- Haskell-Cafe - Haskell.org

In which format should GHC expose documentation via its .hi-files?
by Simon Jakobi 24 Apr '18

24 Apr '18

Hi! In preparation for the coding phase of the Hi Haddock proposal that I want to implement, I need a decision on the format in which we should expose haddocks in GHC's .hi interface files. Please visit https://github.com/haskell/haddock/issues/805, ask questions, make suggestions and help me find a timely decision! :) Cheers, Simon

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Selda: confused about type signtures
by Marc Busqué 24 Apr '18

24 Apr '18

Hi! I'm using [selda](https://hackage.haskell.org/package/selda) package in order to deal with database backends. Selda uses `TypeOperators` language extension, and it introduces `:*:` type operator constructor which take concrete types like `RowID` or `Text` . It also has a `Table` type constructor which takes anything built with `:*:`). On the other hand, `SeldaM` is selda custom monad transformer with `IO` at the bottom. I have following helper function which just wraps a call to the SQLite backend. `dBPath` is just the database file path: ``` withDB :: SeldaM a -> IO a withDB act = do path <- dBPath withSQLite path act ``` I want to wrap selda API in custom functions to be more resilient to changes. Right now I'm trying to abstract selecting all rows for a given table (maybe it seems brittle, but it is just a toy project in order to learn Haskell): ``` list table = withDB $ query (select table) ``` Not adding a type signature to `list` produces following compilation error: ``` • Non type-variable argument in the constraint: selda-0.1.12.1:Database.Selda.Column.Columns (selda-0.1.12.1:Database.Selda.Column.Cols s a) (Use FlexibleContexts to permit this) • When checking the inferred type list :: forall s a. (selda-0.1.12.1:Database.Selda.Column.Columns (selda-0.1.12.1:Database.Selda.Column.Cols s a), selda-0.1.12.1:Database.Selda.Compile.Result (selda-0.1.12.1:Database.Selda.Column.Cols s a)) => Table a -> IO [selda-0.1.12.1:Database.Selda.Compile.Res (selda-0.1.12.1:Database.Selda.Column.Cols s a)] ``` If I try to add what I think would be the correct signature: ``` list :: Table a -> IO [a] ``` The error changes to: ``` • Couldn't match type ‘a’ with ‘selda-0.1.12.1:Database.Selda.Compile.Res (selda-0.1.12.1:Database.Selda.Column.Cols s0 a)’ ‘a’ is a rigid type variable bound by the type signature for: list :: forall a. Table a -> IO [a] at src/Hedger/Category.hs:35:1-25 Expected type: SeldaM [a] Actual type: selda-0.1.12.1:Database.Selda.Backend.Internal.SeldaT IO [selda-0.1.12.1:Database.Selda.Compile.Res (selda-0.1.12.1:Database.Selda.Column.Cols s0 a)] • In the second argument of ‘($)’, namely ‘query (select table)’ In the expression: withDB $ query (select table) In an equation for ‘list’: list table = withDB $ query (select table) • Relevant bindings include table :: Table a (bound at src/Hedger/Category.hs:36:6) list :: Table a -> IO [a] (bound at src/Hedger/Category.hs:36:1) | 36 | list table = withDB $ query (select table) ``` However, if I constraint the type signature to act just for a given table, it compiles without errors: ``` type CategoriesSchema = RowID:*:Text list :: Table CategoriesSchema -> IO [CategoriesSchema] ``` Why is that it works with concrete types but not with a type variable that takes the same concrete type in its both placeholders? Thanks in advance, Marc Busqué http://waiting-for-dev.github.io/about/

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Munich Haskell Meeting, 2018-04-26 @ 19:30
by Leah Neukirchen 23 Apr '18

23 Apr '18

Dear all, This week, our monthly Munich Haskell Meeting will take place again on Thursday, April 26 at Augustiner-Gaststätte Rumpler at 19h30. For details see here: http://muenchen.haskell.bayern/dates.html If you plan to join, please add yourself to this dudle so we can reserve enough seats! It is OK to add yourself to the dudle anonymously or pseudonymously. https://dudle.inf.tu-dresden.de/haskell-munich-apr-2018/ Everybody is welcome! cu, -- Leah Neukirchen <leah(a)vuxu.org> http://leah.zone

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New Version of hmatrix (more ODE solvers)
by dominic＠steinitz.org 22 Apr '18

