- Haskell-Cafe - Haskell.org

ANN: TestExplode 0.1.0.0: testscript generator for all target languages
by Hans-Juergen Guth 13 Jun '15

13 Jun '15

My new Haskell program/library is released! What it does: ------------- Instead of writing one test-script after another, you define the structure of test-sets. Advantages: ----------- More test-scripts with less code, better overview, easier review of the testcases, easier change and addition of testcases. Details: -------- TestExplode generates testcases out of program snippets. The snippets are treated as text, with variables, that are subsituted (thanks the package perl6-interpolated-strings). The text-snippets are annotated and can form a directed acyclic graph. Each path and each variable substitution is a testcase. Subgraphs can be defined and used in many testcases. The graph can be printed with graphviz. The expected results of the testcase can partially defined by changing the state of the testcase by each testsnippet. So you can define a state-machine in haskell and the annotations to the text-snippets give the input to the state-machine, dependant of the test-input and which path through the graph is chosen. The program/library is designed with users in mind, who have little or no knowledge of haskell at the beginning. It's my first bigger haskell program, so it is probably relativ simple haskell code in the library and in the user interface. Examples can be found in the documentation. Further info: ------------- A "Getting Started": https://hackage.haskell.org/package/TestExplode-0.1.0.0/readme The Library which generates the testcases https://hackage.haskell.org/package/TestExplode-0.1.0.0/docs/TestExplode-Te… The Library which makes FGL-Graphs for the module grahpviz out of my own definition of directed acyclic graphs https://hackage.haskell.org/package/TestExplode-0.1.0.0/docs/TestExplode-Di… I would be happy about comments!

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To "instance C D", GHCI responds that C "is applied to too many type arguments".
by Jeffrey Brown 13 Jun '15

13 Jun '15

jeff@jeffLenovUbu:~$ ghci -XNullaryTypeClasses GHCi, version 7.8.4: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. Prelude> class C Prelude> data D = DConstructor Prelude> instance C D <interactive>:4:10: ‘C’ is applied to too many type arguments In the instance declaration for ‘C D’ Prelude>

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Lazy patterns
by Jon Schneider 13 Jun '15

13 Jun '15

Good day, I am trying to understand the lazy patterns as explained at https://www.haskell.org/tutorial/patterns.html If I type in the example without putting a ~ round the pattern sure enough I do not get an answer as the text says. What I completely fail to understand is why there is no CPU usage, increasing memory usage, stack overflow or other runtime error before I use ^C to stop it. What is going on under the hood ? Jon

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Replacing cpp?
by Mike Meyer 12 Jun '15

12 Jun '15

I know the subject of cpp not working well with Haskell comes up every so often, usually with the conclusion that we need a preprocessor that understands Haskell code. If someone is working on that, this should probably be ignored. Maybe gpp could be used instead of having to write one from scratch? it allows you to define the syntax it uses for macros - both user and "meta". Possibly it's flexible enough to do what's needed?

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Proposal: Shorter Import Syntax
by Anthony Cowley 12 Jun '15

12 Jun '15

Hi Everyone, I didn't think this would see any resistance as it doesn't break anything, but this is the internet, so, if you haven't already, take a look at this GHC feature request <https://ghc.haskell.org/trac/ghc/ticket/10478> The idea is that rather than writing, import Data.Map (Map) import qualified Data.Map as M you could instead write, import Data.Map (Map) as M The Map identifier would imported unqualified, while M would be introduced as an alias for the qualified import of Data.Map. Note that, currently, following a parenthesized group with the "as" keyword is a parse error. So allowing this syntax would not affect any currently working programs. I've mentioned this proposal before and gotten good response, so I finally wrote it up last night after people on IRC responded positively to it. As well as IRC and trac, I put the link up on Twitter to get it in front of a large audience, and here's what we have after a bit over 12 hours (not counting the handful of supporters in IRC): +20 -2 You can see the tweet at <https://twitter.com/a_cowley/status/605909841148702720> I'll try to summarize the two negative votes: 1) This isn't that big a burden, and we should instead focus on controlled export of class instances; and 2) We should instead focus on exporting name spaces, and that the ordering of parentheses and the "as" keyword is too close to the existing "import Foo as F(foo)" syntax. Since there are these negative votes, I think it best if as many people as possible at least see the proposal so the GHC developers can get a better sense for the overall response. Anthony

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GHC.Generics: how to derive a Generic instance for an existential type?
by Andreas Abel 12 Jun '15

12 Jun '15

I wonder whether GHC.Generics supports existential types yet... {-# LANGUAGE DeriveGeneric #-} {-# LANGUAGE ExistentialQuantification #-} {-# LANGUAGE StandaloneDeriving #-} import GHC.Generics data U = forall a. (Generic a) => U a -- deriving (Generic) -- Can't make a derived instance of ‘Generic U’: -- Constructor ‘U’ has existentials or constraints in its type -- Possible fix: use a standalone deriving declaration instead -- deriving instance Generic U -- Can't make a derived instance of ‘Generic U’: -- U must be a vanilla data constructor -- In the stand-alone deriving instance for ‘Generic U’ data D1Ser data C1_0Ser instance Generic U where type Rep U = D D1Ser (C1 C1_0Ser (S1 NoSelector (Rep a))) -- Not in scope: type variable ‘a’ -- How to bring the existential type `a' into scope? -- Andreas Abel <>< Du bist der geliebte Mensch. Department of Computer Science and Engineering Chalmers and Gothenburg University, Sweden andreas.abel(a)gu.se http://www2.tcs.ifi.lmu.de/~abel/

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Lecturer position in Programming Languages / Security at Kent
by Simon Thompson 12 Jun '15

12 Jun '15

We’re looking for excellent PL/security people to recruit as faculty to join us in the School of Computing at Kent. Do apply if you’re interested, or mail me if you want to find out more. Simon Simon Thompson | Professor of Logic and Computation School of Computing | University of Kent | Canterbury, CT2 7NF, UK s.j.thompson(a)kent.ac.uk | M +44 7986 085754 | W www.cs.kent.ac.uk/~sjt

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TH to generate imports?
by Corentin Dupont 11 Jun '15

11 Jun '15

Hi guys, I wanted to know if it's possible to use TH on an "import" statement. What I want to do is to generate the names of the packages I need to import. For example: import $(genImport "MyPackage") genImport :: String -> Q TH.Exp genImport s = ... Is that possible? Thanks! Corentin

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Questions about concurrency and laziness
by Tony Garcia 11 Jun '15

11 Jun '15

Hi, First of all, apologies if this is not the right place to ask this question. I'm reading the book "Parallel and concurrent programming in Haskell" and I'm having some issues reasoning about the impact of laziness. Looking at the first example in chapter 8 [1] main = do m1 <- newEmptyMVar m2 <- newEmptyMVar forkIO $ do r <- getURL "http://www.wikipedia.org/wiki/Shovel" putMVar m1 r forkIO $ do r <- getURL "http://www.wikipedia.org/wiki/Spade" putMVar m2 r r1 <- takeMVar m1 r2 <- takeMVar m2 print (B.length r1, B.length r2) I don't understand why this function is not just putting an unevaluated thunk in the MVar. Well, I assume that getURL is an eager function, but looking at its code [2] or at the documentation of Network.Browser [3] I don't see why... Am I looking at this wrong? Is there any rule of thumb to be used in these cases? I have the impression that it's really easy to end up creating thunks in parallel threads and evaluating them in the main one... Thanks, Jose. [1] http://chimera.labs.oreilly.com/books/1230000000929/ch08.html [2] https://github.com/simonmar/parconc-examples/blob/master/GetURL.hs [3] http://hackage.haskell.org/package/HTTP-4000.0.8/docs/Network-Browser.html

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US Haskell Developer Position
by Christopher Lewis 11 Jun '15

11 Jun '15

Karamaan Group, a principal investment firm based in Manhattan, is looking for an outstanding software developer to develop tools for financial analysis and knowledge management. We are a growth oriented firm that values people who take a craftsman's pride in their work. Our ideal candidate is an experienced software developer with strong analytical, organizational, and communication skills. A candidate who demonstrates an intense focus on quality, but has the ability to recognize and make the tradeoffs that are a necessary part of day-to-day software development. Candidates should have at least a degree in a quantitative field and a keen interest in building robust and elegant computer programs. This is a high-impact, high-visibility position where successful candidates will be entrusted with a lot of responsibility for products that have a direct effect on the P&L of the firm and influences our workflow. The ideal candidate will have experience with Haskell, relational database technologies, and Java. All new development is performed in Haskell, but our legacy Java code base occasionally requires maintenance. Unlike most finance companies, our atmosphere is informal and intellectual. We don't require previous experience in finance or business, but knowledge in those areas is a plus. Karamaan Group is situated in Manhattan, we are an investment adviser to hedge funds and family offices. We value innovative thinking, encourage an entrepreneurial atmosphere, and enjoy spirited conversations. We also offer lunch, and every once in a while we go bowling. Telecommuting is an option, but you must be located in the Continental United States and authorized to work in the United States. Please send your CV and cover letter to recruitment at karamaan dot com. Please do not reply to the original poster.

