- Haskell-Cafe - Haskell.org

Re: [Haskell-cafe] Relationship between ((a -> Void) -> Void) and (forall r. (a -> r) -> r)
by Olaf Klinke 17 May '20

17 May '20

> 2. Double negation enables a translation of classical logic in > intuitionistic logic, here embedded in Haskell: > Below I'll give a variant of Li-yao's logic language in Haskell98. Again, the open set model may help to visualize CPS translation: Consider all open sets A on the real line which are fixed points of double negation. All intervals are of this kind, but poking holes in them destroys this property: ---(====)(====)--- A ===)----------(=== Not A ---(==========)--- Not (Not A) Denote the collection of open sets invariant under double negation by X. As the example above demonstrates, X is not closed under set union. But set intersection preserves membership of X. So we have a funny logic that has (&&) but no (||). The CPS trick is now to define a new (||) by means of double negation: ---(====)--------- A ---------(====)--- B ---(====)(====)--- set union of A and B ---(==========)--- A || B, double negation of set union. In fact X is the largest Boolean algebra contained in the Heyting algebra of open sets. In terms of Haskell, we define a new Either: Either' a b = ((Either a b) -> Void) -> Void Looks familiar? Indeed, in this thread we already proved Either' a (Not a). This says that Not A is the Boolean complement of A in X. Using (,) and Either' for (&&) and (||) you can realize all proofs of classical propositional logic in Haskell without GADTs. Olaf

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Re: [Haskell-cafe] Decorating exceptions with backtrace information
by Carter Schonwald 14 May '20

14 May '20

On Fri, May 8, 2020 at 9:32 AM Carter Schonwald <carter.schonwald(a)gmail.com> wrote: > ben, could you please email the libraries list with this too? This seems > like a core libraries / base change rather than a ghc-the-compiler change > > On Thu, May 7, 2020 at 6:57 PM Michael Sloan <mgsloan(a)gmail.com> wrote: > >> Thanks so much for making a proposal for this, Ben!! It's great to see >> progress here. >> >> I'm also glad that there is now a proposal process. I made a fairly >> similar proposal almost exactly 5 years ago to the libraries list - >> https://mail.haskell.org/pipermail/libraries/2015-April/025471.html - >> but without the subtlety of particular backtrace representations. Skimming >> the ensuing thread may still be informative. >> >> In particular, there is one thing I would like to highlight from that old >> proposal. I think it'd be good to have a standard way to represent a chain >> of exceptions, and build this into `catch` and `finally`. Python and Java >> both have a mechanism for this, and both refer to it as a "cause" >> exception. When an exception is thrown during exception handling, the >> exception being handled is preserved as its "cause". I find this mechanism >> to be incredibly useful in Java, it has made the underlying issue much >> clearer in many cases, and in other cases at least provides helpful >> context. I have no doubt such a mechanism would have saved me many hours >> of debugging exceptions in Haskell systems I've worked on in the past. >> >> I considered commenting about that directly on the proposal, but I figure >> this is a better place to suggest expanding the scope of the change :) . >> Totally understandable if you want to keep this proposal focused on >> stacktraces, but I think it'd be good to consider this as a potential >> future improvement. >> >> -Michael >> >> On Thu, May 7, 2020 at 3:55 PM Ben Gamari <ben(a)well-typed.com> wrote: >> >>> >>> Hi everyone, >>> >>> After a nice discussion on IRC about the unfortunate state of error >>> reporting in Haskell, I felt compelled to write down some long-lingering >>> thoughts regarding backtraces on exceptions. The result is GHC proposal >>> #330 [1]. I think the approach is viable and perhaps even >>> straightforward. I have the sketch of an implementation here [2]. >>> >>> Please have a look at the proposal and leave your comments. If there is >>> consensus it is possible that we could have this done for 8.12. >>> >>> Cheers, >>> >>> - Ben >>> >>> >>> [1] https://github.com/ghc-proposals/ghc-proposals/pull/330 >>> [2] https://gitlab.haskell.org/ghc/ghc/-/merge_requests/3236 >>> _______________________________________________ >>> ghc-devs mailing list >>> ghc-devs(a)haskell.org >>> http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs >>> >> _______________________________________________ >> ghc-devs mailing list >> ghc-devs(a)haskell.org >> http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs >> >

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CFP: GPCE 2020 − 19th International Conference on Generative Programming: Concepts & Experiences
by Erwan BOUSSE 13 May '20

13 May '20

----------------------------------------------------------------------- GPCE 2020: 19th International Conference on Generative Programming: Concepts & Experiences November 16-17, 2020 Chicago, IL (USA) (a part of the SPLASH series of conferences) https://conf.researchr.org/home/gpce-2020 ----------------------------------------------------------------------- --------------------------- CALL FOR PAPERS --------------------------- The ACM SIGPLAN International Conference on Generative Programming: Concepts & Experiences is a programming languages conference focusing on techniques and tools for code generation, language implementation, and product-line development. GPCE seeks conceptual, theoretical, empirical, and technical contributions to its topics of interest, which include but are not limited to: program transformation, staging, macro systems, preprocessors, program synthesis, and code-recommendation systems, domain-specific languages, language embedding, language design, and language workbenches, feature-oriented programming, domain engineering, and feature interactions, applications and properties of code generation, language implementation, and product-line development. Authors are welcome to check with the PC chair whether their planned papers are in scope. --------------------------- IMPORTANT DATES --------------------------- - Abstract submission: July 20, 2020 (Monday), AoE - Paper submission: July 27, 2020 (Monday), AoE - Author notification: September 9, 2020 (Wednesday) - Conference: November 16-17, 2020 --------------------------- PAPER SELECTION --------------------------- The GPCE program committee will evaluate each submission according to the following selection criteria: - Novelty. Papers must present new ideas or evidence and place them appropriately within the context established by previous research in the field. - Significance. The results in the paper must have the potential to add to the state of the art or practice in significant ways. - Evidence. The paper must present evidence supporting its claims. Examples of evidence include formalizations and proofs, implemented systems, experimental results, statistical analyses, and case studies. - Clarity. The paper must present its contributions and results clearly. --------------------------- PAPER CATEGORIES --------------------------- GPCE solicits three kinds of submissions: - Full Papers reporting original and unpublished results of research that contribute to scientific knowledge in any GPCE topic listed above. Full paper submissions must not exceed 12 pages excluding bibliography. - Short Papers presenting unconventional ideas or visions about any GPCE topic listed above. Short papers do not always require complete results as in the case of a full paper. In this way, authors can introduce new ideas to the community and get early feedback. Please note that short papers are not intended to be position statements. Short papers are included in the proceedings and will be presented at the conference. Short paper submissions must not exceed 6 pages excluding bibliography. Short papers must have the text “(Short Paper)” appended to their title, though any papers of 6 or fewer pages that are not tool demonstration papers will be considered as short papers. - Tool Demonstrations presenting tools for any GPCE topic listed above. Tools must be available for use and must not be purely commercial. Submissions must provide a tool description not exceeding 6 pages excluding bibliography and a separate demonstration outline including screenshots also not exceeding 6 pages. Tool demonstrations must have the keywords “Tool Demo” or “Tool Demonstration” appended in their title. If the submission is accepted, the tool description will be published in the proceedings. The demonstration outline will only be used by the program committee for evaluating the submission. --------------------------- PAPER SUBMISSION --------------------------- All submissions must use the ACM SIGPLAN Conference Format “acmart” (http://sigplan.org/Resources/Author/#acmart-format) Please be sure to use the latest LaTeX templates and class files. the SIGPLAN sub-format, and 10 point font. Consult the sample-sigplan.tex template and use the document-class \documentclass[sigplan,anonymous,review]{acmart}. Please do not make any changes to this format! To increase fairness in reviewing, a double-blind review process has become standard across SIGPLAN conferences. GPCE will follow a very lightweight model, where author identities are revealed to reviewers after submitting their initial reviews. Hence, the purpose is not to conceal author identities at all cost, but merely to provide reviewers with an unbiased first look at a submission. Author names and institutions should be omitted from submitted papers, and references to the authors’ own related work should be in the third person. No other changes are necessary, and authors will not be penalized if reviewers are able to infer their identities in implicit ways. Papers must be submitted using HotCRP: https://gpce2020.hotcrp.com/ The official publication date is the date the proceedings are made available in the ACM Digital Library. Papers must describe work not currently submitted for publication elsewhere as described by the SIGPLAN Republication Policy (http://www.sigplan.org/Resources/Policies/Republication/) --------------------------- ORGANIZATION --------------------------- - Steering Committee Chair: Ulrik Pagh Schultz (Denmark) - General Chair: Martin Erwig (USA) - Program Chair: Jeff Gray (USA) - Publicity Chair: Erwan Bousse (France) For additional information, clarification, or answers to questions please contact the program chair at gray(a)cs.ua.edu --------------------------- PROGRAM COMMITTEE --------------------------- - Juliana Alves Pereira (Brazil) - Ira Baxter (USA) - Don Batory (USA) - Sandrine Blazy (France) - Sheng Chen (USA) - Shigeru Chiba (Japan) - Thomas Degueule (France) - Robert Glück (Denmark) - Aniruddha Gokhale (USA) - Elisa Gonzalez Boix (Belgium) - Julia Lawall (France) - Geoffrey Mainland (USA) - Marjan Mernik (Slovenia) - Bruno Oliveira (Hong Kong) - Alex Potanin (New Zealand) - Suman Roychoudhury (India) - Christoph Seidl (Denmark) - Michel Steuwer (UK) - Eli Tilevich (USA) - Naoyasu Ubayashi (Japan) - Tijs van der Storm (Netherlands) - Vadim Zaytsev (Netherlands)

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Way to prevent a value from being garbage collected too early
by YueCompl 13 May '20

13 May '20

Folks, I need a little help, I remember I had read some where that, there's a function to fake a use of a value, so it won't be garbage collected too early, as GHC has an optimization to forget a value as early as possible. But I can't remember the function name or even the keyword to google it out, hint me please! Best regards, Compl

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Relationship between ((a -> Void) -> Void) and (forall r. (a -> r) -> r)
by Ruben Astudillo 13 May '20

