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[ANN] Haskell Theory Exploration scripts
by Chris Warburton 10 Nov '17

10 Nov '17

Hello all, I thought I'd make a public announcement about some scripts I've built up over the last few years for "exploring" Haskell code. They can be found at: http://chriswarbo.net/git/haskell-te With mirrors at /ipns/QmWv958VzBJQchGjYKiSaxLC9ugrjvXkqMpVrmjp9AonXq and https://github.com/Warbo/haskell-te These use the excellent QuickSpec library from: https://hackage.haskell.org/package/quickspec This empirically (through brute force and QuickCheck testing) conjectures equations about given Haskell function definitions. I think it's a very under-utilised library, perhaps due to a couple of reasons: - It requires hand-holding from the user, e.g. installing packages, importing modules, naming functions, monomorphising types, picking an arity, etc. - It can be very resource-intensive! I know there's some work to address the efficiency concerns in a newer version, but that's not made its way to Hackage, Nixpkgs, etc. yet: https://github.com/nick8325/quickspec My scripts try to address the usability issues. More details are in the README, but basically we can run: quickspec myHaskellDir | renderEqs This will (hopefully) output conjectures found by QuickSpec about the functions defined in myHaskellDir. This automatically finds all function definitions, looks up their types, arities, etc. and invokes QuickSpec in a subprocess (with optional timeouts and memory limits). If we leave off the 'renderEqs', we get an easily-parsed JSON format, which we can feed into other tools. Note that I make *heavy* use of the Nix language, and it's quite hit-or-miss whether a package will work. My hope is that more widespread knowledge of these tools will alleviate some of the existing pain points. In particular, many packages define `Arbitrary` instances for their types, but they do so in their test suites rather than their libraries, so nobody else can make use of them (I assume this is to keep down library dependencies?) Suggestions and contributions are welcome! -- Thanks, Chris Warburton

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Re: [Haskell-cafe] Proposal: Instance apartness guards
by Anthony Clayden 09 Nov '17

09 Nov '17

Surprisinlgy little comment on this proposal. Perhaps it landed when yous were busy elsewhere. I'd like to push it to the committee soon; this is an invitation for more feedback. Richard E wrote a nice brief summary https://github.com/ghc-proposals/ghc-proposals/pull/56#issuecomment-3114214… Thank you AntC ----- Original Message Follows ----- > After years of pondering this idea (in various forms), > and several rounds of discussion on several forums, > I've written it up. > > "This proposal tackles the thorny topic of Overlapping > instances, > for both type classes and Type Families/Associated types, > by annotating instance heads with type-level apartness > Guards. > Type-level disequality predicates appear in Sulzmann & > Stuckey 2002; > in the type-level case selection in HList 2004; > and in various guises in Haskell cafe discussions in > following years. > This proposal builds on the apartness testing implemented > as part of the Closed Type Families work." > > All feedback welcome. > > https://github.com/AntC2/ghc-proposals/blob/instance-apartness-guards/propo… > > AntC

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Functional programming job opportunity
by Nathan van Doorn 08 Nov '17

08 Nov '17

Myrtle is looking for skilled developers to join our growing development team and assist with the development of our core compiler technology. Ideally candidates would have an interest in either efficient hardware design and/ or machine learning models and frameworks. We use the functional language, Haskell, for every aspect of our work and developers will get the chance to be involved in every part of the project. Familiarity and interest with hardware design would be viewed positively but is not essential. Applicants should be familiar with Unix based systems as all our development is done on Ubuntu or NixOS. More information at https://www.myrtlesoftware.com/vacancies/

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ANN: nice-html, an all-around "nice" html templating library, similar but different to lucid, blaze, type-of-html, etc.
by Mike Ledger 08 Nov '17

08 Nov '17

Hi Haskell-Cafe, I've been working on and off on a HTML templating library that is slightly novel for a few months. Here's the good parts: * it's faster than lucid and blaze * it has a valid monad instance * it has another valid monad instance, that is slightly different! * it paramaterises templates by the data they require * the HTML5 combinators don't rely on any highly overloaded/overlapping typeclasses; type errors in nice-html templates tend to be nice monomorphic type mismatches To achieve this, it has a distinct compilation phase (at runtime) for templates. This means the actual rendering function does very little beyond concatenate Text and escape dynamic input. It also severely restricts usage to the built-in combinators available -- at least, for dynamic data -- and can make writing a nice-html template difficult, though littering _ throughout type signatures helps. Check it out! Benchmark results, a README, and an example: https://github.com/TransportEngineering/nice-html Hackage: https://hackage.haskell.org/package/nice-html The only thing on the roadmap right now is to have some nice :-) way to assign JavaScript event handlers to Markup. This is something that I really appreciate when using React.js, so my eventual aim is for nice-html to be like a (more) server-side version of React. Right now, you can keep track of element ids with `Text.Html.Writer.note`, but it's neither a very good nor very useful interface. Cheers, Mike

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Re: [Haskell-cafe] First annual Haskell users survey
by Taylor Fausak 07 Nov '17

07 Nov '17

Today is the last day to fill out the survey! To the over 1,200 of you have already filled it out: thank you! For those of you that haven't, please take 10 minutes today to fill it out. Thanks! https://haskellweekly.news/surveys/2017.html

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[ANNOUNCE] GHC 8.2.2 release candidate 2
by Ben Gamari 06 Nov '17

06 Nov '17

Hello everyone, The GHC team is very pleased to announce the second candidate of the 8.2.2 release of the Glasgow Haskell Compiler. Source and binary distributions are available at https://downloads.haskell.org/~ghc/8.2.2-rc2/ This is the second and hopefully last of two release candidates leading up the final 8.2.2 release. This release fixes approximately fifty bugs present in 8.2.1. These most notably include, * A correctness issue resulting in segmentation faults in some FFI-users (#13707, #14346) * A correctness issue resulting in undefined behavior in some programs using STM (#14171) * A bug which may have manifested in segmentation faults in out-of-memory condition (#14329) * clearBit of Natural no longer bottoms (#13203) * A specialisation bug resulting in exponential blowup of compilation time in some specialisation-intensive programs (#14379) * ghc-pkg now works even in environments with misconfigured NFS mounts (#13945) * GHC again supports production of position-independent executables (#13702) * Better error messages around kind mismatches (#11198, #12373, #13530, #13610) See [1] for the full list. As always, please report any issues you encounter. Happy testing, - Ben [1] https://ghc.haskell.org/trac/ghc/query?status=closed&milestone=8.2.2&col=id…

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Replace list item with list.
by mirone 06 Nov '17

06 Nov '17

Hello everyone! If I have a list A : ["x", "y", "z"] and a list B: ["f", "g"]. Here is my question: What's the simplest way to replace "z" in the list A by list B, and get ["x", "y", "f", "g"] ?

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asking for feedback: "Magical Haskell" book in progress
by J Ho 05 Nov '17

05 Nov '17

Dear Haskellers, I've systematized my notes from the last 3 years, bit the bullet and started working on the visual Haskell teaching book that had been my desire for a long time now :) The goal is to build it on the proper math foundation of type and category theory to structure concepts and functional patterns as they apply to real world problems neatly from the start, without relying on imperative language experience too much, but make it *visual*, easy to grasp, and not *math-technically* heavy. Basically, I was thinking *"how do I teach Haskell to my kids?"* and *"how would people who learned only functional programming from school, without any imperative exposure, think?"* (I don't think such people exist yet but I sure would like to speak to those if / when they appear :) - but the goal is to make it accessible to, hopefully, teens with some reasonable math and / or programming background, and also, in the later chapters, make it useful to adults who better grasp concepts visually (as I do) to have a neat and systematized Haskell concepts building in their brain. Here's the direct link to the first chapter: Wizards, Types and Functions <https://leanpub.com/magicalhaskell/read#leanpub-auto-chapter-1-wizards-type…> Here's the book's page on leanpub <https://leanpub.com/magicalhaskell> that gets gradually updated as usual, and it will of course always stay free. Would very much appreciate your feedback, suggestions and thoughts! If you have another 7 minutes to spare and want more context, here's a more expanded rationale to this approach to teaching haskell - "Haskell Is Not Programming <https://superstrings.io/haskell-is-not-programming-3246779f2ef6>" - to which I received some very valuable feedback (and some understandable imperative camp critique :)) ) on reddit already, which helped me adjust the book itself. Thanks a lot! Best regards, - J.

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approxRatio and Ratio Word
by Vilem-Benjamin Liepelt 04 Nov '17

04 Nov '17

I have been experiencing curious behaviour with approxRatio: Prelude Data.Ratio> let a = 0 :: Word Prelude Data.Ratio> let b = 10 :: Word Prelude Data.Ratio> a%b 0 % 1 Prelude Data.Ratio> approxRational (a%b) 0.01 1 % 1 Prelude Data.Ratio> approxRational (0%1) 0.01 0 % 1 Is this expected? If so, why?

