March 2017 - Haskell-Cafe

Request for white paper
by Michael Litchard 29 Mar '17

29 Mar '17

I'm looking for a freely available version of the below paper. Google Scholar points me to 7 versions, all of which are behind a paywall. Stefan Kahrs, "Red-black trees with types", Journal of functional programming, 11(04), pp 425-432, July 2001

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Call for participation - Programming Languages Mentoring Workshop (PLMW)
by Tobias Grosser 29 Mar '17

29 Mar '17

We invite early graduate students and undergraduates to apply for a scholarship and to participate in the Programming Languages Mentoring Workshop (PLMW) in Barcelona, Spain on Sunday, June 18, 2017. The purpose of this mentoring workshop is to encourage and prepare graduate students and senior undergraduate students to pursue careers in programming language research. The workshop also includes specific mentoring opportunities for students who belong to traditionally underrepresented populations, including women, people of color, students with disabilities, and first-generation students. Throughout the day, we’ll have technical sessions, to discuss cutting-edge research in programming languages; and mentoring sessions, to help you learn about, prepare, and succeed in your research career. You’ll also have the opportunity to talk with top programming language researchers and other students interested in programming language research. This workshop is part of the activities surrounding the Programming Languages Design and Implementation (PLDI) conference and takes place the day before the main conference. One goal of the workshop is to make the PLDI conference more accessible to newcomers. We hope that participants will stay through the entire conference. Other events that follow the workshop include the Programming Languages Summer School and the European Conference on Object-Oriented Programming (ECOOP). Scholarship funds are available for students to attend PLMW@PLDI. These scholarships should cover reasonable expenses (airfare, hotel, and registration fees) for attendance at both the workshop and an associated event (such as the PLDI conference or the summer school), but are limited. We welcome students with alternative sources of funding to attend as well; but everyone who plans to attend needs to apply. Apply for a scholarship and to participate at PLMW@PLDI: https://goo.gl/forms/80aacyFiXNFNnPqA3 The deadline for full consideration of funding is SATURDAY, APRIL 15. Selected participants will be notified by APRIL 29 and will need to pre-register and commit to attending the workshop by MAY 10. Applicants who apply after April 15 but by May 15 may be eligible to receive funding, if funds remain, and will be notified by by May 29. Additionally, if you are currently a student in the United States who cannot (or will not) travel due to concerns about US travel restrictions, we encourage you to apply and let us know. We may be able to arrange another form of mentoring for you. We look forward to seeing you! The PLMW@PLDI Organizers Michael Carbin, MIT Stephen Chong, Harvard University Işıl Dillig, The University of Texas at Austin Ben Wiedermann, Harvey Mudd College

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Queue which tracks actual in-memory element size?
by Bardur Arantsson 29 Mar '17

29 Mar '17

Hi all, So we have the TBQueue[1] which tracks the number of elements, but does anyone know if there's an equivalent queue which the actual combined size of the elements? (I'd be quite happy if it used "gsize" or similar, but I suppose the ideal would be if I could supply a "size estimator" function.) It seems like an obvious generalization of TBQueue[2], but my Google-fu is letting me down at the moment. Regards, --- [1] https://hackage.haskell.org/package/stm-2.4.4.1/docs/Control-Concurrent-STM… [2] TBQueue would be the equivalent of estimating every element to have size 1.

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[ANN] wiringPi-1.0.1
by Patrick Pelletier 28 Mar '17

28 Mar '17

https://hackage.haskell.org/package/wiringPi https://github.com/ppelleti/hs-wiringPi The most significant addition in this version is the wiringPiISR function, which makes it possible to get a callback when an input pin changes, thus avoiding the need to poll. In a future version, I might add a higher-level facility on top of this, such as the ability to update a TVar when an input changes. (Let me know if you have any thoughts on this.) New in 1.0.1: * Examples for Pi-LITEr and Fish Dish boards. * Added piGpioLayout as a synonym for piBoardRev, to reflect the name change in version 2.36 of the C library. * Addition of wiringPiISR function. (Contributed by Satoshi Ogata.) * Improvements to documentation and examples. About wiringPi: This is a Haskell binding to the wiringPi library, which allows you to interface with the GPIO pins on the Raspberry Pi. Unlike some other solutions for using the Raspberry Pi's GPIO pins, wiringPi provides access to more advanced features, such as enabling the internal pull-up or pull-down resistors. --Patrick