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Benchmarking two versions of the same package?
by Alp Mestanogullari 11 Jun '15

11 Jun '15

Hi -cafe, While we can easily benchmark different functions or libraries easily with criterion, I can't think of a reasonably easy (and accurate!) way of benchmarking two versions of the same package. And not necessarily version as in cabal version -- one of the use cases I have in mind would be running a benchmark suite whenever a PR gets merged to the main branch of a library, so the benchmark would need to compare the performance of the library's-code-before-merging and after. This definitely can't be accomplished with something like criterion because we can't have two different instances of a package in scope for a module, even with -XPackageImports. If we separately build the same program against two instances of the same library and run the benchmarks separately, this might happen far apart enough that the machine running this might be under a different load. This does however seem to be the only actual solution? Run separately at two different commits, diff the numbers, report. I have felt the need for a solution to this for quite some time, but have never needed it bad enough that it became a priority. Thinking about it again today, I thought I should drop an email to the list and see if fellow haskellers have an easy-to-use solution for this? I may very well be overlooking something. -- Alp Mestanogullari

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haskell team at megacorp is hiring!
by Carter Schonwald 11 Jun '15

11 Jun '15

Dear Haskell Colleagues, my team at megacorp, one of the largest for profit organizations on the planet, is looking to grow! about the haskell team: currently 3 engineers, though a 4th on the way already, in the past 6 months alone we've deployed customer facing APIs into production (no small feat in megacorp), built self serve generative sampling based data anonymization tools that promise to transform how megacorp customer data gets shared within and without. And thats just to start! We've repeatedly demonstrated that our use of haskell helps us develop and evolve great software for the business. the haskell team is part of a larger "new product / R&D" group whose mission is to build new tools, products and services to help pull megacorp kicking and screaming into the 2010's in terms of technology and services. The new product group is broadly seeking competent thoughtful technologists across a range of skill sets -- frontend, python, java, and haskell -- to join the engineering team. (so even if you're not the perfect fit for the team, we may know great folks we can point you to) *The Perks* *• *our management fully supports upstreaming bugfixes and improvements to the open source tools that we use • amazing view out of our Brooklyn and/or NYC offices (you will need to be in NYC to work with us, though I believe we can help with relocation) • by dint of the sheer scale of megacorp, every successful project will have pretty huge impact • because we're awesome, we will make sure you get your own mac along with the standard megacorp workstation. • picking a new project to work on every 3-6 months is the norm, owing to our role as a sort of prototyping / R&D group. *The Caveats* • it is megacorp, its a huge organization, and while the new product group is shielded from a lot of that, it can still be overwhelming • you will have to live somewhere within commuting distance of Brooklyn New York. • you have to deal with me and a few other interesting colleagues on a daily basis. *Should you apply?* • we welcome candidates of all experience levels, junior or senior. • we'd love it if you have some portfolio work/open source/work experience that demonstrates your ability to tackle engineering projects headon or master some nontrivial technical problem domain. • colleagues who take teaching/learning new things seriously • you'd enjoy working out of Brooklyn NYC. • you enjoy programming / software engineering and try to do it well. *how to apply* •*email me with the subject "functional megacorp job"*, attaching your resume/cv and if you want, a teeny cover letter that articulates your unique strengths and interests as a professional software engineer. •I can tell you more about the company and team via email. I or another member of the engineering team will be screening resumes first before kicking off any interview processes, so there will not be any HR limbo going on. Every email will get a reply. (NB: actual org name is easy to figure out with some googling, but for various reasons cannot be used publicly by its employees without an authorization process that befits a megacorp) also megacorp is is an equal opportunity and affirmative action employer

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Thank you to new maintainers
by Fred Ross 10 Jun '15

10 Jun '15

The libraries I used maintain have all be handed off. Thank you to: Adam Bergmark for taking over hscamwire. Lana Black for taking over hdaemonize. Jose Calderon for taking over serial. And goodbye for now to this wonderful community. Maybe I'll be back in a few years. Unless I become a field ecologist or a florist or something. Fred Ross http://madhadron.com

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Haskell developer role with Strats at Standard Chartered London
by Don Stewart 10 Jun '15

10 Jun '15

Hi all, I'm hiring (another) Haskell dev to join our team in London. Details here: https://donsbot.wordpress.com/2015/06/10/haskell-dev-role-in-strats-at-stan…

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[ANN] Lentil - a frugal issue tracker
by Francesco Ariis 10 Jun '15

10 Jun '15

lentil - a frugal issue tracker If you litter your code with TODOs and FIXMEs, lentil [1] can help you make sense of that big mess into something nice and readable like: src/Lentil/File.hs 49 does upMan work windows? [feature:intermediate] src/Lentil/Parse.hs 26 eliminate lookahead? refactor parsing? [lint] [refactor] 54 add test for rotoscope [test] It supports tags, filtering, exporting and much more. Check the user manual [2] for more information. [1] http://hackage.haskell.org/package/lentil [2] http://www.ariis.it/static/articles/lentil-manual/page.html

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[Haddock] Alphabetize record accessors
by aditya siram 09 Jun '15

09 Jun '15

How would people feel about Haddock alphabetizing record accessors by default? For example: data T { b :: ..., c :: ... , a :: ... } displays as: data T { a :: ..., b :: ... , c :: ... } Seems as though projects that have large records like Cabal & Parsec would benefit from this. Thanks! -deech

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And, Or, Not, and [something atomic] data types: What am I doing?
by Jeffrey Brown 09 Jun '15

09 Jun '15

I am considering the problem of representing search results in a graph database. (Define such a database as a collection N of nodes and E of labeled edges, where a labeled edge consists of a triplet (l, n1, n2), where l is some kind of label and n1 and n2 are members of N.) I think a type something like the following would be useful: data Search = Fan { origin :: Node , label :: Label , members :: [Node] } | And Search Search | Or Search Search | Not Search In a Fan with origin o, members would be the set of Nodes p such that (label, o, p) is an edge. A Search is ultimately built out of Fans. I've seen this kind of thing before, and I believe it will lead me to some substantial problems that other people have already solved. What should I study?