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Re: [Haskell-cafe] 回复： Replace list item with list.
by David Lilue 04 Nov '17

04 Nov '17

There is a function "concatMap" to simplify "concat $ map". Furthermore, you could use inline notation as follow: concatMap (\x -> x == "z" then B else [x]) A On Sat, Nov 4, 2017 at 7:50 AM, Денис Мирзоев <nolane16(a)gmail.com> wrote: > If you sure "z" is the last element of A you can use > > init A ++ B > > if it's somwhere in the middle consider > > cocat $ map f A > where > f x > | x == "z" = B > | otherwise = [x] > > On Sat, Nov 4, 2017 at 2:41 PM, Денис Мирзоев <nolane16(a)gmail.com> wrote: > >> If you sure "z" is the last element of A you can use >> >> init A ++ B >> >> if it's somwhere in the middle consider >> >> cocat $ map f A >> where >> f x >> | x == "z" = B >> | otherwise = [x] >> >> On Sat, Nov 4, 2017 at 1:57 PM, FlyingSheep <flyingsheep(a)qq.com> wrote: >> >>> (take 2 A) ++ B >>> >>> ------------------ >>> 李鹏 >>> 四川大学计算机学院 >>> >>> >>> >>> ------------------ 原始邮件 ------------------ >>> *发件人:* "mirone"<saul-mirone(a)qq.com>; >>> *发送时间:* 2017年11月4日(星期六) 晚上6:48 >>> *收件人:* "haskell-cafe"<haskell-cafe(a)haskell.org>; >>> *主题:* [Haskell-cafe] Replace list item with list. >>> >>> Hello everyone! >>> If I have a list A : ["x", "y", "z"] >>> and a list B: ["f", "g"]. >>> Here is my question: >>> What's the simplest way to replace "z" in the list A by list B, and get >>> ["x", "y", "f", "g"] ? >>> >>> _______________________________________________ >>> Haskell-Cafe mailing list >>> To (un)subscribe, modify options or view archives go to: >>> http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe >>> Only members subscribed via the mailman list are allowed to post. >>> >> >> > > _______________________________________________ > Haskell-Cafe mailing list > To (un)subscribe, modify options or view archives go to: > http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe > Only members subscribed via the mailman list are allowed to post. > ᐧ

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Re: [Haskell-cafe] 回复： Replace list item with list.
by Денис Мирзоев 04 Nov '17

04 Nov '17

If you sure "z" is the last element of A you can use init A ++ B if it's somwhere in the middle consider cocat $ map f A where f x | x == "z" = B | otherwise = [x] On Sat, Nov 4, 2017 at 2:41 PM, Денис Мирзоев <nolane16(a)gmail.com> wrote: > If you sure "z" is the last element of A you can use > > init A ++ B > > if it's somwhere in the middle consider > > cocat $ map f A > where > f x > | x == "z" = B > | otherwise = [x] > > On Sat, Nov 4, 2017 at 1:57 PM, FlyingSheep <flyingsheep(a)qq.com> wrote: > >> (take 2 A) ++ B >> >> ------------------ >> 李鹏 >> 四川大学计算机学院 >> >> >> >> ------------------ 原始邮件 ------------------ >> *发件人:* "mirone"<saul-mirone(a)qq.com>; >> *发送时间:* 2017年11月4日(星期六) 晚上6:48 >> *收件人:* "haskell-cafe"<haskell-cafe(a)haskell.org>; >> *主题:* [Haskell-cafe] Replace list item with list. >> >> Hello everyone! >> If I have a list A : ["x", "y", "z"] >> and a list B: ["f", "g"]. >> Here is my question: >> What's the simplest way to replace "z" in the list A by list B, and get >> ["x", "y", "f", "g"] ? >> >> _______________________________________________ >> Haskell-Cafe mailing list >> To (un)subscribe, modify options or view archives go to: >> http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe >> Only members subscribed via the mailman list are allowed to post. >> > >

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Amazonka: How to generate S3:// uri from Network.AWS.S3.Types.Object?
by Cody Goodman 02 Nov '17

02 Nov '17

Question: How can I generate an S3://absolute/url from an Object ( https://hackage.haskell.org/package/amazonka-s3-1.4.5/docs/Network-AWS-S3-T… )? Context: I've been using turtle to call "aws s3 ls" and I'm trying to figure out how to replace that with amazonka. Absolute s3 urls were central to how my program worked. I KNow how to get objects now and filter them, but I don't know how to convert an object to an S3 url to integrate with my existing program. I came across the getFile function ( https://github.com/brendanhay/amazonka/blob/4fdd746346323aac7de6e82752b1660…) and tried downloading a file from s3. Perhaps I had something wrong, but it didn't seem like just the S3 Bucket and S3 Object key were enough to download a given file. If I'm wrong about that I need to double check my configuration. Thanks, Cody Goodman

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First annual Haskell users survey
by Taylor Fausak 01 Nov '17

01 Nov '17

https://haskellweekly.news/surveys/2017.html I am super excited to announce this survey! It is inspired by Rust’s recent surveys and Johan Tibell’s state of Haskell surveys from a few years ago. The goal of this survey is to better understand how Haskell users feel about the language, ecosystem, and community. So please, if you’re reading this: Take 10 minutes and fill out the survey. Thanks!

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[ANNOUNCE] GHC 8.2.2 release candidate 3
by Ben Gamari 31 Oct '17

31 Oct '17

Hello everyone, The GHC team is very pleased to announce the third candidate of the 8.2.2 release of the Glasgow Haskell Compiler. Source and binary distributions are available at https://downloads.haskell.org/~ghc/8.2.2-rc3/ This is the third and last of three release candidates leading up the final 8.2.2 release. This release fixes approximately fifty bugs present in 8.2.1. These most notably include, * A correctness issue resulting in segmentation faults in some FFI-users (#13707, #14346) * A correctness issue resulting in undefined behavior in some programs using STM (#14171) * A bug which may have manifested in segmentation faults in out-of-memory condition (#14329) * clearBit of Natural no longer bottoms (#13203) * A specialisation bug resulting in exponential blowup of compilation time in some specialisation-intensive programs (#14379) * ghc-pkg now works even in environments with misconfigured NFS mounts (#13945) * GHC again supports production of position-independent executables (#13702) * Better error messages around kind mismatches (#11198, #12373, #13530, #13610) See [1] for the full list. Release candidate three contains only two patches not present in -rc2 but fixes a packaging issue which lead to unintentional downgrading the process and Cabal libraries. As always, please report any issues you encounter. Happy testing, - Ben [1] https://ghc.haskell.org/trac/ghc/query?status=closed&milestone=8.2.2&col=id…

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What Does Graham Hutton Mean by Effect
by Steven Leiva 31 Oct '17

31 Oct '17

Hi Everyone, I am reading the 2nd edition of Graham Hutton's Programming in Haskell. I'm not reading the entire book, just the parts of Haskell that I am still iffy on. Anyway, in Chapter 12, Section 3, Hutton introduces monads. He start off with the following code: first module Expr wheredata Expr = Val Int | Div Expr Expreval :: Expr -> Inteval (Val n) = neval (Div el er) = eval el `div` eval er Not using Mixmax yet? And then he points out that the second clause of eval will raise an error if eval er evaluates to 0. One solution is that, instead of using the div function, we use a safeDiv:: Int -> Int -> Maybe Int function, which evaluate to Nothing if the divisor is 0. This means that expr's type changes from eval :: Eval -> Int to eval :: Eval -> Maybe Int, and this means that implementing eval becomes very verbose: second module Expr wheredata Expr = Val Int | Div Expr Expreval :: Expr -> Maybe Inteval (Val n) = Just neval (Div el er) = case eval el of Nothing -> Nothing Just y -> case eval er of Nothing -> Nothing Just x -> y `safeDiv` xsafeDiv :: Int -> Int -> Maybe IntsafeDiv x y | y == 0 = Nothing | otherwise = Just (x `div` y) Not using Mixmax yet? In order to make eval more concise, we can try the applicative style, where the second clause of the eval function becomes pure safeDiv <*> eval el <*> eval er. Of course, that doesn't work because pure safeDiv has the type Int -> Int -> Maybe Int, and what we need is a function of type Int -> Int -> Int. Anyways, this is all setup / context to what Hutton says next: The conclusion is that the function eval does not fit the pattern of effectful programming that is capture by applicative functors. The applicative style restricts us to applying pure functions to effectful arguments: eval does not fit this pattern because the function safeDiv that is used to process the resulting values is not a pure function, but may itself fail. I am confused by Hutton's use of the word effectful and by his description of safeDiv as "not a pure function". I tried skimming the other sections of the book to see if he provided a definition of this somewhere, but if he did, I couldn't find it. So my question is, in what way does Hutton mean for the reader to understand the words effect / effectful, and why does he describe the function safeDiv as not a pure function? Thank you! Steven Leiva 305.528.6038 leiva.steven(a)gmail.com http://www.linkedin.com/in/stevenleiva

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Call for Sponsorships: ICFP 2018
by Lindsey Kuper 31 Oct '17

