- Haskell-Cafe - Haskell.org

Getting both the value and name of a variable in one expression
by Christopher Done 23 Aug '19

23 Aug '19

Hi all, Do we have already a syntax for ‘foo that also contains the typed value like TExp? I have e.g. an AST that I want to do more static checks on it that aren’t as convenient to do in the type system. Here’s an example: -- | Check the grammar spec to produce a grammar.checkGrammar :: (SchemaName, [(SchemaName, Schema)]) -> Q ExpcheckGrammar (toplevel, rules) = if M.size rulesMap /= length rules then error "Duplicate rule names in grammar." else lift (Grammar {grammarToplevel = toplevel, grammarRules = rulesMap}) where rulesMap = M.fromList rules -- | Grammar for Haskell.grammar :: Grammargrammar = $(checkGrammar $ runDefine $ mdo -- General expression expression <- rule "Expression" (ChoiceSchema [variable, constructor, parentheses ,tuple, let', application, string]) application <- rule "Application" (CompositeSchema [expression, expression]) parentheses <- rule "Parentheses" (CompositeSchema [openParenSchema, expression, closeParenSchema]) ... pure expression) Here I do a trivial check for duplicates. After I’ve checked the expression at compile-time, I Lift it so that it can be used at runtime. That’s pretty good. But some types like (a -> b) don’t Lift. So an alternative would be: grammar = $(checkGrammar_take2 thename 'thename) In which checkGrammar_take2 would: 1. Use thename at compile-time for a check. 2. If the check passes, then return (VarE thename) E.g. checkGrammar_take2 value name = if valueFine value then varE name elseerror "value isn't fine" That’s actually quite a good solution because it avoids a lift, and I didn’t transform the AST. It’s also more efficient than lifting. But there’s no checked relationship between thename and ‘thename. checkGrammar_take2 has no way of knowing that they refer to the same thing. See? Hence, if I could get e.g. `thename to produce both the value and a name for the value, that would be cool. My gist doesn’t go this far. It might look like this: checkGrammar_take2 namedValue = if valueFine (getValue namedValue) then getExp namedValue else error "value isn't fine" and call it like: mygrammar = checkGrammar_take2 `thename So the semantics would be roughly similar to [|| thename ||] :: TExp a but you’d get `thename :: Named a where data Named a = { namedThing :: a, nameOfThing :: Name } I feel like the more DSLs I design, the more I’d like something like this to perform my static checks. Cheers, Chris

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Question on inductive type-families
by Sylvain Henry 22 Aug '19

22 Aug '19

Hi, Suppose I have the following type family: type family Index (n :: Nat) (l :: [k]) = (r :: k) where Index 0 (x ': _ ) = x Index n (_ ': xs) = Index (n-1) xs I'm using it to define a Variant type: data V2 (vs :: [k]) where V2 :: forall vs (n :: Nat). KnownNat n => Proxy n -> Index n vs -> V2 vs Now I want to match on the Variant retrieving either the head or the tail (another Variant): splitV2 :: forall v vs. V2 (v:vs) -> Either v (V2 vs) splitV2 = \case V2 (n1 :: Proxy n) v -> case sameNat n1 (Proxy @0) of Just Refl -> Left v Nothing -> Right (V2 (Proxy @(n-1)) v) It fails to typecheck with: Couldn't match type ‘Index n (v : vs)’ with ‘Index (n - 1) vs’ Expected type: Index (n - 1) vs Actual type: Index n vs1 NB: ‘Index’ is a non-injective type family Is there a way to make this work? This is highly speculative as I'm not very familiar with type-checking: would it be sound to make the inductive Index family "weakly" injective? I.e. somehow produce a typing rule for the type family: forall n n' x xs xs'. Solving: Index n (x ': xs) ~ Index (n'-1) xs' Is equivalent to solving: if n /= 0 then (n ~ n', xs ~ xs') else (x ~ Index (n'-1) xs') Here we could know that "n /= 0" thanks to the match on "sameNat n1 (Proxy @0)". ("weakly" because we have some form of injectivity on the shape of the list but not on its contents) Any pointer on this kind of stuff in research papers, Coq, Agda, etc. ? Thanks, Sylvain

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Optimizations for list comprehension
by Emilio Francesquini 21 Aug '19

21 Aug '19

Hello Cafe, While investigating a performance problem I stumbled upon what I eventually reduced to the example below: module Main where import Data.Time.Clock outside :: Int -> Int outside n = sum [i + j | i <- range, j <- range] where range = [0..n-1] inside :: Int -> Int inside n = sum [i + j | i <- [0..n-1], j <- [0..n-1]] main :: IO () main = do t0 <- getCurrentTime print $ inside 10000 t1 <- getCurrentTime print $ outside 10000 t2 <-getCurrentTime print (diffUTCTime t1 t0) print (diffUTCTime t2 t1) Compiling with -O2, up to GHC 8.2.2, both `inside` and `outside` functions would take the same amount of time to execute. Somewhere between GHC 8.2.2 and 8.6.4 something changed (possibly some new optimization) making `inside` run ~4x faster on my machine. With LLVM the difference is even bigger. It is not that `outside` got slower, but that `inside` got much faster. I'm curious to what optimizations might be happening to the `inside` function that would not fire on the outside function. Any hints? Best regards, Emilio