13 May '20

Hello cafe I've been doing some basic logic exercises on Haskell the last few days. Along the way I learned about intuitionistic logic and double negation. On STLC types are propositions and terms of those types are proofs of those propositions. The standard way to negate a proposition/type ~A~ in Haskell is via the type type Not a = a -> Void So far so good. On classical logic we have an equivalence between a proposition and it's double negation. On intuitionistic logic I only have a single implication that ~forall a. a -> Not (Not a)~, this is easy to see as we have the proof/term proof :: forall a. a -> Not (Not a) proof a = \cont -> cont a But we don't have the implication between ~forall a. Not (Not a) -> a~, there is no way to obtain an ~a~ from the void continuations. Worse than that, to even use such continuation I would need to have an ~a~ already. On blogposts I've seen the claim that the types ~Void~ and ~forall a. a~ are almost the same. I can see the second one only has bottom as a term. H98 versions of Void also had that term. The curious fact is that people treat them as if they were the same without questioning. But they are not the same, we can see this as there is an implication ~(forall r. (a -> r) -> r) -> a~ given by the proof proof1 :: forall a. (forall r. (a -> r) -> r) -> a proof1 cont = cont id That leads me to believe that given that ~(forall r. (a -> r) -> r)~ and ~(a -> Void) -> Void)~ are not obviously the same, the relationship between ~Void~ and ~forall a. a~ is not as people say. So cafe, how can I reconcile these facts? -- -- Rubén -- pgp: 4EE9 28F7 932E F4AD

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ML Family Workshop 2020: Deadline extension
by Leo White 12 May '20

12 May '20

Due to the COVID-19 pandemic, ML 2020 will take place online. The submission deadline has been extended to the end of the month to accommodate for the change in situation. ============================================================= We are happy to invite submissions to the ML Family Workshop 2020, to be held during the ICFP conference week on Thursday, August 27th. The ML family workshop warmly welcomes submission touching on the programming languages traditionally seen as part of the "ML family" (Standard ML, OCaml, F#, CakeML, SML#, Manticore, MetaOCaml, etc.). The scope of the workshop includes all aspects of the design, semantics, theory, application, implementation, and teaching of the members of the ML family. We also encourage presentations from related languages (such as Haskell, Scala, Rust, Nemerle, Links, Koka, F*, Eff, ATS, etc), to exchange experience of further developing ML ideas. Due to the COVID-19 pandemic, the workshop will take place online. See our detailed CFP online on the ICFP website: https://icfp20.sigplan.org/home/mlfamilyworkshop-2020 Important dates --------------- - Friday 29th May (any time zone): Abstract submission deadline - Friday 17th July: Author notification - Thursday 27th August: ML Family Workshop Program committee ----------------- - Youyou Cong (Tokyo Institute of Technology) - Ivan Gotovchits (Carnegie Mellon University) - Gowtham Kaki (Purdue University) - Neel Krishnaswami (University of Cambridge) - Daan Leijen (Microsoft Research) - Koko Muroya (Kyoto University) - Atsushi Ohori (Tohoku University) - Jonathan Protzenko (Microsoft Research) - Gabriel Radanne (INRIA) - Claudio Russo (Dfinity) - Leo White (Jane Street) (Chair) - Jeremy Yallop (University of Cambridge) Submission details ------------------ See the online CFP for the details on the expected submission format. Submissions must be uploaded to the workshop submission website https://ml2020.hotcrp.com/ before the submission deadline.

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Re: [Haskell-cafe] When did it become so hard to install Haskell on Windows?
by Gershom B 12 May '20

12 May '20

On May 11, 2020 at 10:52:36 AM, Michal J Gajda (mgajda(a)mimuw.edu.pl) wrote: > 3. To enter relevant command line in admin PowerShell so that Chocolatey installs GHC. > 4. Ditto for Mingw (3 attempts, since I think default Mingw package lacks full build environment.) > 4. Ditto for every dependency of my Haskell package. (Is there a way to get standardized library dependencies across the platforms? Drop me a line if it is at all possible without Nix.) I’m confused. When you used chocolatey, why did you not run “choco install haskell-dev” as suggested on https://www.haskell.org/platform/#windows (and i imagine everywhere else)? This should have installed ghc, msys2 and cabal for you at once. Further what do you mean "Ditto for every dependency of my Haskell package” — do you mean haskell dependencies or c dependencies? If the latter, how is this different than any other OS? Also, why were you installing c dependencies with chocolatey, if that’s indeed what you were doing? With msys2 the way to do things is install them with pacman. Perhaps what is needed is just better and more centralized documentation? -g

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Coercing newtype-wrapped monad transformers?
by Viktor Dukhovni 11 May '20

11 May '20

The code sample below my signature wraps RWS.CPS in a newtype, with a view to presenting a more constrained interface with specific functions that internall use the underlying RWST features. It is straightforward to construct the appropriate variants of "ask", "get", "put", "tell", ... via appropriate applications of the FooT constructor and accessor, but I expected to be able to use "coerce", along the lines of: ask :: Monad m => RWST r w s m r ask = coerce M.ask but that fails with: foo.hs:34:7: error: • Couldn't match representation of type ‘m0’ with that of ‘m’ arising from a use of ‘coerce’ ‘m’ is a rigid type variable bound by the type signature for: ask :: forall (m :: * -> *) r w s. Monad m => FooT r w s m r at foo.hs:21:1-34 • In the expression: coerce M.ask In an equation for ‘ask’: ask = coerce M.ask • Relevant bindings include ask :: FooT r w s m r (bound at foo.hs:34:1) Somehow between the various constraints and quantifiers in play, coerce is unable to do what I expected. Should it have worked? If had worked, would the effect have been in fact any different (more efficient?) than what I get with the explicit applications of the constructor and accessor? -- Viktor. {-# LANGUAGE GeneralizedNewtypeDeriving, FlexibleInstances, MultiParamTypeClasses, StandaloneDeriving #-} import Control.Monad.Trans.Class import qualified Control.Monad.Trans.RWS.CPS as M newtype FooT r w s m a = FooT { unFooT :: M.RWST r w s m a } deriving instance (Functor m) => Functor (FooT r w s m) deriving instance (Monad m) => Applicative (FooT r w s m) deriving instance (Monad m) => Monad (FooT r w s m) deriving instance MonadTrans (FooT r w s) runFooT :: (Monoid w, Monad m) => FooT r w s m a -> r -> s -> m (a, s, w) execFooT :: (Monoid w, Monad m) => FooT r w s m a -> r -> s -> m (s, w) evalFooT :: (Monoid w, Monad m) => FooT r w s m a -> r -> s -> m (a, w) ask :: (Monad m) => FooT r w s m r asks :: (Monad m) => (r -> a) -> FooT r w s m a get :: (Monad m) => FooT r w s m s gets :: (Monad m) => (s -> a) -> FooT r w s m a modify :: (Monad m) => (s -> s) -> FooT r w s m () pass :: (Monoid w, Monoid w', Monad m) => FooT r w' s m (a, w' -> w) -> FooT r w s m a put :: (Monad m) => s -> FooT r w s m () tell :: (Monoid w, Monad m) => w -> FooT r w s m () runFooT = M.runRWST . unFooT evalFooT = M.evalRWST . unFooT execFooT = M.execRWST . unFooT ask = FooT $ M.ask asks = FooT . M.asks get = FooT $ M.get gets = FooT . M.gets modify = FooT . M.modify pass = FooT . M.pass . unFooT put = FooT . M.put tell = FooT . M.tell main :: IO () main = runFooT mfoo "read me" "set me" >>= print where mfoo = do ask >>= modify . (. (" or " ++)) . (++) tell "Hello" pass $ do tell "World!" pure (42, (' ':))

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Re: [Haskell-cafe] When did it become so hard to install Haskell on Windows?
by Michal J Gajda 11 May '20

11 May '20

Hi, I recently installed GHC using Chocolatey, since an user requested Windows support. I understand that the new installation mode allows to use package manager on the platform that lacks native package manager support. It took me approximately an hour in total, since the process of compiling a Haskell program required: 1. To install and find powershell, and run it as administrator. (If you try to install Chocolatey as non-admin user you get a long error message.) 2. To use powershell console to change some default Chocolatey options so that it does not stop each time that configuration change is requested. 3. To enter relevant command line in admin PowerShell so that Chocolatey installs GHC. 4. Ditto for Mingw (3 attempts, since I think default Mingw package lacks full build environment.) 4. Ditto for every dependency of my Haskell package. (Is there a way to get standardized library dependencies across the platforms? Drop me a line if it is at all possible without Nix.) 5. Installing Haskell Stack, since I gave up making it work with Cabal install+chocolatey at this point. 6. Building everything with Haskell stack in powershell to acquire environment settings that expose the dependent libraries. Maybe we need a wizard app that takes us through the process? I am certain that part of the pain was due to the fact that I rarely use Windows platform. Did we diverge so far from Windows, because everybody moved to the cloud-based Linux builds? I guess, if not for using Stack, I was about half-way to get install process running. I wonder how to support Windows build environment best. Is there maybe Ansible setup script for barebones Windows environment? -- Cheers Michał

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When did it become so hard to install Haskell on Windows?
by Anthony Clayden 11 May '20

11 May '20

Thanks Irfon-Kim for your research, but you're making this a lot harder than it needs to be. I'll try to redress the balance -- not that I'm claiming it's Windows-friendly. > Yes, you need to know several magic things to go this route. > You don't need to install any extra tools, nor use a command line, nor have a bar of chocolatey. > The first is to have that link that Ben Gamari provided in the first place, which is not an obvious thing at all. I think Ben gave the wrong link[**], and it's not obvious why you'd go to such an unfriendly page at all. Should be: https://www.haskell.org/ghc/download_ghc_8_10_1.html, section Windows 64-bit (x86_64). This is to download ghc-8.10.1-x86_64-unknown-mingw32.tar.xz <https://downloads.haskell.org/~ghc/8.10.1/ghc-8.10.1-x86_64-unknown-mingw32…> it's not the 'unknown' that tells you it's Windows, it's the 'w' in 'w32'. I don't know why you're downloading a `.lz`: the `.tar.xz` is a format that Windows Explorer understands [***]; just use Explorer to find that in the Downloads folder; it needs several clicks to unpack it, but by all means unpack into the Downloads folder; navigate to `\bin`; there is ghci.exe; double-click. (No need to set your path, it'll default to `\bin`; no need to call from a command line.) Indeed you can copy ghci.exe and paste as shortcut on your desktop/taskbar; set the 'Start in' as the `\bin` folder. Now fair warning: this is only the ghci executable/base compiler; no libraries/packages; no cabal, no stackage. But at least a beginner can sit at a GHCi prompt and enter some expressions; they can call `:edit` to create a `.hs` module. They can `:set editor` for a more programmer-friendly editor. [**] How did I get to that download link? (Just to show there's nothing up my sleeves) * haskell.org -> Downloads -> Minimal Installers * -> 'the GHC compiler' in the section's opening sentence * you're at haskell.org/ghc/; latest release -> download * scan down the page for Windows I'm saying to ignore all that stuff about 'proper Haskell distribution'; 'Distribution packages' (not available for Windows). For a beginner it's all cruft; they can figure it out after they're using Haskell. Really this could all be a lot easier and a lot better explained and a lot more in-your-face for a beginner. I continue to take away the impression that ghc doesn't want beginners. [***] I agree a `.xz` needs 7-zip to unpack. I don't think I went so far as to obtain that first; I think it came with my Windows (8.1) install. Perhaps later Windows have it better integrated(?)

