August 2020 - Haskell-Cafe

Counterintuitive ScopedTypeVariables behaviour
by Tom Ellis 09 Sep '20

09 Sep '20

I have just noticed that with ScopedTypeVariables one can write the following bizarre definition -- Inferred signature: -- add :: Num a => a -> a -> a add (x :: a) (y :: b) = x + y The reason for this behaviour is that > in all patterns other than pattern bindings, a pattern type > signature may mention a type variable that is not in scope; in this > case, the signature brings that type variable into scope. https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/glasgow_ext… and the justification is that > Bringing type variables into scope is particularly important for > existential data constructors But this leads to a rather puzzling user experience. Was it really not possible to design this extension in a way that would allow bringing new type variables into scope locally but not allow them to unify with other type variables from outer scopes? To be more concrete, what I would like to see is a design where `k` is allowed because the type `a` (bound within a pattern) does not need to unify with anything but `k2` is not allowed because `a` is forbidden to unify with `b`. `k3` would also be forbidden (the type variables called `a` are actually distinct) as would `k4` (the type variable for the inferred signature would be fresh and distinct from `a`). I believe this design might lead to much less puzzling behaviour. Would it be possible (within a new extension, of course) or have I overlooked something? Tom data T = forall a. MkT [a] k :: T -> T k (MkT [t::a]) = MkT t3 where (t3::[a]) = [t,t,t] data T2 a = MkT2 [a] k2 :: T2 b -> T2 b k2 (MkT2 [t::a]) = MkT2 t3 where (t3::[a]) = [t,t,t] k3 :: T2 a -> T2 a k3 (MkT2 [t::a]) = MkT2 t3 where (t3::[a]) = [t,t,t] k4 (MkT2 [t::a]) = MkT2 t3 where (t3::[a]) = [t,t,t]

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Constraints as a moveable feast?
by Anthony Clayden 02 Sep '20

02 Sep '20

There's the makings of a cute feature in a very old Haskell Language report. (With thanks to the 'Partial Type Constructors' 2019 paper for ferreting this out.) type Num a => Point a = (a, a) 1990 v1.0 Haskell report. Yes that's a type synonym with a constraint. I'll give their examples, though they don't really demonstrate the potential: origin :: Point Integer -- we get Num a => wanted then discharged for free origin = (0, 0) -- although the appearance of zero wants Num a anyway scale :: (Num a) => a -> Point a -> Point a -- seems wrong to give explicit Num a scale w (x,y) = (w*x, w*y) -- the above sig would be inferred anyway The feature disappeared without trace in the next year's Language report. I'm thinking we should be able to write scale :: a -> Point a -> Point a -- get Num a for free by: scale :: a -> (Num a => (a, a)) -> (Num a => (a, a)) -- expand the synonym -- this sig currently illegal scale :: Num a => a -> (a, a) -> (a, a) -- float the constraints out to the implicit forall So this isn't specific to type synonyms: in general allow prefixing constraints to any sub-term in a type; type inference floats them to the front of the signature. You can kinda get the effect today (so I don't think this is entirely whacky), by cunning use of dummies and term-level combinators: typePoint :: Num a => (a, a) typePoint = undefined scale w xy = const (undefined `asTypeOf` xy `asTypeOf` typePoint) (w `asTypeOf` fst xy) -- inferred scale :: Num a => a -> (a,a) -> (a,a) (Putting a dummy undefined on RHS to show there's nothing up my sleeves, because the actual expression would unify to the right type anyway.) You can get maybe cleaner code with PatternSignatures. But it's all workarounds in terms/expressions. I want to do the Right Thing and get it accepted as a stand-alone signature. A couple of caveats: * Haskell 1990 didn't have MultiParam Type Classes; * nor existential quant. So if a nested constraint mentions both an existential tyvar with narrow scope and a universally quant'd tyvar, can't float out that constraint, have to reject the sig. The constraint in `type Num a => Point a = ...` appearing to the left of the introduced type syn, is also reminiscent of DatatypeContexts, so we could similarly float out the constraints from those appearing within a sig(?) And this would be a way to put constraints on newtypes similarly(?) (Can't put constraints on those currently, because there's not really a data constructor to bind the class dictionary to. Haskell 1990 didn't have newtypes.) Or is there something deeply anti-System FC here? AntC

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contribute to GHC by adding documentation
by Richard Eisenberg 01 Sep '20

