September 2016 - Haskell-Cafe

Stable Random Number Generator?
by Christopher Howard 10 Sep '16

10 Sep '16

Hi. What would be a good library if I want a pseudo-RNG which is stable across platforms and compiler versions? By "stable" I mean always produces the same values after being given a particular seed. I have a number of projects which involve generating some kind of pattern or graphic, where the algorithm is either fed values from a RNG, or starts with initial conditions generated by an RNG. So, I want an RNG which which produce the same results when different users on different systems enter the same seed. -- https://qlfiles.net My PGP public key ID is 0x340EA95A (pgp.mit.edu)

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Haskell conferences in Europe
by Damian Nadales 10 Sep '16

10 Sep '16

Hi, Do you know of any Haskell conferences in Europe for 2016/2017? Also, do you know if there is a centralized place where this information can be found? Thanks in advance, Damian.

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Haskell eXchange and related events -- Call for Participation
by Andres Löh 09 Sep '16

09 Sep '16

Hi everyone. As Tom Ellis has correctly pointed out in another thread, I should have announced the Haskell eXchange on this list. Let me do that now: ===================================================================== Haskell eXchange 2016 London, 6-7 October 2016 ===================================================================== The Haskell eXchange is a general Haskell conference aimed at Haskell enthusiasts of all skill levels. The Haskell eXchange is organized annually, and 2016 is its fifth year. For the second year in a row, the venue will be Skills Matter’s CodeNode, where we have space for three parallel tracks. New this year: a large number of beginner-focused talks. At all times, at least one track will be available with a talk aimed at (relative) newcomers to Haskell. Of course, there are also plenty of talks on more advanced topics. Keynote speakers: Don Stewart, Conor McBride, Graham Hutton, Simon Peyton Jones Talks and workshops (some changes may still occur) by: Julian Arni, Christiaan Baaij, Thomas van Binsbergen, Mathieu Boespflug, Lars Brünjes, Ben Clifford, Philip Cunningham, Tom Ellis, Matthias Fischmann, Oleg Grenrus, Nickolay Kudasov, Robert Henderson, Csaba Hruska, Robin Kay, Csongor Kiss, Steven Kutsch, Kris Jenkins, David Luposchainsky, Neil Mitchell, Andrey Mokhov, Romeo Moura, Jann Müller, Boldizsár Németh, Andor Penzes, Ivan Perez, Michał Płachta, Nicolas Pouillard, Arian van Putten, Andrzej Rybczak, Jeremy Singer, Nicolas Wu Homepage and registration: https://skillsmatter.com/conferences/7276-haskell-exchange-2016 If you're interested in the Haskell eXchange, you might also be interested in the Haskell eXchange Haskell Hackathon and/or in the Haskell courses (2-day introductory, 1-day on types, 2-day on performance) offered by Well-Typed on the surrounding days. More info here: http://www.well-typed.com/blog/125/ Cheers, Andres -- Andres Löh, Haskell Consultant Well-Typed LLP, http://www.well-typed.com Registered in England & Wales, OC335890 250 Ice Wharf, 17 New Wharf Road, London N1 9RF, England

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Aeson parsing of GeoJSON geometries
by mike thomas 09 Sep '16

