- Haskell-Cafe - Haskell.org

DSLDI 2018: Extended Call for Talk Proposals
by Sam Tobin-Hochstadt 21 Aug '18

21 Aug '18

We've extended the deadline for DSLDI talk proposals until the 24th. ********************************************************************* EXTENDED CALL FOR TALK PROPOSALS DSLDI 2018 Sixth Workshop on Domain-Specific Language Design and Implementation November 6th, 2018 Boston, USA Co-located with SPLASH http://2018.splashcon.org/track/dsldi-2018-talks https://twitter.com/wsdsldi ********************************************************************* Deadline for talk proposals: 24th of August, 2018, AoE Well-designed and implemented domain-specific languages (DSLs) can achieve both usability and performance benefits over general-purpose programming languages. By raising the level of abstraction and exploiting domain knowledge, DSLs can make programming more accessible, increase programmer productivity, and support domain-specific optimizations. ## Workshop Goal Domain-Specific Language Design and Implementation (DSLDI) is a workshop intended to bring together researchers and practitioners interested in discussing how DSLs should be designed, implemented, supported by tools, and applied in realistic contexts. The focus of the workshop is on all aspects of this process, from soliciting domain knowledge from experts, through the design and implementation of the language, to evaluating whether and how a DSL is successful. More generally, we are interested in continuing to build a community that can drive forward the development of modern DSLs. ## Workshop Format DSLDI is a single-day workshop and will consist of an invited speaker followed by moderated audience discussions structured around a series of short talks. The role of the talks is to facilitate interesting and substantive discussion. Therefore, we welcome and encourage talks that express strong opinions, describe open problems, propose new research directions, and report on early research in progress. Proposed talks should be on topics within DSLDI’s area of interest, which include but are not limited to: * solicitation and representation of domain knowledge * DSL design principles and processes * DSL implementation techniques and language workbenches * domain-specific optimizations * human factors of DSLs * tool support for DSL users * community and educational support for DSL users * applications of DSLs to existing and emerging domains * studies of usability, performance, or other benefits of DSLs * experience reports of DSLs deployed in practice ## Call for Talk Proposals We solicit talk proposals in the form of short abstracts (max. 2 pages). A good talk proposal describes an interesting position, open problem, demonstration, or early achievement. The submissions will be reviewed on relevance and clarity, and used to plan the mostly interactive sessions of the workshop day. Publication of accepted abstracts and slides on the website is voluntary. * Deadline for talk proposals: August 17th, 2018 * Notification: September 17th, 2018 * Workshop: November 6th, 2018 * Submission website: https://dsldi18.hotcrp.com/ ## Workshop Organization Co-chairs: * Sam Tobin-Hochstadt (samth(a)indiana.edu) Indiana University * Lindsey Kuper (lindsey(a)composition.al) University of California, Santa Cruz Follow us on Twitter at https://twitter.com/wsdsldi Program committee: * Eric Van Wyk (University of Minnesota) * Katherine Ye (Carnegie Mellon University) * Nate Foster (Cornell University) * Spencer Florence (Northwestern University) * Phitchaya Mangpo Phothilimthana (University of California, Berkeley) * Jeremy Gibbons (University of Oxford) * Edwin Brady (University of St. Andrews)

1 0

Six new assistant/associate professor positions in Nottingham
by Graham Hutton 21 Aug '18

21 Aug '18

Dear all, The School of Computer Science at the University of Nottingham is seeking to make multiple new appointments (potentially up to 6) at the Assistant or Associate Professor level: https://www.nottingham.ac.uk/jobs/currentvacancies/ref/SCI190718 Applications in the area of the Functional Programming (FP) lab would be most welcome. The FP lab is keen to receive applications from candidates with an excellent publication record and the ability to secure external funding to support their research. Further information about the FP lab is available from: https://www.nottingham.ac.uk/research/groups/fp-lab/ The deadline for applications is 30th September 2018. The advert mentions some specific research areas, but the positions are open to applicants from any area of Computer Science. Best wishes, Graham Hutton — Professor Graham Hutton School of Computer Science University of Nottingham, UK http://www.cs.nott.ac.uk/~pszgmh This message and any attachment are intended solely for the addressee and may contain confidential information. If you have received this message in error, please contact the sender and delete the email and attachment. Any views or opinions expressed by the author of this email do not necessarily reflect the views of the University of Nottingham. Email communications with the University of Nottingham may be monitored where permitted by law.

1 0

Re: [Haskell-cafe] Is there a library that has a strict version of `sum`?
by Damian Nadales 19 Aug '18

19 Aug '18

Ok, so if understood the bottom line of the answers I got from Vanessa and Hiromi the take away seems to be: if you're prototyping then stick to the lazy version of sum, otherwise use more advanced data structures (Vectors or Folds). On Tue, Aug 14, 2018 at 5:18 PM Vanessa McHale <vanessa.mchale(a)iohk.io> wrote: > Actually, vector has a sum function already: > http://hackage.haskell.org/package/vector-0.12.0.1/docs/Data-Vector.html#v:… > > It's strict, and I'd guess more efficient than foldl' on lists. > > On 08/14/2018 09:44 AM, Vanessa McHale wrote: > > To be more explicit: unless you've actually benchmarked sum vs sum' this > seems to be a premature optimization and you'd be better off just using the > stock sum with a sensible data structure. > > On 08/14/2018 09:40 AM, Damian Nadales wrote: > > > > On Tue, Aug 14, 2018 at 4:25 PM Vanessa McHale <vanessa.mchale(a)iohk.io> > wrote: > >> If you'd rather import a custom prelude instead of writing `sum = foldl' >> (+) 0` you might want to think about how dependencies affect maintaining a >> package. >> >> What's the actual problem you're trying to solve? >> > Just add a list of values. I was expecting such a function to be already > defined somewhere else. > > Personally, in terms of cognitive complexity I find `sum` less complex > than `fold' (+) 0`... > > >> >> On 08/14/2018 04:09 AM, Damian Nadales wrote: >> >> I thought the answer would be easy to find on the internet, but I cannot >> find any package that features a strict version of `sum`. >> >> Do you know if such a function is defined anywhere? (like a custom >> prelude for instance) >> >> I know it is trivial to define, but I would prefer not to repeat myself >> (and somebody else). >> >> Thanks! >> >> >> _______________________________________________ >> Haskell-Cafe mailing list >> To (un)subscribe, modify options or view archives go to:http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe >> Only members subscribed via the mailman list are allowed to post. >> >> _______________________________________________ >> Haskell-Cafe mailing list >> To (un)subscribe, modify options or view archives go to: >> http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe >> Only members subscribed via the mailman list are allowed to post. > > > -- > > > > *Vanessa McHale* > Functional Compiler Engineer | Chicago, IL > > Website: www.iohk.io <http://iohk.io> > Twitter: @vamchale > PGP Key ID: 4209B7B5 > > [image: Input Output] <http://iohk.io> > > [image: Twitter] <https://twitter.com/InputOutputHK> [image: Github] > <https://github.com/input-output-hk> [image: LinkedIn] > <https://www.linkedin.com/company/input-output-global> > > > This e-mail and any file transmitted with it are confidential and intended > solely for the use of the recipient(s) to whom it is addressed. > Dissemination, distribution, and/or copying of the transmission by anyone > other than the intended recipient(s) is prohibited. If you have received > this transmission in error please notify IOHK immediately and delete it > from your system. E-mail transmissions cannot be guaranteed to be secure or > error free. We do not accept liability for any loss, damage, or error > arising from this transmission > > > -- > > > > *Vanessa McHale* > Functional Compiler Engineer | Chicago, IL > > Website: www.iohk.io <http://iohk.io> > Twitter: @vamchale > PGP Key ID: 4209B7B5 > > [image: Input Output] <http://iohk.io> > > [image: Twitter] <https://twitter.com/InputOutputHK> [image: Github] > <https://github.com/input-output-hk> [image: LinkedIn] > <https://www.linkedin.com/company/input-output-global> > > > This e-mail and any file transmitted with it are confidential and intended > solely for the use of the recipient(s) to whom it is addressed. > Dissemination, distribution, and/or copying of the transmission by anyone > other than the intended recipient(s) is prohibited. If you have received > this transmission in error please notify IOHK immediately and delete it > from your system. E-mail transmissions cannot be guaranteed to be secure or > error free. We do not accept liability for any loss, damage, or error > arising from this transmission >

