- Haskell-Cafe - Haskell.org

HiW'19: Call for Talks
by Niki Vazou 21 Jun '19

21 Jun '19

Hey all, Haskell Implementors Workshop is calling for talk proposals. Co-located with ICFP it is an ideal place to describe a Haskell library, a Haskell extension, works-in-progress, demo a new Haskell-related tool, or even propose future lines of Haskell development. Deadline is June 28th. Call for Talks The 11th Haskell Implementors’ Workshop is to be held alongside ICFP 2019 this year in Berlin. It is a forum for people involved in the design and development of Haskell implementations, tools, libraries, and supporting infrastructure, to share their work and discuss future directions and collaborations with others. Talks and/or demos are proposed by submitting an abstract, and selected by a small program committee. There will be no published proceedings. The workshop will be informal and interactive, with open spaces in the timetable and room for ad-hoc discussion, demos and lightning talks. Scope and Target Audience It is important to distinguish the Haskell Implementors’ Workshop from the Haskell Symposium which is also co-located with ICFP 2019. The Haskell Symposium is for the publication of Haskell-related research. In contrast, the Haskell Implementors’ Workshop will have no proceedings – although we will aim to make talk videos, slides and presented data available with the consent of the speakers. The Implementors’ Workshop is an ideal place to describe a Haskell extension, describe works-in-progress, demo a new Haskell-related tool, or even propose future lines of Haskell development. Members of the wider Haskell community encouraged to attend the workshop – we need your feedback to keep the Haskell ecosystem thriving. Students working with Haskell are specially encouraged to share their work. The scope covers any of the following topics. There may be some topics that people feel we’ve missed, so by all means submit a proposal even if it doesn’t fit exactly into one of these buckets: - Compilation techniques - Language features and extensions - Type system implementation - Concurrency and parallelism: language design and implementation - Performance, optimization and benchmarking - Virtual machines and run-time systems - Libraries and tools for development or deployment Talks We invite proposals from potential speakers for talks and demonstrations. We are aiming for 20-minute talks with 5 minutes for questions and changeovers. We want to hear from people writing compilers, tools, or libraries, people with cool ideas for directions in which we should take the platform, proposals for new features to be implemented, and half-baked crazy ideas. Please submit a talk title and abstract of no more than 300 words. Submissions can be made via HotCRP at https://icfp-hiw19.hotcrp.com/ until June 28th (anywhere on earth). We will also have lightning talks session. These have been very well received in recent years, and we aim to increase the time available to them. Lightning talks be ~7mins and are scheduled on the day of the workshop. Suggested topics for lightning talks are to present a single idea, a work-in-progress project, a problem to intrigue and perplex Haskell implementors, or simply to ask for feedback and collaborators. Invited Speakers - Lennart Augustsson & Satnam Singh Program Committee - Jose Calderon (Galois, Inc) - Jasper Van der Jeugt (Fugue) - Niki Vazou (IMDEA Software Institute) - Ningning Xie (The University of Hong King) - Brent Yorgey (Hendrix College) Best, Niki Vazou

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ANN: network-3.1.0.1
by Evan Borden 20 Jun '19

20 Jun '19

Hello, I'm happy to announce the release of network-3.1.0.1. This release includes a few maintenance changes. http://hackage.haskell.org/package/network-3.1.0.1 Contributors ============ * Evan Borden @eborden * Jack Kelly @endgame * Kazu Yamamoto @kazu-yamamoto * Leif Metcalf @leifmetcalf Changes ======= You can view the changes on github. https://github.com/haskell/network/releases/tag/v3.1.0.1 Thank you to network's contributors for their continued support. -- Evan Borden

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functional languages - pretty good for students!
by Dennis Raddle 19 Jun '19

19 Jun '19

I've been doing computer science tutoring for a few years in languages like Java and Python. I started teaching a 10th grade student Elm recently. I'd like to use Haskell at some point, but because he likes websites and games, Elm seemed more apropos. What I'm discovering is that a functional language like Elm is fairly natural to him and he's learning quickly! Our lessons prior to Elm (including Python, Java, and JavaScript at this point) are extracurricular and he hasn't put much study in-between lessons, so his progress has been slow. I don't think he's developed many "programming muscles." But now, his progress in Elm is faster. One factor, I think, is how concise functional code is. He hasn't developed much "programmer's eye" up to now (by which I mean scanning a lot of code quickly) but Elm, being much shorter, is easier for him to grasp at a glance. He also thinks the algebraic data model is cool. He sees how it's related to classes and subclasses, but he appreciates how relatively simple it is. Mike