31 Oct '17

ICFP 2018 The 23rd ACM SIGPLAN International Conference on Functional Programming https://icfp18.sigplan.org Call for Sponsorships Web version of this call for sponsorships: https://icfp18.sigplan.org/attending/supporting-icfp ## Why Sponsor ICFP 2018? ICFP is the premier conference on functional programming languages, covering all aspects of theory, implementation, and application. Every year, we bring together over 500 world-leading researchers, practitioners, and students to discuss the latest findings, collaborate on new ideas, and meet new people. By sponsoring ICFP, your organization can demonstrate its commitment to supporting high quality research and to developing the next generation of functional programming experts. Most of our sponsorship funds are used to help students from around the world afford to attend the conference and get the most out of their experience. We believe that this commitment will pay dividends for our students, our sponsors, and the public for years to come. If you're interested in becoming a sponsor, we'd love to hear from you: get in touch with our sponsorship team at sponsorship-2018(a)icfpconference.org ## Sponsorship Opportunities and Benefits ### Bronze - $750 * Your logo on the ICFP 2018 website * Your name listed in the proceedings ### Silver - $3,000 * All of the benefits of Bronze sponsorship * One complimentary 3-day ICFP registration * A table at the industrial reception * Your logo in the proceedings * Your logo on publicity materials such as banners and posters ### Gold - $6,000 * All of the benefits of Silver sponsorship * One additional complimentary 3-day ICFP registration (2 in total) * A named supporter of the industrial reception * An opportunity to include branded merchandise in participants' swag bag ### Platinum - $10,000 * All the benefits of Gold sponsorship * One additional complimentary 3-day ICFP registration (3 in total) * A named supporter of ICFP 2018 * An opportunity to speak to the audience at the industrial reception * A table/booth-like space in the coffee break areas ## Additional Sponsorship Opportunities We offer some additional sponsorship options to sponsors at the silver level or above. ### Lanyard Sponsor - $4,000 You provide the lanyards that every attendee will wear around their neck. ### Video Sponsor - $4,000 ICFP makes videos available for free to non-attendees following the conference. As a video sponsor, you support the recording and release of these videos. In exchange, your logo will be displayed as part of every ICFP video. ### Banner - $1,000 Post a free-standing banner (up to 2m high and 1m wide, provided by you) on the ICFP main stage throughout the conference.

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newtype and access control
by Steven Leiva 30 Oct '17

30 Oct '17

Hello everyone, I am watching a talk on YouTube calledWrangling Monad Transformer Stacks. The talk has the following code: code{-# LANGUAGE GeneralizedNewtypeDeriving -#} module DBTrans where newtype WithTrans a = WithTrans (WithDBConn a) deriving (Functor, Applicative, Monad) inDBTrans :: WithTrans a -> WithConn a inDBTrans = ... NOT USING MIXMAX YET? The speaker mentions that by using a newtype, we can perform "access control". The functioninDBTrans is "aware" that the type constructorWithTrans is simply a wrapper forWithDBConn, and that functions outside of the module are not privy to that information. Out of necessity, the speaker has to assume a certain level of Haskell knowledge that I don't have, because I am confused as to what language principles would allow forinDBTrans to be used for access control. I understand that a newtype declaration does create a new type from a "type safey" point of view; that a newtype declaration is required to have a single data constructor with a single (not named) field; and that, once the type-checking is done, the compiler can strip out the newtype wrapper and leave the underlying type (informally speaking, of course), but that still doesn't get me toinDBTrans can do access control. Thank you for any help! Steven Leiva 305.528.6038 leiva.steven(a)gmail.com http://www.linkedin.com/in/stevenleiva

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ANN: parameterized, indexed monads with a single "parameter" type variable
by Louis Pan 29 Oct '17

29 Oct '17

Hi Haskell-Cafe, I've been experimenting with indexed/parameterized monads with a single "parameter" type variable. The difference from my variation from Conor McBride's IMonad (which also uses one "parameter") is that the "parameter" doesn't have postcondition semanatics. (FYI, a good intro to indexed monads is https://stackoverflow.com/questions/28690448/what-is-indexed-monad) Instead, in the instance of PApplicative, you have to specify the rule of how to combine the "parameter" `t` and `u` into the final parameter `u`. That way, I can create "parameterized" versions of Semigroup, Monoid, as well as Applicative, Alternative, Monad. It also allows for create different flavours of "parameterized" kinds for ReaderT. For example, I've created OverlappingWhichReader, DistinctWhichReader, and ManyReader newtype wrappers around ReaderT. OverlappingWhichReader and DistinctWhichReader can combine reader that read `Which '[Int, Bool]` (polymorphic variant, analogous to 'Either Int Bool') with another reader that reader `Which '[Bool, String]` into a reader that can read `Which '[Int, Bool, String]` ManyReader can combine reader that read `Many '[Int, Bool]` (heterogenous record, analogous to tuple (Int, Bool)) with another reader that reader `Many '[Bool, String]` into a reader that can read `Many '[Int, Bool, String]` See https://github.com/louispan/parameterized/blob/master/src/Parameterized/Con… https://github.com/louispan/parameterized/blob/master/test/Parameterized/Co… ----- The single "parameter" still allows for the traditional changing state type for StateT. For example, I've created ChangingState newtype wrappers around StateT, which changes the state parameter. I've also created ManyState, which can combine StateT that modifies `Many '[Int, Bool]` with another StateT that modifes `Many '[Bool, String]` into a StateT that can modify `Many '[Int, Bool, String]` See https://github.com/louispan/parameterized/blob/master/src/Parameterized/Con… https://github.com/louispan/parameterized/blob/master/test/Parameterized/Co… --- I'd would appreciate if I could get any feedback on this approach. Cheers, Louis

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Munich Haskell Meeting, 2017-10-30 @ 19:30
by Christian Neukirchen 28 Oct '17

28 Oct '17

Dear all, Next week, our monthly Munich Haskell Meeting will take place again on **Monday, October 30** at Augustiner-Gaststätte Rumpler (Baumstraße 21) at 19h30. For details see here: http://muenchen.haskell.bayern/dates.html Please note the changed date! Everybody is welcome. cu, -- Christian Neukirchen <chneukirchen(a)gmail.com> http://chneukirchen.org

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Adding a DB Connection Pool to a Web Application
by Steven Leiva 27 Oct '17

27 Oct '17

Hi Everyone, I am trying to set up a simple web application, but with all the trappings of a production app. (This is strictly for learning purposes). At the moment, I am trying to get a database pool up and running that the rest of the application can use. You can see my current progress at this repository. You may want to look at only the code in the back-end folder, as there is a whole front-end application. So, again, my goal is to get a database pool up and running. At the moment, if I try to build the executable target, I get the following error: Error /Users/stevenleiva/Repos/todo/back-end/app/Main.hs:10:13: error: • Couldn't match type ‘persistent-2.7.0:Database.Persist.Class.PersistStore.BaseBackend backend0’ with ‘Database.Persist.Sql.Types.Internal.SqlBackend’ arising from a use of ‘createPostgresqlPool’ The type variable ‘backend0’ is ambiguous • In a stmt of a 'do' block: pool <- createPostgresqlPool connStr 1 In the expression: do { pool <- createPostgresqlPool connStr 1; startApp } In an equation for ‘main’: main = do { pool <- createPostgresqlPool connStr 1; startApp } Not using Mixmax yet? I tried helping out the compiler by adding pool :: Pool Connection, after importing those type constructors / constants. At that point, I got a different error: Error 2 /Users/stevenleiva/Repos/todo/back-end/app/Main.hs:11:32: error: • No instance for (monad-logger-0.3.25.1:Control.Monad.Logger.MonadLogger IO) arising from a use of ‘createPostgresqlPool’ • In a stmt of a 'do' block: pool :: Pool Connection <- createPostgresqlPool connStr 1 In the expression: do { pool :: Pool Connection <- createPostgresqlPool connStr 1; startApp } In an equation for ‘main’: main = do { pool :: Pool Connection <- createPostgresqlPool connStr 1; startApp } Not using Mixmax yet? I understand that createPostgresqlPool eventually gives me a value of type m (Pool Connection), where there are some constraints on m. I also understand that at the moment I am getting back a value IO (Pool Connection), and that IO does not have an instance of MonadLogger. >From here, I am at a bit of a lose on how to proceed. I thought implementing an instance of MonadLogger for IO that would simply typecheck (and not log), but that would lead to an orphaned instance, correct? What's the easiest next step to make forward progress here? P.S. I am aware that there are a lot of advanced (for me anyway) Haskell that can be done via monad transformers, natural transformations, etc., in order to wire up a DB with Servant, but I am happy with the "next simplest step" until I learn some of those concepts better. (Then Haskell makes refactoring a breeze!). Steven Leiva 305.528.6038 leiva.steven(a)gmail.com http://www.linkedin.com/in/stevenleiva Steven Leiva 305.528.6038 leiva.steven(a)gmail.com http://www.linkedin.com/in/stevenleiva

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Haskell CI with many repositories/packages
by Jeroen Bransen 27 Oct '17

27 Oct '17

Hi cafe, Does anyone know of a good setup for doing continuous integration with a set of Haskell packages, each in its own repository? Just building everything upon every commit is not so hard, but to speed up building times I'd like to build and test only the minimal set of packages. In particular, at a commit for some package A, I would like to build and test A and all packages that depend on A. The problem is that most CI tools use some notion of 'build artefact', which Stack doesn't really seem to give me. Ideally building a package results in some object file, which can then be used by the other packages. When building failed, packages that depend on it can still use the last succesful build. I've tried to look up some Haskell projects, but most of them seem to use some ad hoc setup. Some pointers are appreciated, as we are using Gitlab a gitlab-runner specific option would be great, but I am also open to use Jenkins or other tools. And I guess my main struggle now is on the stack/Haskell side. Regards, Jeroen Bransen

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Multi-param typeclass as superclass of Single-param typeclasses
by Tomasz Chronowski 27 Oct '17

27 Oct '17

This is my first post here, so Hello everyone! In haskell it is possible to express a constraint "if 'a' and 'b' are instance of 'AB' then 'a' is instance of 'A' and 'b' is instance of 'B'": class (A a, B b) => AB a b ... Is it possible to express the converse - "if 'a' is instance of 'A' and 'b' is instance of 'B' then 'a' and 'b' are instance of 'AB'"? I want to create a list of shapes that can be tested for intersection. I think that possibility of expressing such constraints would allow to doing this in a "Object Oriented" way: data Circle = Circle { ... } data Square = Square { ... } class Shape a class Intersect a b where intersect :: a -> b -> Bool -- pseudo haskell - "If 'a' is instance of 'Shape' and 'b' is instance of 'Shape' then 'a' and 'b' are instance of 'Intersect'" -- maybe some type hackery allows to express this? constraint Shape a, Shape b => Intersect a b instance Shape Circle instance Shape Square instance Intersect Circle Circle ... instance Intersect Circle Square ... ... data ShapeBox = forall a. Shape a => ShapeBox a foo = let x:y:_ = [ShapeBox Circle { ... }, ShapeBox Square { ... }, ...] in intersect x y -- this should work because we know for sure that there is an instance of Intersect for type of 'x' and type of 'y' Is such idea already described somewhere?

