- Haskell-Cafe - Haskell.org

Parsing a matrix table with Data.Frames
by Tyson Whitehead 06 Jun '18

06 Jun '18

I have a case where some of my data is stored in CSV matrix form like this example of a table telling whether person i has pet j ,cat,dog,goldfish bill,true,true,false sue,false,true,true fred,false,false,true I can't see any obvious way to read this sort of thing in with the Data.Frames package. Would love to be told differently. Thanks! -Tyson

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Wanted: New Admin for Planet.Haskell.Org
by Gershom B 05 Jun '18

05 Jun '18

Dear all — the planet.haskell.org server is a low-maintenance feed-aggregator that uses the planet venus software to pull together feeds of haskell-related content. I think it is a great resource, and I’ve used it for years. For some time the planet(a)haskell.org address hadn’t been well-monitored, meaning that requests for adding new rss feeds had languished. I’ve gone through and tried to act on the outstanding requests, and will be doing so for the time being. However, I have a lot on my plate, and would like to pass this responsibility on to a volunteer. It basically means, on the occasional recipt of an email, logging into the server, updating a config file, running a command, and logging back out again. If you’re interested in helping out with this, please contact me. Cheers, Gershom

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Re: [Haskell-cafe] No Enum for (,), no Enum or Bounded for Either
by Olaf Klinke 04 Jun '18

04 Jun '18

>Also note that what you're talking about is a special type of objects, >namely > > type BoundedEnum a = (Bounded a, Enum a) -- using ConstraintKinds > >(I'm sure the mathematicians have a better name for this) Indeed the Prelude states no laws for Bounded. But maybe we all agree on this: forall a. fromEnum minBound <= fromEnum a <= fromEnum maxBound With this law and the rule fromEnum.toEnum=id (for those integers that this type's toEnum accepts) we can say that (Bounded a,Enum a) is the class of all types which have a distinguished finite subset. If one assumes further the law toEnum.fromEnum=id (which is violated e.g. by Double) then (Bounded a,Enum a) are precisely the constructively finite types, that is, the types for which one can exhibit an isomorphism with a finite cardinal. Note that in constructive mathematics, a set may be known to be finite in the sense that the assumption it is infinite is absurd, yet one can not name an isomorphism with a finite cardinal. [*] As Jon Fairbairn pointed out, (Int,Int) can be given an Enum instance. The same is true for every type that is build from types that already have an Enum instance using only sum and product. >This is all true, but I was implicitly assuming that Bounded and Enum ought >to agree with Ord. Consider this: From every type's Enum instance you can derive an Ord instance: compare = compare `Data.Function.on` fromEnum -- Olaf [*] In the text [1] by Andrej Bauer I found this funny theorem: Excluded middle is equivalent to the statement that subsets of finite sets are finite. [1] http://www.ams.org/journals/bull/2017-54-03/S0273-0979-2016-01556-4/S0273-0…

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WST 2018 - Call for Participation (July 18-19, 2018)
by Salvador Lucas 04 Jun '18

04 Jun '18

========================================================================== WST 2018 - Call for Participation 16th International Workshop on Termination part of FLoC 2018 July 18-19, 2018, Oxford, United Kingdom http://wst2018.webs.upv.es/ ========================================================================== REGISTRATION (June 6th is the last day for early registration!) http://www.floc2018.org/register/ INVITED SPEAKERS: James Worrell - University of Oxford "Termination Checking and Invariant Synthesis for Affine Programs" Akihisa Yamada - NII Japan "Towards a Unified Method for Termination" PROGRAMME (see also the list of accepted papers attached below): https://easychair.org/smart-program/FLoC2018/WST-program.html TERMINATION AND COMPLEXITY COMPETITION: In 2018, the Termination and Complexity Competition (TERCOMP) will run in parallel with FLoC 2018 as part of the FLoC Olympic Games. The results of the competition will be presented and discussed during a special session at WST 2018. ACCEPTED PAPERS: Eric Hehner. Objective and Subjective Specifications. Guillaume Genestier and Frédéric Blanqui. Termination of Lambda-Pi modulo rewriting using the size-change principle Jera Hensel, Florian Frohn and Jürgen Giesl. Complexity Analysis for Bitvector Programs Alfons Geser, Dieter Hofbauer and Johannes Waldmann. Comparing on Strings: Semantic Kachinuki Order Nachum Dershowitz and Jean-Pierre Jouannaud. GPO: A Path Ordering for Graphs Jose Divasón, Sebastiaan Joosten, René Thiemann and Akihisa Yamada. A Perron-Frobenius Theorem for Jordan Blocks for Complexity Proving Jonas Schöpf and Christian Sternagel. TTT2 with Termination Templates for Teaching Cristina David, Daniel Kroening and Peter Schrammel. Procedure-Modular Termination Analysis Salvador Lucas. Well-founded models in proofs of termination Aalok Thakkar, Balaji Krishnamurthy and Piyush Gupta. Verification of Rewriting-based Query Optimizers Jesús J. Doménech, Samir Genaim and John P. Gallagher. Control-Flow Refinement via Partial Evaluation Alicia Merayo Corcoba and Samir Genaim. Inference of Linear Upper-Bounds on the Expected Cost by Solving Cost Relations Dieter Hofbauer. Embracing Infinity - Termination of String Rewriting by Almost Linear Weight Functions Carsten Fuhs and Cynthia Kop. Improving Static Dependency Pairs for Higher-Order Rewriting

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A law for MonadReader
by Li-yao Xia 03 Jun '18

03 Jun '18

Hello Café, While trying to document laws for mtl classes (https://github.com/haskell/mtl/issues/5) I have been puzzling over the subtle logical relationships between a few variants. Consider the following four possible laws for MonadReader's ask: (1) (ask >> ask) = ask (2) (ask >>= \r1 -> ask >>= \r2 -> return (r1, r2)) = (ask >>= \r -> return (r, r)) (3) (ask >> return ()) = return () Let reader :: MonadReader r m => (r -> a) -> m a reader f = fmap f ask (4) reader is a monad homomorphism from ((->) r) to m, i.e.: reader (\_ -> a) = return a (reader m >>= \x -> reader (k x)) = reader (m >>= k) Question: which ones imply which ones? Note that (1) and (2) do not imply (3) or (4). Intuitively, (1) and (2) say that ask is idempotent ("asking twice is the same as asking once"), but (3) and (4) imply the stronger property of nullipotence ("ask has no side effects"). --- Any help with the following conjectures is welcome (we're assuming the monad laws hold in the first place): A. (1) and (2) are equivalent. I can prove that (2) implies (1). However the converse eludes me. B. (3) and (4) are equivalent. Again, I can prove that (4) implies (3), but I had no success with the converse. C. ((3) or (4)) implies ((1) and (2)) ((3) implies (1)), and ((4) implies ((1) and (2))) are straightforward, but without the above equivalence, the missing bit is ((3) implies (2)) It seems that parametricity plays an important role in the conjectured implications. I didn't manage to apply free theorems to this problem, but perhaps you will have better luck. Regards, Li-yao