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Re: [Haskell-cafe] Decorating exceptions with backtrace information
by Doug McIlroy 10 May '20

10 May '20

> in theory nobody wants non-terminating programs, but in practice > everybody gets them sometimes Without impugning the basic point, I am amused by the premise. What hooked me on Haskell was the clean way in which the nonterminating programs I did want to write could be expressed in terms of lazy lists. Doug

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Problem with Model entity (persistent-template)
by Hadruki 10 May '20

10 May '20

Hi I have a project built around servant + persistent(-postgresql) + esqueleto (and many other libraries). I recently upgraded it to newer versions of GHC and my dependencies and I'm running into a problem which I don't fully understand. ``` • Couldn't match a lifted type with an unlifted type arising from the coercion of the method ‘==’ from type ‘ghc-prim-0.5.3:GHC.Prim.Int# -> ghc-prim-0.5.3:GHC.Prim.Int# -> Bool’ to type ‘Key ActivationCode -> Key ActivationCode -> Bool’ • When deriving the instance for (Eq (Key ActivationCode)) + other similar ``` (full trace + Model.hs below) I get these errors on all my model entities. First of all I don't really understand the error message and I don't understand why I am getting it. I am using these entities (and their key) in esqueleto database operations: ``` findActivationCodeById :: ActivationCodeId -> IO (Entity ActivationCode) ``` I don't get the error if I build with `stack build --fast` (at least the Model.hs builds). I've tried to reproduce this on a smaller project with nothing but this Model file however it compiles there. Additionally even if I remove the `deriving Eq` it doesn't change the error (it's still deriving Eq, Ord, ..). Could anyone give me some hints where I should start looking? Is it due to a language extension? In my (generated) cabal ``` default-extensions: TypeOperators OverloadedStrings DeriveGeneric RecordWildCards DuplicateRecordFields StrictData ``` Thanks! Model.hs (trimmed down to just one entity) {-# LANGUAGE EmptyDataDecls #-} {-# LANGUAGE FlexibleContexts #-} {-# LANGUAGE GADTs #-} {-# LANGUAGE GeneralizedNewtypeDeriving #-} {-# LANGUAGE MultiParamTypeClasses #-} {-# LANGUAGE OverloadedStrings #-} {-# LANGUAGE QuasiQuotes #-} {-# LANGUAGE TemplateHaskell #-} {-# LANGUAGE TypeFamilies #-} {-# LANGUAGE FlexibleInstances #-} -- allow json in data model module Prolix.Model.Model where import qualified Data.Aeson as Aeson import qualified Data.Fixed as Fixed import Data.Text (Text) import Data.Time import Data.Typeable (Typeable) import Database.Persist.Postgresql import Database.Persist.TH import GHC.Generics share [ mkPersist sqlSettings , mkDeleteCascade sqlSettings , mkMigrate "migrateAll" ] [persistLowerCase| ActivationCode json productCode Int sql=accd_product_code code Text sql=accd_code provider Text sql=accd_provider exported Bool sql=accd_exported exportedOn UTCTime Maybe sql=accd_exported_on createdOn UTCTime sql=accd_created_on default=now() modifiedOn UTCTime sql=accd_modified_on default=now() UniqueActivationCode code provider deriving Eq Ord Show Generic |] Full trace [ 54 of 155] Compiling Prolix.Model.Model ( src/Prolix/Model/Model.hs, .stack-work/dist/x86_64-osx/Cabal-2.4.0.1/build/Prolix/Model/Model.o ) [Optimisation flags changed] /src/hadruki/prolix/src/Prolix/Model/Model.hs:35:1: error: • Couldn't match a lifted type with an unlifted type arising from the coercion of the method ‘==’ from type ‘ghc-prim-0.5.3:GHC.Prim.Int# -> ghc-prim-0.5.3:GHC.Prim.Int# -> Bool’ to type ‘Key ActivationCode -> Key ActivationCode -> Bool’ • When deriving the instance for (Eq (Key ActivationCode)) | 35 | share [ mkPersist sqlSettings | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^... /src/hadruki/prolix/src/Prolix/Model/Model.hs:35:1: error: • Couldn't match a lifted type with an unlifted type arising from the coercion of the method ‘compare’ from type ‘ghc-prim-0.5.3:GHC.Prim.Int# -> ghc-prim-0.5.3:GHC.Prim.Int# -> Ordering’ to type ‘Key ActivationCode -> Key ActivationCode -> Ordering’ • When deriving the instance for (Ord (Key ActivationCode)) | 35 | share [ mkPersist sqlSettings | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^... /src/hadruki/prolix/src/Prolix/Model/Model.hs:35:1: error: • Couldn't match a lifted type with an unlifted type arising from the coercion of the method ‘path-pieces-0.2.1:Web.PathPieces.toPathPiece’ from type ‘ghc-prim-0.5.3:GHC.Prim.Int# -> Text’ to type ‘Key ActivationCode -> Text’ • When deriving the instance for (path-pieces-0.2.1:Web.PathPieces.PathPiece (Key ActivationCode)) | 35 | share [ mkPersist sqlSettings | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^... /src/hadruki/prolix/src/Prolix/Model/Model.hs:35:1: error: • Couldn't match a lifted type with an unlifted type arising from the coercion of the method ‘Web.Internal.HttpApiData.toUrlPiece’ from type ‘ghc-prim-0.5.3:GHC.Prim.Int# -> Text’ to type ‘Key ActivationCode -> Text’ • When deriving the instance for (Web.Internal.HttpApiData.ToHttpApiData (Key ActivationCode)) | 35 | share [ mkPersist sqlSettings | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^... /src/hadruki/prolix/src/Prolix/Model/Model.hs:35:1: error: • No instance for (Web.Internal.HttpApiData.FromHttpApiData ghc-prim-0.5.3:GHC.Prim.Int#) arising from the 'deriving' clause of a data type declaration Possible fix: use a standalone 'deriving instance' declaration, so you can specify the instance context yourself • When deriving the instance for (Web.Internal.HttpApiData.FromHttpApiData (Key ActivationCode)) | 35 | share [ mkPersist sqlSettings | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^... /src/hadruki/prolix/src/Prolix/Model/Model.hs:35:1: error: • Couldn't match a lifted type with an unlifted type arising from the coercion of the method ‘toPersistValue’ from type ‘ghc-prim-0.5.3:GHC.Prim.Int# -> PersistValue’ to type ‘Key ActivationCode -> PersistValue’ • When deriving the instance for (PersistField (Key ActivationCode)) | 35 | share [ mkPersist sqlSettings | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^... /src/hadruki/prolix/src/Prolix/Model/Model.hs:35:1: error: • Couldn't match a lifted type with an unlifted type arising from the coercion of the method ‘sqlType’ from type ‘Data.Proxy.Proxy ghc-prim-0.5.3:GHC.Prim.Int# -> SqlType’ to type ‘Data.Proxy.Proxy (Key ActivationCode) -> SqlType’ • When deriving the instance for (PersistFieldSql (Key ActivationCode)) | 35 | share [ mkPersist sqlSettings | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^... /src/hadruki/prolix/src/Prolix/Model/Model.hs:35:1: error: • Couldn't match a lifted type with an unlifted type arising from the coercion of the method ‘Aeson.toJSON’ from type ‘ghc-prim-0.5.3:GHC.Prim.Int# -> Aeson.Value’ to type ‘Key ActivationCode -> Aeson.Value’ • When deriving the instance for (Aeson.ToJSON (Key ActivationCode)) | 35 | share [ mkPersist sqlSettings | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^... /src/hadruki/prolix/src/Prolix/Model/Model.hs:35:1: error: • No instance for (Aeson.FromJSON ghc-prim-0.5.3:GHC.Prim.Int#) arising from the 'deriving' clause of a data type declaration Possible fix: use a standalone 'deriving instance' declaration, so you can specify the instance context yourself • When deriving the instance for (Aeson.FromJSON (Key ActivationCode)) | 35 | share [ mkPersist sqlSettings | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^...

1 0

When did it become so hard to install Haskell on Windows?
by José Pedro Magalhães 10 May '20

10 May '20

Hi, I haven't used Haskell in my personal computer in a while. I decided to install it again. I used the Haskell Platform in the past, so I went for that again - and a quick Google search on "install haskell windows" brings up the HP page, so I thought I was on the right track. At the HP page for Windows, I'm greeted with this: [image: image.png] In the past I'd just download an installer which would take care of things - now it seems to be more complicated. But fine, I followed the link to configure Chocolatey. That's where it starts getting really scary: [image: image.png] First, I have to subscribe to a newsletter? Really? I guess this is entirely optional, but the instructions don't make it sound so. Then I have to know what powershell.exe is, use an administrative prompt, and enter scary commands in it. I gave up at this stage. But going back to the HP page, it appears that even this wouldn't be enough, because I would still need to follow "the instructions at haskellstack.org to install stack". The link to haskellstack.org takes me to a 403 Forbidden <https://docs.haskellstack.org/en/stable/README/>. I honestly don't want this to sound like a rant. I genuinely would like to understand why this multi-step, multi-tool, multi-website process was introduced, how it is superior to a single installer, and whether this is really the process we want newcomers to the language have to follow. Thanks, Pedro

22 33

Re: [Haskell-cafe] Decorating exceptions with backtrace information
by Michal J Gajda 09 May '20

09 May '20

Ben, I agree with you that is a great idea! I can add a few more real world examples: * we get exception from a foreign library that we bound, * we get an exception from a platform (I believe Windows supports throwing exceptions to programs), * user presses CTRL-C and we want to know where our program hanged. * we get infamous <<loop>>, because in theory nobody wants non-terminating programs, but in practice everybody gets them sometimes. I also use `ExceptT`, `EitherT` for processing large sets of data, because that allows me to contain the errors efficiently. However from time to time, I get an error to blow up **and I cannot even locate which library was guilty**. It would be nice to extract them automatically and put them into error database before they are prioritized. -- Cheers Michał

1 0

network's example echo server seems leaking socket on error binding
by Compl Yue 09 May '20