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parsec or attoparsec for 40-50MB text files ?
by briand＠aracnet.com 09 Jun '15

09 Jun '15

Hi, My file is pretty straightforward text file with a small amount of somewhat annoying state: comments* config line comments* data line* if there is no config line it's an error. the data lines can have a variable number of values and it matters how many values there are (hey- it's not my file format !). the data lines can also have a comment at the end. My initial thought was to go with parsec but the data files could be as large as 40-50MB and upon further reading it really seemed like attoparsec would be better. Error handling wouldn't be too sophisticated. if a data line has something other than 1 or more floating point values and the optional comment, failing out with "error line X" is fine. parse time is somewhat critical only because i'll have multiple files to parse, so while 5-10 seconds is ok for one file, i have to multiply that by 5-10. I've seen several comments talking about the fact that parsec can be slow, but so far unable to find anything the quantifies "slow". Any opinions on which would be better for my application (although i think i've just talked myself into using attoparsec) ? In particular- am i going to get at least reasonable "error on line X" error handling using attoparsec ? Thanks, Brian

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Re: [Haskell-cafe] zipWith and divide
by Rehno Lindeque 07 Jun '15

07 Jun '15

Hi! > zipWith(\) z1 z2 ........ this fails but works when using * or + or - > ....... I also tried using div and `div` but they failed to. > The divide operator is slash (/) not backslash (\): > zipWith (/) [0.1, 0.2] [0.2, 0.4] [0.5,0.5] Also, div should work as long you're using a Integral type like Int (you may be using Float or Double): > zipWith div [1, 2] [2, 2] [0,1] Cheers

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ANN: nonce package
by Felipe Lessa 07 Jun '15

07 Jun '15

(Please forgive me if you received multiple copies of this e-mail.) Hello, The nonce package [1] contains functions to easily generate cryptographic nonces for many situations. Some places where these generated nonces can be used include: - Password recovery e-mail tokens. - XSRF protection tokens. - Session IDs sent on cookies. - Initialization vectors. It uses an AES CPRNG periodically reseeded from /dev/urandom (or equivalent). It has no frills, no knobs, so it's hard to misuse. It's been available for an year but I just realized I've never properly announced it. Regrettably, I've seen many uses of the random package (System.Random) when generating nonces. It's a bad choice: it is not a cryptographically secure PRNG, contains low entropy (64-bit state), and its default usage is seeded predictably (using a constant seed). Please avoid using the random package for generating nonces at all costs. In its stead, use the nonce package or something similar. Cheers, [1] http://hackage.haskell.org/package/nonce -- Felipe.

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Re: [Haskell-cafe] Attacking Attachment "Cancelled Event" Bayhac 15
by Byron Hale 07 Jun '15

07 Jun '15

Bayhac 15 has not been cancelled. See http://bayhac.org/ @bayhac2015 Do NOT open the attachment to the cancelling email. It is likely a sophisticated attack. The listed "organizer" is not. We may, however, have modifications. For example, we are still seeking donors for food. We are still seeking speakers, maybe even some repeats of last year's speakers. You might just dust off last year's presentation, until we run out of room. Best Regards, Byron Hale byron.hale(a)einfo.com @Hale_ByronL

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ANN: bytestring-lexing 0.5.0
by wren romano 07 Jun '15

07 Jun '15

-------------------------------------------- -- bytestring-lexing 0.5.0 -------------------------------------------- The bytestring-lexing package offers extremely efficient bytestring parsers for some common lexemes: namely integral and fractional numbers. In addition, it provides efficient serializers for (some of) the formats it parses. As of version 0.3.0, bytestring-lexing offers the best-in-show parsers for integral values. (According to the Warp web server's benchmark of parsing the Content-Length field of HTTP headers.) And as of this version (0.5.0) it offers (to my knowledge) the best-in-show parser for fractional/floating numbers. -------------------------------------------- -- Changes since 0.4.3 (2013-03-21) -------------------------------------------- I've completely overhauled the parsers for fractional numbers. The old Data.ByteString.Lex.Double and Data.ByteString.Lex.Lazy.Double modules have been removed, as has their reliance on Alex as a build tool. I know some users were reluctant to use bytestring-lexing because of that dependency, and forked their own version of bytestring-lexing-0.3.0's integral parsers. This is no longer an issue, and those users are requested to switch over to using bytestring-lexing. The old modules are replaced by the new Data.ByteString.Lex.Fractional module. This module provides two variants of the primary parsers. The readDecimal and readExponential functions are very simple and should suffice for most users' needs. The readDecimalLimited and readExponentialLimited are variants which take an argument specifying the desired precision limit (in decimal digits). With care, the limited-precision parsers can perform far more efficiently than the unlimited-precision parsers. Performance aside, they can also be used to intentionally restrict the precision of your program's inputs. -------------------------------------------- -- Benchmarks -------------------------------------------- For the Criterion output of the benchmark discussed below, see <http://community.haskell.org/~wren/bytestring-lexing/bench/html/readExponen…>. The main competitors we compare against are the previous version of bytestring-lexing (which already surpassed text and attoparsec/scientific) and bytestring-read which was the previous best-in-show. The unlimited-precision parsers provide 3.3x to 3.9x speedup over the readDouble function from bytestring-lexing-0.4.3.3, as well as being polymorphic over all Fractional values. For Float/Double: these functions have essentially the same performance as bytestring-read on reasonable inputs (1.07x to 0.89x), but for inputs which have far more precision than Float/Double can handle these functions are much slower than bytestring-read (0.30x 'speedup'). However, for Rational: these functions provide 1.26x to 1.96x speedup compared to bytestring-read. The limited-precision parsers do even better, but require some care to use properly. For types with infinite precision (e.g., Rational) we can pass in an 'infinite' limit by passing the length of the input string plus one. For Rational: doing so provides 1.5x speedup over the unlimited-precision parsers (and 1.9x to 3x speedup over bytestring-read), because we can avoid intermediate renormalizations. Whether other unlimited precision types would see the same benefit remains an open question. For types with inherently limited precision (e.g., Float/Double), we could either pass in an 'infinite' limit or we could pass in the actual inherent limit. For types with inherently limited precision, passing in an 'infinite' limit degrades performance compared to the unlimited-precision parsers (0.51x to 0.8x 'speedup'). Whereas, passing in the actual inherent limit gives 1.3x to 4.5x speedup over the unlimited-precision parsers. They also provide 1.2x to 1.4x speedup over bytestring-read; for a total of 5.1x to 14.4x speedup over bytestring-lexing-0.4.3.3! -------------------------------------------- -- Links -------------------------------------------- Homepage: http://code.haskell.org/~wren/ Hackage: http://hackage.haskell.org/package/bytestring-lexing Darcs: http://community.haskell.org/~wren/bytestring-lexing Haddock (Darcs version): http://community.haskell.org/~wren/bytestring-lexing/dist/doc/html/bytestri… -- Live well, ~wren

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Re: [Haskell-cafe] Proposal: Shorter Import Syntax
by Anthony Cowley 05 Jun '15

05 Jun '15

> On Jun 5, 2015, at 4:14 PM, Marcin Mrotek <marcin.jan.mrotek(a)gmail.com> wrote: > > +1, though something alleviating the burden of importing the same module twice, like: > > import qualified Text as Text > import Text ( {- something -} ) > > would be even better. That is what this does. You will be able to write: import Text ( {- something -} ) as Text Anthony

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Re: [Haskell-cafe] Proposal: Shorter Import Syntax
by malcolm.wallace 05 Jun '15

05 Jun '15

Here are some more numbers. In our private code base of ~12,400 modules totalling >2,300,000 LoC, there are ~143,800 import lines, of which ~65,000 use the "as" syntax. Of those, ~8,000 have no "qualified", and the remaining ~57,000 use "qualified". So I make that about 5.6% of our imports. I think this is quite a lot higher than the Stackage figure, because this code is mostly applications rather than libraries, so it tends to use a larger range of imported functionality from lots of different semi-independent domains. The chance of naming-overlap for separate concepts is therefore much greater, leading to our common idiom for optional qualification of names with short module synonyms (often single letters). Regards, Malcolm On 03 Jun, 2015,at 08:08 PM, Anthony Cowley <acowley(a)seas.upenn.edu> wrote: I grepped through the source of all the packages in the Stackage Nightly package set and found that less 0.3% of imports use that syntax. It's a small number, but it's not zero, so the breakage would be unacceptable.

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Rounding
by ok＠cs.otago.ac.nz 05 Jun '15

05 Jun '15

The Haskell 2010 report defines, in chapter 9, round :: (Real a, Fractional a, Integral b) => a -> b round x = let (n, r) = properFraction x -- n = truncate x, r = x-n (same sign as x) m = if r < 0 then n - 1 else n + 1 in case signum (abs r - 0.5) of -1 -> n -- round in if |r| < 0.5 1 -> m -- round out if |r| > 0.5 0 -> if even n then n else m (commented and slightly rearranged). The traditional definition of rounding to integer, so traditional that it is actually given in the OED, is basically round x = truncate (x + signum x * 0.5) There was a discussion of rounding recently in another mailing list and I put together this table: * Round x.5 OUT Ada, Algol W, C, COBOL, Fortran, Matlab, Pascal, PL/I, Python, Quintus Prolog, Smalltalk. The pre-computing tradition. * Round x.5 to EVEN Common Lisp, R, Haskell, SML, F#, Wolfram Language. * Round x.5 UP to positive infinity Java, JavaScript, ISO Prolog, Algol 60 * Rounding of x.5 UNSPECIFIED Algol 68, IMP 77 What I was wondering was whether anyone on this list knew why Haskell has the break-ties-to-even definition instead of the traditional break-ties-out one. (And please don't say that it is to get statistical unbiasedness, because given the kinds of data distribution I see, it _isn't_ unbiased.)