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(names for) invariants for Eq and Ord?
by Johannes Waldmann 02 Jun '18

02 Jun '18

Dear Cafe, I am somewhat surprised that the Haskell Standard (*) https://www.haskell.org/onlinereport/haskell2010/haskellch6.html#x13-127000… does not contain any notation for (intended) properties of Eq and Ord instances, and their relation. For Java (standard library), the API spec uses wording like "consistent with equals" https://docs.oracle.com/javase/10/docs/api/java/lang/Comparable.html I am well aware that semantics (transitivity, etc.) cannot be enforced statically, in either language. But both standards still speak of "total order". Do we (Haskell) need something similar to "consistent with equals"? It depends. E.g., looking at a (random) source in containers (Data.Map.Internal), it seems that it will always use the result of compare, never of (==), on keys. That's not obvious from the docs. I just checked -- I can make a type where (==) = undefined, but write a proper Ord instance, and use it as key type. I am asking this because I will be teaching type classes, with Eq and Ord as examples. I detected the funny situation that the Java specification looks "more mathematical" than the corresponding Haskell one. What will the students think ... - J.W. (*) The Haskell standard is what you find when you scroll down, down, down to the very bottom of https://www.haskell.org/documentation . So, probably not that important...

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No Enum for (,), no Enum or Bounded for Either
by Tom Ellis 02 Jun '18

02 Jun '18

I'm a bit surprised that whilst `Either` and `(,)` have instances for `Ord` * `(,)` has no instance for `Enum` * `Either` has no instance for `Enum` or `Bounded` Is there a particular reason for that? It might be tricky to implement toEnum :: Int -> a fromEnum :: a -> Int but in the presence of `Bounded` that should be possible. Tom

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[ANN] karya, music editor
by Evan Laforge 01 Jun '18

01 Jun '18

This is an announcement for Karya, which is a music editor written in Haskell. There are more details at: http://ofb.net/~elaforge/karya/doc/overview.md.html or if you just want to listen to music: http://ofb.net/~elaforge/karya/doc/examples.md.html Source is at: https://github.com/elaforge/karya Even if you're not interested in the music part, it's an example of a haskell program with a GUI, REPL based interface, tests, profiling, and a shake-based build system, although probably idiosyncratic implementations of all of the above. It should run on OS X (well tested) and X11 (poorly tested). If you are interested in the music part, this is basically in the "score writing" genre of software, in that it's suitable for writing a score in a textual non-realtime way. But though non-realtime, it's still interactive, so you can hear what you just wrote. And though textual and language-oriented, it's also semi-graphical, so it's closer to the sequencer / music editor type application than a music-oriented DSL like Haskore. It's hard to describe because as far as I know there's nothing else similar. If you have used a DAW but wanted to define higher level abstractions, or if you used Haskore (or similar) but found it difficult to write music purely textually, you might be interested in this. Or if you want to use your own tunings or scales, or do expressive control of pitch and dynamics. You'll need an external MIDI synthesizer though. There is a direct synthesis backend, but it's not practically usable yet. It's somewhat mature, in that I've been using for all my own music for the last 5 years or so, but of course also has many areas of active development, from graphics, to language design and interpretation, to music notation design, to audio synthesis and numerics. There is some documentation, but it's undoubtedly sparse since I'm the only user. Ask if you have questions! Details will be materialized on demand. Some things not mentioned on the "official" page since they're in progress: - A DSL to notate solkattu, which is Carnatic percussion syllables, and is a sort of instrument-independent notation for rhythmic structures, along with rules for realizing on various instruments. This is closer to a traditional music DSL like Haskore, but winds up different since it's rhythmically oriented. There's a small library of compositions. - An offline synthesis framework, including a sampler and FAUST-based synthesizer, built on a streaming audio library. I'm currently working on implementing off-line incremental audio rendering, which as far as I know is unique.

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PLMW@ICFP: CALL FOR SCHOLARSHIP APPLICATIONS and PARTICIPATION (deadline June 30!)
by Niki Vazou 31 May '18

31 May '18

PLMW@ICFP: CALL FOR SCHOLARSHIP APPLICATIONS and PARTICIPATION (deadline June 30!) ACM SIGPLAN Programming Languages Mentoring Workshop Co-located with ICFP'18 PLMW web page: https://icfp18.sigplan.org/track/PLMW-ICFP-2018 The purpose of the programming languages mentoring workshop (PLMW) is to encourage senior undergraduate and early career (first or second year) to pursue careers in programming language research. We are specifically interested in attracting groups who have traditionally not had the opportunity to participate in research in functional programming. This workshop will be a combination of learning about the work being done in several areas of programming language research and mentoring with respect to helping students prepare for graduate school and the rest of their career. We will bring together leaders in programming language research from academia and industry to give talks on the kind of research typically performed after obtaining a Ph.D. The workshop will engage students, specifically interested in programming language research, in a process of imagining how they might contribute to the world. We especially encourage women and underrepresented minority students to attend PLMW. This workshop is part of the activities surrounding ICFP, the International Conference on Functional Programming, and takes place the day before the main conference. One goal of the workshop is to make ICFP conference more accessible to newcomers. We hope that participants will stay through the entire conference. ## Travel Scholarship Applications (Due 30 June) Please fill out this form https://goo.gl/forms/7o4nENLxzAo2zwYy1. by 30 June to apply for travel funding. See the PLMW web page for additional details. The workshop registration is open to all. Students with alternative sources of funding are welcome. Best, Niki Vazou

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ANN: fast-arithmetic 0.6.0.7
by Vanessa McHale 31 May '18

31 May '18

Hi all, I have just published a new version of fast-arithmetic <https://hackage.haskell.org/package/fast-arithmetic>. It provides fast primitives for number theory and combinatorics on Linux and Mac. The latest release has few dependencies and thus builds should be quite robust.