09 May '20

Hi folks, I just filed an issue https://github.com/haskell/network/issues/447 <https://github.com/haskell/network/issues/447> with the network package, I'd like to post here too in seeking your wisedom at the same time. In top-level doc, the minimal example echo server uses Control.Exception.bracket like this: E.bracket (open addr) close loop where resolve = do let hints = defaultHints { addrFlags = [AI_PASSIVE] , addrSocketType = Stream } head <$> getAddrInfo (Just hints) mhost (Just port) open addr = do sock <- socket (addrFamily addr) (addrSocketType addr) (addrProtocol addr) setSocketOption sock ReuseAddr 1 withFdSocket sock setCloseOnExecIfNeeded bind sock $ addrAddress addr listen sock 1024 return sock loop sock = forever $ do (conn, _peer) <- accept sock void $ forkFinally (server conn) (const $ gracefulClose conn 5000) I happened to copy the configuration with another machine's IP address to run on my dev machine, then of course it failed binding to the IP, but in this case I suspect sock above is leaked without close, as open failed at all, so sock is not given to bracket for it to do the cleanup. Is my suspicion correct ? Or I missed something that the example actually won't leak on binding errors ? Best regards, Compl

3 2

Windows Haskell CI
by Ben Franksen 06 May '20

06 May '20

Like ghc itself, we (the Darcs team) also have trouble providing adequate support for Windows. I personally haven't (seriously) used Windows for decades. So I thought we could at least do some automated building and testing for Windows on some CI system, perhaps even automatically build Windows binaries. Is there a free CI service that you could recommend that ideally supports at least Windows, MacOS, and Linux and that has good support for Haskell programs, i.e. some recent ghc and cabal pre-installed? If not, what could be an alternative solution? Any kind of comments or hints are appreciated. Cheers Ben

7 9

FMBC 2020: 2nd Workshop on Formal Methods for Blockchains (3rd CfP, Deadline Extension)
by Bruno Bernardo 06 May '20

06 May '20

[ Please distribute, apologies for multiple postings. ] ======================================================================== 2nd Workshop on Formal Methods for Blockchains (FMBC) 2020 - Third Call https://fmbc.gitlab.io/2020 July 19, 2020 Co-located with the 32nd International Conference on Computer-Aided Verification (CAV 2020) http://i-cav.org/2020/ *Due do the COVID-19 pandemic, the workshop is likely to be held online.* ------------------------------------------------------------- IMPORTANT DATES -------------------------------- Abstract submission: May 12, 2020 (extended) Full paper submission: May 19, 2020 (extended) Notification: June 23, 2020 (extended) Camera-ready: July 14, 2020 (extended) Conference: July 19, 2020 Deadlines are Anywhere on Earth: https://en.wikipedia.org/wiki/Anywhere_on_Earth -------------------------------- -------------------------------- TOPICS OF INTEREST -------------------------------- Blockchains are decentralized transactional ledgers that rely on cryptographic hash functions for guaranteeing the integrity of the stored data. Participants on the network reach agreement on what valid transactions are through consensus algorithms. Blockchains may also provide support for Smart Contracts. Smart Contracts are scripts of an ad-hoc programming language that are stored in the Blockchain and that run on the network. They can interact with the ledger’s data and update its state. These scripts can express the logic of possibly complex contracts between users of the Blockchain. Thus, Smart Contracts can facilitate the economic activity of Blockchain participants. With the emergence and increasing popularity of cryptocurrencies such as Bitcoin and Ethereum, it is now of utmost importance to have strong guarantees of the behavior of Blockchain software. These guarantees can be brought by using Formal Methods. Indeed, Blockchain software encompasses many topics of computer science where using Formal Methods techniques and tools are relevant: consensus algorithms to ensure the liveness and the security of the data on the chain, programming languages specifically designed to write Smart Contracts, cryptographic protocols, such as zero-knowledge proofs, used to ensure privacy, etc. This workshop is a forum to identify theoretical and practical approaches of formal methods for Blockchain technology. Topics include, but are not limited to: * Formal models of Blockchain applications or concepts * Formal methods for consensus protocols * Formal methods for Blockchain-specific cryptographic primitives or protocols * Design and implementation of Smart Contract languages * Verification of Smart Contracts -------------------------------- -------------------------------- SUBMISSION -------------------------------- Submit original manuscripts (not published or considered elsewhere) with a maximum of twelve pages (full papers), six pages (short papers), and two pages (extended abstract) describing new and emerging ideas or summarizing existing work). Each paper should include a title and the name and affiliation of each author. Authors of selected extended-abstracts are invited to give a short lightning talk. At least one author of an accepted paper is expected to present the paper at the workshop as a registered participant. Submission link: https://easychair.org/conferences/?conf=fmbc2020 The authors are encouraged to use LaTeX and the EasyChair style files: https://easychair.org/publications/for_authors -------------------------------- -------------------------------- PROCEEDINGS -------------------------------- All submissions will be peer-reviewed by at least three members of the program committee for quality and relevance. Accepted regular papers (full and short papers) will be included in the workshop proceedings, published as a volume of the OpenAccess Series in Informatics (OASIcs) by Dagstuhl. -------------------------------- -------------------------------- INVITED SPEAKER -------------------------------- Grigore Rosu, Professor at University of Illinois at Urbana-Champaign, USA and Founder of Runtime Verification http://fsl.cs.illinois.edu/index.php/Grigore_Rosu https://runtimeverification.com/ -------------------------------- -------------------------------- PROGRAM COMMITTEE -------------------------------- PC CO-CHAIRS * Bruno Bernardo (Nomadic Labs, France) (bruno(a)nomadic-labs.com) * Diego Marmsoler (University of Exeter, UK) (D.Marmsoler(a)exeter.ac.uk) PC MEMBERS * Wolfgang Ahrendt (Chalmers University of Technology, Sweden) * Lacramioara Astefanoei (Nomadic Labs, France) * Massimo Bartoletti (University of Cagliari, Italy) * Bernhard Beckert (Karlsruhe Institute of Technology, Germany) * Achim Brucker (University of Exeter, UK) * Silvia Crafa (Universita di Padova, Italy) * Zaynah Dargaye (Nomadic Labs, France) * Jérémie Decouchant (University of Luxembourg, Luxembourg) * Ansgar Fehnker (University of Twente, Netherlands) * Georges Gonthier (Inria, France) * Florian Kammueller (Middlesex University London, UK) * Maurice Herlihy (Brown University, USA) * Igor Konnov (Informal, Austria) * Andreas Lochbihler (Digital Asset, Switzerland) * Anastasia Mavridou (NASA Ames, USA) * Simão Melo de Sousa (Universidade da Beira Interior, Portugal) * Andrew Miller (University of Illinois at Urbana-Champaign, USA) * Karl Palmskog (KTH, Sweden) * Vincent Rahli (University of Birmingham, UK) * Andreas Rossberg (Dfinity Foundation, Germany) * Claudio Russo (Dfinity Foundation, USA) * César Sanchez (Imdea, Spain) * Clara Schneidewind (TU Wien, Austria) * Ilya Sergey (Yale-NUS College/NUS, Singapore) * Bas Spitters (Aarhus University/Concordium, Denmark) * Mark Staples (CSIRO Data61, Australia) * Meng Sun (Peking University, China) * Simon Thompson (University of Kent, UK) * Philip Wadler (University of Edinburgh / IOHK, UK)

1 0

Journal of Functional Programming - Call for PhD Abstracts
by Graham Hutton 04 May '20

04 May '20

============================================================ CALL FOR PHD ABSTRACTS Journal of Functional Programming Deadline: 31st May 2020 http://tinyurl.com/jfp-phd-abstracts ============================================================ PREAMBLE: Many students complete PhDs in functional programming each year. As a service to the community, twice per year the Journal of Functional Programming publishes the abstracts from PhD dissertations completed during the previous year. The abstracts are made freely available on the JFP website, i.e. not behind any paywall. They do not require any transfer of copyright, merely a license from the author. A dissertation is eligible for inclusion if parts of it have or could have appeared in JFP, that is, if it is in the general area of functional programming. The abstracts are not reviewed. Please submit dissertation abstracts according to the instructions below. We welcome submissions from both the PhD student and PhD advisor/supervisor although we encourage them to coordinate. ============================================================ SUBMISSION: Please submit the following information to Graham Hutton <graham.hutton(a)nottingham.ac.uk> by 31st May 2020: o Dissertation title: (including any subtitle) o Student: (full name) o Awarding institution: (full name and country) o Date of PhD award: (month and year; depending on the institution, this may be the date of the viva, corrections being approved, graduation ceremony, or otherwise) o Advisor/supervisor: (full names) o Dissertation URL: (please provide a permanently accessible link to the dissertation if you have one, such as to an institutional repository or other public archive; links to personal web pages should be considered a last resort) o Dissertation abstract: (plain text, maximum 350 words; you may use \emph{...} for emphasis, but we prefer no other markup or formatting; if your original abstract exceeds the word limit, please submit an abridged version within the limit) Please do not submit a copy of the dissertation itself, as this is not required. JFP reserves the right to decline to publish abstracts that are not deemed appropriate. ============================================================ PHD ABSTRACT EDITOR: Graham Hutton School of Computer Science University of Nottingham Nottingham NG8 1BB United Kingdom ============================================================ 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

cost of Text.Parsec.Expr
by Johannes Waldmann 03 May '20

03 May '20

Dear Cafe, I use `buildExpressionParser` https://hackage.haskell.org/package/parsec-3.1.14.0/docs/Text-Parsec-Expr.h… with a table similar to https://hackage.haskell.org/package/haskell-src-exts-1.23.0/docs/Language-H… and I find it's somewhat expensive. I think the algorithm makes me pay for each operator that is in the table even when just a few of them are actually in the input. (I removed from the table those that don't appear in my test cases, and it got faster.) Certainly this is known? What could be done about it? What does Megaparsec do? It seems that Text.Megaparsec.Expr vanished from version 6 to version 7. A classical shift/reduce algorithm would help (?) but not with the given type of table, which does not allow to parse an operator just once, and then look up its precedence. - J.W.