22 Apr '18

I am pleased to announce a new version of hmatrix: 0.19.0.0. This is not intended to be a breaking change but a lot of modules have been modified to ensure that continuous integration (which has now been set up) is green. Support for SUNDIALS <https://computation.llnl.gov/projects/sundials> has been added. It should be possible to replace Numeric.GSL.ODE with Numeric.Sundials.ARKode.ODE and have your program work as before bearing in mind that the methods and error control might differ (even for those with the same names!). The packages that comprise hmatrix are: hmatrix hmatrix-gsl hmatrix-special hmatrix-sparse hmatrix-glpk hmatrix-tests hmatrix-sundials I am planning to split these up into separate projects to make it easier for contributors. To this end, I have created an hmatrix <https://github.com/haskell-numerics> organisation in github. Please contact me if you wish to become a member. I strongly recommend anyone solving ODEs to look as hmatrix-sundials as the underlying package provides many more methods, more documentation and more diagnostic information (great when your 200 variable model fails). It is a truism that any sufficiently sophisticated ODE solver package is a domain specific language (I include the ODE solving part of GSL here). The current interface both to GSL and to SUNDIALS is somewhat rudimentary but will hopefully evovle to a nicer DSL (something at which Haskell excels). Dominic Steinitz dominic(a)steinitz.org http://idontgetoutmuch.wordpress.com Twitter: @idontgetoutmuch

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Selda, type operators and heterogeneous lists
by Marc Busqué 20 Apr '18

20 Apr '18

Hi there! I'm using [selda](https://github.com/valderman/selda) package to deal with relational databases. This package represents database tables in haskell in a type-safe way. And to do so it leans on type operators. For example, here they are two different tables: ``` categories :: Table (RowID:*:Text) expenses :: Table (RowID:*:Text:*:Double:*:RowID) ``` As both of them belong to different types, I can't put them in a single list out of the box: ``` [categories, expenses] --- not valid!! ``` I'm aware of [different solutions](https://wiki.haskell.org/Heterogenous_collections) to deal with it, but they don't convince me: - Algebraic data types: I think in this situation it is specially a burden having to play type-switching... There can exist a lot of different table schemas and they may change at any moment. - Universal type: I see it as a kind of hack - Existential types: I have read that usually it is an anti-pattern, and in this case I don't understand how it should work, because types are constructed with `:*:` operator. So my question if it exists some natural way to treat types built with type operators as a single type. I have to recognize I don't understand well how all this stuff about type operators works. I have read something but I still don't get it. So any direction with it would be very gratifying, too :) Thanks in advance. Marc Busqué http://waiting-for-dev.github.io/about/

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ANN: stack-1.7 RELEASE CANDIDATE
by Emanuel Borsboom 20 Apr '18

20 Apr '18

This is the second release candidate for stack-1.7 (the first was short-lived and never widely announced). Binaries for supported platforms are available here: https://github.com/commercialhaskell/stack/releases/tag/v1.7.0.3 <https://github.com/commercialhaskell/stack/releases/tag/v1.7.0.3>. You easily upgrade to it using: `stack upgrade --binary-version=1.7.0.3` ## Changes since v1.6.5: Release notes: * aarch64 (64-bit ARM) bindists are now available for the first time. * Statically linked Linux bindists are no longer available, due to difficulty with GHC 8.2.2 on Alpine Linux. * 32-bit Linux GMP4 bindists for CentOS 6 are no longer available, since GHC 8.2.2 is no longer being built for that platform. Major changes: * Upgrade from Cabal 2.0 to Cabal 2.2 Behavior changes: * `stack setup` no longer uses different GHC configure options on Linux distributions that use GCC with PIE enabled by default. GHC detects this itself since ghc-8.0.2, and Stack's attempted workaround for older versions caused more problems than it solved. * `stack new` no longer initializes a project if the project template contains a stack.yaml file. Other enhancements: * `stack unpack` now supports a `--to /target/directory` option to specify where to unpack the package into * `stack hoogle` now supports a new flag `--server` that launches local Hoogle server on port 8080. See [#2310](https://github.com/commercialhaskell/stack/issues/2310) * A new sub command `ls` has been introduced to stack to view local and remote snapshots present in the system. Use `stack ls snapshots --help` to get more details about it. * `list-dependencies` has been deprecated. The functionality has to accessed through the new `ls dependencies` interface. See [#3669](https://github.com/commercialhaskell/stack/issues/3669) for details. * Specify User-Agent HTTP request header on every HTTP request. See [#3628](https://github.com/commercialhaskell/stack/issues/3628) for details. * `stack setup` looks for GHC bindists and installations by any OS key that is compatible (rather than only checking a single one). This is relevant on Linux where different distributions may have different combinations of libtinfo 5/6, ncurses 5/6, and gmp 4/5, and will allow simpifying the setup-info metadata YAML for future GHC releases. * The build progress bar reports names of packages currently building. * `stack setup --verbose` causes verbose output of GHC configure process. See [#3716](https://github.com/commercialhaskell/stack/issues/3716) * Improve the error message when an `extra-dep` from a path or git reference can't be found See [#3808](https://github.com/commercialhaskell/stack/pull/3808) * Nix integration is now disabled on windows even if explicitly enabled, since it isn't supported. See [#3600](https://github.com/commercialhaskell/stack/issues/3600) * `stack build` now supports a new flag `--keep-tmp-files` to retain intermediate files and directories for the purpose of debugging. It is best used with ghc's equivalent flag, i.e. `stack build --keep-tmp-files --ghc-options=-keep-tmp-files`. See [#3857](https://github.com/commercialhaskell/stack/issues/3857) * Improved error messages for snapshot parse exceptions Bug fixes: * When a package contained sublibraries, stack was always recompiling the package. This has been fixed now, no recompilation is being done because of sublibraries. See [#3899](https://github.com/commercialhaskell/stack/issues/3899). * The `get-stack.sh` install script now matches manual instructions when it comes to Debian/Fedora/CentOS install dependencies. * Compile Cabal-simple with gmp when using Nix. See [#2944](https://github.com/commercialhaskell/stack/issues/2944) * `stack ghci` now replaces the stack process with ghci. This improves signal handling behavior. In particular, handling of Ctrl-C. To make this possible, the generated files are now left behind after exit. The paths are based on hashing file contents, and it's stored in the system temporary directory, so this shouldn't result in too much garbage. See [#3821](https://github.com/commercialhaskell/stack/issues/3821). * The script interpreter's implicit file arguments are now passed before other arguments. See [#3658](https://github.com/commercialhaskell/stack/issues/3658). In particular, this makes it possible to pass `-- +RTS ... -RTS` to specify RTS arguments used when running the script. * Don't ignore the template `year` parameter in config files, and clarify the surrounding documentation. See [#2275](https://github.com/commercialhaskell/stack/issues/2275). * Benchmarks used to be run concurrently with other benchmarks and build steps. This is non-ideal because CPU usage of other processes may interfere with benchmarks. It also prevented benchmark output from being displayed by default. This is now fixed. See [#3663](https://github.com/commercialhaskell/stack/issues/3663). * `stack ghci` now allows loading multiple packages with the same module name, as long as they have the same filepath. See [#3776](https://github.com/commercialhaskell/stack/pull/3776). * `stack ghci` no longer always adds a dependency on `base`. It is now only added when there are no local targets. This allows it to be to load code that uses replacements for `base`. See [#3589](https://github.com/commercialhaskell/stack/issues/3589#issuecomment) * `stack ghci` now uses correct paths for autogen files with [#3791](https://github.com/commercialhaskell/stack/issues/3791)