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Permenent Faculty Position in PL at University of Bristol
by Meng Wang 20 Aug '19

20 Aug '19

Dear Haskellers, As part of a strategic expansion, the Department of Computer Science at the University of Bristol is hiring in the area of programming languages at the Lecturer (Assistant Professor) or Senior Lecturer/Reader (Associate Professor) level: http://www.bristol.ac.uk/jobs/find/details.html?nPostingID=51136&nPostingTa… Applications in the area of functional programming are especially encouraged. The deadline for application is 8 September 2019. Best wishes, Meng Wang, PhD Senior Lecturer (Associate Professor) Department of Computer Science University of Bristol Merchant Venturers Building, Woodland Road, Clifton BS8 1UB +44 (0) 117 954 5145 meng.wang(a)bristol.ac.uk

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Re: [Haskell-cafe] Unfolding alongside folding
by David Banas 19 Aug '19

19 Aug '19

I’m not certain, but think you might be describing hylomorphism: https://medium.com/@JosephJnk/hylomorphisms-43a5494729b7 <https://medium.com/@JosephJnk/hylomorphisms-43a5494729b7> -db > On Aug 19, 2019, at 5:00 AM, haskell-cafe-request(a)haskell.org wrote: > > Does a function of this general nature exist in some package already?

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Unfolding alongside folding
by David Feuer 19 Aug '19

19 Aug '19

Thinking about fusing indexed maps for lists led me to consider the following function, which generalizes mapWithIndex to allow arbitrary stateful computation foldWithUnfold :: (a -> s -> Maybe (b, s)) -> [a] -> s -> [b] foldWithUnfold f = go where go (a : as) s | Just (b, s') <- f a s = b : go as s' go _ _ = [] which we can fit into fold/build fusion like so: foldWithUnfold f as s0 = build $ \c n -> let go a k s | Just (b, s') <- f a s = b `c` k s' | otherwise = n in foldr go (const n) as s0 Does a function of this general nature exist in some package already? Also, I see that the type can be expressed foldWithUnfoldS :: (a -> StateT s Maybe b) -> [a] -> s -> [b] Is that a better or worse way to say it?

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MPC 2019 - Call for Participation
by Graham Hutton 19 Aug '19

19 Aug '19

Dear all, Registration is now open for the Mathematics of Program Construction (MPC) conference in Portugal. MPC 2019 will feature 15 research papers and 4 keynotes, and is co-located with Formal Methods 2019 and many other events. See you in Porto! https://tinyurl.com/MPC-Porto Best wishes, Graham Hutton Program Chair, MPC 2019 ====================================================================== *** CALL FOR PARTICIPATION -- MPC 2019 *** 13th International Conference on Mathematics of Program Construction 7-9 October 2019, Porto, Portugal Co-located with Formal Methods 2019 https://tinyurl.com/MPC-Porto ====================================================================== PROGRAM: https://tinyurl.com/yxvvc5vb ACCEPTED PAPERS: https://tinyurl.com/yyuhy8ze REGISTRATION AND TRAVEL: https://tinyurl.com/y4uetlsr KEYNOTE SPEAKERS: Assia Mahboubi (MPC) INRIA, France Annabelle McIver (MPC) Macquarie University, Australia Tony Hoare (UTP) Oxford University, UK Shriram Krishnamurthi (FM) Brown University, USA BACKGROUND: The International Conference on Mathematics of Program Construction (MPC) aims to promote the development of mathematical principles and techniques that are demonstrably practical and effective in the process of constructing computer programs. MPC 2019 will be held in Porto, Portugal from 7-9 October 2019, and is co-located with the International Symposium on Formal Methods, FM 2019. Previous conferences were held in Königswinter, Germany (2015); Madrid, Spain (2012); Québec City, Canada (2010); Marseille, France (2008); Kuressaare, Estonia (2006); Stirling, UK (2004); Dagstuhl, Germany (2002); Ponte de Lima, Portugal (2000); Marstrand, Sweden (1998); Kloster Irsee, Germany (1995); Oxford, UK (1992); Twente, The Netherlands (1989). PROGRAM COMMITTEE: Patrick Bahr IT University of Copenhagen, Denmark Richard Bird University of Oxford, UK Corina Cîrstea University of Southampton, UK Brijesh Dongol University of Surrey, UK João F. Ferreira University of Lisbon, Portugal Jennifer Hackett University of Nottingham, UK William Harrison University of Missouri, USA Ralf Hinze University of Kaiserslautern, Germany Zhenjiang Hu National Institute of Informatics, Japan Graham Hutton (chair) University of Nottingham, UK Cezar Ionescu University of Oxford, UK Mauro Jaskelioff National University of Rosario, Argentina Ranjit Jhala University of California, USA Gabriele Keller Utrecht University, The Netherlands Ekaterina Komendantskaya Heriot-Watt University, UK Chris Martens North Carolina State University, USA Bernhard Möller University of Augsburg, Germany Shin-Cheng Mu Academia Sinica, Taiwan Mary Sheeran Chalmers University of Technology, Sweden Alexandra Silva University College London, UK Georg Struth University of Sheffield, UK For any queries about the program please contact the program chair, Graham Hutton <graham.hutton(a)nottingham.ac.uk>. CONFERENCE VENUE: The conference will be held at the Alfândega Porto Congress Centre, a 150 year old former custom's house located in the historic centre of Porto on the bank of the river Douro. The venue was renovated by a Pritzer prize winning architect and has received many awards. LOCAL ORGANISER: José Nuno Oliveira University of Minho, Portugal For any queries about local issues please contact the local organiser, José Nuno Oliveira <jno(a)di.uminho.pt>. ====================================================================== 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.