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How to define classy lenses for polymorphic types that involve singletons?
by Peter Simons 18 Jun '19

18 Jun '19

Hi, I am trying to combine the lens library's 'makeClassy' feature with a type that's polymorphic over a singleton type: > {-# LANGUAGE DataKinds, FlexibleInstances, FunctionalDependencies, GADTs, > KindSignatures, RankNTypes, TemplateHaskell, TypeFamilies > #-} > > import Control.Lens > import Data.Singletons > import Data.Singletons.TH > > data Sex = Male | Female > deriving (Show, Eq, Ord, Bounded, Enum) > > genSingletons [''Sex] > > data Person (sex :: Sex) = Person { _name :: String, _email :: String } > deriving (Show, Eq) > > makeClassy ''Person Lens generates a class definition that looks sensible to me: class HasPerson a (sex :: Sex) | a -> sex where person :: Lens' a (Person sex) email :: Lens' a String name :: Lens' a String {-# MINIMAL person #-} Furthermore, I also need a type SomePerson to hide the phantom type so that I can store people of different sexes in the same container, i.e. [SomePerson]: > data SomePerson where > SomePerson :: Sing sex -> Person sex -> SomePerson > > fromPerson :: SingI sex => Person sex -> SomePerson > fromPerson p = SomePerson Sing p > > toPerson :: SomePerson -> (forall sex. Sex -> Person sex -> a) -> a > toPerson (SomePerson s p) f = f (fromSing s) p Here is where I've run into trouble. In theory, I should be able to make SomePerson an instance of HasPerson, define person :: Lens' SomePerson (Person sex) to access the Person type inside of it, and that would allow me to use 'name' and 'email' for SomePerson just the same as for Person. However, it seems impossible to define that function because it leaks the universally quantified 'sex', so function does not type-check. I have a somewhat awkward work-around that translates lenses on Person to SomePerson > somePerson :: (forall sex. Lens' (Person sex) a) -> Lens' SomePerson a > somePerson l = lens (\(SomePerson _ p) -> view l p) > (\(SomePerson s p) x -> SomePerson s (set l x p)) and that allows me to define: > type SomePerson' (sex :: Sex) = SomePerson > > instance HasPerson (SomePerson' sex) sex where > person = undefined -- cannot type check because 'sex' would leak > name = somePerson name > email = somePerson email I'm not particularly happy with that solution, though. Is there maybe a way to make this work such that I can avoid defining 'name' and 'email' manually? Or is there a clever alternative way to define HasPerson such a 'person' method for SomePerson is possible? Best regards Peter

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Lenses and Nested Maps
by Henry Laxen 18 Jun '19

18 Jun '19

Hi Cafe, Suppose I have: s :: Map String (Map Int Char) s = M.fromList [("alice", M.fromList [(5,'a')]), ("bob", M.fromList [(6,'b')])] Is there any way to modify the 'b' entry easily using lenses? Thanks in advance. Best wishes, Henry Laxen

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[ANNOUNCE] GHC 8.8.1-alpha2 is now available
by Ben Gamari 17 Jun '19