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Call for Workshop Proposals: ICFP 2018
by Lindsey Kuper 27 Oct '17

27 Oct '17

[ Please disregard previous version sent with the wrong subject line. ] CALL FOR WORKSHOP AND CO-LOCATED EVENT PROPOSALS ICFP 2018 23rd ACM SIGPLAN International Conference on Functional Programming September 23-29, 2018 St. Louis, Missouri, United States http://conf.researchr.org/home/icfp-2018 The 23rd ACM SIGPLAN International Conference on Functional Programming will be held in St. Louis, Missouri, United States on September 23-29, 2018. ICFP provides a forum for researchers and developers to hear about the latest work on the design, implementations, principles, and uses of functional programming. Proposals are invited for workshops (and other co-located events, such as tutorials) to be affiliated with ICFP 2018 and sponsored by SIGPLAN. These events should be less formal and more focused than ICFP itself, include sessions that enable interaction among the attendees, and foster the exchange of new ideas. The preference is for one-day events, but other schedules can also be considered. The workshops are scheduled to occur on September 23 (the day before ICFP) and September 27-29 (the three days after ICFP). ---------------------------------------------------------------------- Submission details Deadline for submission: November 20, 2017 Notification of acceptance: December 18, 2017 Prospective organizers of workshops or other co-located events are invited to submit a completed workshop proposal form in plain text format to the ICFP 2017 workshop co-chairs (Christophe Scholliers and David Christiansen), via email to icfp-workshops-2018(a)googlegroups.com by November 20, 2017. (For proposals of co-located events other than workshops, please fill in the workshop proposal form and just leave blank any sections that do not apply.) Please note that this is a firm deadline. Organizers will be notified if their event proposal is accepted by December 18, 2017, and if successful, depending on the event, they will be asked to produce a final report after the event has taken place that is suitable for publication in SIGPLAN Notices. The proposal form is available at: http://www.icfpconference.org/icfp2018-files/icfp18-workshops-form.txt Further information about SIGPLAN sponsorship is available at: http://www.sigplan.org/Resources/Proposals/Sponsored/ ---------------------------------------------------------------------- Selection committee The proposals will be evaluated by a committee comprising the following members of the ICFP 2018 organizing committee, together with the members of the SIGPLAN executive committee. Workshop Co-Chair: Christophe Scholliers (University of Ghent) Workshop Co-Chair: David Christiansen (Indiana University) General Chair: Robby Findler (Northwestern University) Program Chair: Matthew Flatt (University of Utah) ---------------------------------------------------------------------- Further information Any queries should be addressed to the workshop co-chairs (Christophe Scholliers and David Christiansen), via email to icfp-workshops-2018(a)googlegroups.com

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Call for Participation: ICFP 2017
by Lindsey Kuper 27 Oct '17

27 Oct '17

CALL FOR WORKSHOP AND CO-LOCATED EVENT PROPOSALS ICFP 2018 23rd ACM SIGPLAN International Conference on Functional Programming September 23-29, 2018 St. Louis, Missouri, United States http://conf.researchr.org/home/icfp-2018 The 23rd ACM SIGPLAN International Conference on Functional Programming will be held in St. Louis, Missouri, United States on September 23-29, 2018. ICFP provides a forum for researchers and developers to hear about the latest work on the design, implementations, principles, and uses of functional programming. Proposals are invited for workshops (and other co-located events, such as tutorials) to be affiliated with ICFP 2018 and sponsored by SIGPLAN. These events should be less formal and more focused than ICFP itself, include sessions that enable interaction among the attendees, and foster the exchange of new ideas. The preference is for one-day events, but other schedules can also be considered. The workshops are scheduled to occur on September 23 (the day before ICFP) and September 27-29 (the three days after ICFP). ---------------------------------------------------------------------- Submission details Deadline for submission: November 20, 2017 Notification of acceptance: December 18, 2017 Prospective organizers of workshops or other co-located events are invited to submit a completed workshop proposal form in plain text format to the ICFP 2017 workshop co-chairs (Christophe Scholliers and David Christiansen), via email to icfp-workshops-2018(a)googlegroups.com by November 20, 2017. (For proposals of co-located events other than workshops, please fill in the workshop proposal form and just leave blank any sections that do not apply.) Please note that this is a firm deadline. Organizers will be notified if their event proposal is accepted by December 18, 2017, and if successful, depending on the event, they will be asked to produce a final report after the event has taken place that is suitable for publication in SIGPLAN Notices. The proposal form is available at: http://www.icfpconference.org/icfp2018-files/icfp18-workshops-form.txt Further information about SIGPLAN sponsorship is available at: http://www.sigplan.org/Resources/Proposals/Sponsored/ ---------------------------------------------------------------------- Selection committee The proposals will be evaluated by a committee comprising the following members of the ICFP 2018 organizing committee, together with the members of the SIGPLAN executive committee. Workshop Co-Chair: Christophe Scholliers (University of Ghent) Workshop Co-Chair: David Christiansen (Indiana University) General Chair: Robby Findler (Northwestern University) Program Chair: Matthew Flatt (University of Utah) ---------------------------------------------------------------------- Further information Any queries should be addressed to the workshop co-chairs (Christophe Scholliers and David Christiansen), via email to icfp-workshops-2018(a)googlegroups.com

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Automatically using boxed or unboxed vectors
by Jake Waksbaum 26 Oct '17

26 Oct '17

I am trying to create a data type that is an vector of values, but internally uses an unboxed vector when possible and a boxed array otherwise. If this already exists, let me know. For example, any easily unboxed type is stored in an unboxed array, but also any type that is a product type of only easily unboxed types like (Int, Int) is stored in multiple unboxed arrays. I don’t think there is a smart way of doing this for sum types, so those are stored in boxed arrays. I’m trying to create a typeclass class VectorElement a where data Vector a replicate :: Int -> a -> Vector a length :: Vector a -> Int to represent things that can be stored in Vectors. I can then implement it for specific types that I know can be stored in unboxed vectors: instance VectorElement Int where newtype Vector Int = VectorInt (U.Vector Int) deriving Show replicate len x = VectorInt $ U.replicate len x length (VectorInt v) = U.length v I also want to automatically derive instances of this class for other types using the Generic typeclass. Ideally these instances would be the most efficient possible, so that for example the instance for (Int, Int) used two unboxed arrays but the instance for Maybe Int uses a boxed array. To that end I created another typeclass and wrote instances for the Generic data constructors: class VectorElementG (r :: * -> *) where data VectorG r replicateG :: Int -> r a -> VectorG r lengthG :: VectorG r -> Int instance VectorElementG V1 where data VectorG V1 replicateG = undefined lengthG = undefined instance VectorElementG U1 where newtype VectorG U1 = VectorGUnit Int replicateG i U1 = VectorGUnit i lengthG (VectorGUnit i) = i instance VectorElement a => VectorElementG (K1 i a) where newtype VectorG (K1 i a) = VectorGK (Vector a) replicateG i (K1 x) = VectorGK $ replicate i x lengthG (VectorGK v) = length v instance (VectorElementG r1, VectorElementG r2) => VectorElementG (r1 :*: r2) where data VectorG (r1 :*: r2) = VectorGProd (VectorG r1) (VectorG r2) replicateG i (a :*: b) = VectorGProd (replicateG i a) (replicateG i b) lengthG (VectorGProd v _) = lengthG v instance VectorElement ((r1 :+: r2) p) where newtype Vector ((r1 :+: r2) p) = VectorSum (V.Vector ((r1 :+: r2) p)) replicate i x = VectorSum $ V.replicate i x length (VectorSum v) = V.length v instance VectorElementG f => VectorElementG (M1 i c f) where newtype VectorG (M1 i c f) = VectorGMeta (VectorG f) replicateG i (M1 f) = VectorGMeta $ replicateG i f lengthG (VectorGMeta v) = lengthG v I’m not sure if these are correct, especially the one for :+:. I want basically base cases to be any type that already has an instance of VectorElement or a sum type which is automatically boxed, and the recursive case to basically just use parallel vectors for product types. I think this sort of worked insofar as it allowed me to write an instance for tuples: instance (VectorElement a, VectorElement b) => VectorElement (a,b) where newtype Vector (a, b) = VectorTuple (VectorG (Rep (a, b))) replicate i x = VectorTuple $ replicateG i (from x) length (VectorTuple v) = lengthG v Ideally, however, the compiler would automatically derive this instance using the Generic instance. Is there a way to do that also? I would welcome any thoughts on this entire idea and approach. Thanks, Jake Waksbaum