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SublimeHaskell on SublimeText 2
by Scott Michel 28 Mar '17

28 Mar '17

Is there anyone out there who uses the SublimeHaskell plugin for SublimeText 2? Reason for asking: I would sincerely like to remove all vestiges of SublimeText 2 from the plugin. I suspect (know) that it hasn't worked for SublimeText 2 for a long time and the amount of work to backport the current plugin would involve significant effort. For those of you who gave up on SublimeHaskell: Consider giving it another chance. It's more stable now and I've personally been working off issues to make it, in the words of one of the testers, "suck less." (*) It's still not perfect -- known issues are: - The hsdev backend starts up always. I'm refactoring the backend management code, which will take a while, but this will get fixed. It will also enable an enterprising individual to integrate intero and other backends. - Can't connect to a remote hsdev backend. Planned enhancement. - Can't use multiple stack.yamls. Planned enhancement that could get released in the next two weeks. -scooter (*) Paraphrase of Michael Elkin's comment about the Mutt mail client: "All mail users agents suck, Mutt just sucks less."

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Proposal for an extension: in-module namespaces
by Aura Kelloniemi 28 Mar '17

28 Mar '17

Hello I've been thinking about an extension to GHC which would allow creating new module-like namespaces within normal Haskell modules. Because this kind of namespacing system seems quite obvious to me, I ask if it has already been proposed or discussed in detail? The motivation: 1. It would assist in solving the 'pollution of namespace' -problem, of which many Haskellers have complained. 2. It would allow building bundles of related modules which can be exported from one module and then imported into others. This could result in radically fewer import statements. Rant: I'm tired of writing import qualified Data.ByteSstring as B import qualified Data.ByteString.Builder as B import qualified Data.ByteString.Lazy as BL import qualified Data.ByteString.Lazy.Builder as BL import qualified Data.HashMap.Strict as HM import qualified Data.Text as T import qualified Data.Text.Lazy as TL import qualified Data.Text.Lazy.Builder as BL A huge portion of this appears in almos all my Haskell source files. If I wrote my code in Python, I could condense all these imports to: So, you see, it was zero lines of code. I'm also tired of these patterns: (personName p), (companyName c), (objectName o), ... and I hardly prefer (name (p :: Person)), (name (c :: Company)) or (name (o ::Object)) using the DuplicateRecordFields syntax. Proposal: I propose that modules can contain named submodules. They can be exported and imported just like anything else within a module, like this: {-# LANGUAGE Submodules #-} module Library.Things (qualified module Person, qualified module Company, personWorksInCompany ) where -- import ... qualified module Person (Person (..)) where -- This is the module Library.Things.Person. Submodules could possibly -- use some other keywords than 'qualified module'. Maybe just 'module', -- 'submodule' or 'namespace'. data Person = Person { name :: {-# UNPACK #-} !T.Text, id :: {-# UNPACK #-} !Unique, ... } -- Back in Library.Things qualified module Company where data Company = Company { name :: {-# UNPACK #-} !T.Text, employees :: {-# UNPACK #-} !V.Vector Unique, ... } -- back in Library.Things again: personWorksIn :: Person.Person -> Company.Company -> Bool personWorksIn p c = Person.id p `elem` Company.employees c -- ... end of Library.Things And in another file: {-# LANGUAGE Submodules #-} -- The following demonstrates some ways of importing submodules. They are -- not meant to be used like this in real code: import Library.Things import qualified Library.Things as Things -- Now the submodules are also in scope and can be referred to like this: person1 :: Person.Person person2 :: Things.Person.Person company :: Library.Things.Company.Company -- And we also could have written: after importing Library.Things: import Person -- And now data type Person, selector name, etc. are in scope -- If we also imported the Company module: import Things.Company -- selector 'name' would now be ambiguous and need qualification at use site. -- Import lists would also work when bringing submodule contents to current -- namespace. Like this: import Library.Things hiding (module Company) -- or import qualified Library.Things (module Person) -- Which would allow Library.THings.Person.Person to be used requiring use of -- the fully qualified name. -- But the following would not work if Library.Things was not already -- imported: import Library.Things.Person -- because there is no such module, just submodule. -- ... end of other file Some projects probably would like to create a module like this: module Imports (qualified module B, qualified module HM, qualified module T ) where import Data.ByteString as B -- ... and so forth ... Open questions: - Should there be a separate keyword for importing submodules, for example 'use' or 'import namespace'? - Would imports be allowed within submodules? I'd say no for importing other modules, but importing submodules could be ok. - Would it be possible to extend submodules after their initial definition by just writing a new 'qualified module' block? I'd say yes, because submodules are just about namespacing. It is today possible to import many modules qualified and put them in the same namespace, like is often done. - What do you fellow Haskellers think about this all? -- Aura