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Re: [Haskell-cafe] ANN: primitive 0.6.4.0
by Dr.Koster 31 May '18

31 May '18

I have make some comments on reddit, but still I want to record them here: After reading the new code, I got a feeling that there're some API lacking considerations. Here is some of my personal concerns: The generalized MVar is basically useless because ST is meant to seal its state token, you won't want to add a forkST. You can argue that the atomic operations on MutVar is already added, but still I want to justify its usefulness. There're too much combinators added to PrimArray, is there a plan to port these to other arrays? If we ever provide toList operations, then I would really want it to be a good producer in base's fold-build fusion' view, it's robust and easy to implement, and folding is basically the same speed with List.fold and toList. Anyway it seems I missed the bike shedding, but still I want to thank for all the efforts, I'll try to follow new GitHub workflow more closely. Cheers Winter 发自我的iPhone ------------------ Original ------------------ From: Carter Schonwald <carter.schonwald(a)gmail.com> Date: Thu,May 31,2018 2:55 AM To: haskell-cafe <haskell-cafe(a)haskell.org>, Haskell Libraries <libraries(a)haskell.org> Subject: Re: [Haskell-cafe] ANN: primitive 0.6.4.0 This is a pretty exciting (albeit minor version) I'd like to especially thank David Feuer (treeowl ) and Andrew Martin (andrewthad) for their contributions the change log has a huge number of bug fixes relative to 0.6.2-3 also we added some new modules of features PrimArray --- unboxed nonsliceable vectors MVar -- Mvars that run in any PrimMonad augmented the Data.Primitive exports for increased consistency (the new .Ptr and .Mvar modules aren't exported in this minor version so users can easily migrate) Changes in version 0.6.4.0 Introduce Data.Primitive.PrimArray, which offers types and function for dealing with a ByteArray tagged with a phantom type variable for tracking the element type. Implement isByteArrayPinned and isMutableByteArrayPinned. Add Eq1, Ord1, Show1, and Read1 instances for Array and SmallArray. Improve the test suite. This includes having property tests for typeclasses from base such as Eq, Ord, Functor, Applicative, Monad, IsList, Monoid, Foldable, and Traversable. Fix the broken IsList instance for ByteArray. The old definition would allocate a byte array of the correct size and then leave the memory unitialized instead of writing the list elements to it. Fix the broken Functor instance for Array. The old definition would allocate an array of the correct size with thunks for erroring installed at every index. It failed to replace these thunks with the result of the function applied to the elements of the argument array. Fix the broken Applicative instances of Array and SmallArray. The old implementation of <*> for Arrayfailed to initialize some elements but correctly initialized others in the resulting Array. It is unclear what the old behavior of <*> was for SmallArray, but it was incorrect. Fix the broken Monad instances for Array and SmallArray. Fix the implementation of foldl1 in the Foldable instances for Array and SmallArray. In both cases, the old implementation simply returned the first element of the array and made no use of the other elements in the array. Fix the implementation of mconcat in the Monoid instance for SmallArray. Implement Data.Primitive.Ptr, implementations of Ptr functions that require a Prim constraint instead of a Storable constraint. Add PrimUnlifted instances for TVar and MVar. Use compareByteArrays# for the Eq and Ord instances of ByteArray when building with GHC 8.4 and newer. Add Prim instances for lots of types in Foreign.C.Types and System.Posix.Types. Reexport Data.Primitive.SmallArray and Data.Primitive.UnliftedArray from Data.Primitive. Add fold functions and map function to Data.Primitive.UnliftedArray. Add typeclass instances for IsList, Ord, and Show. Add defaultSetByteArray# and defaultSetOffAddr# to Data.Primitive.Types.

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Config Files and monoids
by Olaf Klinke 30 May '18

30 May '18

Dear cafe, a recent post here [1] mentioned that configurations, such as the ones read from a config file, can be given Monoid instances, where mempty is the empty or default configuration and mappend merges two partial configurations, producing a more complete one. The vgrep package explicitly does this, for instance. Although the ConfigParser type from the ConfigFile package has a binary 'merge' operation, it does define neither a Monoid not a Semigroup instance. I'm struggling to make the concept of monoidal configuration work when there is no sensible default configuration. Suppose my configuration type is data Config = Config {foo :: Bool, bar :: Int} with no reasonable default, e.g. emptyConfig = Config { foo = error "you did not specify option foo", bar = error "you did not specify option bar" } Some configuration monoids seem to have the second operand override the first, or the other way around. However, I wish that when cfg1 = emptyConfig {foo = True} cfg2 = emptyConfig {bar = 4} then cfg1 <> cfg2 == Config {foo = True, bar = 4}. So it seems that for mappend to work as intended one needs a terminating function that tells me if a record field is already defined, e.g. when all fields are Maybes. Vgrep.Environment.Config.Monoid does it this way. My solution so far was to resort to the monoid of endofunctions (as the getflag package does), that is, define cfg1, cfg2 :: Config -> Config cfg1 = \cfg -> cfg {foo = True} cfg2 = \cfg -> cfg {bar = 4} And then build (cfg1.cfg2) emptyConfig. (Alternatively, one might structure these as lenses instead of endofunctions, see e.g. Data.Monoid.Endo.Fold in the endo package.) Thus I arrived at class Config cfg where emptyConfig :: cfg -- may contain some defaults configOptions :: [Parser (cfg -> cfg)] Do you think every other concept of configuration parsing can be cast into this typeclass? -- Olaf [1] https://mail.haskell.org/pipermail/haskell-cafe/2018-May/129063.html

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Re: [Haskell-cafe] (names for) invariants for Eq and Ord?
by Olaf Klinke 30 May '18

30 May '18

Prelude> import qualified Data.Map as M Prelude M> let k = 0/0 in let v = 0 in M.lookup k (M.insert k v M.empty) == Just v False This one is not surprising because the presence of 0/0 destroys the total order of Double, so strictly speaking this should not have an Ord instance. Olaf

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ANN: primitive 0.6.4.0
by Carter Schonwald 30 May '18

