- Haskell-Cafe - Haskell.org

Coercing newtype-wrapped monad transformers?
by Viktor Dukhovni 12 May '20

12 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 11 May '20

11 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 | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^...

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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

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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

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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

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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

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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)

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