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Call for Haskell.org Committee Nominations
by Alan & Kim Zimmerman 25 Oct '17

25 Oct '17

Dear Haskellers, It is time to put out a call for new nominations (typically but not necessarily self-nominations) to the haskell.org committee. We have three members of our committee due for retirement -- Ryan Trinkle, John Wiegley and Alan Zimmerman. To nominate yourself, please send an email to committee at haskell.org by December 1, 2017. The retiring members are eligible to re-nominate themselves. Please feel free to include any information about yourself that you think will help us to make a decision. The Haskell.org committee serves as a board of directors for Haskell.org, a 501(c)3 nonprofit which oversees technical and financial resources related to Haskell community infrastructure. Being a member of the committee does not necessarily require a significant amount of time, but committee members should aim to be responsive during discussions when the committee is called upon to make a decision. Strong leadership, communication, and judgement are very important characteristics for committee members. The role is about setting policy, providing direction/guidance for Haskell.org infrastructure, planning for the long term, and being fiscally responsible with the Haskell.org funds (and donations). As overseers for policy regarding the open source side of Haskell, committee members must also be able to set aside personal or business related bias and make decisions with the good of the open source Haskell community in mind. We seek a broad representation from different segments of the Haskell world -- including but not limited to those focused on education, those focused on industrial applications, those with background in organizing users-groups, and those focused directly on our technical infrastructure. More details about the committee's roles and responsibilities are on https://wiki.haskell.org/Haskell.org_committee If you have any questions about the process, please feel free to e-mail us at committee at haskell.org, or contact one of us individually. Regards, Alan Zimmerman

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[ANN] Call for Contributions: BOB 2018 - Berlin, Feb 23, 2018
by Stefan Wehr 24 Oct '17

24 Oct '17

Call for Contributions BOB Conference 2018 "What happens when we use what's best for a change?" http://bobkonf.de/2018/en/cfp.html Berlin, February 23 Deadline: October 29, 2017 You are actively engaged in advanced software engineering methods, implement ambitious architectures and are open to cutting-edge innovation? Attend this conference, meet people that share your goals, and get to know the best software tools and technologies available today. We strive to offer a day full of new experiences and impressions that you can use to immediately improve your daily life as a software developer. If you share our vision and want to contribute, submit a proposal for a talk or tutorial! Topics ------ We are looking for talks about best-of-breed software technology, e.g.: - functional programming - persistent data structures and databases - types - formal methods for correctness and robustness - abstractions for concurrency and parallelism - metaprogramming - probabilistic programming - math and programming - controlled side effects - beyond REST and SOAP - effective abstractions for data analytics - ... everything really that isn’t mainstream, but you think should be. Presenters should provide the audience with information that is practically useful for software developers. We’re especially interested in experience reports. But this could also take other forms, e.g.: - introductory talks on technical background - overviews of a given field - demos and how-tos Requirements ------------ We accept proposals for presentations of 45 minutes (40 minutes talk + 5 minutes questions), as well as 90 minute tutorials for beginners. The language of presentation should be either English or German. Your proposal should include (in your presentation language of choice): - an abstract of max. 1500 characters. - a short bio/cv - contact information (including at least email address) - a list of 3-5 concrete ideas of how your work can be applied in a developer’s daily life - additional material (websites, blogs, slides, videos of past presentations, ...) Submit here: https://docs.google.com/forms/d/e/1FAIpQLSdjgwulSMpaITJ6q6cK_ndrfR1FlEs_HQl… Organisation - direct questions to contact at bobkonf dot de - proposal deadline: October 29, 2017 - notification: November 13, 2017 - program: December 1, 2017 NOTE: The conference fee will be waived for presenters, but travel expenses will not be covered. Speaker Grants -------------- BOB has Speaker Grants available to support speakers from groups under-represented in technology. We specifically seek women speakers and speakers who are not be able to attend the conference for financial reasons. Details are here: http://bobkonf.de/2018/en/speaker-grants.html Shepherding ----------- The program committee offers shepherding to all speakers. Shepherding provides speakers assistance with preparing their sessions, as well as a review of the talk slides. Program Committee ----------------- (more information here: http://bobkonf.de/2018/en/programmkomitee.html) - Matthias Fischmann, zerobuzz UG - Matthias Neubauer, SICK AG - Nicole Rauch, Softwareentwicklung und Entwicklungscoaching - Michael Sperber, Active Group - Stefan Wehr, factis research Scientific Advisory Board - Annette Bieniusa, TU Kaiserslautern - Torsten Grust, Uni Tübingen - Peter Thiemann, Uni Freiburg

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Why is Haskell so slow (comparing to Java/Scala)?
by Станислав Черничкин 23 Oct '17

23 Oct '17

I've wrote simple Haskell benchmark program, which populated primitive vector from vector package: import Data.Vector.Primitive.Mutable as P vectorBench :: Benchmark vectorBench = bgroup "vector" [ primitive ] where primitive = bgroup "primitive" [ write1M ] where write1M = bench "write1M" $ nfIO $ do v <- P.unsafeNew 1000000 void $ for [0..1000000 - 1] $ flip (P.unsafeWrite v) (1 :: Int) return () I use `unsafeNew` to skip memory initialization and `unsafeWrite` to skip boundary checks, I guess it's fastest possible way to write something to vector. My result was about 40 ms. I wrote similar program in Scala: for (_ <- 1 to 5) { val start = System.currentTimeMillis() val a = new Array[Long](1000000) for (i <- 0 until 1000000) { a(i) = 1L } val end = System.currentTimeMillis() println(s"${end - start} ms") } I skip neither boundary checks nor memory initialization, I also used generic array here (suitable for any types of objects, not just for primitive types), so I expected longer run time. But what I got was shocking: 7 ms 3 ms 2 ms 1 ms 2 ms This program runs 20-40 times faster than Haskell after warm-up. 20-40 times, Carl! Why is Haskell soo slooooow? How can it be? -- Sincerely, Stanislav Chernichkin.

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[ANN] Haskell ITA meetup in Rome, Italy (2017-11-18)
by Francesco Ariis 23 Oct '17

23 Oct '17

Dear -cafe, the Italian Haskell Community (http://haskell-ita.it/) is happy to announce its autumn Haskell meetup! The event is going to be hosted in Rome, on November 18th. The meeting will be in Italian; we are going to form small groups/projects, to see Haskell in action and to share knowledge and opinions on functional programming. Everyone, experienced or not, is welcome, just make sure to have a laptop and a working Haskell environment! For a more detailed description of the event and to register, check: https://www.eventbrite.it/e/biglietti-haskell-day-roma-aperitech-38962491862

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[ANN] The Disciplined Disciple Compiler (DDC) v0.5.1
by Ben Lippmeier 23 Oct '17

23 Oct '17

# The Disciplined Disciple Compiler (DDC) v.0.5.1 The Disciple language is an experimental dialect of Haskell which investigates static typing and program transformation in the presence of computational effects. Version 0.5.1 is “working alpha” quality, meaning there is a complete system that can be hacked around with, but it’s not yet industrial strength. Homepage: http://disciple.ouroborus.net/ <http://disciple.ouroborus.net/> Github: https://github.com/DDCSF/ddc <https://github.com/DDCSF/ddc> ## Features: * Haskell-like source language, so Haskell-like programs should work with minor modifications. * Modal region and effect system using ‘box’ and ‘run’ to suspend and force computations. * Higher rank polymorphism with bidirectional type inference. * Simple two space copying garbage collection. * Default call-by-value evaluation. * Typed external core language. ## New in This Release: * Copying garbage collection. We now do simple two space Cheney-scan garbage collection, and grow the heap as needed. We use LLVM shadow stack support to retrieve the GC roots. * Implicit parameters. We now support Agda-style implicit parameters, where arguments are construct at use-sites using whatever bindings are currently in scope. For example, we can perform Haskell-style ad-hoc overloading using implicit dictionaries: elem {Eq a} (k: a) (xx: List a): Bool = case xx of Nil -> False Cons x xs | x == k -> True | otherwise -> elem k xs * Floating point primitives. The addition of floating point primitives combined with proper storage management now lets us write more realistic example programs, like the ray tracer demo, which was also described on the blog [1]. * Travis continuous integration. Every push to the GitHub repo now gets tested by the Travis buildbot [2]. Branches can also be tested individually before being merged. * Support for LLVM versions 3.1 - 5.0 We query the version of the installed LLVM compiler and generate LLVM code with the required syntax. This process now works for versions 3.1 through to 5.0, which is the latest at the time of this release. * New home page with the start of a language specification. The home page now consists of the user manual generated from Sphinx / Restructured Text source. The manual includes grammars for source and core lanuages, as well as previous release notes. The bug tracker is still accessible via the development wiki. * Bug fixes for offside rule, compilation of nested pattern matching, string escapes, unsolved meta variables. Now with more bake. [1] http://disciple-devel.blogspot.com.au/2017/07/ray-tracer-demo.html <http://disciple-devel.blogspot.com.au/2017/07/ray-tracer-demo.html> [2] https://travis-ci.org/DDCSF/ddc <https://travis-ci.org/DDCSF/ddc> [3] http://disciple.ouroborus.net/ <http://disciple.ouroborus.net/> ## Work In Progress We are moving to a new indexed binary format for interface files, as re-parsing interface files is currently a bottleneck. The file format is to be provided by the Shimmer project [4], which has been split out into a separate repository. [4] https://github.com/DDCSF/shimmer <https://github.com/DDCSF/shimmer> ## People The following people contributed to DDC since the last release: * Chris Hall - Travis continuous integration. * Amos Robinson - Build system fixes, start on machine fusion transform. * Ben Sinclair - Build system fixes. * Jacob Stanley - Copying garbage collection. * Ben Lippmeier - Copying garbage collection, implicit parameters, floating point, new home page. ## Previous Releases * 2016/09 DDC 0.4.3: Nested patterns and guards, simple type synonyms. * 2016/04 DDC 0.4.2: Compilation of higher order functions, run and box casts. * 2014/03 DDC 0.4.1: Added bi-directional type inference and region extension. * 2013/07 DDC 0.3.2: Added Tetra and Flow language fragments. ## More Information Home Page: http://disciple.ouroborus.net <http://disciple.ouroborus.net/> The GitHub site: http://github.com/DDCSF/ddc <http://github.com/DDCSF/ddc> Development Wiki: http://trac.ouroborus.net/ddc <http://trac.ouroborus.net/ddc> Development Blog: http://disciple-devel.blogspot.com.au/ <http://disciple-devel.blogspot.com.au/> Mailing List: http://groups.google.com/group/disciple-cafe <http://groups.google.com/group/disciple-cafe>