30 May '18

This is a pretty exciting (albeit minor version) I'd like to especially thank David Feuer (treeowl ) and Andrew Martin (andrewthad) for their contributions the change log has a huge number of bug fixes relative to 0.6.2-3 also we added some new modules of features PrimArray --- unboxed nonsliceable vectors MVar -- Mvars that run in any PrimMonad augmented the Data.Primitive exports for increased consistency (the new .Ptr and .Mvar modules aren't exported in this minor version so users can easily migrate) Changes in version 0.6.4.0 - Introduce Data.Primitive.PrimArray, which offers types and function for dealing with a ByteArray tagged with a phantom type variable for tracking the element type. - Implement isByteArrayPinned and isMutableByteArrayPinned. - Add Eq1, Ord1, Show1, and Read1 instances for Array and SmallArray. - Improve the test suite. This includes having property tests for typeclasses from base such as Eq, Ord, Functor, Applicative, Monad, IsList, Monoid, Foldable, and Traversable. - Fix the broken IsList instance for ByteArray. The old definition would allocate a byte array of the correct size and then leave the memory unitialized instead of writing the list elements to it. - Fix the broken Functor instance for Array. The old definition would allocate an array of the correct size with thunks for erroring installed at every index. It failed to replace these thunks with the result of the function applied to the elements of the argument array. - Fix the broken Applicative instances of Array and SmallArray. The old implementation of <*> for Arrayfailed to initialize some elements but correctly initialized others in the resulting Array. It is unclear what the old behavior of <*> was for SmallArray, but it was incorrect. - Fix the broken Monad instances for Array and SmallArray. - Fix the implementation of foldl1 in the Foldable instances for Array and SmallArray. In both cases, the old implementation simply returned the first element of the array and made no use of the other elements in the array. - Fix the implementation of mconcat in the Monoid instance for SmallArray. - Implement Data.Primitive.Ptr, implementations of Ptr functions that require a Prim constraint instead of a Storable constraint. - Add PrimUnlifted instances for TVar and MVar. - Use compareByteArrays# for the Eq and Ord instances of ByteArray when building with GHC 8.4 and newer. - Add Prim instances for lots of types in Foreign.C.Types and System.Posix.Types. - Reexport Data.Primitive.SmallArray and Data.Primitive.UnliftedArray from Data.Primitive. - Add fold functions and map function to Data.Primitive.UnliftedArray. Add typeclass instances for IsList, Ord, and Show. - Add defaultSetByteArray# and defaultSetOffAddr# to Data.Primitive.Types.

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Selecting a transformer in a monad transformer stack
by Paul Brauner 30 May '18

30 May '18

Hello, I have a newtype for a monad transformer stack: newtype MyMonad a = MyMonad { runMyMonad :: ((ReaderT Env (ExceptT String (StateT Store (ListT Identity)))) a) } and I'd like to it derive MonadPlus. The default instance for MonadPlus I get via GeneralizedNewtypeDeriving is the ExceptT one because it is the outermost transformer implementing MonadPlus. I would like to use the ListT instance instead. The best solution I have come up with is to define: instance MonadPlus MyMonad where mzero = MyMonad $ ReaderT $ \e -> ExceptT $ StateT $ \s -> ListT $ Identity [] mplus a b = MyMonad $ ReaderT $ \e -> ExceptT $ StateT $ \s -> ListT $ Identity $ let run x = runIdentity (runListT (runStateT (runExceptT (runReaderT (runAbstract x) e)) s)) as = run a bs = run b in as ++ bs My questions are: - Is there a shorter way to define mplus? - Is there maybe some variation of monad transformer stacks where the layers are named and can be accessed by name? (I know of effects handlers but I'd like to stick to monad transformers for now.) Paul

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8.4.2 FreeBSD bindist not working with gold & lld
by Sumit Raja 30 May '18

30 May '18

Hi, Just got 8.4.2 running in my FreeBSD 11 environment and am getting the following: cabal: Failed to build entropy-0.4.1.1 (which is required by exe:test-ghoa from oauth2-simple-0.1.0.0). The failure occurred during the configure step. The exception was: dieVerbatim: user error (cabal: '/usr/local/bin/ghc' exited with an error: /usr/local/bin/ld.gold: error: /usr/local/lib/ghc-8.4.2/rts/libHSrts_thr.a(RTS.thr_o): unexpected reloc 8 in object file LLD fails with configure step. The exception was: dieVerbatim: user error (cabal: '/usr/local/bin/ghc' exited with an error: /usr/bin/ld.lld: error: do not know how to handle relocation 'R_X86_64_RELATIVE' (8) Saw another thread that discussed this but there was no resolution. Any ideas on how to fix this? Thanks Sumit

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Fwd: Safe TCP accept loop
by Sumit Raja 30 May '18

30 May '18

Hi, I sent this to beginners but got no responses so I'm hoping someone here can help. I’ve looked at https://wiki.haskell.org/Concurrency_demos/Graceful_exit and used(2) Using throwTo without the use of block and unblock. It runs on 8.2.1 with my limited testing. I can’t find out much about what the > 7.x GHC replacement for block/unblock is other than mask. What does the unblock do in acceptConnections'? Does this method of handling an accept loop still need masking of async exceptions ? If so where does this need to be done? I've got a version with async running for my specific application (https://bitbucket.org/sumitraja/hvrr/src/6927216597a35a9a0d7f2e55cca83fa5b7…) but I don't know if I need to unmask at any point so was hoping for some guidance. Thanks Sumit

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test
by Gershom B 29 May '18

29 May '18

Testing this list, due to a report. -g

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Munich Haskell Meeting, 2018-05-28 @ 19:30
by Leah Neukirchen 26 May '18

26 May '18

Dear all, Next week, our monthly Munich Haskell Meeting will take place again on Monday, May 28 at Hirschgarten (self-service area) at 19h30. (The weather should be fine for sitting outside, else can move inside.) 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-may-2018/ Everybody is welcome! cu, -- Leah Neukirchen <leah(a)vuxu.org> http://leah.zone

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Optimizing nearest-k code
by Richard Evans 25 May '18

25 May '18

Dear Haskell Cafe, Given a set of sets, and a particular target set, I want to find the sets that are nearest (in terms of Hamming distance) to the target set. I am using the following code: import Data.List import qualified Data.Set as Set nearest_k :: Ord a => Int -> [(Set.Set a, v)] -> Set.Set a -> [(Set.Set a, v)] nearest_k k bs b = take k bs' where bs' = sortOn (hamming b) bs hamming :: Ord a => Set.Set a -> (Set.Set a, v) -> Int hamming x (y, _) = hamming_distance x y hamming_distance :: Ord a => Set.Set a -> Set.Set a -> Int hamming_distance xs ys = Set.size (Set.difference xs ys) + Set.size (Set.difference ys xs) subsets :: [a] -> [[a]] subsets [] = [[]] subsets (x:xs) = subsets xs ++ map (x:) (subsets xs) int_lists :: [[Int]] int_lists = subsets [1..20] values :: [(Set.Set Int, Int)] values = map f (zip [1..] int_lists) where f (i, x) = (Set.fromList x, i) test = nearest_k 8 values (Set.fromList [1,2,3]) ---- This works ok for the test above (with sets of ints), but is rather slow in my actual application (in which the sets are large sets of ground atoms of first-order logic). Is there some major optimization I should be doing here? thanks, Richard