3 2

ICFP 2020 will be held ONLINE Aug 23-28
by Sam Tobin-Hochstadt 01 May '20

01 May '20

The ICFP 2020 organizers would like to announce that the conference and co-located events, originally scheduled for August 23-28, in Jersey City, New Jersey, will now be held online during the same dates. Further information for presenters, authors, attendees, sponsors, and the ICFP community will be provided as it becomes available. The ICFP Organizing Committee

1 0

[Haskell-Cafe] How to deal with looped references inside instances?
by Daneel Yaitskov 01 May '20

01 May '20

{- Hi, I am trying to translate a Java sample with looped references below to Haskell. I've found 2 solutions, but both of them look ugly, because they require a helper class. So I am looking for an advice for best practices on working with looped references in polymorphic types. interface Engine { String showEng(); } class Car<E extends Engine> { int id; E eng; String show() { return "Car with " + eng.showEng(); } int getId() { return id; } } class Steam implements Engine { Car<Steam> car; Stream(Car<Steam> car) { this.car = car; } String showEng() { return "id " + car.getId(); } } Car<Steam> car = new Car<>(); car.id = 42; car.eng = new Stream(car); car.show(); -} {-# LANGUAGE UndecidableInstances #-} -- Fix2 shortcoming class Car c where getId :: c -> Int class Engine e where showEng :: e -> String data SteamEng c = SteamEng c instance (Car c) => Engine (SteamEng c) where showEng (SteamEng c) = "id " ++ show (getId c) {- First solution is via ShowEng class. I don't know how to tell type checker that the type which is got after application of type function is instantiating Engine class?? instance (Engine eng) => Show (Car1 eng) where show (Car1 _ eng) = "Car1 with " ++ showEng eng -} data Car1 eng = Car1 Int (eng (Car1 eng)) class ShowEng f where showEng2 :: (Car a) => f a -> String instance ShowEng SteamEng where showEng2 = showEng {- A side note. Intuitive alternative for ShowEng which is not working! showEng2 = showEng means showEng2 gets Engine, but class HaveEngine f where cast :: (Car a) => f a -> f a instance HaveEngine SteamEng where cast = id ... instance (HaveEngine eng) => Show (Car1 eng) where show (Car1 _ eng) = "Car1 with " ++ (showEng (cast eng)) nor instance (HaveEngine eng) => Show (Car1 eng) where show (Car1 _ eng) = "Car1 with " ++ (showEng eng) -} instance Car (Car1 e) where getId (Car1 id _) = id instance (ShowEng eng) => Show (Car1 eng) where show (Car1 _ eng) = "Car1 with " ++ (showEng2 eng) c1 = Car1 42 (SteamEng c1) {- Second solution is removing argument from engine type parameter. -} data Car0 eng = Car0 Int eng instance Car (Car0 e) where getId (Car0 id _) = id instance (Engine eng) => Show (Car0 eng) where show (Car0 _ eng) = "Car0 with " ++ showEng eng {- in this case I have to avoid infinite type and introducing an extra wrapper and instantiating business logic classes for him. λ c0 = Car0 42 (SteamEng c0) <interactive>:30:6: error: • Occurs check: cannot construct the infinite type: car ~ Car0 (SteamEng car) • In the expression: Car0 (SteamEng c0) In an equation for ‘c0’: c0 = Car0 42 (SteamEng c0) • Relevant bindings include c0 :: car (bound at <interactive>:30:1) -} newtype Fix2 f g = Fix2 (f (g (Fix2 f g))) instance (Show (f (g (Fix2 f g)))) => Show (Fix2 f g) where show (Fix2 a) = show a {- λ c0 = Car0 42 (SteamEng (Fix2 c0)) <interactive>:62:1: error: • No instance for (Car (Fix2 Car0 SteamEng)) arising from a use of ‘print’ • In a stmt of an interactive GHCi command: print it -} instance (Car (f (g (Fix2 f g)))) => Car (Fix2 f g) where getId (Fix2 a) = getId a c0 = Car0 42 (SteamEng (Fix2 c0)) {- Thanks, Daniil -}

2 1

A quick poll about GHC's -with-rtsopts flag
by Merijn Verstraaten 01 May '20

01 May '20

Who here is: 1) Aware that multiple invocations of -with-rtsopts clobber each other, and 2) Actually relies on this behaviour? This quick and informal poll in relation to this GHC issue: https://gitlab.haskell.org/ghc/ghc/issues/18117 Cheers, Merijn

4 3

ANN: stack-2.3.1
by Emanuel Borsboom 30 Apr '20

30 Apr '20

See https://haskellstack.org/ for installation and upgrade instructions. Release notes: - We have reduced the number of platforms that we support with binary releases. The reason behind this is that we've been slowed down in our release process until now with issues trying to build binaries for less common platforms. In order to make sure we can address issues more quickly (like supporting new GHC versions), we're limiting support from the Stack team to: - Linux 64-bit (static) - macOS - Windows 64-bit If others want to provide additional binaries, we will definitely be happy for the support. But since our CI system is currently able to produce these three bindists only, that's what we will be providing with the next release. - Since we no longer have dynamically linked Linux binaries, we are removing removing the -static suffix from the static Linux binaries. If you have scripts to download the latest stable Linux binary, update them to use linux-x86_64 instead of linux-x86_64-static (if you are already using the former, nothing needs to change). For this release, both are supported, but the next release will no longer have the -static variant. - We are also deprecating the download links at https://stackage.org/stack. See this page for the current installation instructions: https://docs.haskellstack.org/en/stable/install_and_upgrade/. - These are the canonical locations to download the latest stable binaries from, and will continue to be supported going forward: - Linux 64-bit (static): https://get.haskellstack.org/stable/linux-x86_64.tar.gz - macOS: https://get.haskellstack.org/stable/osx-x86_64.tar.gz - Windows 64-bit: https://get.haskellstack.org/stable/windows-x86_64.zip As always, binaries for specific versions are available from the Github releases: https://github.com/commercialhaskell/stack/releases. *Changes since v2.1.3.1* Major changes: - setup-info-locations yaml configuration now allows overwriting the default locations of stack-setup-2.yaml. #5031 <https://github.com/commercialhaskell/stack/pull/5031> #2983 <https://github.com/commercialhaskell/stack/issues/2983> #2913 <https://github.com/commercialhaskell/stack/issues/2913> - The setup-info configuration key now allows overwriting parts of the default setup-info - The --setup-info-yaml command line flag now may be used in all stack commands such as stack build, and not only in stack setup - The --setup-info-yaml may specify multiple locations for stack-setup.yaml files. - The stack upload can read first reads environment Variable $HACKAGE_USERNAME and $HACKAGE_PASSWORD if they are missing only then asks for username or password - Fully remove GHCJS support. - Remove the freeze command. It has been replaced by lock files. Behavior changes: - Remove the deprecated --stack-setup-yaml command line argument in favor of --setup-info-yaml #2647 <https://github.com/commercialhaskell/stack/issues/2647> - We now recommend checking in generated cabal files for repos. When generating lock files for extra-deps that only include package.yaml files, a deprecation warning will be generated. Also, those packages will no longer be included in the generated lock files. See #5210 <https://github.com/commercialhaskell/stack/issues/5210>. Other enhancements: - Add build-output-timestamps flag in yaml. Setting it to true prefixes each build log output line with a timestamp. - Show warning about local-programs-path with spaces on windows when running scripts. See #5013 <https://github.com/commercialhaskell/stack/pull/5013> - Add ls dependencies json which will print dependencies as JSON. ls dependencies --tree is now ls dependencies tree. See #4424 <https://github.com/commercialhaskell/stack/pull/4424> - Remove warning for using Stack with GHC 8.8-8.10, and Cabal 3.0-3.2. - Allow relative paths in --setup-info-yaml and tool paths #3394 <https://github.com/commercialhaskell/stack/issues/3394> - Added the --only-locals flag. See #5272 <https://github.com/commercialhaskell/stack/issues/5272> Bug fixes: - Upgrade pantry: module mapping insertions into the database are now atomic. Previously, if you SIGTERMed at the wrong time while running a script, you could end up with an inconsistent database state. - --resolver global doesn't retrieve snapshots list from the internet beause doesn't need it. See #5103 <https://github.com/commercialhaskell/stack/issues/5103> - Fix using relative links in haddocks output. See #4971 <https://github.com/commercialhaskell/stack/issues/4971>. - Do not include generated cabal file information in lock files. See #5045 <https://github.com/commercialhaskell/stack/issues/5045>. - Use proper Hoogle executable path when installed automatically. See #4905 <https://github.com/commercialhaskell/stack/issues/4905> - Fix GHC version for batched package unregistration. See #4951 <https://github.com/commercialhaskell/stack/issues/4951> - Use Hoogle from the snapshot used and not the latest version. See #4905 <https://github.com/commercialhaskell/stack/issues/4905> - Resolve "'stty' is not recognized". See #4901 <https://github.com/commercialhaskell/stack/issues/4901> - Fix missing reconfigure check causing errors when a package gets used in multiple projects. See #5147 <https://github.com/commercialhaskell/stack/issues/5147> *Thanks to all our contributors for this release:* - Adam McCullough - Akshay Mankar - Brian Wignall - Cheah Jer Fei - Chris Done - David Baynard - davit nalchevanidze - Dino Morelli - Emanuel Borsboom - Felix Yan - flip111 - Flowers Safety - Fred Miller - Fyrbll - gdziadkiewicz - Good boy - Hypnoes.Liu - Iisus - Iustin Pop - ivanbakel - Javier Neira - Jeff Happily - jeffhappily - jneira - Justin Le - Kirill Zaborsky - Leif Metcalf - Leonhard Markert - Lokesh Mohanty - Matt Audesse - Michael Snoyman - Mihai Maruseac - NightRa - Phil Ruffwind - qinzhenlong - Robert Hensing - Sasha Bogicevic - Sibi - Sibi Prabakaran - Simon Michael - Steve Mao - Vlad Ki - Xia Li-yao

2 2

Re: [Haskell-cafe] Please repair Hugs-for-Windows download links
by Anthony Clayden 30 Apr '20

30 Apr '20

Hi Vanessa, Thorkil, Yes I'm afraid Hugs is unmaintained. If you look at that Hugs-Bugs thread, Neil was kind enough to reply, but it seems everyone is locked out of maintaining the site. > Isn’t hugs unmaintained? Cool stuff but definitely vintage! (Nearly) gone, but not forgotten. There's a few people over the years have built from source, there's a few repos on github. With a bit of time on my hands lately ... I've built the wonderful WinHugs from source, so that I can fix a long-standing bit of bogusness in GHC. (GHC's behaviour is just not worth trying to make sense of.) And ref the discussion of GHC's awkwardness wrt Windows, WinHugs is just so nicely Windows-y, with a few-clicks install. Persuading (Win)Hugs to compile wasn't plain sailing, and there's a few of the libraries/packages have bitrotted beyond repair. But it is sooooo worth it, compared to the ugly GHC experience; plus you get a half-decent records system with Trex. (I've just been adding field label puns to it, to catch up with GHC.) AntC >* On Apr 26, 2020, at 10:25 AM, Thorkil Naur via Haskell-Cafe <haskell-cafe at haskell.org <http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe>> wrote: *> >* Dear Cafe, *> >* Having recently had occasion to suggest the use of Hugs, I found that the *>* links provided for WinHugs and MinHugs on *> >* https://www.haskell.org/hugs/pages/downloading.htm <https://www.haskell.org/hugs/pages/downloading.htm> *> >* are broken. In the discussion "When did it become so hard to install *>* Haskell on Windows?" *> >* https://mail.haskell.org/pipermail/haskell-cafe/2020-April/132129.html <https://mail.haskell.org/pipermail/haskell-cafe/2020-April/132129.html> *> >* Anthony Clayden *> >* https://mail.haskell.org/pipermail/haskell-cafe/2020-April/132155.html <https://mail.haskell.org/pipermail/haskell-cafe/2020-April/132155.html> *> >* referred to *> >* https://mail.haskell.org/pipermail/hugs-bugs/2018-July/001914.html <https://mail.haskell.org/pipermail/hugs-bugs/2018-July/001914.html> *> >* which provides working links: *> >* https://www.haskell.org/hugs/downloads/2006-09/WinHugs-Sep2006.exe <https://www.haskell.org/hugs/downloads/2006-09/WinHugs-Sep2006.exe> *>* https://www.haskell.org/hugs/downloads/2006-09/MinHugs-Sep2006.exe <https://www.haskell.org/hugs/downloads/2006-09/MinHugs-Sep2006.exe> *> >* If someone with suitable access repaired these links, I would be very *>* grateful. I am an occasional user of both GHC and Hugs, and although GHC *>* shines when it comes to efficiency, I also often appreciate the modest *>* and simple Hugs. *> >* Thanks very much in advance. *> >* Best regards *>* Thorkil *