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The evil GADTs extension in ghci 7.8.4 (maybe in other versions too?)
by Ki Yung Ahn 05 Jun '15

05 Jun '15

\y -> let x = (\z -> y) in x x is a perfectly fine there whose type is a -> a. (1) With no options, ghci infers its type correctly. (2) However, with -XGADTs, type check fails and raises occurs check. (3) We can remedy this by supplying some additional options (4) Howver, if you put -XGADTs option at the end, it fails again :( kyagrd@kyahp:~$ ghci GHCi, version 7.8.4: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. Prelude> :t \y -> let x = (\z -> y) in x x \y -> let x = (\z -> y) in x x :: t -> t Prelude> :q Leaving GHCi. kyagrd@kyahp:~$ ghci -XGADTs GHCi, version 7.8.4: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. Prelude> :t \y -> let x = (\z -> y) in x x <interactive>:1:30: Occurs check: cannot construct the infinite type: t0 ~ t0 -> t Relevant bindings include x :: t0 -> t (bound at <interactive>:1:11) y :: t (bound at <interactive>:1:2) In the first argument of ‘x’, namely ‘x’ In the expression: x x Prelude> :q Leaving GHCi. ~$ ghci -XGADTs -XNoMonoLocalBinds -XNoMonomorphismRestriction GHCi, version 7.8.4: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. Prelude> :t \y -> let x = (\z -> y) in x x \y -> let x = (\z -> y) in x x :: t -> t Prelude> :q Leaving GHCi. ~$ ghci -XNoMonoLocalBinds -XNoMonomorphismRestriction -XGADTs GHCi, version 7.8.4: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. Prelude> :t \y -> let x = (\z -> y) in x x <interactive>:1:30: Occurs check: cannot construct the infinite type: t0 ~ t0 -> t Relevant bindings include x :: t0 -> t (bound at <interactive>:1:11) y :: t (bound at <interactive>:1:2) In the first argument of ‘x’, namely ‘x’

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Consecutive FFI calls
by David Turner 03 Jun '15

03 Jun '15

Hi, If I make a sequence of FFI calls (on a single Haskell thread) but which end up being called from different OS threads, is there any kind of ordering guarantee given? More specifically, is there a full memory barrier at the point where a Haskell thread migrates to a new OS thread? Many thanks, David

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Pixel by pixel image processing
by Geraldus 03 Jun '15

03 Jun '15

Hi cafe! I want to do some trivial masking manipulation on PNG images: take a picture, crop it to square shape and then make a circular mask, i.e. take all pixels that lies outside circle with radius equal to half of image width and placed at image center and replace that pixels with zero opacity once (hope this description is clear enough). I've found two packages for image processing so far: `friday` [1] and `unm-hip` [2], but can't figure out which of them suits my needs, maybe there are some other packages I miss? [1] https://hackage.haskell.org/package/friday [2] https://hackage.haskell.org/package/unm-hip

4 4

Haskell dev role in Strats at Standard Chartered (Singapore)
by Don Stewart 03 Jun '15

03 Jun '15

I have an open position for a Haskell developer to join Standard Chartered's Strats team in Singapore. More details at: https://donsbot.wordpress.com/2015/06/03/haskell-dev-role-in-strats-at-stan… -- Don

1 0

Blog platform written in Haskell
by Nicola Gigante 02 Jun '15

02 Jun '15

Hi all, Is there a blog platform written in Haskell? I mean something dynamic (as opposed to a static site generator like hakyll) like WordPress but written in Haskell. Does something like that exist? Greetings, Nicola

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Four Designs for a Monad
by Zemyla 02 Jun '15

02 Jun '15

I'm working on writing a Haskell wrapper for the GNU Lightning code generation library (http://www.gnu.org/software/lightning/) which in C consists of a series of macros that write to a certain global state in a pointer _jit, such as jit_add or jit_call. Now, I figure that can be wrapped fairly simply in a monad, which we'll call the Lightning monad: -- syntax given here is just a sample. incr = lightningEmit $ do prolog in <- arg getArg R0, in addi R0, R0, 1 retr R0 However, I've come up with four possible designs for the Lightning monad. All but one would be reader monad transformers holding the pointer to the JIT state. 1) Lightning m a, where m is a MonadIO. Pros: Users have the power of the monads they are using within. Cons: Complexity, especially with having to handle monads with multiple returns like ListT. 2) Lightning a, which just wraps IO, and is a MonadIO. Pros: Allows the use of other IO functions in the monad. Cons: Allows the use of other IO functions in the monad. 3) Lightning a, which just wraps IO, and is not a MonadIO. Pros: Simplicity; possibly permitting the code generation to be unwrapped with unsafePerformIO. Cons: Referential integrity could still be violated. 4) Lightning a, which would be a free monad over the Lightning operations allowed. Pros: Evaluation (in the IO monad) would be certain to be pure, which would permit or accommodate it being a monad transformer, or converted to a pure function on architectures where the Lightning library is not available. Cons: Assembling, then disassembling to reassemble, a long list of instructions produces significant overhead, rendering it less suitable for JIT purposes. Interacting with the generated code is more difficult. Are there any other options I am missing? Are there any possibly game-changing pros or cons I have failed to notice? Thanks, Paul Drees

1 0

I think someone had a complicated program to use brackets for array indexing - is it possible to use a DSL for this?
by KC 02 Jun '15

02 Jun '15

I think someone had a complicated program to use brackets for array indexing - is it possible to use a DSL for this? That is, to use myArray [5] and have a DSL convert it to standard Haskell syntax -- -- Sent from an expensive device which will be obsolete in a few months! :D Casey

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ANN: Bayhac 2015 June 12, 13, 14 in Mountain View, CA
by Byron Hale 02 Jun '15

02 Jun '15

Bayhac 2015 will be held June 12, 13, 14 at Hackers Dojo in Mountain View, CA. See http://bayhac.org/ to register. Light rail service is available from Caltrain to near Hackers Dojo. In addition, there can be shuttle service from Caltrain first thing in the morning and in the evening. Some known speakers are: Conal Elliott, Phil Freeman, Greg Weber, and Dan Burton You! If you would like to give a talk, send an email to bayhac2015-admin(a)googlegroups.com with the title and a brief explanation of what the talk will be on. It can be on anything Haskell related! We are also looking out for people who might be interested in conducting a more hands-on workshop. Twitter: https://twitter.com/bayhac2015 Byron Hale byron.hale(a)einfo.com @Hale_ByronL

1 0

ANN: toysolver 0.3.0 released
by Masahiro Sakai 01 Jun '15

01 Jun '15

Dear all, I'm announcing the release of the toysolver package. http://hackage.haskell.org/package/toysolver https://github.com/msakai/toysolver It provides solver implementations of various problems including SAT, Max-SAT, PBS (Pseudo Boolean Satisfaction), PBO (Pseudo Boolean Optimization), MILP (Mixed Integer Linear Programming) and non-linear real arithmetic. In particular it contains moderately-fast pure-Haskell SAT solver 'toysat'. It can also be used as a backend for ersatz and satchmo. http://hackage.haskell.org/package/ersatz-toysat http://hackage.haskell.org/package/satchmo-toysat Regards, -- Masahiro Sakai

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A Dockerfile to provision a build environment for an Android game written in Haskell
by Sean Seefried 01 Jun '15