09 Sep '16

Hi all. Having returned to commercial software development after a 10 year break in environmental regulation and travel, it was not long before I was using my favorite language Haskell, fortunately, as part of my job this time. Haskell has changed in many ways since last I used it, and I am having an internal (lack of) knowledge collision with the problem below: As set out in the example below, given a polygon as GeoJSON from a Postgis enabled Postgres datasbase, I want to get to a list of Double tuples: [(Double,Double)], using Aeson, to draw some polygons. I am stopped at the point of dismembering the nested coordinate arrays. With Stackage nightly-2016-09-01 ghci, and the series of functions defined at the end of this message, I get a runtime type error regarding Vectors, which baffles me. Question 1. Why does Aeson nest the arrays using list syntax around the Array type constructor arguments? Question 2. How do I best convert this part of the AST: "(Array [Array [Array [Number 4.5305923601,Number 50.6585120423],Array [Number 4.5307511543,Number 50.657719833],Array [Number 4.5310580333,Number 50.657539732],Array [Number 4.5309023972,Number 50.6584422261],Array [Number 4.5308797482,Number 50.6586166629],Array [Number 4.5305923601,Number 50.6585120423]]])" to a list of Double tuples? I've tried several variations of the function dropOuterArray to try and resolve this run-time error, with no success. All help welcome, and thanks for your time Mike. ========= GHCI Session With Error ====================== p = parseGeoJSONGeometry "{\"type\":\"Polygon\",\"coordinates\":[[[4.5305923601,50.6585120423],[4.5307511543,50.657719833],[4.5310580333,50.657539732],[4.5309023972,50.6584422261],[4.5308797482,50.6586166629],[4.5305923601,50.6585120423]]]}" p (Just (String "Polygon"),Just (Array [Array [Array [Number 4.5305923601,Number 50.6585120423],Array [Number 4.5307511543,Number 50.657719833],Array [Number 4.5310580333,Number 50.657539732],Array [Number 4.5309023972,Number 50.6584422261],Array [Number 4.5308797482,Number 50.6586166629],Array [Number 4.5305923601,Number 50.6585120423]]])) p1 = dropOuterArrayJ (snd p) p1 [Array [Array [Number 4.5305923601,Number 50.6585120423],Array [Number 4.5307511543,Number 50.657719833],Array [Number 4.5310580333,Number 50.657539732],Array [Number 4.5309023972,Number 50.6584422261],Array [Number 4.5308797482,Number 50.6586166629],Array [Number 4.5305923601,Number 50.6585120423]]] p2 = dropOuterArray p1 :151:21: error: • Couldn't match type ‘Data.Vector.Vector Value’ with ‘Value’ Expected type: Value Actual type: Array • In the first argument of ‘dropOuterArray’, namely ‘p1’ In the expression: dropOuterArray p1 In an equation for ‘p2’: p2 = dropOuterArray p1 ============ Code ==================== {-# LANGUAGE OverloadedStrings, DeriveGeneric, DeriveAnyClass, ScopedTypeVariables, LambdaCase, FlexibleContexts #-} module GeoJSONGeometry ( parseGeoJSONGeometry ,Coordinates ,dropOuterArrayJ ,dropOuterArray ,Pt ,gTypeString ) where import Data.Aeson import Data.Aeson.Types --(parse, parseMaybe, parseEither, Value(..)) import GHC.Generics import Data.ByteString.Lazy.Char8 import Data.HashMap.Strict import qualified Data.Vector as V import Data.Either.Extra (fromRight) import Data.Maybe (fromJust) import Control.Applicative import Codec.Picture import Graphics.Rasterific import Graphics.Rasterific.Texture import GHC.Generics parseGeoJSONGeometry gjg = let eresult = (eitherDecode (pack gjg)) :: Either String Object -- Value -- Object result = (fromRight eresult) gType = Data.HashMap.Strict.lookup "type" result gCoords = Data.HashMap.Strict.lookup "coordinates" result in (gType, gCoords) gTypeString (Just (Data.Aeson.Types.String s)) = s dropOuterArrayJ (Just (Array u)) = u dropOuterArray (Array u) = u

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many beautiful things
by dek5 08 Sep '16

08 Sep '16

Dear! Just take a look at this stuff! There are so many beautiful things here, you should visit the store <http://puquytrygy.kylejjackson.com/aeodqar> Sincerely yours, dek5

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regex and Unicode
by Brian Sammon 08 Sep '16

08 Sep '16

I tried to write a program using Text.Regex.PCRE to search through a UTF8-encoded document. It appears that the presence of non-breaking-space characters (code point 160) triggers some weird behavior in my program. This is using the Debian stable(Jessie) packages of ghc 7.6.3 and libraries. Now I find myself at a fork in the road, not sure which direction to head in. Do I: 1) Continue looking (or get help with looking) for bugs in my code? (I have this reduced to a pretty small test case) 2) Assemble a bug-report against debian? 3) Assemble a bug-report against Text.Regex.PCRE (or Text.Regex.Base) for "upstream" 4) Uninstall Text.Regex.PCRE (and/or some other packages) and switch to something that works with Unicode/UTF8? Any ideas?