13 20

Possibly an endless loop in my benchmark code?
by Michail Pevnev 18 Aug '18

18 Aug '18

Hello everyone! At the moment I'm working on a function that computes the rank of a matrix. The testing I've performed doesn't seem to reveal anything obviously wrong with what I have so far, so I've decided to see if it can be improved performance-wise. I've sketched up a minimalist criterion benchmark, and run it with profiling options on. But something in the code causes the benchmark executable to loop endlessly. I've stuck a few 'trace's into the code, and it seems that it just keeps on producing and processing more and more matrices. I can kill the process via 'Ctrl-C', and if I do, a profiling report appears. I have attached one here, if you want to see it in full, but the gist is that 70% of time the program does nothing, 10% it does something labeled 'SYSTEM', and another 9% are garbage collection, with the rest of the functions taking less than 2% each. This can go on for more than an hour. At first I've attributed this to the function in question being grossly inefficient, but in the test executable 100 runs of this function on 100x100 matrices take only 18-20 seconds (well, it's actually a test to see if the transpose of a matrix has the same rank as the original, so there's some extra overhead here). What could cause this behaviour? One weak point I can identify myself is 'NFData' (from deepseq package) instance for my matrices - I'm not 100% certain it's correct and does what it should. I don't really think it's the case, though - 'rnf' method of 'NFData' class would then feature prominently in the profiling report, which it doesn't. 'NFData' instance, benchmarking code and the function in question are below ('rank' lacks a type signature since it's actually a method on 'Matrix' class'): ``` ---- the code that works ---- -- | A generic matrix. May or may not be square. data GenericM e = GenericM Int Int (U.Vector e) deriving (Show, Eq) instance NFData (GenericM e) where rnf (GenericM nrows ncols dat) = rnf (nrows, ncols, dat) rank tolerance m@(GenericM nr nc _) = D.trace "new matrix" $ go 0 0 $ V.map rawData $ rows m where go total col allRows | null allRows = total | col >= nc = total | col >= nr = total | abs (maxRow U.! col) < tolerance = go total (col + 1) allRows | otherwise = go (total + 1) (col + 1) nonZeroed where maxIndex = V.maxIndexBy (comparing (abs . (U.! col))) allRows maxRow = allRows V.! maxIndex allRows' = V.imap multiplyAndAdd allRows nonZeroed = V.ifilter notZero allRows' notZero i v = i /= maxIndex && not (isZeroV tolerance v) multiplyAndAdd i v | i == maxIndex = v | otherwise = U.imap (multAddOne i) v multAddOne i j y | j < col = y | otherwise = y - (maxRow U.! j) * coef i coef i = (allRows V.! i U.! col) / maxRow U.! col ---- the code that benchmarks ---- genericDenseMatrixRank :: Benchmark genericDenseMatrixRank = bgroup "linear/matrix/dense/generic/rank" [ env (genRandomGenericMatrix 100) (b "small") --, env (genRandomGenericMatrix 1000) (b "medium") --, env (genRandomGenericMatrix 5000) (b "big") ] where b name m = bench name $ whnf (rank 1e-6) m genRandomGenericMatrix :: Int -> IO (GenericM Double) genRandomGenericMatrix size = do rng <- getStdGen let v = randoms rng :: [Double] return $ mkGenericMFromList size size v ``` Thanks in advance. -- Michail.

1 0

multiline mode on ghci 8.2.2
by Fabien R 16 Aug '18

16 Aug '18

I thought I could use the directive ":set +m" to define a function with its signature. Prelude> :set +m Prelude> square :: Num a=>a->a <interactive>:3:1: error: Variable not in scope: square :: a1 -> a1 Instead, I have to type: Prelude> :unset +m Prelude> :{ Prelude| square :: Num a=>a->a Prelude| square = (^2) Prelude| :} Prelude> Any hint ? -- Fabien

3 2

First CfP: JFP Special Issue on The Theory and Practice of Algebraic Effects and Handlers
by KC Sivaramakrishnan 16 Aug '18

16 Aug '18

============================================================ CALL FOR PAPERS JFP Special Issue on The Theory and Practice of Algebraic Effects and Handlers Submission Deadline: 18 January 2019 Expected Publication Date: December 2019 ============================================================= ## Scope An important aspect of real-world languages is their support for computational effects such as raising exceptions, printing to the screen, accessing a database, non-determinism, and concurrency. In order to reason about the semantics of a programming language with computational effects, it is necessary to separate the effects out from the rest of the language. To this end, algebraic effects permit a wide class of computational effects to be specified in a pure setting using only operations that give rise to them and equations that the operations satisfy. The algebraic treatment of operations naturally leads to a novel treatment of handlers for all computational effects, not just for exceptions. Algebraic effect handlers have been steadily gaining attention as a programming language feature since they generalise many control-flow abstractions such as exception handling, iterators, async/await, or backtracking, while ensuring that the composition of various features remains well-behaved. Indeed, there are implementations of algebraic effects and effect handlers as libraries in C, Clojure, F#, Haskell, OCaml, Scala, JavaScript, as well as full-fledged languages such as Eff, Frank, Links, Koka, and Multicore OCaml. Algebraic effect handlers have also influenced the design of software tools in industry including Facebook's React UI library and Uber's Pyro probabilistic programming language. To recognise and encourage the publication of mature research contributions in this area, a special issue of the Journal of Functional Programming (JFP) will be devoted to the same theme. ## Topics Full-length, archival-quality submissions are solicited on theoretical and practical aspects of algebraic effects and handlers. Examples include, but are not limited to: * Reasoning about algebraic effects and handlers (denotational semantics, dependent types, logical relations, language support for equational reasoning) * Effect typing (subtyping, row-polymorphism, generativity, encapsulation) * Implementation of effect handlers (dynamic effects, selective CPS translations, delimited continuations) * Applications of algebraic effect handlers (probabilistic programming, event correlation, meta-programming, asynchronous I/O, debugging) Reports on applications of these techniques to real-world problems are especially encouraged, as are submissions that relate ideas and concepts from several of these topics, or bridge the gap between theory and practice. Papers will be reviewed as regular JFP submissions, and acceptance in the special issue will be based on both JFP's quality standards and relevance to the theme. The special issue also welcomes high-quality survey and position papers that would benefit a wide audience. Authors are encouraged to indicate interest in submitting by December 14, 2018, to aid in identifying suitable reviewers. The submission deadline is January 18, 2019. The expected submission length is 25-35 pages, excluding bibliography and appendices. Shorter submissions are encouraged; prospective authors of longer submissions should discuss their plans with the special issue editors in advance. Submissions that are based on previously-published conference or workshop papers must clearly describe the relationship with the initial publication, and must differ sufficiently that the author can assign copyright to Cambridge University Press. Prospective authors are welcome to discuss such submissions with the editors to ensure compliance with this policy. ## Submissions Submissions should be sent through the JFP Manuscript Central system at https://mc.manuscriptcentral.com/cup/jfp_submit. Choose “Effects and Handlers” as the paper type, so that it gets assigned to the special issue. For other submission details, please consult an issue of the Journal of Functional Programming or see the Journal's web page at http://journals.cambridge.org/jid_JFP. ## Tentative Schedule * 14 December 2018: Expression of interest * 18 January 2019: Submission deadline * 22 April 2019: First round of reviews * 23 August 2019: Revision deadline * 15 November 2019: Second round of reviews * 13 December 2019: Final accepted versions due ## Guest Editors * Andrej Bauer, Faculty of Mathematics and Physics, University of Ljubljana * KC Sivaramakrishnan, Department of Computer Science and Technology, University of Cambridge ## Editors in Chief * Jeremy Gibbons, Department of Computer Science, University of Oxford * Matthias Felleisen, College of Computer and Information Science, Northeastern University

1 0

WG:
by Stefan Chacko 16 Aug '18

16 Aug '18

Dear Haskell supporters, I have a some question about the definitions in haskell. 1. What is the use of making definitions compared to having no definitions? Is it just like a comment for documation or does it really make a difference in compiling the code? 2. What does the arrow(->) mean in such a definition? Is it a binary operation as *, +, -, <,>,= or is it something else? For example there is sumWithL :: [Int] -> Int -> Int Does this mean in other words [Int]+Int=Int 1. Why do we use clinches in such definitions. I concluded you need clinches if a function is not associative such as (a-b)+c . (Int->Int)->Int->Int But also if a higher order function needs more than one argument. (a->b)->c . Can you please explain it ? Thank you Kind regards. Gesendet von Mail<https://go.microsoft.com/fwlink/?LinkId=550986> für Windows 10

5 4

Re: [Haskell-cafe] How to turn Integer into Proxy (n :: KnownNat) with n equal to the integer?
by Johannes Waldmann 15 Aug '18

15 Aug '18

> How can I convert an Integer into a Proxy (n :: KnownNat) > with n equal to the integer? Data.Reflection.reifyNat ? https://mail.haskell.org/pipermail/haskell-cafe/2017-November/128152.html - J.W.

2 1

A function synonym is not (by default) as polymorphic as the original.
by Jeffrey Brown 15 Aug '18

15 Aug '18

Today I was writing a lot of Hashable instances, and found it hard to read code littered long lines mostly consisting of `hashWithSalt`, so I made a synonym: infixl 5 ## (##) = hashWithSalt But when I replaced all the `hashWithSalt`s with ##, I got lots of weird errors -- GHC kept wanting things to be Strings that were Ints, or vice verse. The problem vanished when I added this type signature: (##) :: Hashable a => Int -> a -> Int That's exactly the same type signature that hashWithSalt has. Is this expected behavior? Desirable? -- Jeff Brown | Jeffrey Benjamin Brown Website <https://msu.edu/~brown202/> | Facebook <https://www.facebook.com/mejeff.younotjeff> | LinkedIn <https://www.linkedin.com/in/jeffreybenjaminbrown>(spammy, so I often miss messages here) | Github <https://github.com/jeffreybenjaminbrown>

2 1

Status of the Hi Haddock project
by Simon Jakobi 14 Aug '18

14 Aug '18

Hi! I have summarized the status of my GSoC project at https://sjakobi.github.io/blog/2018/08/14/hi-haddock-3/ My WIP patches for GHC and haddock are at https://phabricator.haskell.org/D5067 and https://github.com/haskell/haddock/pull/906. Cheers, Simon

1 0

Is there a library that has a strict version of `sum`?
by Damian Nadales 14 Aug '18

14 Aug '18

I thought the answer would be easy to find on the internet, but I cannot find any package that features a strict version of `sum`. Do you know if such a function is defined anywhere? (like a custom prelude for instance) I know it is trivial to define, but I would prefer not to repeat myself (and somebody else). Thanks!