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Help evaluating TQueue variants
by David Feuer 25 May '18

25 May '18

I've written three related variations on TQueue (from the stm package). All three are designed to avoid the reader starvation problem described in GHC Trac 9539 [1] while limiting contention. Two are based on Okasaki's scheduled (i.e., real-time) banker's queues, while the third is based on his amortized-time banker's queue. I am hoping one or more people with programs that use TQueue heavily would be willing and able to try out these alternatives and see how they perform in their applications. I'm interested in how they compare to the current TQueue implementation, and also how they compare to each other. The queues are currently in a GitHub repository [2], but if it would be helpful I could put them up on Hackage. Thank you for your time. [1] https://ghc.haskell.org/trac/ghc/ticket/9539 [2] https://github.com/treeowl/stm-queues

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haskell in the cloud
by Dennis Raddle 24 May '18

24 May '18

I have a CPU-intensive Haskell application. I have it working with simple multi-core concurrency. I'm wondering if I can run this on a cloud virtual machine with 128 cores or so, paying by the CPU minute. I'll run it for maybe 15 minutes a day so I'm probably best off paying just for the CPU used. What platform would be recommended? For ease of use? For best price/performance? It's a backtracking optimization algorithm that builds data, one element at a time. It's not hard at all to make it concurrent: at the first 3 or so levels of element choices would be about 100 to 500 combinations. Even if the simplest method of running concurrent Haskell on multiple cores doesn't work, I could just divide these first cases into batches and run them on individual machines. D

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how do cabal internal libraries work?
by Evan Laforge 24 May '18

24 May '18

The example at https://www.haskell.org/cabal/users-guide/developing-packages.html#library doesn't seem to work as I expect. The doc for the library field is out of date, but if you scroll down it mentions "internal libraries". But depending on the internal library doesn't seem to have any effect, in that cabal still complains I didn't mention Foo.Internal, and doesn't see the build-depends from it. Am I misinterpreting how the feature is supposed to work? This is with Cabal 2.2.0.1 and cabal-install 2.2.0.0, ghc 8.4.1: % cat testing.cabal name: foo version: 1.0 license: BSD3 cabal-version: >= 1.23 build-type: Simple library foo-internal exposed-modules: Foo.Internal build-depends: base, text library exposed-modules: Foo.Public build-depends: foo-internal, base % cat Foo/Internal.hs module Foo.Internal where import Data.Text % cat Foo/Public.hs module Foo.Public where import Foo.Internal % cabal build Resolving dependencies... Configuring foo-1.0... Warning: Packages using 'cabal-version: >= 1.10' must specify the 'default-language' field for each component (e.g. Haskell98 or Haskell2010). If a component uses different languages in different modules then list the other ones in the 'other-languages' field. Preprocessing library 'foo-internal' for foo-1.0.. Building library 'foo-internal' for foo-1.0.. [1 of 1] Compiling Foo.Internal ( Foo/Internal.hs, dist/build/foo-internal/Foo/Internal.o ) [1 of 1] Compiling Foo.Internal ( Foo/Internal.hs, dist/build/foo-internal/Foo/Internal.p_o ) Preprocessing library for foo-1.0.. Building library for foo-1.0.. <no location info>: warning: [-Wmissing-home-modules] These modules are needed for compilation but not listed in your .cabal file's other-modules: Foo.Internal [1 of 2] Compiling Foo.Internal ( Foo/Internal.hs, dist/build/Foo/Internal.o ) Foo/Internal.hs:2:1: error: Could not find module ‘Data.Text’ Use -v to see a list of the files searched for. | 2 | import Data.Text | ^^^^^^^^^^^^^^^^

3 3

profunctorial vs vanlaarhoven lenses
by Paolino 23 May '18

23 May '18

Hello, I'm trying to write a lens for a datatype which seems easy in the Twan van Laarhoven encoding but I cannot find it as easy in the profunctorial one data Q5 a b = Q51 a (Identity b) | Q52 [b] lq5Twan :: Applicative f => (b -> f b') -> Q5 a b -> f (Q5 a b') lq5Twan f (Q51 a bs) = Q51 a <$> traverse f bs lq5Twan f (Q52 bs) = Q52 <$> traverse f bs data BT tt tt' b t t' a = BT1 (tt -> b) (t a) | BT2 (tt' -> b) (t' a) deriving (Functor,Foldable,Traversable) runBT (BT1 f x) = f x runBT (BT2 f x) = f x lq5Profunctor :: forall p a b b' . Traversing p => p b b' -> p (Q5 a b) (Q5 a b') lq5Profunctor = dimap pre post . second' . traverse' where pre (Q51 a x) = ((), BT1 (Q51 a) x) pre (Q52 bs) = ((), BT2 Q52 bs) post ((),x) = runBT x The issue here is that I need to project type 'b' into 2 different Traversable (sketched [] and Identity) so the traverse' must handle both together. Is the lq5Twan correct ? Which simpler ways to write the lq5Profunctor we have ? Thanks for help. paolino

3 8

Question of defining Type Class
by Shinichi Higashino 22 May '18

22 May '18

I am confusing since I am trying to define Abstract sense of Matrix; class Mat - is a more abstract concept since 1x1 Matrix is isomorphic to Num - is a more abstract concept since 1xn Matrix is isomorphic to Vector So, class Mat a => Num a where … class Mat a => Vec a where … should be . However, I know that the Initial object of Ring Category is Integer . So, I should start with Z and then I should define class ( Module R on Z ) But I could not find that kind of definition in Haskell. Is it Data.Ring ? I am so confusing now… Please help me if you are drinking some tea just now!

1 1

Tezos is hiring 40 functional programmers
by Ryan Trinkle 21 May '18

21 May '18

Hi everyone, I received this from Tezos - one of Obsidian's clients - and I thought it would be of interest to people here. Tezos is a blockchain and a platform of distributed applications written in OCaml, which brings scalability, formal verification and self-amendment to the blockchain space. Tezos has 40 open positions in the world including 25 in France. Remote and part-time accepted. We are looking for profiles ranging from masters (with possible funding for PhD) to PhD with extensive experience. Expertise in a functional language such as OCaml, Haskell, Clojure or equivalent is required, as well as skills in one of the following topics: algorithms, compilers, design of programming languages, cryptography, p2p, web programming / UI in OCaml, SAT / SMT, linear and integer programming, formal verification, stochastic processes, computer algebra or numerical optimization. Highly competitive salaries. If you are interested, please send your CV (resumé) to careers(a)tezos.com Best, Ryan

1 0

tutorials, Freer or "data a la carte"
by Dennis Raddle 21 May '18

21 May '18

I'm interested in Control.Monad.Freer, but I would need some kind of tutorial aimed at a low-intermediate level. Also I think "data a la carte" might be applicable to me, but again I would need some kind of tutorial. D

2 1

Archival of community.haskell.org
by Mario Rogic 19 May '18

19 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(a)h.o or hop on the #haskell-infrastructure freenode channel to provide feedback / concerns. Thanks - Mario

1 2

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

3 3

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

4 6

2nd CfP Workshop on Functional High-Performance Computing 2018
by Kei Davis 17 May '18

17 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.