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building ghc-8.4.2 from source: undefined reference to 'mpz_powm_sec'
by Johannes Waldmann 20 Apr '18

20 Apr '18

Dear Cafe, I am getting this when building ghc-8.4.2 from source, with ghc-8.2.2: ghc-8.4.2/libraries/integer-gmp/dist-install/build/libHSinteger-gmp-1.0.2.0_p.a(wrappers.p_o):wrappers.c:function integer_gmp_powm_sec: error: undefined reference to 'mpz_powm_sec' collect2: error: ld returned 1 exit status `gcc' failed in phase `Linker'. (Exit code: 1) make[1]: *** [iserv/ghc.mk:104: iserv/stage2_p/build/tmp/ghc-iserv-prof] Error 1 This may be due to some mis-config on my part, because I'm getting this on one machine only, and it works on others. Still, perhaps you have seen this before, and could help me with an educated guess on what may have caused this. It is especially puzzling because this install of ghc-8.2.2 seems to work perfectly fine for all of my projects. Thanks - J.W.

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Integer representation
by David Feuer 19 Apr '18

19 Apr '18

Is there any way to determine whether Integer comes from integer-gmp or integer-simple? I'm playing with the idea of using the underlying representation to get more compact/efficient tries, but I need to be able to find out what that representation is.

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ANN: RISC-V ISA Formal Spec (written in Haskell)
by Rishiyur Nikhil 19 Apr '18

19 Apr '18

Title: RISC-V ISA Formal Specification Speaker: Thomas Bourgeat, MIT Venue: 8th RISC-V Workshop, Barcelona Date: Monday May 7, 2018 Abstract: In this tutorial we will demonstrate several flavors of a formal specification of the RISC-V ISA, written in Haskell. We will present the important part of the code, use it as a software simulator, automatically transform it into a coq specification (used to prove the correctness of a small imperative language), and automatically synthesize it to circuit (to model check against other designs). If time permits, we will show how the same code can be used to explore some non-determinism in the specification. Registration info: https://riscv.org/workshops/

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Is there a reason that template-haskell does not expose its own parser for using in quasi-quoters?
by Christopher Done 16 Apr '18

16 Apr '18

I.e. something like parseExpr :: String -> Either String Exp It seems that a lot of trivial extensions could be made nicely if this was so. An example might be: [ne| 1, 2, 3, ... |] for a non-empty list statically checked to be non-empty. In there I could put any Haskell code. But presently I need to use HSE which isn't quite GHC Haskell and has a large foot-print. The alternative splice $(ne [| [1, 2, 3, ... ] |]) is rather unwieldy.

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