- Haskell-Cafe - Haskell.org

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 | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^...

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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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