1 0

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

6 11

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

1 0

state monad transformer that adds or combines state
by Dennis Raddle 15 May '18

15 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

6 8

template haskell library for OO-like selector syntax?
by Dennis Raddle 15 May '18

15 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

5 6

Failing build for HaXml
by Vanessa McHale 15 May '18

15 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>

5 4

GHC TV station?
by Sylvester Hazel 15 May '18

15 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

9 11

Haskell eXchange 2018 CFP (London, Thursday, 11th - Friday, 12th October)
by Oliver Charles 15 May '18

15 May '18

Hi all, The Haskell eXchange is an annual conference created for and by the Skills Matter community. An opportunity for Haskellers to meet, learn and share skills, discover emerging technologies and help evolve the Haskell ecosystem. Everyone is welcome to join, whether you are an expert or a beginner, whether you are a commercial user, an academic or a hobbyist, we'd love it if you join us this year at the Haskell eXchange! For 2018, we are planning to have three types of sessions: 1. Regular talks (35 minutes) 2. Hands-on workshops (approximately 90 minutes) 3. Lightning talks (5-10 minutes to be determined in the formal confirmation) The Haskell eXchange is a broad conference, and we'll consider anything with a clear Haskell connection. For example, talks about Haskell libraries and applications, experience reports of using Haskell for work, and tutorials on programming techniques are all fantastic proposals - but please feel free to submit anything that you think is interesting. The committee are happy to work with you if you're unsure if your talk is a good fit, and can be reached at haskellx(a)skillsmatter.com Most importantly, we're looking for talks that inspire and educate. If your proposal doesn't fit into any of the above categories, it doesn't matter! To submit, please see the form in https://skillsmatter.com/conferences/10237-haskell-exchange-2018#get_involv… We look forward to your submissions!

1 0

Maintaining cpphs
by Markus Ongyerth 15 May '18

15 May '18

Hi Malcolm, Are you still interested in maintaining cpphs? I have posted an issue to your github tracker [1] (linked at [2]) and didn't get any response in a week. While that's not a terribly long time, the reason for this patch is trickling down to different distributions repositories (arch and debian-sid) which is making it harder for me to support users because of a dependency. If you are not planing to maintain cpphs in future, I'd like to take over the package on hackage. Cheers, ongy [1] https://github.com/malcolmwallace/cpphs/issues/16 [2] http://hackage.haskell.org/package/cpphs

1 0

PSA for Cabal 2.2 new-* users regarding .ghc.environment files
by Gershom B 14 May '18

14 May '18

There is an important change in the cabal new- commands for 2.2 that the release docs should have highlighted more significantly. Cabal new-* commands now produce a .ghc.environment file by default. These files [1] are picked up by ghc and ghci automatically (since 8.0.1), and allow them to operate directly with the same package environment used by the new-* commands. This lets you, for example, run `ghci` in a project where you are using `new-build` and get the proper dependencies in scope. Herbert has written an experimental tool to make it easier to create and manipulate these environments [2]. However: there is a drawback (on which some discussion at [3] and [4]. In particular, there is not good information provided by ghc about when these files are picked up and used. So if you are admixing new-* commands and other commands for the time being, jumping between the two, or admixing ghc and ghcjs, etc., then you may run into unexpected behavior! [5] The simplest solution for now is to delete the local .ghc.environment file in those cases (i.e. where you're mixing commands and get unexpected behavior). A particular gotcha is that these files are picked up not just in the current directory but also in any parent directory. Cheers, Gershom [1] documented at https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/packages.ht… [2] https://github.com/hvr/cabal-env [3] https://github.com/haskell/cabal/issues/4542 [4] https://ghc.haskell.org/trac/ghc/ticket/13753 [5] Error messages may be like ".cabal/store/ghc-8.0.2/package.db/package.cache: openBinaryFile: does not exist (No such file or directory)" or complaints about missing inplace dependencies.

8 9

Re: [Haskell-cafe] Problem with submitting papers for haskell 2018
by Nicolas Wu 14 May '18

14 May '18

Hello Winter, Thanks for getting in touch, I’m glad to hear that you’re preparing a submission to the Haskell Symposium this year. You haven’t missed any deadlines: we’re still reviewing the early track papers and so EasyChair is closed for regular submissions until notifications for those papers are released. This will happen on 18 May 2018. Best wishes, Nick Dr Nicolas Wu Department of Computer Science University of Bristol > On 14 May 2018, at 14:19, Brent Yorgey <byorgey(a)gmail.com> wrote: > > Hi Nick, > > Did you see this message that was sent to haskell-cafe a few hours ago? > > -Brent > > ---------- Forwarded message --------- > From: winter <drkoster(a)qq.com> > Date: Mon, May 14, 2018 at 4:15 AM > Subject: [Haskell-cafe] Problem with submitting papers for haskell 2018 > To: <haskell-cafe(a)haskell.org> > > > Dear all, > > Currently i and my friends @sighingnow are writing a paper on our > new I/O manager based on libuv. We planed to submit it to Haskell 2018, > and according to the official site's info, the deadline should be next > month: https://icfp18.sigplan.org/track/haskellsymp-2018-papers > > But here is my problem, i registered an account on easychair as Haskell > 2019 required and logged in, but when i click the submit link, a page > with following info are provided: > > > Paper submission for Haskell 2018 is closed.,,You seem to have no > previous roles at Haskell 2018, so > you cannot enter its pages. If you > have questions about the conference, please contact Haskell 2018 > chairs. > > What does this mean? Did i miss the submission window? Or do i have to > do some extra preparations? Please help! > > > Cheers~ > > Winter > > > > _______________________________________________ > 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.

2 1

Current state of dependent types in Haskell
by Serguey Zefirov 14 May '18

14 May '18

Can anyone kindly point me at blog posts and/or articles on the subject? I would like to brush off my Haskell skills in that area with whatever new things that are hot. I once implemented type-level arithmetic in Haskell for sizing various hardware things but it is ages ago now. I would like to experiment with current state of affairs with type-level arithmetic and possibly experiment with adding structured values into the constraints.