01 Jun '15

Fellow Haskell users! For the last few months I've been working on a small game targeting Android devices. It's still in development and will probably change radically from what I've already done. However, in the process of developing this game I've managed to develop a Dockerfile that will provision a build environment to build the game, which I think is worth releasing to the Haskell community right away. I'd appreciate some assistance in validating that it, in fact, works. The repo containing the Dockerfile is hosted at: https://github.com/sseefried/docker-epidemic-build-env The repo containing the game is: https://github.com/sseefried/open-epidemic-game Please use the latest version of Docker when you attempt to build this, at least version 1.6. Be aware that this will take a long time to build. You will need to download an image from THe Docker Hub Registry that is about 1.1G and then pull down even more files when building all the C and Haskell libraries. Be patient and if you have any issues please let me know via GitHub. Oh, and if you want to see a demo of what you are building click here: https://goo.gl/FZFPsm (It's a very simple game at present. Don't expect too much.) Enjoy! Sean

1 0

.a files not found
by Henk-Jan van Tuyl 30 May '15

30 May '15

L.S., I get messages about .a files not being found, when I try to install package data-default-instances-base, data-default-instances-containers, data-default-instances-dlist or data-default-instances-old-locale. How can I solve this? I am using GHC 7.10.1 on Windows. Sample output: C:\X\data-default-instances-old-locale-0.0.1> cabal install Resolving dependencies... Configuring data-default-instances-old-locale-0.0.1... Building data-default-instances-old-locale-0.0.1... Failed to install data-default-instances-old-locale-0.0.1 Build log ( C:\Users\X\AppData\Roaming\cabal\logs\data-default-instances-old-locale-0.0.1.log ): Building data-default-instances-old-locale-0.0.1... Preprocessing library data-default-instances-old-locale-0.0.1... [1 of 1] Compiling Data.Default.Instances.OldLocale ( Data\Default\Instances\OldLocale.hs, dist\build\Data\Default\Instances\OldLocale.o ) C:\Programs\Haskell\MinGHC-7.10.1\ghc-7.10.1\mingw\bin\ar.exe: dist\build\libHSdata-default-instances-old-locale-0.0.1-6jcjjaR25tK4x3nJhHHjFM.a-2192\libHSdata-default-instances-old-locale-0.0.1-6jcjjaR25tK4x3nJhHHjFM.a: No such file or directory cabal: Error: some packages failed to install: data-default-instances-old-locale-0.0.1 failed during the building phase. The exception was: ExitFailure 1 Regards, Henk-Jan van Tuyl -- Folding@home What if you could share your unused computer power to help find a cure? In just 5 minutes you can join the world's biggest networked computer and get us closer sooner. Watch the video. http://folding.stanford.edu/ http://Van.Tuyl.eu/ http://members.chello.nl/hjgtuyl/tourdemonad.html Haskell programming --

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foldt fix proposal (HaskellWiki)
by Michael V. Antosha 30 May '15

30 May '15

hi ALL, I think I've found a mistake in foldt implementation on the Fold page[1] of the HaskellWiki. foldt :: (a -> a -> a) -> a -> [a] -> a foldt f z [] = z foldt f z [x] = x -- mistake? -- foldt f z [x] = f x z -- fix proposal foldt f z xs = foldt f z (pairs f xs) pairs :: (a -> a -> a) -> [a] -> [a] pairs f (x:y:t) = f x y : pairs f t pairs f t = t Without the fix, foldt just ignores z value in all cases except for the case of empty input list. (Which is an inconsistent behaviour I suppose.) The Examples page[2] also needs corresponding fix. Is this mail list a right place to ask for the HaskellWiki page fix? Thanks! Michael. [1] Tree-like folds @ Fold @ HaskellWiki https://wiki.haskell.org/Fold#Tree-like_folds [2] Examples @ Fold @ HaskellWiki https://wiki.haskell.org/Fold#Examples -- Michael V. Antosha http://mivael.in.ua xmpp:m@mivael.in.ua (Jabber ID)

2 1

Why is vector sort slower than list sort?
by Holden Lee 30 May '15

30 May '15

>From places like http://programmers.stackexchange.com/questions/180417/fastest-haskell-libra… and http://stackoverflow.com/questions/3655329/how-does-one-sort-with-data-vect… I'm led to believe that sorting mutable vectors is faster than sorting lists. However, when running on my own computer, I'm seeing the opposite effect. What's wrong with what I'm doing, or am I misunderstanding something? What's the fastest way to sort? Here's the code: Sort0.hs (List version) import Control.Monad import System.Random import qualified Data.Vector as IV import qualified Data.Vector.Mutable as MV import qualified Data.Vector.Generic as V import qualified Data.Vector.Algorithms.Intro as VA import qualified Data.List as L randList :: IO [Int] randList = sequence $ replicate 1000000 (randomIO :: IO Int) main = do v <- randList print $ L.sort v Here's Sort1.hs import Control.Monad import System.Random import qualified Data.Vector as IV import qualified Data.Vector.Mutable as MV import qualified Data.Vector.Generic as V import qualified Data.Vector.Algorithms.Intro as VA randVector :: IO (MV.IOVector Int) randVector = do v <- MV.new 1000000 forM [0..999999] $ \x -> do r <- randomIO :: IO Int MV.write v x r return v main = do v <- randVector VA.sort v iv <- V.unsafeFreeze v :: IO (IV.Vector Int) print . V.toList $ iv I'm compiling and running as follows: ghc -prof -fprof-auto -rtsopts -o Sort0 Sort0.hs ./Sort0 +RTS -p -sstderr -K99999999 -RTS Here are the traces; the first one is Sort0.hs (list version) and takes ~11s while the other takes ~17s: Sat May 30 05:06 2015 Time and Allocation Profiling Report (Final) Sort0.exe +RTS -p -sstderr -K99999999 -RTS total time = *11.43 secs* (11434 ticks @ 1000 us, 1 processor) total alloc = 1,966,669,596 bytes (excludes profiling overheads) COST CENTRE MODULE %time %alloc main Main 61.4 49.4 randList Main 38.6 50.6 individual inherited COST CENTRE MODULE no. entries %time %alloc %time %alloc MAIN MAIN 41 0 0.0 0.0 100.0 100.0 CAF GHC.IO.Encoding.CodePage 69 0 0.0 0.0 0.0 0.0 CAF GHC.IO.Encoding 67 0 0.0 0.0 0.0 0.0 CAF System.CPUTime 59 0 0.0 0.0 0.0 0.0 CAF GHC.IO.Handle.FD 58 0 0.0 0.0 0.0 0.0 CAF Data.Fixed 54 0 0.0 0.0 0.0 0.0 CAF Data.Time.Clock.POSIX 50 0 0.0 0.0 0.0 0.0 CAF System.Random 49 0 0.0 0.0 0.0 0.0 CAF Main 48 0 0.0 0.0 100.0 100.0 randList Main 83 1 1.7 3.1 1.7 3.1 main Main 82 1 61.4 49.4 98.3 96.9 randList Main 84 0 36.9 47.6 36.9 47.6 Sat May 30 04:59 2015 Time and Allocation Profiling Report (Final) Sort1.exe +RTS -p -sstderr -K99999999 -RTS total time = * 17.07 secs* (17066 ticks @ 1000 us, 1 processor) total alloc = 5,783,472,528 bytes (excludes profiling overheads) COST CENTRE MODULE %time %alloc main Main 71.9 81.5 randVector.\ Main 25.0 16.5 randVector Main 3.1 2.0 individual inherited COST CENTRE MODULE no. entries %time %alloc %time %alloc MAIN MAIN 60 0 0.0 0.0 100.0 100.0 CAF GHC.IO.Encoding.CodePage 108 0 0.0 0.0 0.0 0.0 CAF GHC.IO.Encoding 106 0 0.0 0.0 0.0 0.0 CAF System.CPUTime 98 0 0.0 0.0 0.0 0.0 CAF GHC.IO.Handle.FD 95 0 0.0 0.0 0.0 0.0 CAF Data.Fixed 87 0 0.0 0.0 0.0 0.0 CAF Data.Time.Clock.POSIX 82 0 0.0 0.0 0.0 0.0 CAF System.Random 81 0 0.0 0.0 0.0 0.0 CAF Main 67 0 0.0 0.0 100.0 100.0 randVector Main 121 1 0.0 0.0 0.0 0.0 main Main 120 1 71.9 81.5 100.0 100.0 randVector Main 122 0 3.1 2.0 28.1 18.5 randVector.\ Main 123 1000000 25.0 16.5 25.0 16.5 Thanks, Holden