5 4

First Call for Participation for IFL 2018 (Implementation and Application of Functional Languages)
by Jurriaan Hage 14 Aug '18

14 Aug '18

Hello, Please, find below the 1st call for participation for IFL 2018. Please forward these to anyone you think may be interested. Apologies for any duplicates you may receive. best regards, Jurriaan Hage Publicity Chair of IFL --- CALL FOR PARTICIPATION: ================================================================================ IFL 2018 30th Symposium on Implementation and Application of Functional Languages University of Massachusetts Lowell, MA, USA September 5th-7th, 2018 http://iflconference.org September 5th: Haskell Mini-Course by Galois, Inc, see details below. ================================================================================ ### Scope The goal of the IFL symposia is to bring together researchers actively engaged in the implementation and application of functional and function-based programming languages. IFL 2018 will be a venue for researchers to present and discuss new ideas and concepts, work in progress, and publication-ripe results related to the implementation and application of functional languages and function-based programming. ### Keynote Speakers * Adam Chlipala, Massachusetts Institute of Technology CSAIL * Arjun Guha, University of Massachusetts Amherst ### Haskell Mini-Course (September 5th, 2018) Mini-course on Haskell by Galois, Inc, presented by David Thrane Christiansen and Jose Manuel Calderon Trilla. Register: Attendance at the course is free, but you must RSVP. (Link to RSVP form can be found at http://iflconference.org) Course Description: Recent versions of the GHC compiler for Haskell feature support for a number of advanced type system features, including pattern-matching functions in the type system (type families), indexed families (generalized algebraic datatypes, or GADTs), type-level data structures (data kinds), and general compile-time metaprogramming (Template Haskell). At Galois, we use these features in a number of our projects, which allows us to build deep embeddings of programming languages and have GHC enforce the target language's type system for us. This style of programming ensures that we only produce well-typed terms, including that we do not forget any of the run-time checks that are necessary to preserve our invariants when accepting input from untyped sources such as files. In the course of developing these projects, some common problems and programming patterns emerged. We developed the `parameterized-utils` library to codify solutions to these problems, and provide necessary generalizations of interfaces from the standard library (Eq, Applicative, Traversable, etc.) We will expect that participants in the course have used Haskell before, but we will not expect everyone to be experts. We will introduce GADTs, type families, and data kinds, and then show how to use them together with the tools from parameterized-utils with an implementation of the simply-typed lambda calculus, including basic AST definitions, evaluation, and parsing. ### Peter Landin Prize The Peter Landin Prize is awarded to the best paper presented at the symposium every year. The honored article is selected by the program committee based on the submissions received for the formal review process. The prize carries a cash award equivalent to 150 Euros. ### Organization and Program committee Chairs: Jay McCarthy & Matteo Cimini, University of Massachusetts Lowell, USA Program Committee: * Arthur Chargueraud, Inria, FR * Ben Delaware, Purdue University, USA * Christos Dimoulas, Northwestern University, USA * David Darais, University of Vermont, USA * Dominic Orchard, University of Kent, UK * Ekaterina Komendantskaya, Heriot-Watt University, UK * Garrett Morris, University of Kansas, USA * Heather Miller, EPFL & Northeastern University, CH & USA * Jeremy Yallop, University of Cambridge, UK * Keiko Nakata, SAP Innovation Center Potsdam, DE * Laura Castro, University of A Coruna, ESP * Magnus Myreen, Chalmers University of Technology, SWE * Natalia Chechina, Bournemouth University, UK * Peter Achten, Radboud Universiteit Nijmegen, NL * Peter-Michael Osera, Grinnell College, USA * Richard Eisenberg, Bryn Mawr College, USA * Trevor McDonell, University of New South Wales, AUS * Yukiyoshi Kameyama, University of Tsukuba, JAP ### Venue The 30th IFL is organized by the University of Massachusetts Lowell. The City of Lowell is located at the heart of the Merrimack Valley just 30 miles northwest of Boston. Lowell can be easily reached by train or taxi. See the website for more information on the venue.

1 0

What is the right way to use unsafeCoerce?
by Cosmia Fu 14 Aug '18

14 Aug '18

Hi all, I have noticed that unsafeCoerce is able to > Prelude> import Unsafe.Coerce > Prelude Unsafe.Coerce> data P = P Int Double deriving Show > Prelude Unsafe.Coerce> data Q a b = Q a b deriving Show > Prelude Unsafe.Coerce> unsafeCoerce (P 23 2.33) :: Q Int Double > Q 23 2.33 And it works even for sum type, and the first constructor always wins. > Prelude Unsafe.Coerce> data M a = J a deriving Show > Prelude Unsafe.Coerce> unsafeCoerce (J "qwe") :: Either String String > Left "qwe" > Prelude Unsafe.Coerce> unsafeCoerce (Just "qwe") :: M String > J "qwe" And the documenation for unsafeCoerce# says > Casting between two types that have the same runtime representation. One case is when the two types differ only in "phantom" type parameters, for example Ptr Int to Ptr Float, or [Int] to [Float] when the list is known to be empty. Also, a newtype of a type T has the same representation at runtime as T. > http://hackage.haskell.org/package/base-4.11.1.0/docs/GHC-Exts.html#v:unsaf… But info tables for different constructors are different. Why does the code above work? And is it safe to use unsafeCoerce like this? Thanks, Cosmia Fu

2 2

Learning & Haskell Job Board
by Alex Mesropians 13 Aug '18

13 Aug '18

Hey All, Just wanted to share Workshub <http://t.sidekickopen13.com/e1t/c/5/f18dQhb0S7lC8dDMPbW2n0x6l2B9nMJW7t5XZs5…> with you all for those looking to hire Haskell engineers or want to see for yourself whats out there! We also have a learn section which is forever growing with new FP related content! We are always also looking for people to contribute to! thanks, Alex

1 0

Google Summer of Code : CodeWorld
by Krystal Maughan 13 Aug '18

13 Aug '18

Hi All, I wrote a blog post on features I worked on this summer for Google Summer of Code in CodeWorld. Hope you enjoy it! Thank you for giving me the opportunity to work in Haskell this summer! https://medium.com/@krystal.maughan/breaking-the-space-time-barrier-with-ha… Best, Krystal

1 0

cpphs broken on GHC 8.6.1
by Vanessa McHale 12 Aug '18

12 Aug '18

As of the latest release of GHC 8.6.1, cpphs no longer compiles. Anyone happen to know what's going on? 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> This e-mail and any file transmitted with it are confidential and intended solely for the use of the recipient(s) to whom it is addressed. Dissemination, distribution, and/or copying of the transmission by anyone other than the intended recipient(s) is prohibited. If you have received this transmission in error please notify IOHK immediately and delete it from your system. E-mail transmissions cannot be guaranteed to be secure or error free. We do not accept liability for any loss, damage, or error arising from this transmission

2 1

hs-boot and (micro)lenses
by Francesco Ariis 12 Aug '18

12 Aug '18

Hello cafe, I have some problem setting up an appropriate .hs-boot module. I attach a .tar.gz'd cabal folder, so you can immediately test it with `cabal new-repl`. I had a file like this -- datatypes data A = A { _i :: Int, _b :: B } type AS a = State A a data B = B (AS ()) makeLenses ''A -- functions f :: B f = B $ i .= 18 So I decided to split it up in 2 files to test hs-boot in the wild: -- A.hs ------- data A = A { _i :: Int, _b :: B } type AS a = State A a makeLenses ''A -- B.hs ------- data B = B (AS ()) f :: B f = B $ i .= 18 Well, I don't know how to write A.hs-boot. I tried this: module A where import Lens.Micro.Platform data A b :: Lens' A B but ghc rightfully complains that he doesn't know about B, and if I slap an `import B` in the .hs-boot then I have another import-cycle. What's the way out of that? The project I am dealing with is bigger than this problem and I would really love to keep functions and related datatypes nearby for mental sanity. As I wrote above, attached you can find a .tar.gz with everything set up to replicate this -F