17 Jun '19

Hello everyone, The GHC team is pleased to announce the second and likely last alpha release of GHC 8.8.1. The source distribution, binary distributions, and documentation are available at https://downloads.haskell.org/~ghc/8.8.1-alpha2 A draft of the release notes is also available [1]. This release is the culmination of over 3000 commits by over one hundred contributors and has several new features and numerous bug fixes relative to GHC 8.6: * Profiling now works correctly on 64-bit Windows (although still may be problematic on 32-bit Windows due to platform limitations; see #15934) * A new code layout algorithm for amd64's native code generator * The introduction of a late lambda-lifting pass which may reduce allocations significantly for some programs. * Further work on Trees That Grow, enabling improved code re-use of the Haskell AST in tooling * More locations where users can write `forall` (GHC Proposal #0007) * Further work on the Hadrian build system This release brings a number of fixes since alpha 1: * A number of linker fixes (#16779, #16784) * The process, binary, Cabal, time, terminfo libraries have all been bumped to their final release versions * A regression rendering TemplateHaskell unusable in cross-compiled configurations has been fixed (#16331) * A regression causing compiler panics on compilation of some programs has been fixed (#16449) * -Wmissing-home-modules now handles hs-boot files correctly (#16551) * A regression causing some programs to fail at runtime has been fixed (#16066) Due to on-going work on our release and testing infrastructure this cycle is proceeding at a pace significantly slower than expected. However, we anticipate that this investment will allow us to release a more reliable, easier-to-install compiler on the planned six-month release cadence in the future. As always, if anything looks amiss do let us know. Happy compiling! Cheers, - Ben [1] https://downloads.haskell.org/ghc/8.8.1-alpha2/docs/html/users_guide/8.8.1-…

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Deprecation cycle for classes
by Tom Ellis 16 Jun '19

16 Jun '19

I am unsure how to change the name of a class in a library whilst providing a safe deprecation cycle for the users of the library. Suppose I have version 1 of my library with a class -- Version 1 class Foo a where foo :: a and I want to migrate it to version 3 as follows, but with a deprecation cycle provided by version 2 that allows code to work with either version 1 and 2, or 2 and 3. -- Version 3 class Bar a where bar :: a Then I can try to write version 2 as -- Version 2 {-# ConstraintKinds #-} class Foo a where bar :: a bar = foo foo :: a foo = bar type Bar = Foo This satisfies the following conditions: Users of versions 1 and 2 can use * The constraint Foo * The method foo * The class Foo and method foo for implementing their own instances User of versions 2 and 3 can use * The constraint Bar * The method bar * The method bar for implementing their own instances but they cannot jointly use * The *class* Bar for implementing their own instances (since Bar is not a class, just a Constraint) Is there any way I can write a version 2 that allows users to implement instances that work with both version 2 and version 3? Thanks, Tom

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Posting Request
by Michelle Keppler 14 Jun '19

14 Jun '19

Hello, Could you please post the following to the Haskell Cafe email? Galois is hiring for a variety of roles <https://news.us10.list-manage.com/track/click?u=49a6a2e17b12be2c5c4dcb232&i…> Including but not limited to: Software Engineers/Researchers, Project Managers, Hardware Engineers, and a Software Integration Engineer. We collaborate with organizations like NASA, DARPA, and AWS to explore blue sky ideas and turn them into usable technology. Some of the things we’ve worked on in the past: Formal methods, static analysis, cryptographic algorithms, abstract interpretation, type theory, formal verification, reinforcement learning, autonomous systems assurance, communication security, cyber-deception for network defense, DDoS defense, provable hardware security, and statistical anomaly detection for detecting advanced persistent threats. We think working here is awesome; see lifeatgalois.com <https://news.us10.list-manage.com/track/click?u=49a6a2e17b12be2c5c4dcb232&i…> . Thank you, Michelle Keppler -- Michelle Keppler Human Resources Partner michelle(a)galois.com <michelle(a)galois.com> 571-510-0659 *I* *galois* *I * *GALOIS, INC.* 901 N Stuart Street *I* Suite 501 *I* Arlington, VA 22203 www.galois.com

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ANN: stack-2.1.1 release
by Emanuel Borsboom 14 Jun '19