2 1

Threads and critical sections (threadWaitRead)
by Lars Petersen 30 May '15

30 May '15

Hello cafe, I'm currently trying to understand Haskell's threading model and GHC's implementation of it. Specifically I wondered about the safety of file IO. Consider this piece of code: > newtype Socket = Socket (MVar Fd) > > read :: Socket -> IO ByteString > read (Socket mfd) = do > fd <- readMVar mfd > -- (1) > threadWaitRead fd > -- (2) > withMVar mfd $ \fd'-> do > -- (3) the actual read... Context: - The socket is configured non-blocking. - No other operation will hold the MVar over a blocking call (because this would make it impossible to close it while it's blocking). - My close operation will fill the MVar with -1 after it's closed and will hold the MVar during the close operation. Questions: - Is it theoretically possible that my thread gets paused at (1) or within threadWaitRead, another thread closes the socket (or even worse opens another file with the same fd) _and_ my thread is resumed afterwards (I'm not asking about async exceptions)? - What constructs contain safepoints and which are guaranteed to be executed without interruption? Considerations (so far): - It is unlikely that (1) is a safepoint as no memory allocations occur, but is it guaranteed? - If the socket were closed during (1), threadWaitRead throws an exception complaining about a bad file descriptor which is unexpected but not harmful. - I found this thread (https://mail.haskell.org/pipermail/haskell-cafe/2014-September/115841.html) which is somewhat related, but I couldn't extract a clear answer from it. Best, Lars

2 1

Disambiguating a Num/RealFrac instance
by amindfv＠gmail.com 30 May '15

30 May '15

Given: data GPA = F Float | Excuse String class ToGPA a where giveGPA :: a -> GPA Is there any way (without IncoherentInstances or Rebindablesyntax) that I can let the user write e.g. "giveGPA 4.0" (and "giveGPA 4") and get back "F 4" without getting type errors that "4.0"'s type is ambiguous? I can guarantee there won't be any additional instances to "ToGPA" (For my uses, I really (actually!) don't want to make the user write e.g. "F 4.0") I've gotten this working, once with IncoherentInstances and once with RebindableSyntax, but both of those flags feel pretty dirty. Thanks, Tom

3 3

GHC + musl = easier static linking
by Marios Titas 29 May '15

29 May '15

TL;DR: I made some GHC binaries for linux that rely on musl instead of glibc for easier static linking. Scroll down for the link. Dear *, As most of you know, musl is a linux only C standard library that is optimized for static linking. Statically linked binaries can simplify distribution and deployment. If you have tried to statically link a program with glibc, you will probably know that this does not always work as expected: for example, in many cases the resulting binary will require to be executed in a system that has the exact same version of glibc as the one that was used to compiled it. In essence, static binaries linked with glibc are oftentimes *less* portable than dynamic ones. I decided to see if I could use GHC with musl. The problem was that while I could find some references online, I could not find a precompiled version of GHC that uses musl. So I decided to try and a build one myself. I was able to successfully bootstrap GHC under musl and I am posting it today in case someone else finds it usefull: https://drive.google.com/folderview?id=0B0zktggRQYRkbGJkbmU0UFFSYUE#list I posting this on google drive, I have no better place to host this, hopefully google will not disable my account :-) This is a fully bootstrapped (i.e. stage 2) GHC and *not* a cross compiler. So this will *not* work in a typical glibc based linux distribution. You need a complete musl based environment to use it (see bellow for that). Also, the binaries produced by this GHC will all depend on musl and not work on most glibc based distros. On the other hand, statically compiled binaries should be very portable and will not depend on any particular C standard library. I have done some minimal testing and it seems to work rather well; everything I tried to compile from hackage just worked. Additionally it can compile GHC itself which is always a good test. The size of the resulting static binaries is acceptable. In my 64 bit system, a simple hello world has the following sizes (stripped): 800K glibc dynanic, 1648K glibc static, and 1012K musl static. Why not use a cross-compiler? I have found that in practice it is easier to have a separete environment for producing static binaries. A cross-compiler requires that you also cross-compile all the C libraries that you use. Additionally, cabal has still problems (e.g. [1]) with cross-compiling and will not work out of the box. Note that musl is not 100% compatible with glibc, so some things might work slightly differently. If you do not like using a GHC precompiled by someone else, you can use my binaries to compile GHC under musl yourself; it would be easier than cross-compiling GHC from scratch. So how do you use this? You need a complete musl based environment. There a few musl based distros [2], albeit most of them are geared towards embedded applications. I personally use the experimental musl based gentoo image [3] which feels more like a regular distro. You can use it with chroot, lxc, or systemd-nspawn. For the sake of completeness, I have included at the end of this message some quick and dirty instructions on how to do this. [1] https://github.com/haskell/cabal/issues/1493 [2] http://wiki.musl-libc.org/wiki/Projects_using_musl [3] http://distfiles.gentoo.org/experimental/amd64/musl/ 8<------------------------------------- # gentoo stage3 image instructions CHROOT_PATH=/var/tmp/gentoo-musl cd /var/tmp wget http://distfiles.gentoo.org/experimental/amd64/musl/stage3-amd64-musl-vanil… mkdir "$CHROOT_PATH" tar xjf stage3-amd64-musl-vanilla-20150405.tar.bz2 -C "$CHROOT_PATH" for x in dev proc sys; do mount --bind /$x "$CHROOT_PATH/$x"; done cp -L /etc/resolv.conf "$CHROOT_PATH/etc" chroot "$CHROOT_PATH" /bin/bash -l PS1="(chroot) $PS1" # fetch the gentoo package db for the first time (slow): emerge-webrsync # enable static libraries (*.a), which are disabled by default: sed -ie 's/USE="/USE="static-libs /' /etc/portage/make.conf # reinstall/recompile gmp so that we have its static libs: emerge --oneshot dev-libs/gmp # install ghc: tar xJf /path/to/ghc-7.10.1-x86_64-unknown-linux-musl.tar.xz -C /tmp cd /tmp/ghc-7.10.1; ./configure --prefix=/opt/ghc-7.10.1; make install # compile a hello world: cd /tmp echo 'main = putStrLn "Hello world"' > test.hs /opt/ghc-7.10.1/bin/ghc --make -O -optl-static test.hs ./test file ./test # Of course, using chroot this way is insecure, don't use to install packages from hackage.

4 5

Annoucing Eddie 1.0.0
by Mike Meyer 29 May '15

29 May '15

Eddie is a tool to let you use Haskell code in shell scripts, in a manner similar to the way awk and other scripting languages can be used. The jump to 1.0.0 isn't because it's finished, but because this is a rewrite from the ground up to incorporate more modern Haskell features (the old version was my first non-trivial Haskell program), resulting in changes to various behaviors as well as some option tweaks. Most notably, it no longer uses [Char] for everything. In text mode, it uses Text, with options to specify both input and output encodings. In binary mode, it uses ByteString, with an option to set the output back to Text so you can generate summaries as opposed to simply filter. To make using Text and ByteString sane, it also uses ClassyPrelude instead of Prelude + Data.Char & Data.List so your Haskell list habits work - at least mostly. See CHANGES.md in the source or ChangeLog in the wiki for detailed changes. Otherwise, the manual page is available on the web site.

