of Nothing -> out.append (x, y) Just (x0, y0) -> for (x, y) in interpolate ...: out.append (x, y) state = Just (x, y) The simplest option would be just doing the transformation on a list: `Vector.concat . snd . List.mapAccumL go Nothing . Vector.toList` But unless fusion is more powerful than I imagined, that builds a couple of lists and a bunch of intermediate vectors. I did try to read the core, but it's hard to follow when vector is involved. If they're large, it's probably no worse than a builder, but if they're small, well I'd want a builder to fill up each chunk efficiently... but there's no builder for Data.Vector (I did write one once, maybe I should pull it out and dust it off). unfoldr could also do it, but to emit multiple samples I'd need to make the state `Either ([a], state) state`, and some awkward plumbing, and constant tag checks. So it's looking like converting to a list and back is the most reasonable option... unless there's some way to express this that I'm missing?

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