2 2

Fwd: [Haskell] [ANNOUNCE] GHC 8.6.1-beta1 available
by Dan Burton 11 Aug '18

11 Aug '18

I've generated a stack.yaml which should allow most stack users to install this ghc release via stack. https://gist.github.com/DanBurton/43b5f5155fdd1affd02f6e86f37da9ae e.g. $ mkdir test-new-ghc && cd test-new-ghc $ wget https://gist.github.com/DanBurton/43b5f5155fdd1affd02f6e86f37da9 ae/raw/6cb25068fcfae7d70739b7927248a80518192ad6/stack.yaml $ stack setup $ stack repl Once installed in this way, you can simply use: compiler: ghc-8.6.0.20180810 compiler-check: match-exact In any project's stack.yaml in order to build with this version of ghc. -- Dan Burton ---------- Forwarded message ---------- From: Ben Gamari <ben(a)well-typed.com> Date: Fri, Aug 10, 2018 at 7:31 PM Subject: [Haskell] [ANNOUNCE] GHC 8.6.1-beta1 available To: GHC developers <ghc-devs(a)haskell.org>, GHC users < glasgow-haskell-users(a)haskell.org>, Haskell Cafe < haskell-cafe(a)googlegroups.com>, haskell(a)haskell.org Hello everyone, The GHC development team is very pleased to announce the first beta leading up to GHC 8.6.1 release. The usual release artifacts are available from https://downloads.haskell.org/~ghc/8.6.1-beta1 This beta fixes most of the bugs reported in the first two alphas and brings all of the core libraries up to their final release versions. The 8.6 release fixes over 300 bugs from the 8.4 series and introduces a number of exciting features. These most notably include: * Significantly better handling of macOS linker command size limits, avoiding linker errors while linking large projects * A new deriving mechanism, `deriving via`, providing a convenient way for users to extend Haskell's typeclass deriving mechanism * Quantified constraints, allowing forall quantification in contexts * An early version of the GHCi `:doc` command * The `ghc-heap-view` package, allowing introspection into the structure of GHC's heap * Valid hole fit hints, helping the user to find terms to fill typed holes in their programs * The BlockArguments extension, allowing the `$` operator to be omitted in some unambiguous contexts * The next phase of the MonadFail proposal, enabling -XMonadFailDesugaring by default A full list of the changes in this release can be found in the release notes: https://downloads.haskell.org/~ghc/8.6.1-beta1/docs/html/ users_guide/8.6.1-notes.html This will very likely be the last release before the final 8.6.1 so do give it a thorough testing and, as always, report any issues you encounter. Thanks for your help! Cheers, - Ben _______________________________________________ Haskell mailing list Haskell(a)haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell

2 1

Aeson memory use
by Lana Black 09 Aug '18

09 Aug '18

Hello Cafe, Recently while trying to optimize some code, I noticed that aeson requires quite a substantial amount of memory when parsing map-like structures. The quick example I crafted [1] is quite surprising. In order to parse an 11 megabyte file aeson requires around 300 megabytes of memory (according to the heap profiler), and the total memory footprint (according to htop) jumps to around 700 megabytes. Is there a way to reduce this number to something less insane? I suspect that I might be doing something wrong here, but I can't exactly see what. [1] https://github.com/greydot/aeson-example

5 9

Call for Participation: ICFP 2018
by Lindsey Kuper 07 Aug '18

07 Aug '18

*** Early registration ends 27 August. *** ===================================================================== Call for Participation ICFP 2018 23rd ACM SIGPLAN International Conference on Functional Programming and affiliated events September 23 - September 29, 2018 St. Louis, Missouri, USA http://icfp18.sigplan.org/ ===================================================================== ICFP provides a forum for researchers and developers to hear about the latest work on the design, implementations, principles, and uses of functional programming. The conference covers the entire spectrum of work, from practice to theory, including its peripheries. This year, ICFP is co-located with Strange Loop! Considering attending ICFP for the first time? See our brief explainer: https://icfp18.sigplan.org/attending/introduction-to-icfp * Overview and affiliated events: http://icfp18.sigplan.org/home * Program: http://icfp18.sigplan.org/program/program-icfp-2018 * Accepted papers: http://icfp18.sigplan.org/track/icfp-2018-papers * Registration is available via: https://regmaster4.com/2018conf/ICFP18/register.php Early registration ends 27 August, 2018. * Programming contest results: https://icfpcontest2018.github.io/ * Student Research Competition: https://icfp18.sigplan.org/track/icfp-2018-Student-Research-Competition * Follow us on Twitter for the latest news: http://twitter.com/icfp_conference In addition to Strange Loop (9/26-9/28), there are several events co-located with ICFP: * Erlang Workshop (9/29) * Functional Art, Music, Modeling and Design (9/29) * Functional High-Performance Computing (9/29) * Haskell Implementors' Workshop (9/23) * Haskell Symposium (9/27-9/28) * Higher-order Programming with Effects (9/23) * ICFP Tutorials (9/27-9/29) * ML Family Workshop (9/28) * Numerical Programming in Functional Languages (9/27) * OCaml Workshop (9/27) * Programming Languages Mentoring Workshop (9/23) * Scala Symposium (9/28) * Scheme Workshop (9/28) * Type-Driven Development (9/27) Conference Organizers: General Chair: Robby Findler (Northwestern University, USA) Program Chair: Matthew Flatt (University of Utah, USA) Accessibility Chair: Alan Jeffrey (Mozilla Research, USA) Artefact Evaluation Co-Chair: Simon Marlow (Facebook, UK) Industrial Relations Chair: Alan Jeffrey (Mozilla Research, USA) PLMW Co-Chair: Dan Licata (Wesleyan University, USA) PLMW Co-Chair: David Van Horn (University of Maryland, USA) PLMW Co-Chair: Niki Vazou (University of Maryland, USA) Programming Contest Organizer: Matthew Fluet (Rochester Institute of Technology, USA) Publications Co-Chair: Alex Potanin (Victoria University of Wellington, New Zealand) Publicity Chair: Lindsey Kuper (UC Santa Cruz, USA) Student Research Competition Chair: Ravi Chugh (University of Chicago, USA) Student Volunteer Co-Captain: Jakub Zalewski (University of Edinburgh, UK) Student Volunteer Co-Captain: Spencer P. Florence (Northwestern University, USA) Treasurer and Conference Manager: Annabel Satin (P.C.K., UK) Video Co-Chair Jamie Willis (University of Bristol, UK) Video Co-Chair: Jose Calderon (Galois, USA) Workshops Co-Chair: Christophe Scholliers (Ghent University, Belgium) Workshops Co-Chair: David Christiansen (Galois, USA) Sponsors and industrial partners: Platinum supporters: Ahrefs Jane Street Standard Chartered X Gold supporters: DFINITY Facebook Mozilla McCormick School of Engineering, Northwestern University Silver supporters: Bloomberg Cal Poly Computer Science & Software Engineering Digital Asset Galois Microsoft Research Oracle Labs Tweag I/O Bronze supporters: Google IntelliFactory Kadena Obsidian Systems Systor Vest Well-Typed

1 0

Working around floating-point inaccuracies
by Michail Pevnev 07 Aug '18

07 Aug '18

Hello everyone! For a library I write at the moment, I made a class for approximate geometric equality comparisons (and dubbed it `ApproxEq`). It's not a very lawful class, but there is one invariant that should hold absolutely always - if two entities coincide but have different representations, they should compare "equal". All instances for my geometric entities seem to uphold this invariant (btw, QuickCheck is awesome). Except for the `Plane` type, which gives me a false negative once in about 50000 runs. I've chosen to represent planes by two vectors: an arbitrary point on a plane, and a normal. To test this invariant I translate the plane by a vector parallel to the plane, and then compare the translated plane with the original. The general pattern to these failures is that one of the coordinates of the origin point of the untranslated plane is huge compared to the other two - by 4-6 orders of magnitude, while the translation vector is not particularly big and has about the same magnitude as the two smaller coordinates. I suspect that this difference throws off the function that tests if a point belongs to a plane (which is in turn used in the equality comparison) by filling lower digits of participating numbers with floating-point garbage. I can live with this false negative, but I'd prefer not to. There probably is a clever way to work around it, I just can't find it. Any ideas? Thanks in advance. The relevant code is below. ``` -- | A plane is represented by an arbitrary point on it and a normal vector. data Plane a = Plane (V3 a) (V3 a) deriving (Show) instance (Epsilon a, Floating a) => ApproxEq (Plane a) where p1@(Plane o1 n1) ~== p2@(Plane o2 n2) = res where n1' = normalize n1 n2' = normalize n2 parallel = n1' ~== n2' antiparallel = (-n1') ~== n2' originsAreTheSame = o1 ~== o2 originsBelong = pointOnPlane p1 o2 || pointOnPlane p2 o1 res = (parallel || antiparallel) && (originsAreTheSame || originsBelong) -- | Return True if a point lies on a plane. pointOnPlane :: (Epsilon a, Floating a) => Plane a -> V3 a -> Bool pointOnPlane (Plane o n) v = nearZero $ delta `dot` n' where delta = normalize $ o - v n' = normalize n ``` -- Michail.

3 3

How to increase solver iterations?
by David Banas 07 Aug '18

07 Aug '18

Hi all, I’m getting this ghc compilation error: solveSimpleWanteds: too many iterations (limit = 4) Set limit with -fsolver-iterations=n; n=0 for no limit These are the only plug-ins I’m using: {-# OPTIONS_GHC -fplugin GHC.TypeLits.KnownNat.Solver #-} {-# OPTIONS_GHC -fplugin TypeNatSolver #-} I’ve tried the following, one at a time: {-# OPTIONS_GHC -fplugin-opt=TypeNatSolver:solver-iterations=10 #-} {-# OPTIONS_GHC -fplugin-opt=TypeNatSolver:fsolver-iterations=10 #-} {-# OPTIONS_GHC -fplugin-opt=GHC.TypeLits.KnownNat.Solver:solver-iterations=10 #-} {-# OPTIONS_GHC -fplugin-opt=GHC.TypeLits.KnownNat.Solver:fsolver-iterations=10 #-} But, I always get, “(limit = 4)”. Does anyone know how to change the number of solver iterations? Thanks, -db

2 4

Bizarre GHC error.
by David Banas 05 Aug '18

05 Aug '18

Has anyone seen this kind of error from GHC? dca9047d694f:concat-learn a594349$ stack build :lenna concat-learn-0.2.0.0: build (lib + exe) Preprocessing library concat-learn-0.2.0.0... [6 of 6] Compiling ConCat.Learn.CNN.Conv ( src/ConCat/Learn/CNN/Conv.hs, .stack-work/dist/x86_64-osx/Cabal-1.24.2.0/build/ConCat/Learn/CNN/Conv.o ) user error (Unexpected result from the SMT solver: Expected: success Result: (error "line 1515 column 22: unknown constant tn_1" ) ) -- While building custom Setup.hs for package concat-learn-0.2.0.0 using: /Users/a594349/.stack/setup-exe-cache/x86_64-osx/Cabal-simple_mPHDZzAJ_1.24.2.0_ghc-8.0.2 --builddir=.stack-work/dist/x86_64-osx/Cabal-1.24.2.0 build lib:concat-learn exe:lenna --ghc-options " -ddump-hi -ddump-to-file" Process exited with code: ExitFailure 1 I started seeing it when I started trying to use the someNatVal function.