2 2

Problem with submitting papers for haskell 2018
by winter 14 May '18

14 May '18

Dear all, Currently i and my friends @sighingnow are writing a paper on our new I/O manager based on libuv. We planed to submit it to Haskell 2018, and according to the official site's info, the deadline should be next month: https://icfp18.sigplan.org/track/haskellsymp-2018-papers But here is my problem, i registered an account on easychair as Haskell 2019 required and logged in, but when i click the submit link, a page with following info are provided: > Paper submission for Haskell 2018 is closed.,,You seem to have no previous roles at Haskell 2018, so > you cannot enter its pages. If you have questions about the conference, please contact Haskell 2018 > chairs. What does this mean? Did i miss the submission window? Or do i have to do some extra preparations? Please help! Cheers~ Winter

1 0

Call for Papers: Workshop in OCL and Textual Modeling (OCL 2018)
by Achim D. Brucker 10 May '18

10 May '18

(Apologies for duplicates) CALL FOR PAPERS 18th International Workshop on OCL and Textual Modeling Co-located with MODELS 2018: ACM/IEEE 21th International Conference on Model Driven Engineering Languages and System, October 14 (tbc), 2018, Copenhagen, Denmark http://oclworkshop.github.io Modeling started out with UML and its precursors as a graphical notation. Such visual representations enable direct intuitive capturing of reality, but they have weaknesses: for example, detailed visual representations bear the risk of becoming overcrowded faster than textual models and some of the visual features lack the level of precision required to create complete and unambiguous specifications. These weaknesses of graphical notations encouraged the development of text-based modeling languages that either integrate with or replace graphical notations for modeling. Typical examples of such languages are OCL, textual MOF, Epsilon, and Alloy. Textual modeling languages have their roots in formal language paradigms like logic, programming and databases. The goal of this workshop is to create a forum where researchers and practitioners interested in building models using OCL or other kinds of textual languages can directly interact, report advances, share results, identify tools for language development, and discuss appropriate standards. In particular, the workshop will encourage discussions for achieving synergy from different modeling language concepts and modeling language use. The close interaction will enable researchers and practitioners to identify common interests and options for potential cooperation. ## Topics of interest Topics of interest include (but are not limited to): - Mappings between textual modeling languages and other languages/formalisms - Mathematical models and/or formal semantics for textual modeling languages - Algorithms, evaluation strategies and optimizations in the context of textual modeling languages for: - validation, verification, and testing, - model transformation and code generation, - meta-modeling and DSLs, and - query and constraint specifications - Alternative graphical/textual notations for textual modeling languages - Evolution, transformation and simplification of textual modeling expressions - Libraries, templates and patterns for textual modeling languages - Tools that support textual modeling languages (e.g., verification of OCL formulae, runtime monitoring of invariants) - Model-driven security using textual modeling languages - Complexity results for textual modeling languages - Quality models and benchmarks for comparing and evaluating textual modeling tools and algorithms - Successful applications of textual modeling languages - Case studies on industrial applications of textual modeling languages - Experience reports: - usage of textual modeling languages and tools in complex domains, - usability of textual modeling languages and tools for end-users - Empirical studies about the benefits and drawbacks of textual modeling languages - Innovative textual modeling tools - Comparison, evaluation and integration of modeling languages - Correlation between modeling languages and modeling tasks We particularly encourage submissions describing applications and case studies of textual modeling as well as test suites and benchmark collections for evaluating textual modeling tools. ## Venue This workshop will be organized as a part of MODELS 2018 Conference in Copenhagen, Denmark. Similar to its predecessors, the workshop addresses both people from academia and industry. The aim is to provide a forum for addressing integration of OCL and other textual modeling languages, as well as tools for textual modeling, and for disseminating good practice and discussing the new requirements for textual modeling. ## Workshop Format The workshop will include short (about 15 min) presentations, parallel sessions of working groups, and sum-up discussions. ## Submissions Two types of papers will be considered: * Short contributions (between 5 and 7 pages) describing new ideas, innovative tools or position papers. * Full papers (between 10 and 14 pages). in LNCS format. Submissions should be uploaded to [EasyChair](https://easychair.org/conferences/?conf=ocl2018). The program committee will review the submissions (minimum 2 reviews per paper, usually 3 reviews) and select papers according to their relevance and interest for discussions that will take place at the workshop. Accepted papers will be published online, as part of the MODELS workshop proceedings, in [CEUR](http://www.ceur-ws.org). ## Important Dates - Submission of papers: July 17, 2018 - Notification: August 17, 2018 - Workshop date: October 14, 2018 (tbc) -- Dr. Achim D. Brucker | Software Assurance & Security | University of Sheffield https://www.brucker.ch | https://logicalhacking.com/blog @adbrucker | @logicalhacking

1 0

Taking over `data-default-*`
by Marco Zocca 10 May '18

10 May '18

Dear all, I've been increasingly relying on data-default-class, but sadly it doesn't look like it's currently maintained; there are some tickets on the github issue tracker that would need some attention. I have tried contacting the authro/current maintainer Lukas Mai (CC'd) by email but have not received an answer so far (I hope he's doing well, btw). This is my first public request for taking over the `data-default` family of packages: data-default-class data-default-instances-base data-default-instances-containers data-default-instances-dlist data-default-instances-old-locale data-default Kind regards, Marco Zocca (http://hackage.haskell.org/user/ocramz)

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Re: [Haskell-cafe] How to expose if a constraint is satisfiable
by Anthony Clayden 09 May '18

09 May '18

Thanks David, Oliver, I'll leave it so Clinton whether that helps his use case. I see the write-ups for both the Constraint Unions package and ifCxt mentioned on the reddit thread talk about needing large amounts of boilerplate. That is, an instance for each class and type that effectively tells the class has an instance for that type. So that's the same as the AdvancedOverlap I linked to -- from Oleg + SPJ. IIUC, what Clinton's asking is: since there are already instances for `Show Int`, `Ord Float`, etc (declared in the Prelude), why can't ghc 'just know' that and expose it to type-level logic that can say: if there's `Show a` do this, otherwise do that? AntC > On *Tue May 8 22:29:20 UTC 2018*, David Feuer wrote: > Maybe look at https://www.reddit.com/r/haskell/comments/6k86je/constraint_unions_bringing… > (relating to the repository Oliver links to) and note also Edward Kmett's response. Eurrm. Could somebody explain that response using familiar language. I think the `:-` operator might be referring to Edward's & David's 'constraints' package on github, for which I've asked if there's human-facing documentation. Also asked if it'll be superseded by the 'Qualified Constraints' new development. >> On Tue, May 8, 2018 at 9:52 AM, Oliver Charles <ollie at ocharles.org.uk <http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe>> wrote: >>* I've only skimmed the thread, so sorry if this is a red herring, but could *>>* this be helpful?*