3 2

Re: [Haskell-cafe] When did it become so hard to install Haskell onWindows?
by Anthony Clayden 29 Apr '20

29 Apr '20

> I think the comments regarding users' proficiency with Chocolatey or PowerShell or whatever are beside the point. The truth is that *nobody* expects that anyone would need to be familiar with those tools in order to install software on Windows. As an example of what I'm talking about, consider Microsoft Visual Studio. This is a big, complex piece of software. And, obviously, it's an application that's used by software developers, rather than ordinary end users. During installation, it runs multiple PowerShell and Cmd scripts, and, for all I know, uses Chocolatey under the hood as well. The point is, I don't have to care about any of that. I install it the same way I install any other software in Windows: I click a few buttons and make a few checkbox selections, and it just runs*. Indeed, my experience too. Furthermore MSVC/Visual Studio upgraded the source distro 'Solution' for Hugs/WinHugs from 2006; and I could go ahead and build that with less than a dozen patches to bitrotted code and config. No sight of a command line needed. In comparison, building the Unixy bits was a nightmare. (And I think little of that was Windows-specific or mixed-platform specific.) > I understand perfectly well the issue with resource constraints preventing the creation of a better experience for Windows users. ... I don't understand. SLPJ is at *Microsoft*; his homepage is at *Microsoft*; his email address is *@Microsoft*. It's ironic that half of the threads Simon starts on ghc-devs relate to crashes/failed builds on *Windows*. And here we are (not for the first time) using up disproportionate airspace on Windows. I appreciate the sponsorship (in effect) the GHC community gets from Microsoft. Could their help provide a little more practical benefit?: there are surely Microsofters who know the tools and have the skills to deliver a few-clicks installer for GHC(?) > And I think it's safe to assume that Microsoft has a few more resources at its disposal to support this than the Haskell community does. So you just need to be honest about it, and state up front that there isn't a simple, seamless way to install the Haskell Platform in Windows at this time, but if you want to install it anyway, here's what you need to do. AntC

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Re: [Haskell-cafe] Hugs sources WITH TESTS?
by Anthony Clayden 29 Apr '20

29 Apr '20

Hi Siddharth, Hugs dates from a different age, when testing was something of an afterthought. I can suggest some places to look: * The hugs-bugs mailing list is the only way to (possibly) contact the original developers. * There's a couple of repos on github: Franklin Chen, Chris Done. Franklin has also grabbed the 2009 developer snapshot. But I suspect the answer is: there are no tests/never were. AntC > I got interested in hugs thanks to the surrounding discussion and went spelunking in their codebase. Weirdly, I can't find a test suite! - The November 2003 version <https://github.com/bollu/hugs/blob/master/src/Makefile.in#L280> has a rule in the makefile to cd ../tests . However, such a folder does not exist in the source tree. - the September 2006 version <https://github.com/haskell-implementations/hugs/blob/master/src/Makefile.in>, doesn't have a test / check rule at all. I find this baffling. Where did the tests go?! If you have access to them --- or if you the story behind where the tests are, please do let me know!

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Problem with publishing project/packages
by Dominik Schrempf 28 Apr '20

28 Apr '20

Hello, I am not sure if this is the right place to report/discuss the following issue, but I have been fiddling around with properly publishing a project I am developing in Haskell for quite some time now. I have encountered various difficulties that I would like to share and hear your opinion about, and also your suggestions about what I could improve, and probably also, how the Haskell ecosystem could be improved. First, I use Stack+Hpack to manage `.cabal` files. I know, that this combination has summoned many controversies, but I don't want to spend hours fiddling around in my `.cabal` files and on the version bounds, so decided for this option. I use strict PVP bounds (--pvp-bounds both options), so that at least compilation should be reproducible, albeit only allowing the specific set of library versions present in the Stack resolver. My project does heavy numerical calculations, so I was awaiting the support of LLVM9.0, which will come with GHC 8.10, for a long time. In the meantime, I tried to compile and publish my project in its current state with the newest Stack resolver 15.x with GHC 8.8.3. On my local machine, compilation and the development cycle work reasonably well, but as soon as I try to publish my work, I have serious trouble. First, I encountered an issue with auto generated modules of the packages causing trouble in the `.cabal` files generated by Hpack [1]. I think this is because the Cabal specification was improved (i.e., has changed), and new specifications have to be honored. I try to work around the issue by directly specifying the newest Cabal specification 3.0 and by tinkering around with the flags, until Hpack produces files compatible with the newest version of Cabal. Finally, I can create source tarballs with `stack sdist --pvp-bounds both`. However, I cannot upload these tarballs, because somehow, the created `.cabal` files in the source tarballs include dependencies of the following form: `base : {} >= x.y, <= z.a` (or something similar), and Hackage chokes on this form (is this a new syntax? or is it just a bug?). So I think, maybe this is caused by an incompatibility of Stack with my Cabal or cabal-install version, which are newer than the newest ones officially supported by Stack. I had already lost one day of working time at this point. I did not give up, but decided to install a Cabal version officially supported by Stack. Also, I removed all my dependencies on auto-generated modules from the packages and the `.cabal` files, I reverted other changes I had implemented because of the Cabal specification change (for example, reverting all license identifiers). I managed to compile and publish my project on Hackage. However, Hackage is not able to compile the packages and show the documentation (which is what I need most). Because I am going the easy route, and use strict PVP bounds, Hackage cannot compile my packages because they depend on base >= 4.13, and Cabal version 3.0. It took me another hour or so, to find out that the official package repository of Haskell does not support the official base library which has been published over six months ago. From a users point of view, this is unacceptable. I have found some resources about this issue now, but I was not aware of this discrepancy before experiencing it. So my next thought is: "Let's just put it on Stackage, because the best days of Hackage seem to be over...". However, I cannot do that, because my project needs to compile on Hackage in order to be put on Stackage... I guess all this is consequence of using an operating system with up-to-date packages (Arch Linux), and my inherent demand to use the newest compiler, and so on. However, I feel that it is not entirely my fault, the new compiler with the new base libraries should be supported by the official repository, and in a coherent way by the most widely used build tools Cabal and Stack (which uses Hpack by default, by the way). I sincerely apologize for being so harsh, but I am angry at the ecosystem, and angry at myself, because, obviously, I don't know enough to get things done. Thanks for your feedback, it is appreciated! Dominik Schrempf References: [1] https://github.com/sol/hpack/issues/384 [2] In case of interest, the project I am developing is a set of libraries/binaries to be used in evolutionary biology https://github.com/dschrempf/elynx

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Re: [Haskell-cafe] Problem with publishing project/packages
by Olaf Klinke 28 Apr '20

28 Apr '20

> $ cabal haddock --for-hackage --hyperlink-source > $ cabal upload --documentation --publish dist/pkg-0.1-docs.tar.gz I also have projects where the normal build is fine but `stack haddock` fails on some dependency. I resorted to `stack haddock --no-haddock-deps` and at least obtained documentation for _my_ package, but with missing/broken links to the dependencies. Olaf

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Questions: Using Hackage as a package maintainer?
by Saurabh Nanda 28 Apr '20

28 Apr '20

Hi, I seem to be struggling with Hackage [1], specifically package candidates. Is this the right place to ask the following questions: - Does Hackage generate docs for package candidates? How long does it take. I have waited for 12+ hours and can't seem to find any docs. - If building docs fails, where is the error displayed? (especially for package candidates) - Do **warnings** affect this process? I see the following warning once I upload a package candidate: 'ghc-options: -main-is' is not portable. - Once I upload a package candidate, shouldn't it show-up on this page automatically - https://hackage.haskell.org/packages/candidates/ ? (my package is not showing-up here) - Is there a page which shows all the packages that have been uploaded by my Hackage user account? [1] https://stackoverflow.com/questions/61463139/does-hackage-generate-haddocks… -- Saurabh.

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Re: [Haskell-cafe] When did it become so hard to install Haskell onWindows?
by Gregory Guthrie 28 Apr '20

28 Apr '20

Re: Windows installs Anthony Clayden <anthony_clayden(a)clear.net.nz> writes: > ... > Users who don't want to use Chocolatey are encouraged to use the usual > binary distributions [1], as in the past. > [1] https://downloads.haskell.org/ghc/8.10.1/ Thanks; I didn't see any binary labelled Windows there. Dr. Gregory Guthrie Maharishi International University ----------------------------------------------------------------

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Munich Virtual Haskell Meeting, 2020-04-30 @ 19:30
by Leah Neukirchen 27 Apr '20

27 Apr '20

Dear all, This week, our monthly Munich Haskell Meeting will take place again on Thursday, April 30. **Due to curfew in Bavaria, this meeting will take place online!** For details see here: http://muenchen.haskell.bayern/dates.html A Google Meet link to join the room will be provided on Thursday. Everybody is welcome, especially the Haskellers from Bavaria that do not usually come to our Munich meetings due to travel distance! cu, -- Leah Neukirchen <leah(a)vuxu.org> https://leahneukirchen.org/

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Hugs sources WITH TESTS?
by Siddharth Bhat 27 Apr '20

27 Apr '20

Hello, I got interested in hugs thanks to the surrounding discussion and went spelunking in their codebase. Weirdly, I can't find a test suite! - The November 2003 version <https://github.com/bollu/hugs/blob/master/src/Makefile.in#L280> has a rule in the makefile to cd ../tests . However, such a folder does not exist in the source tree. - the September 2006 version <https://github.com/haskell-implementations/hugs/blob/master/src/Makefile.in>, doesn't have a test / check rule at all. I find this baffling. Where did the tests go?! If you have access to them --- or if you the story behind where the tests are, please do let me know! Thanks a lot, ~Siddharth -- https://bollu.github.io/

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Re: [Haskell-cafe] Windows Haskell CI
by Ben Franksen 27 Apr '20