01 Sep '20

Hi all, Occasionally I get emails asking how a newcomer can best contribute to GHC. I have some stock answers (find a feature in Haskell and implement it in Template Haskell; find an unimplemented proposal and implement it), but these sometimes don't work out. Wait no longer! If you'd like to contribute to GHC, there is a great chance in https://gitlab.haskell.org/ghc/ghc/-/issues/18633, which links to a file listing scores of undocumented compiler flags. Your task, if you choose to accept it: document these flags! Along the way, you will get to learn about various dark corners of GHC (and who doesn't love to lurk in dark corners?) and prod other GHC folks to produce answers to esoteric questions. Hours of fun to be had here! And along the way, you would do us all a wonderful service. Thanks, Richard

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RebindableSyntax Strangeness
by Henry Laxen 31 Aug '20

31 Aug '20

Hi Cafe, I have encountered the following puzzling behaviour from a very simple program: Consider -- {-# LANGUAGE RebindableSyntax #-} module A where import Data.Time running this with ghci gives: A> :i UTCTime type UTCTime :: * data UTCTime = UTCTime {utctDay :: Day, utctDayTime :: DiffTime} -- Defined in ‘time-1.9.3:Data.Time.Clock.Internal.UTCTime’ instance [safe] Eq UTCTime -- Defined in ‘time-1.9.3:Data.Time.Clock.Internal.UTCTime’ instance [safe] Ord UTCTime -- Defined in ‘time-1.9.3:Data.Time.Clock.Internal.UTCTime’ instance [safe] Show UTCTime -- Defined in ‘time-1.9.3:Data.Time.LocalTime.Internal.ZonedTime’ instance [safe] FormatTime UTCTime -- Defined in ‘time-1.9.3:Data.Time.Format.Format.Instances’ instance [safe] Read UTCTime -- Defined in ‘time-1.9.3:Data.Time.Format.Parse’ instance [safe] ParseTime UTCTime -- Defined in ‘time-1.9.3:Data.Time.Format.Parse.Instances’ Now uncomment the first line (RebindableSyntax) and rerun it: A> :i UTCTime type UTCTime :: * data UTCTime = UTCTime {utctDay :: Day, utctDayTime :: DiffTime} -- Defined in ‘time-1.9.3:Data.Time.Clock.Internal.UTCTime’ instance [safe] FormatTime UTCTime -- Defined in ‘time-1.9.3:Data.Time.Format.Format.Instances’ instance [safe] ParseTime UTCTime -- Defined in ‘time-1.9.3:Data.Time.Format.Parse.Instances’ And all of the nice instances for UTCTime have disappeared. Why? Best wishes, Henry Laxen

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augmenting GHC package database
by Sylvester Hazel 29 Aug '20

29 Aug '20

Hi Haskellers, Cabal couldn't resolve dependencies for a generated package so I used Stack that used 2 different GHC versions to build library from this package. Stack stored the library as libHSclient-something.a , HSclient-something.o and helper ".hi" files The dependent libraries Stack stored and registered at C:\sr..\lib\ How to add this information to GHC or Cabal database to be able to use this new library? This is on Chocolatey managed Windows platform;-)

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writing wizards in Yesod
by Olaf Klinke 27 Aug '20

27 Aug '20

Dear Café, TL;DR: What is the canonical way to deal with state in Yesod? IORef? StateT? Routes? Subsites? The Yesod book [1,2] gives examples where state is encoded in an IORef inside the foundational type, like so: data AppState = ... data App = App {appState :: IORef AppState} All handlers would then first read the AppState via getsYesod appState >>= (liftIO . readIORef) and branch on the result. I wonder if there are other ways to deal with state that changes through user interaction. After all, we have instance MonadHandler m => MonadHandler (StateT s m) in Yesod.Core. What is this useful for? From [1]: "By just providing you with a readable environment, you’re able to recreate a StateT transformer by relying on mutable references." also: "In order to run these [monad transformer actions], we can use the standard [...] unwrap functions [...] to run the action and get back a normal Handler." I interpret quote 1 as "StateT is to be emulated (via IORef), not used literally" and quote 2 as "real StateT can not be used directly, but must be unwrapped to be used in a handler". Context for my question: I found that Kleisli maps model wizards nicely as a sequence of fragments, e.g. Program User f :: () -> m a g :: a -> m b h :: b -> m c i :: c -> m d j :: d -> m () wizard :: m () wizard = (f >=> g >=> h >=> i >=> j) () The intermediate types a, c are questions and b, d are answers, and all types together form the program states. If the user side of the wizard is to be provided by websites, we need to model these states explicitly somehow. My current approach is to use the union type AppState = Start | Answer1 b | Answer2 d that tells the handler what step to serve next. Olaf [1] https://www.yesodweb.com/book/yesods-monads#yesods-monads_adding_a_new_mona… [2] https://www.yesodweb.com/book/visitor-counter