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Failing Travis CI Build
by Steven Leiva 22 Oct '17

22 Oct '17

Hi Everyone, I am trying to contribute to a smallHaskell project in order to break out of structured learning via books into some real-world code. Unfortunately, the pull request that I made (linked above) is failing the continuous integration check. You can see a partial log before. The entire logs can be found here. I thought that this would work because: 1. We are using the 8.20 resolver in our project. 2. The 8.20 resolver includes QuickCheck, hspec, and quickcheck-instances. 3. The dependencies are listed in the cabal file. Am I missing something obvious? Is the above not the way it works? Thank you for any help. Logs Reading available packages...Choosing modular solver.Resolving dependencies...Could not resolve dependencies:trying: twilio-0.2.0.0 (user goal)rejecting: twilio-0.2.0.0:!test (global constraint requires opposite flagselection)trying: twilio-0.2.0.0:*testnext goal: quickcheck-instances (dependency of twilio-0.2.0.0:*test)fail (unknown package: quickcheck-instances)Dependency tree exhaustively searched.Configuring twilio-0.2.0.0...cabal: At least the following dependencies are missing:QuickCheck ==2.*, hspec ==2.*, quickcheck-instances -any Not using Mixmax yet? Steven Leiva 305.528.6038 leiva.steven(a)gmail.com http://www.linkedin.com/in/stevenleiva

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Stackage Haddocks extremely slow
by aditya siram 19 Oct '17

19 Oct '17

Hi all, I've noticed that lately Stackage's Haddock searches have been extremely slow; for example https://www.stackage.org/haddock/lts-9.9/base-4.9.1.0/Prelude.html#v:foldl took almost 8 seconds to pull up on my machine. Is anyone else seeing this? Thanks! -deech

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Re: [Haskell-cafe] [Haskell] Hack “Importify” by Serokell During #Hacktoberfest!
by Henning Thielemann 18 Oct '17

18 Oct '17

On Wed, 18 Oct 2017, Gints Dreimanis wrote: > All of us love to read explicit import lists (especially in an unknown > module), but we all hate to write and maintain them. You won't have many explicit import lists if you design the function names for qualified imports.

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Unused type-classes triggering a recompilation?
by Saurabh Nanda 18 Oct '17

18 Oct '17

Hi, We're trying to optimise the way lenses are built and imported in our project (there are 500+ of them!) . In the simplified version that I have presented below, whenever a new HasX class is added to the LensClasses module, both User & Main end up getting recompiled even though they have nothing to do with the newly added type-class. I've read and re-read "Recompilation Avoidance" [1], but haven't been able to figure out why this would be happening. PS: To work-around this issue we've had to give each HasX class an independent module of its own. We specifically import only those which are required (all of this is done via a code-gen tool). This has resulted in an explosion of modules and is probably causing slow-down of our build process [2] *LensClasses.hs:* {-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies, FlexibleInstances #-} module LensClasses where import Control.Lens class HasName s a | s -> a where name :: Lens' s a class HasEmail s a | s -> a where email :: Lens' s a class HasAge s a | s -> a where age :: Lens' s a *User.hs (which defines a few lenses for the User record):* {-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies, TypeSynonymInstances, FlexibleInstances #-} module User ( module User , HasName(..) , HasEmail(..)) where import LensClasses(HasName(..), HasEmail(..), HasVariantName(..)) data User = User { _userName :: String , _userEmail :: String } deriving (Eq, Show) instance HasName User String where {-# INLINE name #-} name fctor (User name_ email_) = fmap (\dragon -> User dragon email_) (fctor name_) instance HasEmail User String where {-# INLINE email #-} email fctor (User name_ email_) = fmap (\dragon -> User name_ dragon) (fctor email_) *Main.hs (the final call site/s):* module Main where import Control.Lens import User main :: IO () main = do let u = User "saurabh" "saurabhnanda(a)gmail.com" putStrLn $ u ^. name [1] https://ghc.haskell.org/trac/ghc/wiki/Commentary/Compiler/RecompilationAvoi… [2] https://www.reddit.com/r/haskell/comments/76zljl/hotswapping_haskell_at_fac…

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NYC Haskell Developer Position
by Christopher Lewis 18 Oct '17

18 Oct '17

Karamaan Group, is an investment firm based in Manhattan. We are looking for an outstanding software developer to develop tools for financial analysis and knowledge management. Since we are investors and not traders, our focus is not on building trading systems, but on business and data analysis. We strongly value people who take pride in their work and have a craftsman’s dedication to building bespoke products. Our ideal candidate is an experienced software developer with a penchant for abstraction and analysis, a solid sense of organization, and an ability to communicate and persuade. While we are looking for a candidate who demonstrates an intense focus on quality, she also needs a master engineer’s intuition on how to make trade offs that are a necessary part of day-to-day software development. Candidates should have some formal training in a quantitative field and a keen interest in building robust and elegant software. 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 influence on our workflow. The ideal candidate will have production experience with Haskell or another functional language, relational databases and relational data modeling, Python, and Java. Additionally, as this is a startup environment, the ideal candidate will be comfortable filling many different technology roles, and contributing to the success of the firm wherever they can, not just in agreement with their job description. Remote work is not an option for this position. Karamaan Group is an investment adviser and manager catering to family offices and institutional investors. We value innovative thinking, encourage an entrepreneurial atmosphere, and enjoy spirited discussions. 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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ANN: New Full Platform Build of Haskell Platform 8.2.1 for Mac
by Gershom B 18 Oct '17

18 Oct '17

Thanks to some intrepid sleuthing by Albert Y. C. Lai, we realized there was a serious problem in the full (not core) builds of HP for Mac and Linux (not Windows). If you've seen lots of "unusable due to shadowed dependencies" errors on packages that came with full 8.2.1 platform installs on those systems, this is likely why. We should have tested better and caught this, and I'm sorry for the trouble it has caused. I've released a new build of the full mac installer, and updated the hash on the site. A new generic-linux build should follow soon. Regards, Gershom P.s. For those interested, the technical details are at https://github.com/haskell/haskell-platform/issues/285 -- in brief, the platform had done builds using inplace conf files but deployed using proper target package-conf files. This worked just fine until the new abi-depends machinery was introduced for better computing dependency mismatches (https://phabricator.haskell.org/D2846 is one place to start reading about that). That new machinery then recognized that different abis were generated for the inplace conf than the target conf, and things went screwy. But the problem here isn't with those changes -- it's with our inadequate testing that should have caught when things went wrong and alerted us that we needed to step back and fix things earlier, and the responsibility for that is all on me.

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Re: [Haskell-cafe] [Call for Contributions] Haskell Communities and Activities Report, November 2017 edition (33rd edition)
by Karim Salah 17 Oct '17

17 Oct '17

Asteroids Name: Kareem salah (FCIH) status: stable, actively developed participants: mostafa hazem, khaled hesham, belal ibrahim Asteroids is Haskell version of the known asteroids game which was developed with many programing languages as startup project the main aim of the game is to score the highest score by shooting the coming asteroids while avoiding them (just remember u have only three spaceships) the game is simulating the space physics so don't think it is so easy. the game following up the original game rules beside that we added a multi-player system to invite your friends to join your campaign. our team found it a good opportunity to develop a version using Haskell and by using the lazy programming techniques in our favor we were able to provide a stable fps a stable version of the game was released and it's free to download and play we are looking forward for active contributions in order to add versus system, rewarding system and progression system so, forward contributions related to code refinement or adding features is welcomed url : https://github.com/kareem2048/Asteroids

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[Announce] ZuriHac 2018 - 8-10 June in Zurich, Switzerland
by Jasper Van der Jeugt 17 Oct '17