27 Apr '20

Am 27.04.20 um 15:26 schrieb J. Garrett Morris: > You might find Tamar's post on Windows packaging of GHC useful. He > includes CI support as one of the motivations: > > https://hub.zhox.com/posts/introducing-haskell-dev/ Yes, very useful indeed. Thanks! Cheers Ben

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Re: [Haskell-cafe] When did it become so hard to install Haskell onWindows?
by Anthony Clayden 27 Apr '20

27 Apr '20

Remember GHC's motto is 'avoid success at all costs'. Then naturally it is prohibitively difficult to get to use GHC. For students/people who you want to encourage to love Haskell, especially on Windows, I'm astonished you're not using Hugs, especially WinHugs (2-click install). Despite being over a dozen years unsupported it is still orders-of-magnitude more friendly than GHC, and has plenty of functionality (in Hugsmode) for undergraduate level. What's more Haskell from the intro texts just works on it; whereas GHC throws all sorts of obscure advanced type errors. I don't think powershell is a 'standard tool'. I use mostly Windows machines, I'm aware of powershell, I've never used it. Chocolatey is an abhorence. Fortunately I've never had to use it; I don't know why GHC would inflict it on anybody. Increasingly, GHC HQ is a cult/elite that doesn't want any new members. The difficulties in trying to use GHC just show how exclusive it has become. AntC > I appreciate that these things are standard tools for Windows developers, but it's worth noting how much harder it can make things for completely new people (either new developers or new to Windows). > At the start of the year, I prepared install instructions for university students who would be using Haskell as part of a first year CS course. We needed to use GHC 8.6.5 because certain libraries were not available for GHC 8.8.x (their base upper bounds hadn't updated, which ruled out haskell-dev), and tried to use Chocolatey as an experiment. > It was remarkably tough to get students set up on their own machines. I was planning on recommending the Haskell Platform installer for Semester 2 this year, and am disappointed to find that it no longer exists. > If it becomes too hard for students to install Haskell on their own Windows machines, it may become too hard for us to use Haskell as an educational tool, and I'd consider that a tragedy.

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how can we fix our license?
by Ben Franksen 27 Apr '20

27 Apr '20

After uploading a new release i found that this and all our earlier release show on hackage as "GPL-2.0-only". What we have in our darcs.cabal is "GPL-2". This was always intended to mean "or later". I wanted to fix this and upload a new release that uses the new variant "GPL-2.0-or-later" that I read about in the docs. But this isn't accepted by hackage. The upload candidate page responds with darcs-2.14.4/darcs.cabal:0:0: version with tags and cabal check says Warning: These warnings may cause trouble when distributing the package: Warning: 'license: GPL-2.0' is not a known version of that license. The known versions are 2, 3. If this is not a mistake and you think it should be a known version then please file a ticket. Warning: darcs.cabal:4:33: version with tags Note we have cabal-version: 1.24 in the cabal file, otherwise we could not support older ghc versions (we currently support ghc-8.0 and later). How can I fix this?

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Installing haddock, cabal-install, ... when building GHC from git source
by Viktor Dukhovni 27 Apr '20

27 Apr '20

The GHC source tree includes sources for haddock, cabal, ... which I expected to be able to somehow install as part of "make install" or "make binary-dist" and installing from that, ... But I've so far failed to find the right set of incantations. Is it possible to install the additional libraries and utilities included with the source when building GHC from source? -- Viktor.

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Please repair Hugs-for-Windows download links
by Thorkil Naur 26 Apr '20

26 Apr '20

Dear Cafe, Having recently had occasion to suggest the use of Hugs, I found that the links provided for WinHugs and MinHugs on https://www.haskell.org/hugs/pages/downloading.htm are broken. In the discussion "When did it become so hard to install Haskell on Windows?" https://mail.haskell.org/pipermail/haskell-cafe/2020-April/132129.html Anthony Clayden https://mail.haskell.org/pipermail/haskell-cafe/2020-April/132155.html referred to https://mail.haskell.org/pipermail/hugs-bugs/2018-July/001914.html which provides working links: https://www.haskell.org/hugs/downloads/2006-09/WinHugs-Sep2006.exe https://www.haskell.org/hugs/downloads/2006-09/MinHugs-Sep2006.exe If someone with suitable access repaired these links, I would be very grateful. I am an occasional user of both GHC and Hugs, and although GHC shines when it comes to efficiency, I also often appreciate the modest and simple Hugs. Thanks very much in advance. Best regards Thorkil

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Re: [Haskell-cafe] When did it become so hard to install Haskell onWindows?
by Anthony Clayden 26 Apr '20

26 Apr '20

Thank you Ben > I'd urge everyone to keep in mind that our Windows support exists almost entirely due to the efforts of Tamar; we all owe Tamar a debt of gratitude for all of the work he has done over the past years. We could certainly use more people who are able and willing to contribute resources to help improve GHC's Windows story. I've tried and tried, to the point where I was clearly considered a nuisance. I've now given up. And GHC has just got worse since. I suggest Tamar downloads/installs Hugs [*], then provides (and documents) something as easy as that. Wasn't there a GHC equivalent to WinHugs? That is: something as easy for casual/recreational users/learners of Haskell, not aimed at Microsoft afficianados. [*] Sadly, I apologise in advance for this: https://mail.haskell.org/pipermail/hugs-bugs/2018-July/001914.html >>* Increasingly, GHC HQ is a cult/elite that doesn't want any new members.* > To the contrary, we both want and need new contributors! Not my experience. I was told in polite but unambiguous terms to stop contributing. > I would ask anyone who feels that GHC development is exclusionary to please be in touch; I am very interested to know how we can improve. I'd like to cite as evidence the survey results Francesco links to: I don't think I ever got invited to contribute -- where would that have been? There wasn't an invite on the Hugs-users list. Note I don't use Reddit because I'm not a brogrammer. The survey is kinda self-fulfilling: most people don't use GHC on Windows, because it's too hard to install. GHC is exclusionary against casual/recreational Haskellers (who are more likely on Windows) and those more interested in using it to explore PL Theory. This is the opposite of how it was when I first grew to love Haskell around a decade ago. Just look at the buzz on the cafe back then. AntC

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Re: [Haskell-cafe] When did it become so hard to install Haskell onWindows?
by Anthony Clayden 26 Apr '20

26 Apr '20

@lonetiger, I take it you've never had to support software installs on shared machines, or students' machines where you can't be too sure of the config? The reasons against using powershell and curl (or equiv) I would have thought are pretty obvious: you don't know what you're getting/what it might be downloading. You might mis-type some command and cause havoc on your machine or on your network. With Hugs you download the packed-up .exe; you disconnect from the internet and run virus checks; you run the install while your machine is still in quarantine. With Hugs install you (usually) don't need to go into Admin mode. Just because some long-winded process is "completely standard on Windows", doesn't mean it's fit for use. That's a general learning about stuff from Microsoft. AntC ➢ Then I have to know what powershell.exe is, use an administrative prompt, and enter scary commands in it. > Powershell has been the standard shell in Windows for well over the past decade. Every single script from Microsoft or third parties come with powershell for automation. It’s understandable that you may not know it since your primary platform isn’t Windows. But it’s been included in every single Windows version for the past 13 years. > An administrative prompt is nothing different than running sudo or clicking on that installer that you *assumed* not to be scary because you didn’t see the actions it was performing. That scary looking command is nothing but a curl command allowing the one time execution of a script from a remote source. As in a script that’s not physically on your machine. > So what exactly makes this scary? Is it because > Set-ExecutionPolicy Bypass -Scope Process -Force; [System.Net.ServicePointManager]::SecurityProtocol = [System.Net.ServicePointManager]::SecurityProtocol -bor 3072; iex ((New-Object System.Net.WebClient).DownloadString('https://chocolatey.org/install.ps1')) > Is more verbose than > curl -sSL https://path.to.some.script/ | sh > or because the technologies used while completely standard on Windows aren’t known to the casual user?

3 2

Need help with advanced type-level programming
by Evgeny Permyakov 26 Apr '20

26 Apr '20

Consider following code data Tag :: (Symbol -> Constraint) -> Symbol -> * where Tag :: (KnownSymbol s, con s) => Tag con s data TagList :: (Symbol -> Constraint) -> [Symbol] -> * where LNil :: TagList con '[] LCon :: Tag con s -> TagList con ss -> TagList con (s ': ss) data TagSet :: (Symbol -> Constraint ) -> [Symbol] -> * where SNil :: TagSet con '[] SSin :: Tag con s -> TagSet con '[s] SCon :: CmpSymbol ht mt ~ LT => Tag con ht -> TagSet con (mt ': ts) -> TagSet con (ht ': mt ': ts) How one would write a function converting a TagList into a TagSet , dropping duplicate tags ? I tried a few approaches, but failed. I clearly lack understanding how to fit it into GHC type system. I know there is a library type-level-sets, but it isn't what I would like and I don't understand how it fits into GHC type system, so it appears to be magical to me. Similarly, it would be greatly appreciated if someone could show how to implement following typeclass class CmpTag (s1 :: Symbol) (s2 :: Symbol) where cmpTag :: Tag con s1 -> Tag con s2 -> OrdTag con (CmpSymbol s1 s2) s1 s2 data OrdTag :: (Symbol -> Constraint) -> Ordering -> Symbol -> Symbol -> * where TLT :: CmpSymbol s1 s2 ~ LT => Tag con s1 -> Tag con s2 -> OrdTag con LT s1 s2 TEQ :: (CmpSymbol s1 s2 ~ EQ, s1 ~ s2) => Tag con s -> Tag con s2 -> OrdTag con EQ s1 s2 TGT :: CmpSymbol s1 s2 ~ GT => Tag con s1 -> Tag con s2 -> OrdTag con GT s1 s2

3 3

Getting the profile results for GHC when running in Cabal.
by Sean Parsons 25 Apr '20

25 Apr '20

Hi there. I've built a GHC with profiling enabled as guided by the wiki and that works fine standalone. I found the option for specifying a certain GHC to Cabal. But I'm a little baffled as to how to get the results of the GHC profile when building a library with Cabal. My goal is to try to get a baseline profile, then have a poke at the innards of GHC and then see if whatever I've changed has made an improvement to the build speed. Does anyone have any idea what the best way to do that is? Thanks, Sean.

2 1

base library on hackage
by Ben Franksen 25 Apr '20

25 Apr '20

It seems the latest version uploaded there is 4.12. Where can I find docs for later versions?