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Munich Haskell Meeting, 2020-08-27 @ 19:30
by Leah Neukirchen 27 Aug '20

27 Aug '20

Dear all, This week, our monthly Munich Haskell Meeting will take place again on Thursday, August 27 at Hirschgarten at 19h30. For details see here: http://muenchen.haskell.bayern/dates.html Please add yourself here if you want to come so we know who to look out for: https://dudle.inf.tu-dresden.de/haskell-munich-aug-2020/ Unfortunately, I almost forgot about the meeting tomorrow, please excuse the short notice. I'm also at Hirschgarten for other reasons, so I may drop by only for a short time. Everybody is welcome! cu, -- Leah Neukirchen <leah(a)vuxu.org> https://leahneukirchen.org/

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Call for participation for IFL 2020: 32nd Symposium on Implementation and Application of Functional Languages
by Simon Thompson 26 Aug '20

26 Aug '20

[apologies for any multiple copies] ======================================================= IFL 2020 32nd Symposium on Implementation and Application of Functional Languages Call for Participation venue: online 2nd - 4th September 2020 https://www.cs.kent.ac.uk/events/2020/ifl20/ ======================================================== ### 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 2020 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. ### Registration The symposium will be run via Zoom (zoom.us<http://zoom.us>). If you can use Zoom, then you can participate. Please register for free via Eventbrite on the symposium webpage: https://www.cs.kent.ac.uk/events/2020/ifl20/ ### Programme Day 1: Wednesday, 2 September 12:45 Welcome 13:00 Nico Naus and Johan Jeuring: End-user feedback in multi-user workflow systems 13:30 Mart Lubbers, Haye Böhm, Pieter Koopman and Rinus Plasmeijer: Asynchronous Shared Data Sources 14:00 Pieter Koopman, Steffen Michels and Rinus Plasmeijer: Dynamic Editors for Well-Typed Expressions 14:30 Bas Lijnse and Rinus Plasmeijer: Asymmetric Composable Web Editors in iTasks 15:00 Social break 15:30 Sven-Olof Nyström: A subtyping system for Erlang 16:00 Andrew Marmaduke, Christopher Jenkins and Aaron Stump: Generic Zero-Cost Constructor Subtyping 16:30 Joris Burgers, Jurriaan Hage and Alejandro Serrano: Heuristics-based Type Error Diagnosis for Haskell - The case of GADTs and local reasoning 17:00 Social break 17:30 Kavon Farvardin and John Reppy: A New Backend for Standard ML of New Jersey 18:00 Chaitanya Koparkar, Mike Rainey, Michael Vollmer, Milind Kulkarni and Ryan R. Newton: A Compiler Approach Reconciling Parallelism and Dense Representations for Irregular Trees 18:30 Hans-Nikolai Vießmann and Sven-Bodo Scholz: Effective Host-GPU Memory Mangement Through Code Generation 20:00 Virtual Pub Day 2: Thursday, 3 September 10:00 Virtual Breakfast 13:00 Michal Gajda: Less Arbitrary waiting time 13:30 Sólrún Halla Einarsdóttir and Nicholas Smallbone: Template-based Theory Exploration: Discovering Properties of Functional Programs by Testing 14:00 Péter Bereczky, Dániel Horpácsi, Judit Kőszegi, Soma Szeier and Simon Thompson: Validating Formal Semantics by Comparative Testing 14:30 Social break 15:00 Gergo Erdi: An Adventure in Symbolic Execution 15:30 Joshua M. Schappel, Sachin Mahashabde and Marco T. Morazan: Using OO Design Patterns in a Functional Programming Setting 16:00 Filipe Varjão: Functional Programming and Interval Arithmetic with High Accuracy 16:30 Social break 17:00 Laith Sakka, Chaitanya Koparkar, Michael Vollmer, Vidush Singhal, Sam Tobin-Hochstadt, Ryan R. Newton and Milind Kulkarni: General Deforestation Using Fusion, Tupling and Intensive Redundancy Analysis 17:30 Benjamin Mourad and Matteo Cimini: A Declarative Gradualizer with Lang-n-Change 18:00 Maheen Riaz Contractor and Matthew Fluet: Type- and Control-Flow Directed Defunctionalization 19:30 Virtual Pub Day 3: Friday, 4 September 10:00 Virtual Breakfast 13:00 Michal Gajda: Towards a more perfect union type 13:30 Folkert de Vries, Sjaak Smetsers and Sven-Bodo Scholz: Container Unification for Uniqueness Types 14:00 Alejandro Díaz-Caro, Pablo E. Martínez López and Cristian Sottile: Polymorphic System I 14:30 Social break 15:00 Michal Gajda: Schema-driven mutation of datatype with multiple representations 15:30 Alexandre Garcia de Oliveira, Mauro Jaskelioff and Ana Cristina Vieira de Melo: On Structuring Pure Functional Programs with Monoidal Profunctors 16:00 Sara Moreira, Pedro Vasconcelos and Mário Florido: Resource Analysis for Lazy Evaluation with Polynomial Potential 16:30 Social break 17:00 Neil Mitchell, Moritz Kiefer, Pepe Iborra, Luke Lau, Zubin Duggal, Hannes Siebenhandl, Matthew Pickering and Alan Zimmerman: Building an Integrated Development Environment (IDE) on top of a Build System 17:30 Evan Sitt, Xiaotian Su, Beka Grdzelishvili, Zurab Tsinadze, Zongpu Xie, Hossameldin Abdin, Giorgi Botkoveli, Nikola Cenikj, Tringa Sylaj and Viktoria Zsok: Functional Programming Application for Digital Synthesis Implementation 18:00 Jocelyn Serot: HoCL: High level specification of dataflow graphs 19:30 Virtual Pub All times are in British Summer Time (BST), the local time in Canterbury, UK. So please translate these into your own time zone, using a service such as time and date. Simon Thompson | Professor Emeritus of Logic and Computation School of Computing | University of Kent | Canterbury, CT2 7NF, UK s.j.thompson(a)kent.ac.uk<mailto:s.j.thompson@kent.ac.uk> | M +44 7986 085754 | W www.cs.kent.ac.uk/~sjt<http://www.cs.kent.ac.uk/~sjt>