14 Jun '19

Announcing the first release in the stack-2.1 series! You can download bindists for all supported platforms from https://github.com/commercialhaskell/stack/releases/tag/v2.1.1 The Stack 2 release represents a series of significant changes to how Stack works internally. For the vast majority of cases, these changes are backwards compatible, in that existing projects will continue to build in the same way with Stack 2 as they did with Stack 1. The large version bump is due to the fundamental internal changes to cache handling, database storage (using SQLite in place of binary files), implicit snapshots (which greatly improve the precompiled cache), and moving to Pantry. We have also removed some less used features, as listed in the release notes below. **Changes since v1.9.3** Major changes: * Switch over to pantry for managing packages. This is a major change to Stack's internals, and affects user-visible behavior in a few places. Some highlights: * Drop support for multiple package indices and legacy `00-index.tar` style indices. See [#4137](https://github.com/commercialhaskell/stack/issues/4137). * Support for archives and repos in the `packages` section has been removed. Instead, you must use `extra-deps` for such dependencies. `packages` now only supports local filepaths. * Add support for Git repositories containing (recursive) submodules. * Addition of new configuration options for specifying a "pantry tree" key, which provides more reproducibility around builds, and (in the future) will be used for more efficient package content downloads. You can also specify package name and version for more efficient config parsing. * __NOTE__ The new `stack freeze` command provides support for automatically generating this additional information. * Package contents and metadata are stored in an SQLite database in place of files on the filesystem. The `pantry` library can be used for interacting with these contents. * Internally, Stack has changed many datatypes, including moving to Cabal's definition of many data types. As a result of such changes, existing cache files will in general be invalidated, resulting in Stack needing to rebuild many previously cached builds in the new version. Sorry :(. * A new command, `stack freeze` has been added which outputs project and snapshot definitions with dependencies pinned to their exact versions. * The `ignore-revision-mismatch` setting is no longer needed, and has been removed. * Overriding GHC boot packages results in any other GHC boot packages depending on it being no longer available as a dependency, such packages need to be added explicitly when needed. See [#4510] (https://github.com/commercialhaskell/stack/issues/4510) * Cabal solver integration was not updated to support newer `cabal-install` versions so `stack solver` command was removed as well as a related option `--solver` from `stack new` and `stack init`. * Upgrade to Cabal 2.4 * Note that, in this process, the behavior of file globbing has been modified to match that of Cabal. In particular, this means that for Cabal spec versions less than 2.4, `*.txt` will match `foo.txt`, but not `foo.2.txt`. * Remove the `stack image` command. With the advent of Docker multistage builds, this functionality is no longer useful. For an example, please see [Building Haskell Apps with Docker](https://www.fpcomplete.com/blog/2017/12/building-haskell-apps-with-…. * Support building GHC from source (experimental) * Stack now supports building and installing GHC from source. The built GHC is uniquely identified by a commit id and an Hadrian "flavour" (Hadrian is the newer GHC build system), hence `compiler` can be set to use a GHC built from source with `ghc-git-COMMIT-FLAVOUR` * `stack.yaml` now supports a `configure-options`, which are passed directly to the `configure` step in the Cabal build process. See [#1438](https://github.com/commercialhaskell/stack/issues/1438) * Remove support for building GHCJS itself. Future releases of Stack may remove GHCJS support entirely. * Support for lock files for pinning exact project dependency versions Behavior changes: * `stack.yaml` now supports `snapshot`: a synonym for `resolver`. See [#4256](https://github.com/commercialhaskell/stack/issues/4256) * `stack script` now passes `-i -idir` in to the `ghc` invocation. This makes it so that the script can import local modules, and fixes an issue where `.hs` files in the current directory could affect interpretation of the script. See [#4538](https://github.com/commercialhaskell/stack/pull/4538) * When using `stack script`, custom snapshot files will be resolved relative to the directory containing the script. * Remove the deprecated `--upgrade-cabal` flag to `stack setup`. * Support the `drop-packages` field in `stack.yaml` * Remove the GPG