- Haskell-Cafe - Haskell.org

Archival of community.haskell.org
by Mario Rogic 20 May '18

20 May '18

Hi all As announced back in 2015 <http://blog.haskell.org/post/7/the_future_of_community.haskell.org/> we have started archiving community.haskell.org services. We've created https://archives.haskell.org/, and the old projects mailing lists have been the first to be archived. Next on the list is the remainder of projects.haskell.org, and the code.haskell.org site. As always, you can email admin@h.o or hop on the #haskell-infrastructure freenode channel to provide feedback / concerns. Thanks - Mario

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typeclasses considered unnecessary/the power of the @
by Anthony Clayden 18 May '18

18 May '18

Haskell 98/2010 keeps a strict distinction between term-level syntax and tokens vs type-level. With ExplicitTypeApplications that's being eased: there's a lightweight way to sneak a type expression into a term. And ghc's internal term language 'core' has had explicit type applications for many years. Then consider a lightweight syntax using type applications in patterns to define typeclass instances by (pattern) 'matching' on types: > (==) @Int x y = primEqInt x y -- or eta-reduced > (==) @Int = primEqInt -- shorthand for > > instance Eq Int where > (==) = primEqInt Given that many typeclasses have a single method (or one base method with others defined in terms of it) [Note **], having to invent a typeclass name is a bit of a pain. Here's a lighweight method decl: > meth :: @a @b. => a -> b -> Bool -- typevar binding to left of => (there might also be superclass constraints) > -- shorthand for > > class Classfor_meth a b where -- generated class name > meth :: a -> b -> Bool Do we need class names at all? We could sneak term-level names into types. Just use the method name in constraints (with some handy syntactic marking -- what could be better than prefix @): > meth2 :: (@meth a b, @show b) => a -> b -> String > meth2 x y = if (meth x y) then show y else "blah" Of course we can infer the constraints on `meth2`, given its equation. Note **: for some of the Prelude's classes with many methods, like Num, there's long-standing complaints that the methods are too much tangled up and should be refactored into a hierarchy with a single method in each class: Additive, Multiplicative, Divisive, etc. With type families we can tackle the classic collections class (with methods empty, insert) as a superclass constraint , er, supermethod method: > type family Elem c :: * > insert @c. => c -> Elem c -> c > > empty :: @c. (@insert c) => c -- entangle empty with insert Likewise for Monad: > (>>=) :: @m. => (m a) -> (a -> m b) -> (m b) > > return :: @m. (@>>= m) => a -> m a The soon-to-arrive Quantified Constraints extension will provide further ways to entangle methods/constraints. That might be an alternative way to express collections: > insert @c @e. (e ~ Elem c) => c -> e -> c > > empty @c. (forall e. @insert c e => @empty c) => c Of course all the above syntax is speculative/bikesheddable. The syntax needs to differentiate tyvars that are parameters/overloadable to the class/method vs parametric free tyvars. With explicit forall: > (>>=) :: forall m a b. @m. => (m a) -> (a -> m b) -> (m b) > > return :: forall m a. @m. (@>>= m) => a -> m a Or perhaps method signatures should look more like methods-as-constraints -- at cost of a bit of repetition > (>>=) :: forall m a b. (@>>= m) => (m a) -> (a -> m b) -> (m b) > > return :: forall m a. (@return m, @>>= m) => a -> m a AntC

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compilation error of tensorflow-proto 0.1.0.0
by Fabien R 18 May '18