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First and Only Call for Participation for IFL 2020 (Implementation and Application of Functional Languages)
by Jurriaan Hage 26 Aug '20

26 Aug '20

Hello, Please, find below the first and final call for participation for IFL 2020. 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 ======================================================= IFL 2020 32nd Symposium on Implementation and Application of Functional Languages Call for Participation venue: online 2nd - 4th September 2020 https://www.cs.kent.ac.uk/events/2020/ifl20/ ======================================================== ### 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 2020 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. ### Registration The symposium will be run via Zoom (zoom.us). If you can use Zoom, then you can participate. Please register for free via Eventbrite on the symposium webpage: https://www.cs.kent.ac.uk/events/2020/ifl20/ ### Programme Day 1: Wednesday, 2 September 12:45 Welcome 13:00 Nico Naus and Johan Jeuring: End-user feedback in multi-user workflow systems 13:30 Mart Lubbers, Haye Böhm, Pieter Koopman and Rinus Plasmeijer: Asynchronous Shared Data Sources 14:00 Pieter Koopman, Steffen Michels and Rinus Plasmeijer: Dynamic Editors for Well-Typed Expressions 14:30 Bas Lijnse and Rinus Plasmeijer: Asymmetric Composable Web Editors in iTasks 15:00 Social break 15:30 Sven-Olof Nyström: A subtyping system for Erlang 16:00 Andrew Marmaduke, Christopher Jenkins and Aaron Stump: Generic Zero-Cost Constructor Subtyping 16:30 Joris Burgers, Jurriaan Hage and Alejandro Serrano: Heuristics-based Type Error Diagnosis for Haskell - The case of GADTs and local reasoning 17:00 Social break 17:30 Kavon Farvardin and John Reppy: A New Backend for Standard ML of New Jersey 18:00 Chaitanya Koparkar, Mike Rainey, Michael Vollmer, Milind Kulkarni and Ryan R. Newton: A Compiler Approach Reconciling Parallelism and Dense Representations for Irregular Trees 18:30 Hans-Nikolai Vießmann and Sven-Bodo Scholz: Effective Host-GPU Memory Mangement Through Code Generation 20:00 Virtual Pub Day 2: Thursday, 3 September 10:00 Virtual Breakfast 13:00 Michal Gajda: Less Arbitrary waiting time 13:30 Sólrún Halla Einarsdóttir and Nicholas Smallbone: Template-based Theory Exploration: Discovering Properties of Functional Programs by Testing 14:00 Péter Bereczky, Dániel Horpácsi, Judit Kőszegi, Soma Szeier and Simon Thompson: Validating Formal Semantics by Comparative Testing 14:30 Social break 15:00 Gergo Erdi: An Adventure in Symbolic Execution 15:30 Joshua M. Schappel, Sachin Mahashabde and Marco T. Morazan: Using OO Design Patterns in a Functional Programming Setting 16:00 Filipe Varjão: Functional Programming and Interval Arithmetic with High Accuracy 16:30 Social break 17:00 Laith Sakka, Chaitanya Koparkar, Michael Vollmer, Vidush Singhal, Sam Tobin-Hochstadt, Ryan R. Newton