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Two PhD positions in Ljubljana starting October 2017
by Matija Pretnar 28 Mar '17

28 Mar '17

We are looking for two PhD students at the Faculty of Mathematics and Physics, University of Ljubljana. The programme starts in October 2017 and lasts three years. The positions will be fully funded (subject to approval by the funding agency). The candidates should have a Master's degree in mathematics or computer science. No knowledge of Slovene is required. The first PhD student will be advised by dr. Andrej Bauer. The topic of research is foundations of type theory. The candidate should have interest in mathematical aspects of type theory, and familiarity with proof assistants is desirable. The second PhD student will be advised by dr. Matija Pretnar. The topic of research is the theory of programming languages with a focus on computational effects. The candidate should have interest in both the mathematical foundations and practical implementation of programming languages. Candidates should send their applications as soon as possible, but no later than the end of April, to Andrej Bauer <andrej.bauer(a)fmf.uni-lj.si> or Matija Pretnar <matija.pretnar(a)fmf.uni-lj.si>, depending on their primary interest. Please include a short CV, academic record, and a statement of interest. Andrej Bauer and Matija Pretnar

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Munich Haskell Meeting, 2017-03-29 @ 19:30
by Christian Neukirchen 26 Mar '17

26 Mar '17

Dear all, Next week, our monthly Munich Haskell Meeting will take place again on Wednesday, March 29 at Max-Emanuel-Brauerei at 19h30. For details see here: http://muenchen.haskell.bayern/dates.html If you plan to join, please add yourself to this dudle so we can reserve enough seats! It is OK to add yourself to the dudle anonymously or pseudonymously. https://dudle.inf.tu-dresden.de/haskell-munich-mar-2017/ Everybody is welcome! cu, -- Christian Neukirchen <chneukirchen(a)gmail.com> http://chneukirchen.org

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Void vs ()
by Bardur Arantsson 25 Mar '17

25 Mar '17

Hi all, [Context: I'm working on a CQRS/ES library and in the following, "i" is an identifier for an aggregate root, "e" is an event type, and the "Int" is a sequence number.] So, I have a data type data Foo i e = Foo i e Int where I want to have a corresponding data Batch i e = Batch i [Foo x e Int] (I'll come back to "x". It's just a metasyntactic variable for now. The point of the "Int" is that it's extra information that I really need to keep, but it's not essential to the Foo data structure itself.) with an operation batchFoos :: [Foo i e] -> [Batch i e] batchFoos = ... (basically a standard Data.List.groupBy where I compare for equality on the i's.) I also have an extraction function toList :: Batch i e -> (i, [Foo x e Int]) My question is then: What's the best thing to use for "x", Void or ()? The idea behind "collapsing" the "i" parameter away to an "x" is to signal to users to the API that they can be absolutely certain that when they call "toList", they can be sure that all the Foo's that they get back have the same i value, namely the one which is the first component of the tuple. An alernative I've been considering is actually to create a Foo' type which actually does eliminate the i parameter, but it seems a bit boilerplatey. (It's not a *lot* of code to duplicate, so if it looks like the best option, I'll do that... but it feels odd.) Anyone have any suggestions on what the best way to approach this would be? (Aside: AFAICT there's no reasonable way to implement anything like Foldable or Traversable to avoid the explicit "toList" function, right? AFAICT Foldable/Traversable constrain you to using the *actual* last type parameter, "e", for folding over. Are there any generic alternative type classes that let you "pick your own element type"? I'm guessing Traversal could possibly be that, but if I have to incur a Lens dependency, I'll just live with a "toList" function. This is just about whether it can be done elegantly -- it's not a big deal.) Regards,

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Literate Haskell w/ markdown?
by David Banas 25 Mar '17

25 Mar '17

Hi all, Would anyone care to share their preferred method for writing/processing literate Haskell w/ markdown? Thanks, -db

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