1 0

ANNOUNCE: Haskell Communities and Activities Report (28th ed., May 2015)
by Mihai Maruseac 29 May '15

29 May '15

On behalf of all the contributors, we are pleased to announce that the Haskell Communities and Activities Report (28th edition, May 2014) is now available, in PDF and HTML formats: http://haskell.org/communities/05-2015/report.pdf http://haskell.org/communities/05-2015/html/report.html Many thanks go to all the people that contributed to this report, both directly, by sending in descriptions, and indirectly, by doing all the interesting things that are reported. We hope you will find it as interesting a read as we did. If you have not encountered the Haskell Communities and Activities Reports before, you may like to know that the first of these reports was published in November 2001. Their goal is to improve the communication between the increasingly diverse groups, projects, and individuals working on, with, or inspired by Haskell. The idea behind these reports is simple: Every six months, a call goes out to all of you enjoying Haskell to contribute brief summaries of your own area of work. Many of you respond (eagerly, unprompted, and sometimes in time for the actual deadline) to the call. The editors collect all the contributions into a single report and feed that back to the community. When we try for the next update, six months from now, you might want to report on your own work, project, research area or group as well. So, please put the following into your diaries now: ======================================== End of September 2015: target deadline for contributions to the November 2015 edition of the HC&A Report ======================================== Unfortunately, many Haskellers working on interesting projects are so busy with their work that they seem to have lost the time to follow the Haskell related mailing lists and newsgroups, and have trouble even finding time to report on their work. If you are a member, user or friend of a project so burdened, please find someone willing to make time to report and ask them to "register" with the editors for a simple e-mail reminder in October (you could point us to them as well, and we can then politely ask if they want to contribute, but it might work better if you do the initial asking). Of course, they will still have to find the ten to fifteen minutes to draw up their report, but maybe we can increase our coverage of all that is going on in the community. Feel free to circulate this announcement further in order to reach people who might otherwise not see it. Enjoy! Mihai Maruseac and Alejandro Serrano Mena <hcar at haskell.org> -- Mihai Maruseac (MM) "If you can't solve a problem, then there's an easier problem you can solve: find it." -- George Polya

1 0

Status of Acid-State and IxSet
by JP Moresmau 29 May '15

29 May '15

Hello, for a project I decided to use Acid-State and IxSet, and was pretty happy with them. I enjoyed working with Haskell structures for storing and querying data, and my data could fit in memory. However, now I've moved to GHC 7.10, and for example syb-with-class, a dependency of ixset, does not build with template-haskell 2.10. So I'm wondering, are these projects still alive and kicking? Are if I want some memory based storage and indexing, should I look at some other projects (maybe VCache http://hackage.haskell.org/package/vcache) Or is it just bad timing and GHC 7.10 compatible versions of these libraries are in the works? Thanks! -- JP Moresmau http://jpmoresmau.blogspot.com/

6 7

recursive datakinds
by Nicholls, Mark 29 May '15

29 May '15

Hello, If I were trying to do something like creating types like.... > {-# LANGUAGE DataKinds #-} > {-# LANGUAGE ExplicitForAll #-} > {-# LANGUAGE FlexibleContexts #-} > {-# LANGUAGE FlexibleInstances #-} > {-# LANGUAGE GADTs #-} > {-# LANGUAGE MultiParamTypeClasses #-} > {-# LANGUAGE PolyKinds #-} > {-# LANGUAGE StandaloneDeriving #-} > {-# LANGUAGE TypeFamilies #-} > {-# LANGUAGE TypeOperators #-} > {-# LANGUAGE UndecidableInstances #-} > {-# LANGUAGE ScopedTypeVariables #-} > data MyList a where > MyEnd :: MyList '[] > MyCons :: a -> MyList b -> MyList (a ': b) > mysimplelist :: MyList '[Integer] > mysimplelist = MyCons 1 (MyEnd) what about > myrecursivelist = MyCons 1 myrecursivelist Cafe2.lhs:23:30: Occurs check: cannot construct the infinite type: b ~ a : b Expected type: MyList b Actual type: MyList (a : b) Relevant bindings include myrecursivelist :: MyList (a : b) (bound at Cafe2.lhs:23:3) In the second argument of 'MyCons', namely 'myrecursivelist' In the expression: MyCons 1 myrecursivelist can I "fix" this somehow? I obviously want the type of myrecursivelist to be MyList '[Integer, Integer, Integer, Integer, Integer, Integer, Integer, Integer,.....] Something like a completely nonsensical... MyList (Fix (Integer ': )) P.S. Yes, I barely understand "fix" as a function, let alone a data type. CONFIDENTIALITY NOTICE This e-mail (and any attached files) is confidential and protected by copyright (and other intellectual property rights). If you are not the intended recipient please e-mail the sender and then delete the email and any attached files immediately. Any further use or dissemination is prohibited. While MTV Networks Europe has taken steps to ensure that this email and any attachments are virus free, it is your responsibility to ensure that this message and any attachments are virus free and do not affect your systems / data. Communicating by email is not 100% secure and carries risks such as delay, data corruption, non-delivery, wrongful interception and unauthorised amendment. If you communicate with us by e-mail, you acknowledge and assume these risks, and you agree to take appropriate measures to minimise these risks when e-mailing us. MTV Networks International, MTV Networks UK & Ireland, Greenhouse, Nickelodeon Viacom Consumer Products, VBSi, Viacom Brand Solutions International, Be Viacom, Viacom International Media Networks and VIMN and Comedy Central are all trading names of MTV Networks Europe. MTV Networks Europe is a partnership between MTV Networks Europe Inc. and Viacom Networks Europe Inc. Address for service in Great Britain is 17-29 Hawley Crescent, London, NW1 8TT.

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Haskell Weekly News
by Kim-Ee Yeoh 29 May '15