signing code during uploads. The GPG signatures have never been used yet, and there are no plans to implement signature verification. * Remove the `--plain` option for the `exec` family of commands * Always use the `--exact-configuration` Cabal configuration option when building (should mostly be a non-user-visible enhancement). * No longer supports Cabal versions older than `1.19.2`. This means projects using snapshots earlier than `lts-3.0` or `nightly-2015-05-05` will no longer build. * Remove the `stack docker cleanup` command. Docker itself now has [`docker image prune`](https://docs.docker.com/engine/reference/commandline/image_prune/) and [`docker container prune`](https://docs.docker.com/engine/reference/commandline/container_prun…, which you can use instead. * Interleaved output is now turned on by default, see [#4702](https://github.com/commercialhaskell/stack/issues/4702). In addition, the `packagename> ` prefix is no longer included in interelaved mode when only building a single target. * The `-fhide-source-paths` GHC option is now enabled by default and can be disabled via the `hide-source-paths` configuration option in `stack.yaml`. See [#3784](https://github.com/commercialhaskell/stack/issues/3784) * Stack will reconfigure a package if you modify your `PATH` environment variable. See [#3138](https://github.com/commercialhaskell/stack/issues/3138). * For GHC 8.4 and later, disable the "shadowed dependencies" workaround. This means that Stack will no longer have to force reconfigures as often. See [#3554](https://github.com/commercialhaskell/stack/issues/3554). * When building a package, Stack takes a lock on the dist directory in use to avoid multiple runs of Stack from trampling each others' files. See [#2730](https://github.com/commercialhaskell/stack/issues/2730). * Stack will check occassionally if there is a new version available and prompt the user to upgrade. This will not incur any additional network traffic, as it will piggy-back on the existing Hackage index updates. You can set `recommend-stack-upgrade: false` to bypass this. See [#1681](https://github.com/commercialhaskell/stack/issues/1681). * `stack list-dependencies` has been removed in favour of `stack ls dependencies`. * The new default for `--docker-auto-pull` is enabled. See [#3332](https://github.com/commercialhaskell/stack/issues/3332). Other enhancements: * Support MX Linux in get-stack.sh. Fixes [#4769](https://github.com/commercialhaskell/stack/issues/4769). * Defer loading up of files for local packages. This allows us to get plan construction errors much faster, and avoid some unnecessary work when only building a subset of packages. This is especially useful for the curator use case. * Existing global option `--color=WHEN` is now also available as a non-project-specific yaml configuration parameter `color:`. * Adopt the standard proposed at http://no-color.org/, that color should not be added by default if the `NO_COLOR` environment variable is present. * New command `stack ls stack-colors` lists the styles and the associated 'ANSI' control character sequences that stack uses to color some of its output. See `stack ls stack-colors --help` for more information. * New global option `--stack-colors=STYLES`, also available as a non-project-specific yaml configuration parameter, allows a stack user to redefine the default styles that stack uses to color some of its output. See `stack --help` for more information. * British English spelling of 'color' (colour) accepted as an alias for `--color`, `--stack-colors`, `stack ls stack-colors` at the command line and for `color:` and `stack-colors:` in yaml configuration files. * New build option `--ddump-dir`. (See [#4225](https://github.com/commercialhaskell/stack/issues/4225)) * Stack parses and respects the `preferred-versions` information from Hackage for choosing latest version of a package in some cases, e.g. `stack unpack packagename`. * The components output in the `The main module to load is ambiguous` message now include package names so they can be more easily copy-pasted. * Git repos are shared across multiple projects. See [#3551](https://github.com/commercialhaskell/stack/issues/3551) * Use en_US.UTF-8 locale by default in pure Nix mode so programs won't crash because of Unicode in their output [#4095](https://github.com/commercialhaskell/stack/issues/4095) * Add `--tree` to `ls dependencies` to list dependencies as tree. [#4101](https://github.com/commercialhaskell/stack/issues/4101) * Add `--pedantic` to `ghci` to run with `-Wall` and `-Werror` [#4463](https://github.com/commercialhaskell/stack/issues/4463) * Add `--cabal-files` flag to `stack ide targets` command. * Add `--stdout` flag to all `stack ide` subcommands. * Use batches when unregistering packages with `ghc-pkg`. (See [#2662](https://github.com/commercialhaskell/stack/issues/2662)) * `get-stack` script now works on Windows CI machines of Appveyor, Travis and Azure Pipelines. See [#4535](https://github.com/commercialhaskell/stack/issues/4535)/ * Show snapshot being used when `stack ghci` is invoked outside of a project directory. See [#3651](https://github.com/commercialhaskell/stack/issues/3651) * The script interpreter now accepts a `--extra-dep` flag for adding packages not present in the snapshot. Currently, this only works with packages from Hackage, not Git repos or archives. * When using the script interpreter with `--optimize` or `--compile`, Stack will perform an optimization of checking whether a newer executable exists, making reruns significantly faster. There's a downside to this, however: if you have a multifile script, and change one of the dependency modules, Stack will not automatically detect and recompile. * `stack clean` will delete the entire `.stack-work/dist` directory, not just the relevant subdirectory for the current GHC version. See [#4480](https://github.com/commercialhaskell/stack/issues/4480). * Add `stack purge` as a shortcut for `stack clean --full`. See [#3863](https://github.com/commercialhaskell/stack/issues/3863). * Both `stack dot` and `stack ls dependencies` accept a `--global-hints` flag to bypass the need for an installed GHC. See [#4390](https://github.com/commercialhaskell/stack/issues/4390). * Add the `stack config env` command for getting shell script environment variables. See [#620](https://github.com/commercialhaskell/stack/issues/620). * Less verbose output from `stack setup` on Windows. See [#1212](https://github.com/commercialhaskell/stack/issues/1212). * Add an optional `ignore-expiry` flag to the `hackage-security` section of the `~/.stack/config.yaml`. It allows to disable timestamp expiration verification just like `cabal --ignore-expiry` does. The flag is not enabled by default so that the default functionality is not changed. * Include default values for most command line flags in the `--help` output. See [#893](https://github.com/commercialhaskell/stack/issues/893). * Set the `GHC_ENVIRONMENT` environment variable to specify dependency packages explicitly when running test. This is done to prevent ambiguous module name errors in `doctest` tests. * `get-stack` script now works on Windows CI machines of Appveyor, Travis and Azure Pipelines. See [#4535](https://github.com/commercialhaskell/stack/issues/4535) * Warn when a Docker image does not include a `PATH` environment variable. See [#2472](https://github.com/commercialhaskell/stack/issues/2742) * When using `system-ghc: true`, Stack will now find the appropriate GHC installation based on the version suffix, allowing you to more easily switch between various system-installed GHCs. See [#2433](https://github.com/commercialhaskell/stack/issues/2433). * `stack init` will now support create a `stack.yaml` file without any local packages. See [#2465](https://github.com/commercialhaskell/stack/issues/2465) * Store caches in SQLite database instead of files. * No longer use "global" Docker image database (`docker.db`). * User config files are respected for the script command. See [#3705](https://github.com/commercialhaskell/stack/issues/3705), [#3887](https://github.com/commercialhaskell/stack/issues/3887). * Set the `GHC_ENVIRONMENT` environment variable to `-` to tell GHC to ignore any such files when GHC is new enough (>= 8.4.4), otherwise simply unset the variable. This allows Stack to have control of package databases when running commands like `stack exec ghci`, even in the presence of implicit environment files created by `cabal new-build`. See [#4706](https://github.com/commercialhaskell/stack/issues/4706). * Use a database cache table to speed up discovery of installed GHCs * You can specify multiple `--test-arguments` options. See [#2226](https://github.com/commercialhaskell/stack/issues/2226) * Windows terminal width detection is now done. See [#3588](https://github.com/commercialhaskell/stack/issues/3588) * On Windows, informs users if the 'programs' path contains a space character and further warns users if that path does not have an alternative short ('8 dot 3') name, referencing the `local-programs-path` configuration option. See [#4726](https://github.com/commercialhaskell/stack/issues/4726) * Add `--docker-mount-mode` option to set the Docker volume mount mode for performance tuning on macOS. Bug fixes: * Ignore duplicate files for a single module when a Haskell module was generated from a preprocessor file. See [#4076](https://github.com/commercialhaskell/stack/issues/4076). * Only track down components in current directory if there are no hs-source-dirs found. This eliminates a number of false-positive warnings, similar to [#4076](https://github.com/commercialhaskell/stack/issues/4076). * Handle a change in GHC's hi-dump format around `addDependentFile`, which now includes a hash. See [yesodweb/yesod#1551](https://github.com/yesodweb/yesod/issues/1551) * Fix `subdirs` for git repos in `extra-deps` to match whole directory names. Also fixes for `subdirs: .