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Scoped Type Variables discussion forum [was: open up the issues tracker on ghc-proposals]
by Anthony Clayden 09 May '18

09 May '18

This thread is a discussion about discussions, not the discussion itself ;-) I'm cc'ing to the cafe; but I'd prefer replies to come to glasgow-haskell-users. >> I can volunteer to at least scrape together all the objections to ScopedTypeVariables as currently. It's not yet a proposal, so not on github. Start a wiki page? A cafe thread? (It'll get lost.) A ghc-users thread? (It'll get ignored.) > ... don’t care what forums or list or whatever. As long as it’s collated and such > It could even be on the prime issue tracker for prime proposals. Just that it’s written down :) Thanks Carter, but I understand Haskell Prime to be to assess mature/stable proposals (preferably already delivered as extensions). This discussion is at first going to be more exploratory: * likes and dislikes about ScopedTypeVariables as currently. * confusions experienced by users (especially newbies) -- although absolute newbies wouldn't be using it(?), so intermediates? * feedback from those teaching Haskell. * wild ideas for possible alternative designs. * possible improvements to the current design. * I think we're all agreed that ScopedTypeVariables should have been in Haskell from the beginning; but it wasn't, so now we have to worry about backwards compatibility for programs that worked around the omission. Or do we? What code would break? How much pain would that cause? * anything else? > We have lots of forums, but your point is that certain sorts of discussions never get going with the right audience – you especially point to “confused beginners”. > ... It’s quite a challenge because beginners tend not to be vocal, and yet they are a crucial set of Haskell users. Every Haskell user started as a beginner. On this particular topic, there's plenty of confused people asking questions on StackOverflow. (Heads up: they're especially asking why they need explicit `forall` whereas in reguar Haskell that 'intermediates' see, the forall is implicit.) Can other people point me to questions/likes/dislikes on other forums? Reddit for example. If you've read this far, you now understand what we're trying to cover. It's going to be random/varied thoughts at first, then perhaps coalescing to an approach or two. At that point a formal proposal on github proper; and the random stuff might be interesting background but will essentially get archived/thrown away. I do agree with David's suggestion that github Issue tracker looks like a suitable solution. We can write formatted code and text. We can add links and references. What do others think? Joachim has opened up Issues tracker, as a try-out. If using it doesn't work out, that's fine and in keeping with my "thrown away" above. Also where else should I post links to this message to 'advertise' the thread? I don't reddit much, so if that's suitable, please someone post there. Thank you AntC

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How to expose if a constraint is satisfiable
by Clinton Mead 08 May '18

08 May '18

It's occurred to me that one could write a class C t which is satisfied whenever (A t) or (B t) is satisfied like so: --- data Satisfied type family IsSatisfiable :: Constraint -> Type class A t class B t class C' t ta tb instance A t => C' t Satisfied tb where ... instance B t => C' t ta Satisfied where ... instance (A t, B t) => C' t Satisfied Satisfied where ... class C' t ( IsSatisfiable (A t)) ( IsSatisfiable (B t)) => C t --- We may need overlapping instances (with all the caveats that come with it) but it should be fine otherwise. "IsSatisfiable" here is defined as follows: IsSatisfiable c = Satisfied -- (if c is satisfiable) IsSatisfiable c = IsSatisfiable c -- (if c is not satisfiable) That's all that's needed. And it seems to be a reasonable type function. I note classes are open, so perhaps it could be dangerous to state that a constraint is not satisfiable (because it might be in another part of a program) but simply not reducing if we can't satisfy the constraint locally should be fine I think. At worst we'll get a compile error, but we shouldn't get inconsistent types. Is there anyway to implement this type function, or alternatively an approach which allows this type of "inherit from two classes" idea?

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Re: [Haskell-cafe] How to expose if a constraint is satisfiable
by Anthony Clayden 08 May '18

08 May '18

> It's occurred to me that one could write a class C t which is satisfied > whenever (A t) or (B t) is satisfied like so: Hi Clinton, this sounds like you might want "Choosing a type-class instance based on the context" https://wiki.haskell.org/GHC/AdvancedOverlap > --- > > data Satisfied > > type family IsSatisfiable :: Constraint -> Type That type family is doing the same job as auxiliary class `ShowPred` on that wiki page. Rather than all the machinery you give for the disjunction, you could use another type family: type family EitherSatisfied :: Type -> Type -> Type instance EitherSatisfied Satisfied tb = Satisfied instance EitherSatisfied ta Satisfied = Satisfied Those two instances do overlap (as you expected); and they exhibit confluence, aka coincident overlap (they produce the same result); so that's fine. But you haven't given any instances for `IsSatisfiable`. How do you expect to get from the Constraint to the `Satisfied` type? You say > IsSatisfiable c = Satisfied -- (if c is satisfiable) What are you going to put for `c`? If you follow that wiki page, you'll need to in effect repeat every instance decl for classes `A, B`: instance A Int where ... type instance IsSatisfiable (A Int) = Satisfied (The wiki page was written before there were type families, so type class `ShowPred` has a Functional Dependency giving a type-level Boolean result.) Your `C t` class becomes class EitherSatisfied ( IsSatisfiable (A t)) ( IsSatisfiable (B t)) ~ Satisfied => C t where ... ---- Nowadays there's a better way: make Associated Types for the two classes, give them a default instance: class A t where type APred t type instance APred t = Satisfied ... class B t where type BPred t type instance BPred t = Satisfied ... Now every instance defined for `A, B` in effect automatically gives you `APred, BPred` instances. Then class EitherSatisifed (APred t) (BPred t) ~ Satisfied => C t where ... AntC

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help with emacs intero
by Dennis Raddle 06 May '18

06 May '18

I started having trouble with Emacs intero mode a week ago. I've tried to get help on the Intero issues list as well as Stack Overflow to no avail. I'm wondering if anyone here knows enough about Intero or Emacs to at least point me to the next debugging step. At this point I don't enough to know what to try or look for. Basically flycheck stopped working, and every time I type a few characters I'm getting "Error while checking syntax automatically: (wrong-type-argument number-or-marker-p "Abort trap: 6") Any clue what this is referring to, or why it would suddenly start last week, when all was fine prior?

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