29 May '15

*Top Picks:* - Tony Day, Brisbane-based investment strategist and high-frequency-trading hacker, rides 100% idiomatic Haskell <http://tonyday567.github.io/blog/mvc-todo/> into the Single Page (web)-App <http://en.wikipedia.org/wiki/Single-page_application> space. How? He spins the GHCJS transpiler on Gabriel Gonzalez's Model-View-Controller library to obtain his own TodoMVC benchmark demo <http://tonyday567.github.io/static/index-auto.html>. Per GHCJS, the production spans multi-megabytes of javascript. Not to be missed: auto-run, i.e. click the QuickCheck-powered checkbox labeled "Let haskell do the work." Github repo <https://github.com/tonyday567/mvc-todo>. Along the way, he discovers how crippled Javascript is without sum types. As noted on HN <https://news.ycombinator.com/item?id=9591868#up_9601842>, "Typos and missing cases represent a very large set of trivial bugs." He believes superior FP features such as sum types makes it "much harder for haskell to avoid success." Woe is us. /r/haskell <http://www.reddit.com/r/haskell/comments/36y9jc/haskell_as_an_mvc_framework/> - Michael Walker, a Ph.D. student at York, reveals <http://www.reddit.com/r/haskell/comments/37dv8y/what_side_projects_have_you…> his personal book-collection management web-app <http://www.barrucadu.co.uk/bookdb/> that runs on top of persistent <https://hackage.haskell.org/package/persistent>, WAI <https://hackage.haskell.org/package/wai>, and web-routes <https://hackage.haskell.org/package/web-routes>. Public domain. <https://github.com/barrucadu/bookdb> - David Christiansen announces on /r/haskell <http://www.reddit.com/r/haskell/comments/36az3n/idris_0918_released_with_fa…> Idris 0.9.18 <http://www.idris-lang.org/idris-0-9-18-released/> with fancier records. Top comment says Idris is more type-friendly than Haskell despite the "esoteric academics behind dependent type theory." Why? Because Idris, helpfully offers suggestions that turn ill- into well-typed code. Haskell doesn't. - Remember JP Moresmau dropping EclipseFP <http://haskell.1045720.n5.nabble.com/Haskell-Weekly-News-tp5809360.html>? Can Leksah take its place? Hamish Mackenzie announces a 7.10-ready Leksah 0.15.0 <https://github.com/leksah/leksah/wiki/Leksah-0.15.0> on /r/haskell <http://www.reddit.com/r/haskell/comments/37emp2/leksah_0150/>. Top new feature? Support for GHCJS, which excites Phil Freeman of PureScript fame. Here's a 2min video clip on How to make a ghcjs-dom application in Leksah <https://www.youtube.com/watch?v=zQnExdDL63c>. - Roman Cheplyaka writes a short 'n sweet tutorial <https://ro-che.info/articles/2015-05-28-force-list> on how a list may be variously forced. He summarizes the similarities and differences in a table that goes from the shallowestly evaluated seq () to forceSpine to forceElements to the deepest rnf. /r/haskell <http://www.reddit.com/r/haskell/comments/37ky62/how_to_force_a_list/> - Justin Leitgeb, Rails developer and Co-Founder / CTO of Stack Builders, a Haskell-enabled software consultancy, deprioritizes learning Clojure, Go, Erlang, and Scala in favor of Agda, Coq, Idris, Elm, and Liquid Haskell <http://www.stackbuilders.com/news/another-personal-programming-language-roa…>. He will invest a couple of weeks at the Oregon PL Summer School starting June 15 <https://www.cs.uoregon.edu/research/summerschool/summer15/>. - Tony Morris announces a three-day Haskell-based Intro to FP course in Melbourne, July 21-23 this year. Pitched at beginners, it features learning by coding. Free; application deadline: July 10 <https://groups.google.com/forum/#!topic/nicta-fp/Dnq-oh4nJbg>. - By popular request <http://haskell.1045720.n5.nabble.com/Workshop-on-Type-Inference-and-Automat…>, František Farka and his team recorded all of a workshop on type inference in Dundee, Scotland <http://staff.computing.dundee.ac.uk/frantisekfarka/tiap/> held a couple of weeks ago. Highly-viewed talks <https://www.youtube.com/user/qmbevents/videos> include Edwin Brady <http://staff.computing.dundee.ac.uk/frantisekfarka/tiap/#edwin> on implementing dependent types in Idris and Conor McBride <http://staff.computing.dundee.ac.uk/frantisekfarka/tiap/#conor> on "Type Inference Needs Revolution." /r/haskell <http://www.reddit.com/r/haskell/comments/36dfcx/workshop_on_type_inference_…> - Michael Hicks at UMD, PC chair of POPL 2012, writes a PL research apologia <http://www.pl-enthusiast.net/2015/05/27/what-is-pl-research-and-how-is-it-u…> cum pitch for new grad students. An informal poll he did shows PL Ph.D.s get good jobs. He explains that "The ethos of PL research is to not just find solutions to important problems, but to find the *best expression of those solutions.*" As ethos specimens, he gives three: probabilistic programming, incremental computation a.k.a. self-adaptive computation, and authenticated data structures (see LambdaAuth <http://amiller.github.io/lambda-auth/>). HN-worthy <https://news.ycombinator.com/item?id=9614178>. - Jan Stolarek gives a glimpse <http://lambda.jstolarek.com/2015/05/injective-type-families-for-haskell/> of GHC's new injective type families feature, which is joint research with SPJ and Richard Eisenberg. It brings Haskell one step closer to having a notion of type functions as opposed to mere 'constructors.' The feature is useful in type-level hacking. It allows the arguments of a type family to be inferred solely by result type. But Lennart Augustsson on /r/haskell <http://www.reddit.com/r/haskell/comments/37bvh0/injective_type_families_for…> gives a trivial example where even when a type family isn't injective, its argument can still be inferred knowing a particular result type. He regrets that GHC doesn't do this. - Joe Nelson uploads the video and a summary <http://begriffs.com/posts/2015-05-24-safe-haskell.html> of Kristen Kozak introducing the Safe Haskell extension to the SF Bay Area Haskell Users Group <http://www.meetup.com/Bay-Area-Haskell-Users-Group/events/220312001/> on May 12. Slides here <http://wordroute.com/grayjay/safe-haskell.pdf>. Redditor beerdude26 <http://www.reddit.com/r/haskell/comments/37a1cq/using_the_safe_haskell_lang…> adds the link to and tidies up the Safe Haskell wiki <https://wiki.haskell.org/Safe_Haskell> entry. - Out of frustration with existing Windows GUI FFIs, Luka Horvat <http://lukahorvat.github.io/programming/2015/05/24/haskellforms/> creates bindings for WinForms <https://github.com/LukaHorvat/HaskellForms>, Microsoft's .NET menus-and-widgets library. He binds to F# instead of lower-level C++/C# because it's easier. /r/haskell <http://www.reddit.com/r/haskell/comments/37464x/haskellforms_winforms_bindi…> - Gabriel Gonzalez <http://www.haskellforall.com/2015/05/the-internet-of-code.html> demoes another piece of Morte, what he describes as his "pandoc for programming languages." He prototypes "distributing typed code over the internet where the unit of compilation is individual expressions." /r/haskell <http://www.reddit.com/r/haskell/comments/36d12v/haskell_for_all_the_interne…> - Taylor Fausak forsakes TagSoup <https://hackage.haskell.org/package/tagsoup> and scrapes websites <http://taylor.fausak.me/2015/05/21/scraping-websites-with-haskell/> the hard way using xml-conduit. He concludes that it's "tougher than doing the same thing in scripting languages, but hopefully easier than [his blog readers] expected." - Stuart Popejoy writes a monad tutorial <http://slpopejoy.github.io/posts/Effectful01.html>. A redditor finds it an "entertaining explanation of monads." Another asks the OOP-ish question: are monads "basically wrappers that contain 'impure' data and actions with some common functions?" See /r/haskell <http://www.reddit.com/r/haskell/comments/374c3o/effectful_haskell_io_monads…> *Announcement:* Semen Trygubenko has stepped down from publishing his edition of HWN. Losing him means there will be no issue next week. *Quotes of the Week:* - UnoOuzo <https://twitter.com/UnoOuzo/status/601658244201848832>: "The type of my love is parametrically polymorphic. It is unbounded." Looking into ways to cite this in my thesis. - Conor McBride <http://www.reddit.com/r/haskell/comments/36y9jc/haskell_as_an_mvc_framework…>: Algebra is a posh way of saying "construction kit". - Michael Hicks <http://www.pl-enthusiast.net/2015/05/27/what-is-pl-research-and-how-is-it-u…>: The ethos of PL research is to not just find solutions to important problems, but to find the *best expression of those solutions. * - There is a fountain of youth: it is your mind, your talents, the creativity you bring to your life and the lives of people you love. When you learn to tap this source, you will truly have defeated age. -- Sophia Loren -- Kim-Ee

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-fdefer-more-errors
by Tom Ellis 29 May '15

29 May '15

Deferring type errors until runtime is a feature which probably helps a lot with the adoption of Haskell. Newcomers can get the encouragement of a mostly-working program even if there are type errors in a few places. I can write foo = 1 + 1 bar = head True main = print foo and still run the program or load it in GHCi. Does anyone think it would also be beneficial to have "-fdefer-name-errors"? If I write foo = 1 + 1 bar = baz main = print foo but baz doesn't exist it would still be nice to let the program run or be loaded in GHCi. This could be achieved by replacing all missing variables by "error 'baz was not defined'" or similar. If a suitable parser could be written it might even be possible to defer syntax errors! This kind of thing would make the experience with Haskell gentler for newcomers, but also more pleasant for veterans! Tom

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Re: [Haskell-cafe] Any recommendations on how to interact with the running state of a program?
by Anatoly Yakovenko 29 May '15

29 May '15

How would the program get updated values? via an IORef? This thing is running until the user decides the output looks good Anatoly On Thu, May 28, 2015 at 12:16 PM Carlos López-Camey <c.lopez(a)kmels.net> wrote: > Cloud Haskell + Event streaming via some Protocol (like json on > websockets) ? > > > > 2015-05-25 15:14 GMT-06:00 Anatoly Yakovenko <aeyakovenko(a)gmail.com>: > >> I am playing around with some deep learning algorithms and I need a way >> to periodically read the intermediate result and change some of the system >> variables. >> >> Any ideas on how to best approach this problem? I was thinking of >> something basic, like having the program periodically read and write from >> some files. >> >> Thanks, >> Anatoly >> >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe >> >> >

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Haskell wiki: user account
by Julien Cretel 29 May '15

29 May '15

Hi there, I'd like to get a user account for the Haskell wiki. Desired username: Jubobs Cheers -- Julien Cretel PhD student University College Cork, Ireland Email: j.cretel(a)umail.ucc.ie

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Anyone using Tidal/emacs like emacs-live/overtone?
by Cody Goodman 29 May '15

29 May '15

Does anyone have a working setup where you enter Haskell code from the tidal library and use a keybinding to execute that statement (or multiple)? Basically wanting to try livecoding Tidal code like all the overtone videos: https://vimeo.com/22798433

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