2 1

Re: [Haskell-cafe] Working around floating-point inaccuracies
by Olaf Klinke 05 Aug '18

05 Aug '18

Michail, Each plane in 3d space is determined by a single linear equation with 3 unknowns. Essentially the equation is what pointOnPlane evaluates. Two planes are equal if the system of linear equations consisting of both equations has rank 1. This is a standard concept of linear algebra, hence the answer to your problem (and code for a good implementation) should be found in the literature on that subject - numerical linear algebra. Google "numerical rank of a matrix". The concepts you are looking for is the "condition number" or "operator norm". You could try to enforce an invariant that the representation is always as nice as possible, numerically. If you are okay with a vector space over the rational numbers, you could represent your planes by an equation with integer coefficients, which makes equality testing independent of floating point issues. Olaf

1 0

Fixity of ($) and (.) vs. (&) and (>>>)
by Frederik Hanghøj Iversen 05 Aug '18

05 Aug '18

With function composition I can easily write something like h . g . f $ expr However if I want to write the equivalent in left-to-right composition style: expr & f >>> g >>> h This is not possible due to a parsing error arising from the fixity of the operators. (>>>) is provided by Control.Arrow and (&) by Data.Function. My question is, would it not be better to *decrement* the fixity of (&) by one or *increment* the fixity of (>>>) by one (probably rather the latter)? -- Regards *Frederik Hanghøj Iversen*

2 1

ANN: lsp-test-0.1.0.0
by Luke Lau 05 Aug '18

05 Aug '18

My GSoC 2018 project (and first package!), lsp-test <https://hackage.haskell.org/package/lsp-test-0.1.0.0>, is now available on Hackage. It allows you to carry out functional end-to-end tests at a high level for LSP <https://microsoft.github.io/language-server-protocol/> servers, and we’re currently using it in haskell-ide-engine <https://github.com/haskell/haskell-ide-engine>. Many thanks to Alan Zimmerman and Zubin Duggal for their invaluable help this summer. Hopefully it will help improve your LSP servers, or maybe entice you to make one for your language!

1 0

Re: [Haskell-cafe] [random-fu] sampleRVar with StdGen in RWS?
by dominic＠steinitz.org 04 Aug '18

04 Aug '18

> > I suppose you could write an orphan instance for RWST if you really > > wanted to. > > That's what I'll most probably do for my small project, but I was > wondering whether it made sense to make a pull request to random-fu. To > me, having a MonadRandom for both StateT and RWST seems very natural. Please do make a PR - I will accept it immediately :) Dominic Steinitz dominic(a)steinitz.org http://idontgetoutmuch.wordpress.com Twitter: @idontgetoutmuch

2 1

A Performance Puzzle
by Gregory Wright 04 Aug '18

04 Aug '18

Hi, Something Haskell has lacked for a long time is a good medium-duty linear system solver based on the LU decomposition. There are bindings to the usual C/Fortran libraries, but not one in pure Haskell. (An example "LU factorization" routine that does not do partial pivoting has been around for years, but lacking pivoting it can fail unexpectedly on well-conditioned inputs. Another Haskell LU decomposition using partial pivoting is around, but it uses an inefficient representation of the pivot matrix, so it's not suited to solving systems of more than 100 x 100, say.) By medium duty I mean a linear system solver that can handle systems of (1000s) x (1000s) and uses Crout's efficient in-place algorithm. In short, a program does everything short of exploiting SIMD vector instructions for solving small subproblems. Instead of complaining about this, I have written a little library that implements this. It contains an LU factorization function and an LU system solver. The LU factorization also returns the parity of the pivots ( = (-1)^(number of row swaps) ) so it can be used to calculate determinants. I used Gustavson's recursive (imperative) version of Crout's method. The implementation is quite simple and deserves to be better known by people using functional languages to do numeric work. The library can be downloaded from GitHub: https://github.com/gwright83/luSolve The performance scales as expected (as n^3, a linear system 10 times larger in each dimension takes a 1000 times longer to solve): Benchmark luSolve-bench: RUNNING... benchmarking LUSolve/luFactor 100 x 100 matrix time 1.944 ms (1.920 ms .. 1.980 ms) 0.996 R² (0.994 R² .. 0.998 R²) mean 1.981 ms (1.958 ms .. 2.009 ms) std dev 85.64 μs (70.21 μs .. 107.7 μs) variance introduced by outliers: 30% (moderately inflated) benchmarking LUSolve/luFactor 500 x 500 matrix time 204.3 ms (198.1 ms .. 208.2 ms) 1.000 R² (0.999 R² .. 1.000 R²) mean 203.3 ms (201.2 ms .. 206.2 ms) std dev 3.619 ms (2.307 ms .. 6.231 ms) variance introduced by outliers: 14% (moderately inflated) benchmarking LUSolve/luFactor 1000 x 1000 matrix time 1.940 s (1.685 s .. 2.139 s) 0.998 R² (0.993 R² .. 1.000 R²) mean 1.826 s (1.696 s .. 1.880 s) std dev 93.63 ms (5.802 ms .. 117.8 ms) variance introduced by outliers: 19% (moderately inflated) Benchmark luSolve-bench: FINISH The puzzle is why the overall performance is so poor. When I solve random 1000 x 1000 systems using the linsys.c example file from the Recursive LAPACK (ReLAPACK) library -- which implements the same algorithm -- the average time is only 26 ms. (I have tweaked ReLAPACK's dgetrf.c so that it doesn't use optimized routines for small matrices. As near as I can make it, the C and haskell versions should be doing the same thing.) The haskell version runs 75 times slower. This is the puzzle. My haskell version uses a mutable, matrix of unboxed doubles (from Kai Zhang's matrices <https://hackage.haskell.org/package/matrices> library). Matrix reads and writes are unsafe, so there is no overhead from bounds checking. Let's look at the result of profiling: Tue Jul 31 21:07 2018 Time and Allocation Profiling Report (Final) luSolve-hspec +RTS -N -p -RTS total time = 7665.31 secs (7665309 ticks @ 1000 us, 1 processor) total alloc = 10,343,030,811,040 bytes (excludes profiling overheads) COST CENTRE MODULE SRC %time %alloc unsafeWrite Data.Matrix.Dense.Generic.Mutable src/Data/Matrix/Dense/Generic/Mutable.hs:(38,5)-(39,38) 17.7 29.4 basicUnsafeWrite Data.Vector.Primitive.Mutable Data/Vector/Primitive/Mutable.hs:115:3-69 14.7 13.0 unsafeRead Data.Matrix.Dense.Generic.Mutable src/Data/Matrix/Dense/Generic/Mutable.hs:(34,5)-(35,38) 14.2 20.7 matrixMultiply.\.\.\ Numeric.LinearAlgebra.LUSolve src/Numeric/LinearAlgebra/LUSolve.hs:(245,54)-(249,86) 13.4 13.5 readByteArray# Data.Primitive.Types Data/Primitive/Types.hs:184:30-132 9.0 15.5 basicUnsafeRead Data.Vector.Primitive.Mutable Data/Vector/Primitive/Mutable.hs:112:3-63 8.8 0.1 triangularSolve.\.\.\ Numeric.LinearAlgebra.LUSolve src/Numeric/LinearAlgebra/LUSolve.hs:(382,45)-(386,58) 5.2 4.5 matrixMultiply.\.\ Numeric.LinearAlgebra.LUSolve src/Numeric/LinearAlgebra/LUSolve.hs:(244,54)-(249,86) 4.1 0.3 primitive Control.Monad.Primitive Control/Monad/Primitive.hs:152:3-16 3.8 0.0 basicUnsafeRead Data.Vector.Unboxed.Base Data/Vector/Unboxed/Base.hs:278:813-868 3.3 0.0 basicUnsafeWrite Data.Vector.Unboxed.Base Data/Vector/Unboxed/Base.hs:278:872-933 1.5 0.0 triangularSolve.\.\ Numeric.LinearAlgebra.LUSolve src/Numeric/LinearAlgebra/LUSolve.hs:(376,33)-(386,58) 1.3 0.1 <snip> A large amount of time is spent on the invocations of unsafeRead and unsafeWrite. This is a bit suspicious -- it looks as if these call may not be inlined. In the Data.Vector.Unboxed.Mutable library, which provides the underlying linear vector of storage locations, the unsafeRead and unsafeWrite functions are declared INLINE. Could this be a failure of the 'matrices' library to mark its unsafeRead/Write functions as INLINE or SPECIALIZABLE as well? On the other hand, looking at the core (.dump-simpl) of the library doesn't show any dictionary passing, and the access to matrix seem to be through GHC.Prim.writeDoubleArray# and GHC.Prim.readDoubleArray#. If this program took three to five times longer, I would not be concerned, but a factor of seventy five indicates that I've missed something important. Can anyone tell me what it is? Best Wishes, Greg