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[ANN] express-0.1.2: manipulate dynamically typed Haskell expressions
by Rudy Matela 18 Aug '19

18 Aug '19

Hello Haskell-Café, I am happy to announce the [Express] library for Haskell. https://github.com/rudymatela/express Express allows manipulation of dynamically typed Haskell expressions. It is similar to Data.Dynamic but with support for encoding applications and variables. It provides an Expr type and over a hundred functions for building, evaluating, comparing, folding, canonicalizing and matching Exprs. [Express' Haddock documentation] is pretty comprehensive. The project [README] has a few examples showing how to use Express to: * create heterogeneous lists; * list valid applications between expressions; * generalize counterexamples; * automatically generate test properties. This library has its origins as an internal module of [Speculate] and [Extrapolate], I am now releasing it separetely hoping that it could be used for other purposes. To install it, just: $ cabal update $ cabal install express Then you're ready to use it: $ ghci > import Data.Express > let true = val True > :t true true :: Expr > print true True :: Bool > eval False true True -- Rudy [Express]: https://github.com/rudymatela/express [Express' Haddock documentation]: https://hackage.haskell.org/package/express/docs/Data-Express.html [README]: https://github.com/rudymatela/express#readme [Speculate]: https://github.com/rudymatela/speculate [Extrapolate]: https://github.com/rudymatela/extrapolate

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PhD position at Imperial College London
by Wu, Nicolas G 16 Aug '19

16 Aug '19

# PhD Position in Functional Programming: Effect Handlers at Imperial College London Applications are invited for a PhD student in Functional Programming under the supervision of Dr Nicolas Wu at Imperial College London. The project aims to enhance the applications of effects and handlers by developing scoped contextual operations and effects. There are two main interrelated topics: 1. Developing the foundations of operations and effects that are confined within a scope and that are sensitive to context, thus widening the applications of effect handlers. 2. Producing efficient implementations and demonstrating applications that allow programmers to combine and use such effects. Addressing these topics will deliver both theoretical insight to the academic community and practical benefits to software engineers. To apply for this position, you will need to have a strong background in at least one of the following areas: functional programming, programming language design, compilers, algebraic effects. A working understanding of Haskell is essential. Applicants are expected to have a First Class or Distinction Masters level degree, or equivalent, in a relevant scientific or technical discipline, such as computer science or mathematics. Applicants must be fluent in spoken and written English. The position is fully funded, covering tuition fees, travel funds and a stipend/bursary. The position is available to home, EU and overseas students. Studentship: Untaxed bursary of £17,009 per annum (2018/19 figure including London weighting plus home/EU fees) For more details on how to apply, please see the advertisement here: http://www.imperial.ac.uk/computing/prospective-students/courses/phd/schola… listed under "PhD Position in Functional Programming: Effect Handlers". Early applications are strongly encouraged. Please be in touch with informal inquiries about the position with Dr Nicolas Wu (n.wu(a)imperial.ac.uk)

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[TFP'20] first call for papers: Trends in Functional Programming 2020, 13-14 February, Krakow, Poland
by Peter Achten 15 Aug '19

15 Aug '19

------------------------------------------------------------------------- First call for papers 21st Symposium on Trends in Functional Programming tfp2020.org ------------------------------------------------------------------------- The symposium on Trends in Functional Programming (TFP) is an international forum for researchers with interests in all aspects of functional programming, taking a broad view of current and future trends in the area. It aspires to be a lively environment for presenting the latest research results, and other contributions. * TFP is moving to new winter dates, to provide an FP forum in between the annual ICFP events. * TFP offers a supportive reviewing process designed to help less experienced authors succeed, with two rounds of review, both before and after the symposium itself. Authors have an opportunity to address reviewers' concerns before final decisions on publication in the proceedings. * TFP offers two "best paper" awards, the John McCarthy award for best paper, and the David Turner award for best student paper. * This year we are particularly excited to co-locate with Lambda Days in beautiful Krakow. Lambda Days is a vibrant developer conference with hundreds of attendees and a lively programme of talks on functional programming in practice. TFP will be held in the same venue, and participants will be able to session-hop between the two events. Important Dates --------------- Submission deadline for pre-symposium review: 15th November, 2019 Submission deadline for draft papers: 10th January, 2020 Symposium dates: 13-14th February, 2020 Visit tfp2020.org for more information.

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