17 Oct '17

It is our pleasure to announce that, like in previous years, ZuriHac will also take place in 2018 -- from Friday June 8th to Sunday the 10th. It will be hosted at the Hochschule Rapperswil right besides the beautiful lake Zurich, like last year. The Zurich Haskell Hackathon is a free (as in beer), international, grassroots collaborative coding festival whose goal is to expand the community and to build and improve Haskell libraries, tools, and infrastructure. This is already the 7th Haskell Hackathon in Zurich! This year, we will enjoy keynotes from: - Niki Vazou - Edward Kmett - Stephen Diehl More keynote speakers will be announced. This event is open to any experience level, from beginners to gurus. This year, Julie Moronuki, co-author of Haskell Programming from first principles [1], has kindly agreed to teach a beginners course in one of the classrooms we have available. Additionally, there will be mentors on site whom you can directly approach during the whole event with any Haskell-related question you have. This is a great opportunity to meet your fellow Haskellers in real life, find new contributors for your project, improve existing libraries and tools or even start new ones! We will have space for 300 attendees. Registration will open on 6th of December. You can find more information on our website [2]. We would also like to thank our sponsors Digital Asset [3] and HSR [4] for supporting this great event! Looking forward to see you there, the Zurich HaskellerZ meetup group [1]: https://www.goodreads.com/book/show/25587599-haskell-programming [2]: https://zurihac.info/ [3]: https://digitalasset.com/careers.html [4]: https://www.hsr.ch/

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Exposing a GADT-provided dictionary
by Kosyrev Serge 17 Oct '17

17 Oct '17

Good day, cafe! I found a way to expose a GADT-provided dictionary, but I don't quite like it: > {-# LANGUAGE GADTs, RankNTypes #-} > > import Data.Function ((&)) > > class HasDict a where > > data GADT a where > G1 :: HasDict a => a -> GADT a > -- ... > Gn :: HasDict a => a -> GADT a > > -- | Execute @body with the dictionary for HasDict extracted from the > -- GADT. > withGADTDict :: > (forall b. (b ~ a, HasDict b) => GADT b -> c) -> GADT a -> c > withGADTDict body x = x & case x of > G1 _ -> body > -- ... > Gn _ -> body The problem is in withGADTDict's proportionality to the number of GADT's clauses, as it is somewhat anti-modular. Is that the only light-weight option? I can imagine there is a way to leverage generic programming, but that's a bit heavy.. -- с уважениeм / respectfully, Kosyrev Serge PGP Key ID: 3F62F118E38BED6D -- “Most deadly errors arise from obsolete assumptions.” -- Frank Herbert, Children of Dune

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Re: [Haskell-cafe] Network Connect Timeouts
by Chris Wong 16 Oct '17

16 Oct '17

Hi Mark, >From reading the source code for *runTCPClient* [1], it looks like a thin wrapper around *getSocketFamilyTCP* and *close*. It should be sufficient to copy that definition into your project and wrap the *getSocketFamilyTCP* call in a *timeout*. You might need to import *Data.Streaming.Network.Internal* to get access to the *AppData* constructor. Hope this helps! [1] https://hackage.haskell.org/package/streaming-commons-0.1.18/docs/src/Data-… On Sun, Oct 15, 2017 at 9:08 AM, Mark Fine <mark.fine(a)gmail.com> wrote: > Hi Chris, > > In my application, I'm using *Data.Conduit.Network* from *conduit-extra* directly > (*Data.Streaming.Network* from *streaming-commons* indirectly). The > timeout I'm interested in managing is the connection establishment timeout > - how long does the connect call wait to establish a connection. I haven't > been able to figure out how to control this outside of applying a > non-conditional timeout to the connect call (which is a timeout on the > lifetime of a connection, not a timeout on the establishment of a > connection). Thanks! > > Mark > > On Sat, Oct 14, 2017 at 3:09 AM, Chris Wong <lambda.fairy(a)gmail.com> > wrote: > >> Hi Mark, >> >> What networking library are you using? >> >> There should be a lower level interface which allows for managing the >> lifetime of a connection by hand. >> >> Chris >> >> On Oct 14, 2017 15:17, "Mark Fine" <mark.fine(a)gmail.com> wrote: >> >>> I'd like a configurable network connect timeout. Has anyone solved this >>> reasonably? >>> >>> *System.Timeout.timeout* times out connections, but works on the >>> granularity of the life of the connection. Is there a reasonable way to >>> configure a timeout around just establishing the connection? Maybe >>> something like a conditional timeout that enables an action to disable the >>> timeout once it expires? >>> >>> As a workaround, I'm spinning trying to successfully connect first >>> before trying to connect for real: >>> >>> -- | Try the TCP connection and see if you can connect... >>> -- >>> tryTCPClient :: Int -> ClientSettings -> IO () >>> tryTCPClient microseconds settings = do >>> ok <- newIORef False >>> void $ timeout microseconds $ runTCPClient settings $ const $ >>> writeIORef ok True >>> ok' <- readIORef ok >>> unless ok' $ >>> tryTCPClient microseconds settings >>> >>> -- | Wrap runTCPClient with a connect timeout. >>> -- >>> -- Tries the TCP connection first, and the runs the regular runTCPClient. >>> -- Of course, this only enforces a TCP connect timeout on the first >>> connect. >>> -- The second TCP connect has no timeout :( >>> -- >>> runTCPClient' :: Int -> ClientSettings -> (AppData -> IO a) -> IO a >>> runTCPClient' microseconds settings action = do >>> tryTCPClient microseconds settings >>> runTCPClient settings action >>> >>> I've also tried running the *tryTCPClient* in its own thread >>> concurrently with *runTCPClient* and throwing an exception if it can't >>> connect in *microseconds*. None of these offer an actual true >>> connection establishment timeout. >>> >>> Has anyone found a way to solve this? Thanks! >>> >>> Mark >>> >>> >>> >>> _______________________________________________ >>> Haskell-Cafe mailing list >>> To (un)subscribe, modify options or view archives go to: >>> http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe >>> Only members subscribed via the mailman list are allowed to post. >>> >> > -- Chris Wong (https://lambda.xyz) "I had not the vaguest idea what this meant and when I could not remember the words, my tutor threw the book at my head, which did not stimulate my intellect in any way." -- Bertrand Russell

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Re: [Haskell-cafe] Your haskell-cafe message [Was: random library]
by jpaugh＠gmx.com 16 Oct '17

16 Oct '17

Ian, Thanks for the note. I work for a secondary school, so that explains the block. I'll keep that in mind, and send mail from home. Thanks, Jon On October 14, 2017 12:29:03 PM CDT, Ian Zimmerman <itz(a)very.loosely.org> wrote: >On 2017-10-10 22:46, jpaugh(a)gmx.com wrote: > >> Interesting library! Here's a link: >> >http://hackage.haskell.org/package/DRBG-0.5.5/docs/Crypto-Random-DRBG.html > >The address from which you posted this is listed by spamhaus: > > ahiker!2 itz$ rblcheck 172.56.6.158 >172.56.6.158 not listed by sbl.spamhaus.org >172.56.6.158 listed by xbl.spamhaus.org >172.56.6.158 listed by pbl.spamhaus.org >172.56.6.158 not listed by bl.spamcop.net >172.56.6.158 not listed by psbl.surriel.com >172.56.6.158 not listed by dul.dnsbl.sorbs.net > >This often happens when people post from cafes and similar, and if this >is the case here not much can be done, unfortunately - other than >perhaps trying a different cafe next time :-) > >-- >Please don't Cc: me privately on mailing lists and Usenet, >if you also post the followup to the list or newsgroup. >Do obvious transformation on domain to reply privately _only_ on >Usenet. -- Sent from my Android device with K-9 Mail. Please excuse my brevity.

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Where to import Id from?
by David Banas 16 Oct '17

16 Oct '17

Hi all, Would someone tell me where I can import Id from, please? I’ve struck out with both Hoogle and Google searches. Thanks! -db

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Re: [Haskell-cafe] haskell-cafe brown out [Was: random library]
by dominic＠steinitz.org 14 Oct '17

14 Oct '17

I get a daily digest from the mailing list and then cut out the bit that seemed relevant fwiw. Well done for caring about mailing lists :) > On 11 Oct 2017, at 17:28, Ian Zimmerman <itz(a)very.loosely.org> wrote: > > On 2017-10-11 13:50, dominic(a)steinitz.org wrote: > >>> Interesting library! Here's a link: >>> http://hackage.haskell.org/package/DRBG-0.5.5/docs/Crypto-Random-DRBG.html >>> <http://hackage.haskell.org/package/DRBG-0.5.5/docs/Crypto-Random-DRBG.html> >>> >>> Thomas, >>> >>> In the linked docs, there's a claim that Hash DRBG is the fastest >>> cryptographically secure RNG on Hackage. Do you have a link to the >>> benchmark results, or perhaps some updated ones? Unlike Viktor, I'm >>> interested in less secure applications, but if the performance is >>> good, it might be worth switching from the defacto random package. >>> >>> Thanks, Jonathan > >> No-one should be using the de facto random package. It is slow and >> produces surprising results. >> >> I think one of the posters suggested using mwc-random and splitting by >> using a different seed. Although mwc-random has reasonably good >> properties, there is no guarantee about how correlated the various >> streams of random numbers will be. QuickCheck uses tf-random to avoid >> such problems. There is also a version of SplitMix >> https://hackage.haskell.org/package/splitmix >> <https://hackage.haskell.org/package/splitmix> but I don’t think this >> has been heavily road-tested. >> >> But in this case, cryptographically secure randomness is required so I >> would follow Thomas’ suggestion. >> >> PS I just noticed my spellchecker changed defacto to defect! Perhaps >> AI is taking over. > > Hello, may I ask how exactly you posted the above message? > > Your message has no threading headers (In-Reply-To or References), but I > _think_ I never received the message to which you appear to be reacting > (that one by "Jonathan"). I cannot prove it beyond doubt because of the > missing headers. Was "Jonathan"'s message posted on the haskell-cafe > mailing list at all? Or what else is going on here? > > Ian, who cares about his mailing lists > > -- > Please don't Cc: me privately on mailing lists and Usenet, > if you also post the followup to the list or newsgroup. > Do obvious transformation on domain to reply privately _only_ on Usenet. Dominic Steinitz dominic(a)steinitz.org http://idontgetoutmuch.wordpress.com