4 6

Re: [Haskell-cafe] A Performance Puzzle
by dominic＠steinitz.org 04 Aug '18

04 Aug '18

> Something Haskell has lacked for a long time is a good medium-duty > linear system solver based on the LU decomposition. There are bindings > to the usual C/Fortran libraries, but not one in pure Haskell. (An > example "LU factorization" routine that does not do partial pivoting has > been around for years, but lacking pivoting it can fail unexpectedly on > well-conditioned inputs. Another Haskell LU decomposition using partial > pivoting is around, but it uses an inefficient representation of the > pivot matrix, so it's not suited to solving systems of more than 100 x > 100, say.) > > By medium duty I mean a linear system solver that can handle systems of > (1000s) x (1000s) and uses Crout's efficient in-place algorithm. In > short, a program does everything short of exploiting SIMD vector > instructions for solving small subproblems. > > Instead of complaining about this, I have written a little library that > implements this. It contains an LU factorization function and an LU > system solver. The LU factorization also returns the parity of the > pivots ( = (-1)^(number of row swaps) ) so it can be used to calculate > determinants. I used Gustavson's recursive (imperative) version of > Crout's method. The implementation is quite simple and deserves to be > better known by people using functional languages to do numeric work. > The library can be downloaded from GitHub: > https://github.com/gwright83/luSolve <https://github.com/gwright83/luSolve> Great news :) Dominic Steinitz dominic(a)steinitz.org http://idontgetoutmuch.wordpress.com Twitter: @idontgetoutmuch

1 1

Need some help figuring out how to use the repl with reflex using only stack
by Michael Litchard 03 Aug '18

03 Aug '18

I've created a minimal example to show you what I've tried. Below is also a truncated error log. Could someone help me figure out how to get ghci working with this example? https://github.com/mlitchard/reflex-todo stack repl --stack-yaml=stack-ghc.yaml Truncated error log Error: -- While building custom Setup.hs for package jsaddle-dom-0.9.2.0 using: /home/mlitchard/.stack/setup-exe-cache/x86_64-linux/Cabal-simple_mPHDZzAJ_1.24.2.0_ghc-8.0.2 --builddir=.stack-work/dist/x86_64-linux/Cabal-1.24.2.0 build lib:jsaddle-dom --ghc-options " -ddump-hi -ddump-to-file" Process exited with code: ExitFailure 1 Logs have been written to: /home/mlitchard/projects/git/reflex-todo/.stack-work/logs/jsaddle-dom-0.9.2.0.log Preprocessing library jsaddle-dom-0.9.2.0... [587 of 599] Compiling JSDOM.Custom.XMLHttpRequest ( src/JSDOM/Custom/XMLHttpRequest.hs, .stack-work/dist/x86_64-linux/Cabal-1.24.2.0/build/JSDOM/Custom/XMLHttpRequest.o ) /home/mlitchard/projects/git/reflex-todo/.stack-work/downloaded/Vheiln5kqwE0/src/JSDOM/Custom/XMLHttpRequest.hs:39:46: error: • Could not deduce (Control.Monad.Catch.MonadThrow Language.Javascript.JSaddle.Types.JSM) arising from a use of ‘throwM’ from the context: MonadDOM m bound by the type signature for: throwXHRError :: MonadDOM m => Maybe XHRError -> m () at src/JSDOM/Custom/XMLHttpRequest.hs:38:1-53 • In the second argument of ‘(.)’, namely ‘throwM’ In the second argument of ‘maybe’, namely ‘(liftDOM . throwM)’ In the expression: maybe (return ()) (liftDOM . throwM) /home/mlitchard/projects/git/reflex-todo/.stack-work/downloaded/Vheiln5kqwE0/src/JSDOM/Custom/XMLHttpRequest.hs:42:20: error: • No instance for (Control.Monad.Catch.MonadMask Language.Javascript.JSaddle.Types.JSM) arising from a use of ‘bracket’ • In the first argument of ‘(.)’, namely ‘bracket aquire id’ In the expression: bracket aquire id . const In an equation for ‘withEvent’: withEvent aquire = bracket aquire id . const /home/mlitchard/projects/git/reflex-todo/.stack-work/downloaded/Vheiln5kqwE0/src/JSDOM/Custom/XMLHttpRequest.hs:45:31: error: • Could not deduce (Control.Monad.Catch.MonadCatch Language.Javascript.JSaddle.Types.JSM) arising from an operator section from the context: MonadDOM m bound by the type signature for: send' :: MonadDOM m => XMLHttpRequest -> Maybe JSVal -> m () at src/JSDOM/Custom/XMLHttpRequest.hs:44:1-62 • In the expression: (`onException` abort self) In the second argument of ‘($)’, namely ‘(`onException` abort self) $ do { result <- liftIO newEmptyMVar; r <- withEvent (onAsync self Generated.error . liftIO $ putMVar result (Just XHRError)) $ withEvent (onAsync self abortEvent . liftIO $ putMVar result (Just XHRAborted)) $ withEvent (onAsync self load . liftIO $ putMVar result Nothing) $ do { void $ case mbVal of { Nothing -> ... Just val -> ... }; liftIO $ takeMVar result }; throwXHRError r }’ In the expression: liftDOM $ (`onException` abort self) $ do { result <- liftIO newEmptyMVar; r <- withEvent (onAsync self Generated.error . liftIO $ putMVar result (Just XHRError)) $ withEvent (onAsync self abortEvent . liftIO $ putMVar result (Just XHRAborted)) $ withEvent (onAsync self load . liftIO $ putMVar result Nothing) $ do { void $ case mbVal of { Nothing -> ... Just val -> ... }; liftIO $ takeMVar result }; throwXHRError r }

4 4

How to turn Integer into Proxy (n :: KnownNat) with n equal to the integer?
by David Banas 03 Aug '18

03 Aug '18

I’d like to use Data.Vector.Sized.slice’, but it wants the starting position and length parameters as Proxys, not Integers. How can I convert an Integer into a Proxy (n :: KnownNat) with n equal to the integer?

2 1

[random-fu] sampleRVar with StdGen in RWS?
by Sergiu Ivanov 03 Aug '18

03 Aug '18

Dear Café, I would like to use random-fu to do some pseudo-random simulations for a given StdGen (so that I can run the same simulation multiple times, if needed). The following works: testState :: StdGen -> Int testState = evalState (sampleRVar $ uniform 1 10) The following doesn't: testRWS :: StdGen -> Int testRWS = fst . evalRWS (sampleRVar $ uniform 1 10) () I get <interactive>:2:26: error: • No instance for (MonadRandom (RWST () b0 StdGen Identity)) arising from a use of ‘sampleRVar’ • In the first argument of ‘evalRWS’, namely ‘(sampleRVar $ uniform 1 10)’ In the second argument of ‘(.)’, namely ‘evalRWS (sampleRVar $ uniform 1 10) ()’ In the expression: fst . evalRWS (sampleRVar $ uniform 1 10) () Indeed, I do see a MonadRandom instance for StateT, but none for RWST [0]. Is there a reason to not have a MonadRandom instance for RWST? Am I looking in the wrong place? - Sergiu [0] https://hackage.haskell.org/package/random-source-0.3.0.6/docs/Data-Random-…

2 4

hp2pretty-0.9
by Claude Heiland-Allen 02 Aug '18

02 Aug '18

Hi all, hp2pretty[0] graphs heap profiles as SVG, much like hp2ps graphs as PostScript. I release hp2pretty-0.9[1], with support for detached keys in a separate text file with individual label images, which lets you construct a single key for multiple graphs, and also workaround the issue of truncated names (running off the edge of the image) when using the +RTS -L option. I applied the same principle to the titles, they can now be omitted from the images and added as captions in a surrounding document if desired. The examples/ folder shows how to embed the output in PDF[2] using the LaTeX 'svg' package (which calls out to 'inkscape'). There's also an HTML5[3] example. Because one can now create PDF output from SVG, the vapourware plans for other output formats have been dropped. Get it from Hackage[4], use `cabal unpack hp2pretty` to access examples. Or browse the git repository[5]. Thanks, Claude [0] https://mathr.co.uk/blog/hp2pretty.html [1] https://mathr.co.uk/blog/2018-08-01_hp2pretty-0.9_released.html [2] https://mathr.co.uk/hp2pretty/0.9/report.pdf [3] https://mathr.co.uk/hp2pretty/0.9/report.html [4] https://hackage.haskell.org/package/hp2pretty [5] https://code.mathr.co.uk/hp2pretty -- https://mathr.co.uk

1 0

why different type variables enforce different types?
by Rui Azevedo 02 Aug '18

02 Aug '18

after using haskell for a while i was surprised by this little thing f::a->b f a=a and this is not making sense to me till now, so please can someone enlighten me, why does haskell assume that b can not be the same type of a? after all having two distinct math variables does not mean they can not have the same value. I know i could use f::a->a, but its not the same thing, on this last case I'm requesting them to be the same type, but on the first I'm not forbidding it, is there a math reason behind it? would type inference be rendered impossible if considering the case of them being equal? thanks Rui Azevedo (neu-rah)