and Milind Kulkarni: General Deforestation Using Fusion, Tupling and Intensive Redundancy Analysis 17:30 Benjamin Mourad and Matteo Cimini: A Declarative Gradualizer with Lang-n-Change 18:00 Maheen Riaz Contractor and Matthew Fluet: Type- and Control-Flow Directed Defunctionalization 19:30 Virtual Pub Day 3: Friday, 4 September 10:00 Virtual Breakfast 13:00 Michal Gajda: Towards a more perfect union type 13:30 Folkert de Vries, Sjaak Smetsers and Sven-Bodo Scholz: Container Unification for Uniqueness Types 14:00 Alejandro Díaz-Caro, Pablo E. Martínez López and Cristian Sottile: Polymorphic System I 14:30 Social break 15:00 Michal Gajda: Schema-driven mutation of datatype with multiple representations 15:30 Alexandre Garcia de Oliveira, Mauro Jaskelioff and Ana Cristina Vieira de Melo: On Structuring Pure Functional Programs with Monoidal Profunctors 16:00 Sara Moreira, Pedro Vasconcelos and Mário Florido: Resource Analysis for Lazy Evaluation with Polynomial Potential 16:30 Social break 17:00 Neil Mitchell, Moritz Kiefer, Pepe Iborra, Luke Lau, Zubin Duggal, Hannes Siebenhandl, Matthew Pickering and Alan Zimmerman: Building an Integrated Development Environment (IDE) on top of a Build System 17:30 Evan Sitt, Xiaotian Su, Beka Grdzelishvili, Zurab Tsinadze, Zongpu Xie, Hossameldin Abdin, Giorgi Botkoveli, Nikola Cenikj, Tringa Sylaj and Viktoria Zsok: Functional Programming Application for Digital Synthesis Implementation 18:00 Jocelyn Serot: HoCL: High level specification of dataflow graphs 19:30 Virtual Pub All times are in British Summer Time (BST), the local time in Canterbury, UK. So please translate these into your own time zone, using a service such as time and date.

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doctest-extract
by Henning Thielemann 22 Aug '20

22 Aug '20

I have started an alternative implementation of 'doctest'. In contrast to the original one, doctest-extract extracts Haskell test-modules from doctest comments. This integrates better with Cabal and its package version resolution and the tests are compiled, not interpreted in GHCi. Disadvantage is that doctest-extract does not support all features of 'doctest', e.g. IO-tests. I uploaded a preview as package candidates to Hackage: $ cabal install $(for pkg in doctest-extract-0.0 doctest-lib-0.0 utility-ht-0.0.16 non-empty-0.3.3; do echo https://hackage.haskell.org/package/$pkg/candidate/$pkg.tar.gz; done) Extract test modules like so: $ doctest-extract -i src/ -o test/ --executable-main=Test/Main.hs --import-tested $$(cat test-module.list) There is still a problem: A failing QuickCheck test does not abort the whole test-suite. 'cabal test' will report success even if one QuickCheck property test fails. I wonder what is the proposed way to make a failing property fail the test-suite? I could use (QC.whenFail exitFailure). Is this the intended way?

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