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Network Connect Timeouts
by Mark Fine 14 Oct '17

14 Oct '17

I'd like a configurable network connect timeout. Has anyone solved this reasonably? *System.Timeout.timeout* times out connections, but works on the granularity of the life of the connection. Is there a reasonable way to configure a timeout around just establishing the connection? Maybe something like a conditional timeout that enables an action to disable the timeout once it expires? As a workaround, I'm spinning trying to successfully connect first before trying to connect for real: -- | Try the TCP connection and see if you can connect... -- tryTCPClient :: Int -> ClientSettings -> IO () tryTCPClient microseconds settings = do ok <- newIORef False void $ timeout microseconds $ runTCPClient settings $ const $ writeIORef ok True ok' <- readIORef ok unless ok' $ tryTCPClient microseconds settings -- | Wrap runTCPClient with a connect timeout. -- -- Tries the TCP connection first, and the runs the regular runTCPClient. -- Of course, this only enforces a TCP connect timeout on the first connect. -- The second TCP connect has no timeout :( -- runTCPClient' :: Int -> ClientSettings -> (AppData -> IO a) -> IO a runTCPClient' microseconds settings action = do tryTCPClient microseconds settings runTCPClient settings action I've also tried running the *tryTCPClient* in its own thread concurrently with *runTCPClient* and throwing an exception if it can't connect in *microseconds*. None of these offer an actual true connection establishment timeout. Has anyone found a way to solve this? Thanks! Mark

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Pattern Matching with Different Types
by Yotam Ohad 11 Oct '17

11 Oct '17

Hello cafe, I am trying to do the following: data Foo = Foo { a1 :: Int -> Int, a2 :: Int -> Char } data Bar = Bar { a1 :: Int -> Int } func :: a -> Maybe (Int -> Int) -- a is either Foo or Bar func (x::(Bar/Foo) = Just $ a1 x func _ = Nothing I'm not sure how to implement this. All I know that the types are matching so I think it could be possible. Thanks for your help -Yotam

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Galois is hiring
by Eric Mertens 11 Oct '17

11 Oct '17

Hello, Galois is growing again! We're looking for researchers, principal investigators, software engineers, including those with expertise in functional programming, formal methods, machine learning, embedded systems, computer security, or networking. For the exact available positions, please have a look at our website: http://galois.com/careers We now have three locations: Portland, Ore.; Arlington, Va.; and Dayton, Ohio. There are positions available at all locations. We are looking for people to work on-site, not remotely. Mostly, we are looking for candidates who are already authorized to work in the US, but in exceptional situations we can work with the candidate to obtain the necessary documentation. If you are interested, please apply through the web site. If you have questions about working at Galois, you're welcome to contact me directly. Best regards, Eric Mertens glguy

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random library
by Kazu Yamamoto 11 Oct '17

11 Oct '17

Hello, I'm preparing for a major release of the dns package version 3.0.0. https://github.com/kazu-yamamoto/dns/issues/88 Before the release, I would like to replace the random package since it is slow. I'm looking for a random library which is - fast - thread-safe (good for concurrent use) Any recommendations? // Kazu

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Re: [Haskell-cafe] random library
by dominic＠steinitz.org 11 Oct '17

11 Oct '17

No-one should be using the de facto random package. It is slow and produces surprising results. I think one of the posters suggested using mwc-random and splitting by using a different seed. Although mwc-random has reasonably good properties, there is no guarantee about how correlated the various streams of random numbers will be. QuickCheck uses tf-random to avoid such problems. There is also a version of SplitMix https://hackage.haskell.org/package/splitmix <https://hackage.haskell.org/package/splitmix> but I don’t think this has been heavily road-tested. But in this case, cryptographically secure randomness is required so I would follow Thomas’ suggestion. PS I just noticed my spellchecker changed defacto to defect! Perhaps AI is taking over. > All, > > Interesting library! Here's a link: http://hackage.haskell.org/package/DRBG-0.5.5/docs/Crypto-Random-DRBG.html <http://hackage.haskell.org/package/DRBG-0.5.5/docs/Crypto-Random-DRBG.html> > > Thomas, > > In the linked docs, there's a claim that Hash DRBG is the fastest cryptographically secure RNG on Hackage. Do you have a link to the benchmark results, or perhaps some updated ones? Unlike Viktor, I'm interested in less secure applications, but if the performance is good, it might be worth switching from the defacto random package. > > Thanks, > Jonathan > > On October 10, 2017 10:23:14 PM CDT, Thomas DuBuisson <thomas.dubuisson(a)gmail.com <mailto:thomas.dubuisson@gmail.com>> wrote: >> The hash drbg from the drbg package should meet your needs. >> Deterministic, >> pure Haskell except the actual hash function. >> >> On Oct 10, 2017 8:13 PM, "Viktor Dukhovni" <ietf-dane(a)dukhovni.org <mailto:ietf-dane@dukhovni.org>> >> wrote: >> >>> >>>> On Oct 10, 2017, at 8:48 PM, Kazu Yamamoto (山本和彦) <kazu(a)iij.ad.jp <mailto:kazu@iij.ad.jp>> >>> wrote: >>>> >>>> Before the release, I would like to replace the random package >> since >>>> it is slow. I'm looking for a random library which is >>>> - fast >>>> - thread-safe (good for concurrent use) >>>> >>>> Any recommendations? >>> >>> Just to make it more interesting, I should mention that the RNG >>> should be not just statistically random, but should in fact be >>> crypto random (resist predictability through cryptanalysis when >>> properly seeded). >>> >>> So indeed there are two more issues here: >>> >>> - Securely seeding the RNG (likely using the OS API for >>> random seeds, and/or the RDSEED/RDRAND instructions on >>> Intel CPUs), IIRC we can that from cryptonite, I hope >>> at a reasonable cost. >>> >>> - Choosing a suitable DRBG based on the seed. Likely again >>> something from cryptonite. >>> >>> Some time back I posted to the cryptography list about the >>> soundness of relying on RDRAND in cryptonite's RNG (uses >>> it instead of /dev/urandom and the like when available). >>> The rough consensus IIRC was not rely solely on RDRAND. >>> I never went back to write a PR to address that... >>> >>> http://www.metzdowd.com/pipermail/cryptography/2016- <http://www.metzdowd.com/pipermail/cryptography/2016-> >>> November/thread.html#30859 >>> >>> -- >>> Viktor. Dominic Steinitz dominic(a)steinitz.org http://idontgetoutmuch.wordpress.com

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parallel strategies for foldMap?
by waldmann 10 Oct '17

10 Oct '17

Dear Cafe, I wonder if there is some accepted wisdom (and implementation) to support parallel foldMap in some uniform way. It should probably be tuneable for the case that mappend is less or more expensive than the function to apply in the elements. I can do the following, but it does not parallelize the mappends: import qualified Data.Foldable as F import Control.Parallel.Strategies foldMapPar :: (Foldable f, Monoid b) => Int -> (a -> b) -> f a -> b foldMapPar block f xs = F.fold ( map f (F.toList xs) `using` parBuffer block rseq ) In my application, the Foldable thingy is an unbalanced tree. - J.W. ( the above is a genuine question, while the following is a genuine announcement: don't miss the early registration deadline for "Haskell in Leipzig" https://hal2017.softbase.org/ )

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Confusing `bind` and (>>=)
by mirone 10 Oct '17

10 Oct '17

Hi there, I'm learning haskell for weeks and I'm trying to write a parser using haskell. First I create a datatype: newtype Parser a = P (String -> [(a, String)]) I create a function called `bind` : bind :: Parser a -> ( a -> Parser b) -> Parser b bind f g = P $ \s -> concatMap (\(a, s') -> parse (g a) s') $ parse f s and then: instance Monad Parser where (>>=) = bind that's looks cool, than I write a function liftToken :: (Char -> [a]) -> Parser a liftToken f = P g where g [] = [] g (c:cs) = f c >>= (\x -> return (x, cs)) It works well, then I change this function to liftToken :: (Char -> [a]) -> Parser a liftToken f = P g where g [] = [] g (c:cs) = f c `bind` (\x -> return (x, cs)) GHC throw errors: regular.hs|153 col 14 error| • Couldn't match expected type ‘Parser t0’ with actual type ‘[a]’ • In the first argument of ‘bind’, namely ‘f c’ In the expression: f c `bind` (\ x -> return (x, cs)) In an equation for ‘g’: g (c : cs) = f c `bind` (\ x -> return (x, cs)) • Relevant bindings include f :: Char -> [a] (bound at regular.hs:151:11) liftToken :: (Char -> [a]) -> Parser a (bound at regular.hs:151:1) That's really confusing, why I can use (>>=) but I can't use `bind`? Is there any difference between them?

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