18 May '18

Hello, Using ghc 8.2.2 (debian) , I got the following error while building the package with cabal 2.0: Building library for tensorflow-proto-0.1.0.0.. creating dist/build /media/travail/ghc/bin/ghc --make -fbuilding-cabal-package -O -static -dynamic-too -dynosuf dyn_o -dynhisuf dyn_hi -outputdir dist/build -odir dist/build -hidir dist/build -stubdir dist/build -i -idist/build -i. -idist/build/autogen -idist/build/global-autogen -Idist/build/autogen -Idist/build/global-autogen -Idist/build -I. -optP-include -optPdist/build/autogen/cabal_macros.h -this-unit-id tensorflow-proto-0.1.0.0-A707TW4oXcjUPzDlNSl3T -hide-all-packages -Wmissing-home-modules -package-db dist/package.conf.inplace -package-id proto-lens-0.2.2.0-4VJbI6HHabd74gTwpFhCiY -package-id proto-lens-protoc-0.2.2.3-KiMlOomTiwkJiIiDqOmBdc -package-id base-4.10.1.0 -XHaskell2010 Proto.Tensorflow.Core.Framework.AttrValue Proto.Tensorflow.Core.Framework.Graph Proto.Tensorflow.Core.Framework.NodeDef Proto.Tensorflow.Core.Framework.OpDef Proto.Tensorflow.Core.Framework.ResourceHandle Proto.Tensorflow.Core.Framework.Summary Proto.Tensorflow.Core.Framework.Tensor Proto.Tensorflow.Core.Framework.TensorShape Proto.Tensorflow.Core.Framework.Types Proto.Tensorflow.Core.Protobuf.Config Proto.Tensorflow.Core.Util.Event Proto.Tensorflow.Core.Framework.AllocationDescription Proto.Tensorflow.Core.Framework.CostGraph Proto.Tensorflow.Core.Framework.Function Proto.Tensorflow.Core.Framework.StepStats Proto.Tensorflow.Core.Framework.TensorDescription Proto.Tensorflow.Core.Framework.Versions Proto.Tensorflow.Core.Protobuf.Debug [ 1 of 18] Compiling Proto.Tensorflow.Core.Framework.AllocationDescription ( dist/build/autogen/Proto/Tensorflow/Core/Framework/AllocationDescription.hs, dist/build/Proto/Tensorflow/Core/Framework/AllocationDescription.o ) dist/build/autogen/Proto/Tensorflow/Core/Framework/AllocationDescription.hs:159:15: error: â¢ Couldn't match expected type âData.ProtoLens.MessageDescriptor AllocationDescriptionâ with actual type âData.Map.Map Prelude.String (Data.ProtoLens.FieldDescriptor AllocationDescription) -> Data.ProtoLens.MessageDescriptor AllocationDescriptionâ â¢ Probable cause: âData.ProtoLens.MessageDescriptorâ is applied to too few arguments In the expression: Data.ProtoLens.MessageDescriptor (Data.Map.fromList [(Data.ProtoLens.Tag 1, requestedBytes__field_descriptor), (Data.ProtoLens.Tag 2, allocatedBytes__field_descriptor), (Data.ProtoLens.Tag 3, allocatorName__field_descriptor), (Data.ProtoLens.Tag 4, allocationId__field_descriptor), ....]) (Data.Map.fromList [("requested_bytes", requestedBytes__field_descriptor), ("allocated_bytes", allocatedBytes__field_descriptor), ("allocator_name", allocatorName__field_descriptor), ("allocation_id", allocationId__field_descriptor), ....]) Does it mean that I have the wrong version of proto-lens ? * proto-lens Synopsis: A lens-based implementation of protocol buffers in Haskell. Default available version: 0.3.1.0 Installed versions: 0.2.2.0, 0.3.1.0 Homepage: https://github.com/google/proto-lens#readme License: BSD3 -- Fabien

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2nd CfP Workshop on Functional High-Performance Computing 2018
by Kei Davis 18 May '18

18 May '18

2nd CfP Workshop on Functional High-Performance Computing 2018 The 7th ACM SIGPLAN Workshop on Functional High-Performance Computing (FHPC 2018) is being held as in previous years in conjunction with the International Conference on Functional Programming (ICFP 2018) together with numerous other workshops/symposia, and as a first, colocated with Strange Loop, in St. Louis, MO, USA. https://icfp18.sigplan.org/track/FHPC-2018-papers#FHPC-2018-Call-for-Papers -- Kei Davis Applied Computer Science Group CCS-7, Mail Stop B287 Los Alamos National Laboratory Los Alamos, NM 87545, U.S.A.

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hiring PureScript coder
by Dennis Raddle 17 May '18

17 May '18

Hello list, I'm have many years of Python/C++ programming experience professionally, and I've used Haskell in my hobbies. But I'm now working full time as a math tutor. Every day I get ideas for a web app that could help me teach math, and maybe could eventually be for sale in the app store. I'd like to write version 0.1 of this in PureScript. And because I have more money than time at this point, I'd like to hire someone to get me started. I'm thinking of asking for the following: - set up a PureScript project that integrates with React - write code that demonstrates using the kinds of features I'll need, like React features, the HTML5 canvas, and JavaScript math notation libraries. - be available for a couple of Skype or Hangouts meetings as I look over this and have questions Can I get some suggestions as to where to find willing PureScript programmers, and a ballpark number of hours for this? I mean just an order of magnitude... is this closer to 5 hours, 10 hours, or 50 hours? Thanks, D

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ANNOUNCE: Haskell Communities and Activities Report (34th ed., May 2018)
by Mihai Maruseac 16 May '18