3 2

JOB | Permanent Java Developer (Montreal, Canada)
by James Tobin 01 Aug '18

01 Aug '18

Hello, I'm working with an employer that is looking to hire (for their Montreal office) a permanent Java developer that has extensive hands-on Java coding experience. Consequently I had hoped that some members of this mailing list may like to discuss with me further; off-list. I can be reached using "JamesBTobin (at) Gmail (dot) Com". Kind regards, James

4 3

JOB | Permanent Java Development Manager (Montreal, Canada)
by James Tobin 01 Aug '18

01 Aug '18

Hello, I'm working with an employer that is looking to hire (for their Montreal office) a permanent development manager that has extensive hands-on Java coding experience. Consequently I had hoped that some members of this mailing list may like to discuss with me further; off-list. I can be reached using "JamesBTobin (at) Gmail (dot) Com". Kind regards, James

1 0

Missing common function for Maybe types
by Marc Busqué 31 Jul '18

31 Jul '18

Hi! I have two functions: ``` foo :: a -> Maybe b bar :: a -> Maybe c ``` >From which I want to build a higher order function: ``` foobar :: a -> (a -> Maybe b) -> (a -> Maybe c) -> Either b c ``` The implementation I need is: ``` foobar x f g = case (f x) of Nothing -> g x Just y -> Just y ``` I'm a bit surprised that looking at hoogle I don't find a built-in solution for this quite common need for `Maybe` types (or perhaps for any monad). Am I looking in the wrong way? Does it exist a similar abstraction but with a different shape? Thanks in advance, Marc Busqué http://waiting-for-dev.github.io/about/

7 16

Testing a collision detection system
by mpevnev＠gmail.com 30 Jul '18

30 Jul '18

As a part of a project, I write a collision detection system. It is in dire need of testing, but designing and writing tests for all possible pairs of types of colliding geometry would be a pretty big effort - not only I would have to calculate the fact of collision manually for 20-something pairs of types of colliding geometry, I would also have to do so multiple times for each pair, since each pair requires several test cases. So the idea is to use an existing collision detection library to generate (a lot of) test cases from random data. I've found two such libraries for Haskell - HODE and Bullet. The problem is, Bullet bindings aren't documented at all, and HODE (which isn't really documented either, but at least lists available functions) is extremely ugly with IO all over the place, and manual tracking of objects' lifetimes (at least that's what I infer from `create :: World -> IO Body` and `destroyBody :: Body -> IO ()`, because again - no documentation). So my question is: does anyone know a library I could use? I'll pretty much settle for whatever. -- Michail.

5 4

using Haskell more like scripting language
by Dennis Raddle 29 Jul '18

29 Jul '18

I'm actually working in Purescript, but the issue is pretty much the same as one would encounter in Haskell. My application (animation of math-related ideas) is changing constantly. It's research, and I don't know where I'll be with it next week, or even tomorrow. For instance, the kinds of "objects" I'm animating (numbers, equations) and the way they move and interact with each other is a research project, and I'm constantly ripping out parts of my application and scavenging them for variations on a theme. My objects need state. At this point I'm thinking of just putting the state into a map with strings as keys and some kind of sum type to represent each data item I might need. So if I want to represent that an object has a particular color and position and velocity, itemState = M.fromList [("color" , DataColor blue ) ,("position", DataPoint 500.0 100.0) ,("velocity", DataVector 50.0 45.0 ) ] Which starts to look more like a dynamically-typed scripting language. But it's not hard to work with. I can uses lenses/prisms to inspect and update state, and it's okay if my program crashes on encountering malformed state. What this lets me do is create new objects that mix and match parts of old objects, and copy and paste a lot of the code. Is there a better way to do this? Is there a compromise I can make that will allow the compiler to help me out more with finding errors at compile time? D

4 4

stack test hangs when using 1.7.1 but completes with 1.4.0
by Olaf Klinke 29 Jul '18

29 Jul '18

Dear cafe, I recently upgraded (with some trouble) to stack-1.7.1 from 1.4.0. I have an older project using lts-9.2 that builds just fine when using the new stack, but it hangs on `stack test`. The difference is shown below. I suspect it is something that needs to be upgraded alongside stack. Maybe Cabal? Upgrading stack was troublesome because of different index formats, which is why using the other stack causes the rebuilds of my extra-deps, I suppose. When using stack --verbosity debug I see messages of the form Error while decoding $HOME/.stack/indices/Hackage/01-index.cache ... (this might not be an error, when switching between stack versions) Any help welcome. Olaf $> /usr/bin/stack-1.7.1 test Populated index cache. $MYPACKAGE: unregistering (missing dependencies: glpk-hs) gasp-1.0.1.0: unregistering (old configure information not found) glpk-hs-0.5: unregistering (missing dependencies: gasp) gasp-1.0.1.0: configure gasp-1.0.1.0: build gasp-1.0.1.0: copy/register glpk-hs-0.5: configure glpk-hs-0.5: build glpk-hs-0.5: copy/register $MYPACKAGE: configure (lib + exe + test) Configuring $MYPACKAGE... $MYPACKAGE: build (lib + exe + test) Preprocessing library $MYPACKAGE... <<< some more output ... >>> Registering $MYPACKAGE... $MYPACKAGE: test (suite: $PACKAGENAME-test) <<< with --verbosity debug >>> [debug] Run process within $PACKAGEDIR/: $PACKAGEDIR/.stack-work/dist/x86_64-linux/Cabal-1.24.2.0/build/$PACKAGENAME-testStub/$PACKAGENAME-testStub @(src/Stack/Build/Execute.hs:1711:27) Progress 1/2: $MYPACKAGE <<< Hangs from here on, does not consume CPU cycles >>> $> /usr/bin/stack-1.4.0 test Populated index cache. $MYPACKAGE: unregistering (missing dependencies: glpk-hs) gasp-1.0.1.0: unregistering (old configure information not found) glpk-hs-0.5: unregistering (missing dependencies: gasp) gasp-1.0.1.0: configure gasp-1.0.1.0: build gasp-1.0.1.0: copy/register glpk-hs-0.5: configure glpk-hs-0.5: build glpk-hs-0.5: copy/register $MYPACKAGE: configure (lib + exe + test) Configuring $MYPACKAGE... $MYPACKAGE: build (lib + exe + test) Preprocessing library $MYPACKAGE... <<< some more output >>> Progress: 3/4+++ OK, passed 100 tests. <<< some more output >>> Completed 4 action(s).

2 1

Monad Reader pubs?
by Donald Sitompul 28 Jul '18

28 Jul '18

I’m looking for past publications of the Monad Reader, where are they now? Donald Sitompul

2 1

Re: [Haskell-cafe] Why does this result in a runtime loop instead of a type error or typechecker loop?
by Anthony Clayden 26 Jul '18

26 Jul '18

On Thurs, Jul 26, 2018 at 05:39:00, Emil Axelsson wrote: >> Den 2018-07-26 kl. 01:02, skrev Anthony Clayden: >> >>> * On Tue, Jul 24, 2018 at 5:52 AM,Ryan Reich wrote: *>*>> This seems like the canonical illustration of the dangers of * >>>* UndecidableInstances.* >> >>* No I don't buy that explanation. The worst UndecidableInstances can do * >>* is send the type checker into a loop.* > I used to have a counter-example to that claim: https://mail.haskell.org/pipermail/haskell-cafe/2010-January/072357.html > That example was causing looping dictionaries at run time on GHC 6.10. > But now, on GHC 8.2, the program terminates. 6.10 was rather a long time ago. The whole type inference/improvement engine has had several overhauls since 2010. To be clear: you can write looping programs without type classes at all -- such as mutually recursive functions. You can write looping programs with type classes -- by turning those functions into methods in classes. You can write looping programs with type classes that need `UndecidableInstances` -- by making those classes mutual superclasses or mutual constraints on instances. That doesn't mean the `UndecidableInstances` are to 'blame' for the looping. I would blame the functions/method calls for not reducing the problem. > But one wonders if there are other ways to create looping dictionaries? With `UndecidableInstances`, yes there's plenty. And I expect that'll make the typechecker loop at compile time. The O.P. here is not getting typechecker looping, so I think the explanation is to do with method calls -- for which we need to see what's going on in the imports. AntC

1 0

Integrate GHC-API with Cabal
by Félix Baylac 26 Jul '18

26 Jul '18

Dear cafe, I am currently working on a project [1] which aims to index the code published on Stackage in order to provide a "code example" database. Basically, for each library, I want to generate a real-world example corpus for each exported symbol. Before generating this database, I need to retrieve the exported symbol of a package. So far, I have been gathering the dependencies and integrating them in the GHC pkg database using an external stack call, I then parse the cabal file and load the exposed modules using the GHC API and gather the exported symbols*. It works pretty reliably on the simple packages, however, I end up with some missing dynamic flags for some more advanced packages (missing c includes, c libraries, ASM flags, etc.). I then started to parse and load these missing attributes to GHC until I stepped back for a moment and realized I was re-implementing cabal build. I then looked for a way to "hook" my GHC API code in cabal build. This would allow cabal to both handle the dependencies gathering as well as setting up the correct GHC dyn-flags. The only resource I found was [2]. It's really clever, however, looks a bit hacky to me. Is there a better way to perform this kind of "hook"? [1] https://git.alternativebit.fr/NinjaTrappeur/Exhs [2] http://blog.ezyang.com/2017/02/how-to-integrate-ghc-api-programs-with-cabal/ * I am aware of the hoogle index generated by haddock. However, this index is missing when the haddock documentation of a package cannot be generated.

