October 2017 - Haskell-Cafe

ANN: parameterized, indexed monads with a single "parameter" type variable
by Louis Pan 28 Oct '17

28 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 26 Oct '17

26 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 26 Oct '17

26 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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