16 May '18

On behalf of all the contributors, we are pleased to announce that the Haskell Communities and Activities Report (34th edition, May 2018) is now available, in PDF and HTML formats: http://haskell.org/communities/05-2018/report.pdf http://haskell.org/communities/05-2018/html/report.html All previous editions of HCAR can be accessed on the wiki at https://wiki.haskell.org/Haskell_Communities_and_Activities_Report Many thanks go to all the people that contributed to this report, both directly, by sending in descriptions, and indirectly, by doing all the interesting things that are reported. We hope you will find it as interesting a read as we did. If you have not encountered the Haskell Communities and Activities Reports before, you may like to know that the first of these reports was published in November 2001. Their goal is to improve the communication between the increasingly diverse groups, projects, and individuals working on, with, or inspired by Haskell. The idea behind these reports is simple: Every six months, a call goes out to all of you enjoying Haskell to contribute brief summaries of your own area of work. Many of you respond (eagerly, unprompted, and sometimes in time for the actual deadline) to the call. The editors collect all the contributions into a single report and feed that back to the community. When we try for the next update, six months from now, you might want to report on your own work, project, research area or group as well. So, please put the following into your diaries now: ======================================== End of September 2018: target deadline for contributions to the October 2018 edition of the HCAR Report ======================================== Unfortunately, many Haskellers working on interesting projects are so busy with their work that they seem to have lost the time to follow the Haskell related mailing lists and newsgroups, and have trouble even finding time to report on their work. If you are a member, user or friend of a project so burdened, please find someone willing to make time to report and ask them to "register" with the editors for a simple e-mail reminder in November (you could point us to them as well, and we can then politely ask if they want to contribute, but it might work better if you do the initial asking). Of course, they will still have to find the ten to fifteen minutes to draw up their report, but maybe we can increase our coverage of all that is going on in the community. Feel free to circulate this announcement further in order to reach people who might otherwise not see it. Enjoy! -- Mihai Maruseac (MM) "If you can't solve a problem, then there's an easier problem you can solve: find it." -- George Polya

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state monad transformer that adds or combines state
by Dennis Raddle 16 May '18

16 May '18

I find myself using the state and exception monads at large scale in my programs. For instance I often keep a pseudorandom generator as the state, and use exception handling. In local computations, I may want to use some additional state. Is there a way to add a little extra state temporarily? Let's say I have data State1 = State1 StdGen type M1 = State State1 data State2 = State2 State1 Int type M2 = State State2 runM2 :: M2 a -> Int -> M1 a someFunc :: M1 Double someFunc = do r <- <compute something pseudorandomly> r2 <- runM2 (deeperFunc r) 3 <..etc..> deeperFunc :: Double -> M2 Double

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template haskell library for OO-like selector syntax?
by Dennis Raddle 16 May '18

16 May '18

In my current application, I'm really missing the ability from OO to do things like someData.field1.field2 = "foo" What's do I mean? - field selectors are scoped so that names don't clash with each other or global names - this way of selecting data can be used both for reading it, and for updating specific fields of some data deep in a data hierarchy (In Haskell, if using immutable data, this would requiring doing a data update on every level). Any attempt to do this in Haskell, for me anyway, results in an explosion of names and a lot of boilerplate code and a resulting heavy syntax. Is there a way of obtaining OO-like concise syntax, perhaps with Template Haskell? Dennis

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Failing build for HaXml
by Vanessa McHale 16 May '18

16 May '18

Does anyone know how to submit patches for HaXml? The .cabal file does not list any upstream source repo and it's currently breaking with GHC 8.4.2, which in turn breaks cabal-debian. Thanks -- *Vanessa McHale* Functional Compiler Engineer | Chicago, IL Website: www.iohk.io <http://iohk.io> Twitter: @vamchale PGP Key ID: 4209B7B5 Input Output <http://iohk.io> Twitter <https://twitter.com/InputOutputHK> Github <https://github.com/input-output-hk> LinkedIn <https://www.linkedin.com/company/input-output-global>

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GHC TV station?
by Sylvester Hazel 16 May '18

16 May '18

White space in "do notation" is no longer supported by GHC but I had to guess it the hard way seeing my solutions to Hutton's book and then his own code produce "Parse error" and irritating hint "Possibly caused by a missing 'do'?". So, Hutton 2ed is outdated, Haskell Platform is dropped exhausted then perhaps GHC should have a TV station? Fun is best when it can be shared. Regards, Sylvester

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