4 4

Re: [Haskell-cafe] Why does this result in a runtime loop instead of a type error or typechecker loop?
by Anthony Clayden 26 Jul '18

26 Jul '18

On Tue, Jul 24, 2018 at 5:52 AM,Ryan Reich wrote: > This seems like the canonical illustration of the dangers of UndecidableInstances. No I don't buy that explanation. The worst UndecidableInstances can do is send the type checker into a loop. For mutually recursive functions to cause looping, you don't need classes at all: foo :: Bool -> () foo x = bar x bar :: () -> Bool bar x = foo x No typechecking will detect that loop. This `HasBool` example is just an obfuscated way to write that loop(?) There's some odd thigs in the OP: Why use OVERLAPPABLE? Neither of those instances need it. Or are there more instances in the imports? (There must at least be the definition for class `Has`.) If so, what are their constraints? And what are their implementations? Perhaps that's causing the looping? Will `runRIO` or `view` use one of those other instances? What does this mean: "I can compile an equivalent program; it loops on execution."? There doesn't need to be a `Bool` in the "environment": the `HasBool a` instance introduces `Bool` in its constraint. I suggest boiling this down to a stand-alone program not needing imports. Then tell the whole story. AntC On Mon, Jul 23, 2018 at 2:10 PM, Theodore Lief Gannon < tanuki at gmail.com <http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe>> wrote: >* λ. :set -XFlexibleContexts -XFlexibleInstances -XMultiParamTypeClasses *>* -XUndecidableInstances *>* λ. import RIO *>* λ. import Data.Has *>* λ. class HasBool a where boolL :: Lens' a Bool *>* λ. instance {-#OVERLAPPABLE#-} HasBool a => Has Bool a where hasLens = *>* boolL *>* λ. instance {-#OVERLAPPABLE#-} Has Bool a => HasBool a where boolL = *>* hasLens *>* λ. runRIO () $ view boolL *>* ^CInterrupted. *>>* The RIO environment () doesn't contain a Bool. I can see how the *>* typechecker might get lost in this, but I can compile an equivalent *>* program; it loops on execution. *>>* _______________________________________________ *>* Haskell-Cafe mailing list *>* To (un)subscribe, modify options or view archives go to: *>* http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe <http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe> *>* Only members subscribed via the mailman list are allowed to post. *>

2 1

Re: [Haskell-cafe] Integrate GHC-API with Cabal
by Daniel Gröber 25 Jul '18

25 Jul '18

Hi, On Wed, Jul 25, 2018 at 08:30:48PM +0200, Félix Baylac wrote: > Thanks a lot for this really helpful message. > > > I think you might be interrested in one of my packages, > > [cabal-helper](https://hackage.haskell.org/package/cabal-helper). It's > > geard more towards editor tooling but I think it should work for your > > use case too. > > Indeed, looks like what I'm looking for. > > > Basically you'd have to do `cabal install --only-dependencies && cabal > > configure && cabal build` for each package you want to inspect and > > then cabal-helpers API boils down to giving you a list of GHC > > flags/module names you need to pass into the GHC API to make it work. > > > > (The `cabal build` step is needed to run preprocessors like hsc2hs and > > compile C dependencies, but once cabal has taken care of that you can > > usually just load the package without problems.) > > I had to make some modifications and jump in some nix hoops to use > cabal-install instead of stack to build the packages, but indeed, it does work > like a charm. Ah, I forgot you said you were using Stack. You can use it too if you prefer. You just have to use `stack path --dist-dir` (run in the project directory) to get the path to the dist-dir after running `stack build` instead of using the hardcoded `$PWD/dist` value. There have been some reports of issues with stack using weird combinations of Cabal-library versions though which so it might need some tweaking. > This is perfect, I just tried your library following this example on the text > package and it seems to do the job. Thanks a lot, fixing this problem without > re-inventing the wheel is a huge relief. > > Do you mind if I write a small article on this integration. No not at all, send me a link when you're done. > It'll probably prevent some other people from using dodgy hacks. Well I'm not sure I would call cabal-helper not a dodgy hack but at least it's a (sort of) well maintained hack :) --Daniel PS: Looks like I messed up the list-reply on my original message so I resent it and I'm CC'ing haskell-cafe on this one. Sorry about that.

1 0

Haskell Opportunity - New York - Blockchain
by Alex Mesropians 25 Jul '18

25 Jul '18

Hey All, All Haskell engineers in the US we have a new and exciting role with a blockchain firm who are rapidly expanding! Check them out - Haskell Role <http://t.sidekickopen13.com/e1t/c/5/f18dQhb0S7lC8dDMPbW2n0x6l2B9nMJW7t5XZs5…> Thanks, Alex

1 0

how to terminate a (Posix) process and all its subprocesses?
by Johannes Waldmann 24 Jul '18

24 Jul '18

Dear Cafe, I am using https://hackage.haskell.org/package/process-1.6.4.0/docs/System-Process.htm… to start an external command, and wait for its completion. Now I would like to time-out this after a while. I can use https://hackage.haskell.org/package/base-4.11.1.0/docs/System-Timeout.html and this works in simple cases - but not in case the external command has spawned child processes. (cleanupProcess sends SIGTERM but only to the process at the top of the tree ?) I guess I need to use https://hackage.haskell.org/package/unix-2.7.2.2/docs/System-Posix-Process.… but that seems rather low-level - and I don't see how I would get the ProcessID of the process started by readProcess, so I'd also have to re-do that. Is there an abstraction/library that would help here? Thanks - J.W.

6 7

Free theorem for `forall z. (A,z) -> (B,z)`?
by Conal Elliott 24 Jul '18

24 Jul '18

Suppose `g :: forall z. (A,z) -> (B,z)`. Is it necessarily the case that `g = first f` for some `f :: A -> B` (where `first f (a,b) = (f a, b)`), perhaps as a free theorem for the type of `g`? Note that `(A,)` and `(B,)` are functors and that `g` is a natural transformation, so `g . fmap h == fmap h . g` for any `h :: u -> v`. Equivalently, `g . second h == second h . g` (where `second h (a,b) = (a, h b)`). Thanks, -- Conal

4 8

Design of an extensible system
by Marc Busqué 24 Jul '18

24 Jul '18

Hi there, I'm currently programming my first non toy project in Haskell. I come from Ruby background, where everything is possible no matter how much entropy the universe gains after you finish your work, so I'm still not sure how to do certain things without this side-effect :) I'll try to be concise. My idea is that the project would consist of several different Haskell packages. The central one, call it Core, would define a certain data type (with info about certain URLs). Other packages would define each one an strategy to do something with that data type (processing the URL info). Each strategy would have its own dependencies, and as a user probably won't need all of the strategies, I would like to keep them in separated packages so she or he has no need to download the unneeded stuff. The problem is that Core should act as the central dashboard, whether through a CLI or even a web frontend. For example, in the CLI case, I would like to be able to install Core package along with package StrategyA and package StrategyB and then do something like the following: ``` bash> core --action=fetch --strategy=A bash> core --action=fetch --strategy=B ``` Two generic ideas come to my mind but I don't know how to translate them to Haskell: - Infer module name from the `--strategy` argument and call it dynamically. However, as far as I know, in Haskell modules are not first class citizens, so no way I can dynamically call them. I have read something to achieve this through Template Haskell or through outdaded [plugins](http://hackage.haskell.org/package/plugins) package, but I'm not sure whether the need is the same and anyway it seems quite unnatural to Haskell (I would not like to go against it, but to learn to do thing in its way). - Define from Core some kind of global list where strategy packages can register a map between CLI argument value and a defined instance of the core data type. But, again, how to do global things in Haskell (and I would prefer not to need external tools like RDBMS, filesystem or whatever)? And, anyway, surely I don't want to do this because concurrency and everything... Any idea? Thanks in advance! Marc Busqué http://waiting-for-dev.github.io/about/

6 9

Expressing disjunctions of constraints
by Olaf Klinke 24 Jul '18

24 Jul '18

Dear cafe, today I wrote a type class with three parameters. class Foo a b c | a b -> c where foo :: a -> b -> c instance Foo A R A where ... instance Foo R A A where ... In addition to the functional dependency I'd like to express that at least one of the parameters a and b is c, that is, Foo is the union of the two classes class Foo1 a b where foo1 :: a -> b -> a class Foo2 a b where foo2 :: a -> b -> b and furthermore the choice between Foo1 and Foo2 determines one parameter to be a fixed type, say R. I understand that logical disjunction poses problems for instance resolution [1] because adding an instance may affect the instance dictionary lookup. This should not happen in my case because in the end this would be a one-parameter type class. The ultimate aim is to use the same symbol for both a function foo :: R -> a -> a and for flip foo. Of course writing flip foo is not that much of a nuisance, I just wonder whether Haskell is expressive enough to do this. Olaf [1] https://stackoverflow.com/questions/10255148/how-can-i-combine-two-type-con…

3 2