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Summer 2017 Functional Programming / Math internship
by Siddharth Bhat 07 Sep '16

07 Sep '16

Hello, I'm a comp. sci undergrad student in my second year. I'm studying at IIIT-Hyderabad in India. I'm looking for companies that want summer interns for functional programming or pure mathematics related to FP (category theory, type theory, HoTT..). My github is here: http://github.com/bollu Math.stackexhcange: http://math.stackexchange.com/ users/261373/siddharth-bhat Thanks, Siddharth -- Sending this from my phone, please excuse any typos!

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[ANN] brittany - haskell source code formatting tool - experimental/alpha
by lennart spitzner 06 Sep '16

06 Sep '16

Greetings, I am happy to announce a first alpha release of brittany (0.7.0.0), a haskell source code formatting tool based on ghc-exactprint [2] (and thus ghc's parser). https://github.com/lspitzner/brittany Two small feature cases that brittany would produce/leave unmodified: > -- Newlines are used sparingly: Only after "do" and when the > -- `liftBaseOpDiscard` application would lead to overflowing 80 columns. > main :: IO () > main = do > pool <- createPostgresqlPool (toS databaseConnectionString) 10 > initiate $ \chan -> forever $ do > flip runDbConn pool $ do > makeSureQueueIsFull chan > void $ liftBaseOpDiscard (consumeMsgs chan responseQueue Ack) > (uncurry processMsg) > threadDelay 1000000 > -- Alignment of patterns > go [] "" = True > go [WildCard ] "" = True > go (WildCard :rest) (c:cs) = go rest (c : cs) || go (WildCard : rest) cs > go (Union globs:rest) cs = any (\glob -> go (glob ++ rest) cs) globs > go [] (_:_) = False > go (_:_) "" = False Yet this is an alpha release, for the following reasons: - Brittany currently only transforms top-level type signatures and (function) bindings. - Not all syntactical constructs (even of plain 2010 haskell) are understood. All common constructs should work, but "common" is purely subjective. - Not all (syntactic) extensions are supported, either. - Needs more testing. (And less than a hundred automated testcases is not very much to explore every syntactic corner of haskell.) On the other hand - I have started using the tool successfully, albeit selectively. - There are checks in place that detect 1) if output is not valid haskell or 2) if source comments are not transcribed to the output. That is, running into non-handled cases should lead to error messages that are harmless, not into any silent it-overwrote-my-code-with-garbage. *How to build?* Brittany requires ghc-8. See https://github.com/lspitzner/brittany#building A hackage release is not planned yet. *Brittany's formatting style* Currently brittany does _not_ give the user many choices. A few flags exist, most prominently to change the indentation (default 2) or the columns (default 80). I am open to suggestions of how to improve the style, but trying to make everyone happy is not a fulfilling goal for me. *Implementation/documentation/performance* The implementation and its core ideas are not documented. The core algorithm in theory has linear complexity in input size; the implementation probably is quadratic dues to some misuses of plain lists or other such implementation details. Providing at least a high-level overview is on my to-do list. [1] https://github.com/lspitzner/brittany [2] https://hackage.haskell.org/package/ghc-exactprint -- lennart

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Applicative Monoid instance
by Brian Hurt 06 Sep '16

06 Sep '16

This may be a stupid question, but I'm wondering why the following instance isn't in prelude: instance (Applicative m, Monoid a) => Monoid (m a) where mempty = pure mempty mappend a b = mappend <$> a <*> b ? I ask, because being able to use a WriterT (IO ()) STM a to debug complex STM expressions would be really nice. But it depends upon IO () being a monoid. Brian

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[ANN] Microspec
by amindfv＠gmail.com 05 Sep '16

05 Sep '16

Happy to announce the release of a new, tiny property-based test framework: Microspec. Microspec has a clean RSpec/HSpec-style API, depends only on base and QuickCheck, runs unit and property-based tests, and is well under 500 lines of code. If you'd like to add tests to your code but don't like increasing the amount of code you depend on, give Microspec a try: https://hackage.haskell.org/package/microspec Happy hacking! Tom

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