3 2

parsing Haskell expressions
by Johannes Waldmann 24 Apr '20

24 Apr '20

Dear Cafe, for a language that has (subset-of) Haskell expression syntax, but different global declaration syntax, we are using parseExp from haskell-src-exts. I have two question on this. I am asking here because https://github.com/haskell-suite/haskell-src-exts#maintenance says that it's no longer active. (1) can we make parseExp self-delimiting? The type is parseExp :: String -> ParseResult (Exp SrcSpanInfo) but I'd want "maximum munch" and the part that was not eaten, as in :: String -> ParseResult (.. , String) I tried parseExp "x } foo bar" ParseFailed (SrcLoc "<unknown>.hs" 1 3) "Unexpected }" so I could extract the error location, and then re-parse that prefix. That's feels a bit strange. (2) perhaps we don't need (1) if we do the chunking on the outside. E.g., we run a lexer and look for delimiters (keywords of the host language that then are forbidden in inner expressions). We want to use lexTokenStream :: String -> ParseResult [Loc Token] and then I'd want parseExp' :: [Token] -> ... Else, we'd have to reconstruct the slice of the original input, which again, feels a bit strange and (because) it duplicates work. It is suggested to use http://hackage.haskell.org/package/ghc-lib-parser instead. Would this help with the above? But then of course (3) is there a transition strategy (e.g., wrapper modules)? Thanks - J.W. NB: this is the issue from the project's tracker: https://gitlab.imn.htwk-leipzig.de/waldmann/cyp/issues/45

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Re: [Haskell-cafe] Better names for
by Olaf Klinke 24 Apr '20

24 Apr '20

> > fmap f. If I could re-define Haskell from scratch, I'd probably > >name > > the Functor type class function 'map' > > As it was in Haskell 1.4 That's interesting, I did not know this. Thanks for sharing. Are there any public records as to why the decision was made to re-name 'map' to 'fmap'? I can not find any mention in the Haskell98 report(s). There is a stackoverflow thread (#6824255) which mentions paedagogical reasons, simplifying error messages. I sometimes use the current naming 'map' to specialize a polymorphic function to work exclusively on lists where any functor could have been used. Other than that, I can not quite see the use in keeping 'map' around when it is equivalent to fmap. What reasons are there apart from backwards compatibility? Are there re-write rules that make 'map's (operational) semantics different to 'fmap'? Olaf

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Does GHC recognise that CmpSymbol s1 s2 ~ EQ implies s1 ~ s2 ?
by Evgeny Permyakov 24 Apr '20

24 Apr '20

By semantic CmpSymbol s1 s2 ~ EQ should be equivalent to s1 ~ s2 for s1 , s2 :: Symbol Does GHC recognize it? If not, is there some trick to obtain evidence of the second constraint from the first ? I'm talking about context of type class instance declarations.

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Re: [Haskell-cafe] Better names for
by Olaf Klinke 23 Apr '20

23 Apr '20

> I've heard several times that some Haskell type classes and methods > are > ill-named. > > For instance, a functor can be both covariant and contravariant, > while > Haskell `Functor` typeclass i just covariant. Would `Cofunctor` be a > better name for it? And what about the `fmap` method? Would it be > better > to call it `comap`? How about `Bifunctor`, `Profunctor` and others? Hi Marc, part of the answer is: For historical reasons and to preserve backwards compatibility. The term "Functor", however, comes from category theory. MacLane [1] defines the term "Functor" to refer to covariant functors. The notion of "contravariant" is introduced via the concept of "opposite category" [2] which has the same objects but direction of arrows reversed. Thus a contravariant functor F: C -> D is just an ordinary functor C -> D^op. Category theory has no name for the action of a functor on arrows, Haskell had to make up a name, which ended up to be "fmap". Sometimes the notation F(f) is used in category theory where in Haskell we write fmap f. If I could re-define Haskell from scratch, I'd probably name the Functor type class function 'map' and omit the map function on lists, since it is the same as the Functor instance. As for your suggestion of 'comap': The prefix co- in category theory is mostly used in conjunction with the opposite construction [3], whence a category theorist would believe 'comap' to mean the opposite of 'map'. Regards, Olaf [1] Saunders MacLane: Categories for the Working Mathematician. ISBN 7- 5062-6008-5 [2] See Op in Data.Functor.Contravariant [3] See categorical concepts like limit vs. colimit, cone vs. cocone.

2 1

Better names for
by Marc Busqué 22 Apr '20

22 Apr '20

Hi there! I've heard several times that some Haskell type classes and methods are ill-named. For instance, a functor can be both covariant and contravariant, while Haskell `Functor` typeclass i just covariant. Would `Cofunctor` be a better name for it? And what about the `fmap` method? Would it be better to call it `comap`? How about `Bifunctor`, `Profunctor` and others? Thanks, Marc Busqué http://waiting-for-dev.github.io/about/

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Where is Show for GHC AST?
by Daneel Yaitskov 22 Apr '20

22 Apr '20

Hi, I am trying to modify LiquidHaskell and want to see how source code is represented in AST format. I think I have AST i.e. HsExpr value, but there is no Show instance for that. I noticed that GHC classes implement similar class Outputable, but it doesn't do what I expected. It just reconstruct AST back to source code. I could probably write a TH function generating Show instances for all types which HsExpr depends on, but I hope there is more straight forward way to do that. And after all I curious what is the reason for not having Show for GHC AST types? It would be so convenient having Show. -- Daneel S. Yaitskov Java Developer

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Re: [Haskell-cafe] Where is Show for GHC AST?
by Michal J Gajda 22 Apr '20

22 Apr '20

Hi Daneel, Did you check the `Outputable` class? http://hackage.haskell.org/package/ghc-8.6.5/docs/Outputable.html#t:Outputa… -- Cheers Michał

1 0

FMBC 2020: 2nd Workshop on Formal Methods for Blockchains (CfP, Deadline Extension)
by Bruno Bernardo 21 Apr '20

21 Apr '20

[ Please distribute, apologies for multiple postings. ] ======================================================================== 2nd Workshop on Formal Methods for Blockchains (FMBC) 2020 - Second Call https://fmbc.gitlab.io/2020 July 19, 2020, Los Angeles, USA Co-located with the 32nd International Conference on Computer-Aided Verification (CAV 2020) http://i-cav.org/2020/ ------------------------------------------------------------- IMPORTANT DATES -------------------------------- Abstract submission: May 5, 2020 (extended) Full paper submission: May 12, 2020 (extended) Notification: June 16, 2020 (extended) Camera-ready: July 7, 2020 Conference: July 19, 2020 -------------------------------- -------------------------------- TOPICS OF INTEREST -------------------------------- Blockchains are decentralized transactional ledgers that rely on cryptographic hash functions for guaranteeing the integrity of the stored data. Participants on the network reach agreement on what valid transactions are through consensus algorithms. Blockchains may also provide support for Smart Contracts. Smart Contracts are scripts of an ad-hoc programming language that are stored in the Blockchain and that run on the network. They can interact with the ledger’s data and update its state. These scripts can express the logic of possibly complex contracts between users of the Blockchain. Thus, Smart Contracts can facilitate the economic activity of Blockchain participants. With the emergence and increasing popularity of cryptocurrencies such as Bitcoin and Ethereum, it is now of utmost importance to have strong guarantees of the behavior of Blockchain software. These guarantees can be brought by using Formal Methods. Indeed, Blockchain software encompasses many topics of computer science where using Formal Methods techniques and tools are relevant: consensus algorithms to ensure the liveness and the security of the data on the chain, programming languages specifically designed to write Smart Contracts, cryptographic protocols, such as zero-knowledge proofs, used to ensure privacy, etc. This workshop is a forum to identify theoretical and practical approaches of formal methods for Blockchain technology. Topics include, but are not limited to: * Formal models of Blockchain applications or concepts * Formal methods for consensus protocols * Formal methods for Blockchain-specific cryptographic primitives or protocols * Design and implementation of Smart Contract languages * Verification of Smart Contracts -------------------------------- -------------------------------- SUBMISSION -------------------------------- Submit original manuscripts (not published or considered elsewhere) with a maximum of twelve pages (full papers), six pages (short papers), and two pages (extended abstract) describing new and emerging ideas or summarizing existing work). Each paper should include a title and the name and affiliation of each author. Authors of selected extended-abstracts are invited to give a short lightning talk. At least one author of an accepted paper is expected to present the paper at the workshop as a registered participant. Submission link: https://easychair.org/conferences/?conf=fmbc2020 The authors are encouraged to use LaTeX and the EasyChair style files: https://easychair.org/publications/for_authors -------------------------------- -------------------------------- PROCEEDINGS -------------------------------- All submissions will be peer-reviewed by at least three members of the program committee for quality and relevance. Accepted regular papers (full and short papers) will be included in the workshop proceedings. -------------------------------- -------------------------------- INVITED SPEAKER -------------------------------- Grigore Rosu, Professor at University of Illinois at Urbana-Champaign, USA and Founder of Runtime Verification http://fsl.cs.illinois.edu/index.php/Grigore_Rosu https://runtimeverification.com/ -------------------------------- -------------------------------- PROGRAM COMMITTEE -------------------------------- PC CO-CHAIRS * Bruno Bernardo (Nomadic Labs, France) (bruno(a)nomadic-labs.com) * Diego Marmsoler (University of Exeter, UK) (D.Marmsoler(a)exeter.ac.uk) PC MEMBERS * Wolfgang Ahrendt (Chalmers University of Technology, Sweden) * Lacramioara Astefanoei (Nomadic Labs, France) * Massimo Bartoletti (University of Cagliari, Italy) * Bernhard Beckert (Karlsruhe Institute of Technology, Germany) * Achim Brucker (University of Exeter, UK) * Silvia Crafa (Universita di Padova, Italy) * Zaynah Dargaye (Nomadic Labs, France) * Jérémie Decouchant (University of Luxembourg, Luxembourg) * Ansgar Fehnker (University of Twente, Netherlands) * Georges Gonthier (Inria, France) * Florian Kammueller (Middlesex University London, UK) * Maurice Herlihy (Brown University, USA) * Igor Konnov (Informal, Austria) * Andreas Lochbihler (Digital Asset, Switzerland) * Anastasia Mavridou (NASA Ames, USA) * Simão Melo de Sousa (Universidade da Beira Interior, Portugal) * Andrew Miller (University of Illinois at Urbana-Champaign, USA) * Karl Palmskog (KTH, Sweden) * Vincent Rahli (University of Birmingham, UK) * Andreas Rossberg (Dfinity Foundation, Germany) * Claudio Russo (Dfinity Foundation, USA) * César Sanchez (Imdea, Spain) * Clara Schneidewind (TU Wien, Austria) * Ilya Sergey (Yale-NUS College/NUS, Singapore) * Bas Spitters (Aarhus University/Concordium, Denmark) * Mark Staples (CSIRO Data61, Australia) * Meng Sun (Peking University, China) * Simon Thompson (University of Kent, UK) * Philip Wadler (University of Edinburgh / IOHK, UK)

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