`. See [#4292](https://github.com/commercialhaskell/stack/issues/4292) * Fix for git packages to update submodules to the correct state. See [#4314](https://github.com/commercialhaskell/stack/pull/4314) * Add `--cabal-files` flag to `stack ide targets` command. * Don't download ghc when using `stack clean`. * Support loading in GHCi definitions from symlinked C files. Without this patch, Stack will try to find object files in the directory pointed to by symlinks, while GCC will produce the object files in the original directory. See [#4402](https://github.com/commercialhaskell/stack/pull/4402) * Fix handling of GitHub and URL templates on Windows. See [commercialhaskell/stack#4394](https://github.com/commercialhaskell/stack/is… * Fix `--file-watch` not responding to file modifications when running inside docker on Mac. See [#4506](https://github.com/commercialhaskell/stack/issues/4506) * Using `--ghc-options` with `stack script --compile` now works. * Ensure the detailed-0.9 type tests work. See [#4453](https://github.com/commercialhaskell/stack/issues/4453). * Extra include and lib dirs are now order-dependent. See [#4527](https://github.com/commercialhaskell/stack/issues/4527). * Apply GHC options when building a `Setup.hs` file. See [#4526](https://github.com/commercialhaskell/stack/issues/4526). * Stack handles ABI changes in FreeBSD 12 by differentiating that version from previous. * Help text for the `templates` subcommand now reflects behaviour in stack 1.9 — that it downloads and shows a help file, rather than listing available templates. * Fix detection of aarch64 platform (this broke when we upgraded to a newer Cabal version). * Docker: fix detecting and pulling missing images with `--docker-auto-pull`, see [#4598](https://github.com/commercialhaskell/stack/issues/4598) * Hackage credentials are not world-readable. See [#2159](https://github.com/commercialhaskell/stack/issues/2159). * Warnings are dumped from logs even when color is enabled. See [#2997](https://github.com/commercialhaskell/stack/issues/2997) * `stack init` will now work for cabal files with sublibraries. See [#4408](https://github.com/commercialhaskell/stack/issues/4408) * When the Cabal spec version is newer than the global Cabal version, build against the snapshot's Cabal library. See [#4488](https://github.com/commercialhaskell/stack/issues/4488) * Docker: fix detection of expected subprocess failures. This fixes downloading a compatible `stack` executable when the host `stack` is not compatible with the Docker image (on Linux), and doesn't show an unnecessary extra error when the in-container re-exec'ed `stack` exits with failure. * The `stack ghci` command's `--ghc-options` flag now parses multiple options. See [#3315](https://github.com/commercialhaskell/stack/issues/3315). **Thanks to all our contributors for this release:** * Adam Bergmark * Akshay Mankar * Aleksey Kozin * ALeX Kazik * Alexey Kozin * Alexey Kuleshevich * Anders Kaseorg * Anders Kiel Hovgaard * Artyom Kazak * Bastian Krol * Björn Gohla * Brad Neimann * Brandon Chinn * Colin Woodbury * Cthulhu.Den * dadepo * Daniel Gröber * David Baynard * David Spies * David Vollbracht * dbaynard * Emanuel Borsboom * favonia * flip111 * Florjan Bartol * Gleb Popov * Hussein Ait-Lahcen * Isumi Feng * Janfel * Jurijs Oniscuks * Kayla Ngan * Kirill Zaborsky * Magicloud * Matt Audesse * Mauricio Fierro * Michael Sloan * Michael Snoyman * Mihai Maruseac * Mike Pilgrem * Neil Mitchell * NeonGraal * Niklas Hambüchen * penteract * Phil de Joux * Raghu Kaippully * Sibi * Sibi Prabakaran * Simon Hengel * skapazzo * Sylvain HENRY * Sylvain Henry * Taekyung * Timothy Clem * Tom Sydney Kerckhove * Valery V. Vorotyntsev * Vance Palacio * waddlaw * 欧阳泽

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How to fix ambiguous type variable?
by Boris 13 Jun '19

13 Jun '19

Hi everyone, I was playing around with some tests and run into code that looks really similar to the following function someCheck :: (Show a, Read a, Eq a) => String -> a -> Bool someCheck s v = (read . show . read $ s) == v But obviously, it's not clear what instance to use in the right-most read and the show functions. So it doesn't compile. Out of curiosity, is it possible to make this function compile? I've struggled some time and could not find an answer by myself, so here I am. Thanks in advance! Cheers, boris(a)d12frosted.io

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