- Haskell-Cafe - Haskell.org

ANN: base-compat updated for AMP + request for collaboration
by Simon Hengel 31 Mar '15

31 Mar '15

Hi, I updated base-compat[1] for the recent AMP/Foldable/Traversable changes and base-4.8.0.0. A new version 0.6.0 is on Hackage. The purpose of base-compat is to provide a way to write code that works with multiple versions of GHC without the need for CPP. The idea is to have .Compat modules (e.g. Control.Exception.Compat) that provide the same interface across all supported versions of GHC (we test GHC 7.* on travis and I test 6.12.3 manually). The main focus of this release is to make Prelude.Compat as complete as possible to help with the AMP/Foldable/Traversable transition. Prelude.Compat now exposes the exact same interface as Prelude from base-4.8.0.0. It can be used together with -XNoImplicitPrelude to write code that works with all supported versions of GHC, e.g.: {-# LANGUAGE NoImplicitPrelude #-} import Prelude.Compat main :: IO () main = mapM_ print (Just 23) Prelude.Compat is complete and we have tests that ensure that the interface is the same for all supported versions of GHC. But other modules still need more work. In the past base-compat did not get the love it deserves, but I'm committed to put more effort into it and make it as complete as possible. Any help with that effort would be greatly appreciated. If you want to help, please open issues and pull requests on GitHub, or send me feedback by email or on IRC (solirc on Freenode). Cheers, Simon [1] http://hackage.haskell.org/package/base-compat

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GHC update from 7.8.4 to 7.10.1 breaks 1, 500 packages in NixOS
by Peter Simons 31 Mar '15

31 Mar '15

Hi guys, the Nixpkgs distribution [1] contains all of Hackage -- 7,898 packages at the time of this writing --, and we regularly build these packages with GHC 7.8.4 in our CI environment [2] running NixOS Linux. About 3,000 of these builds are disabled outright, because we know they won't succeed anyway (i.e. they have dependencies we cannot fulfill), but the remaining 5,000 packages compile successfully most of the time. Now, a few days ago, we created a separate build job [2] that compiles the same package set with GHC 7.10.1, and the result is interesting: with the newer compiler, approximately 1,500 packages are broken compared to the state with 7.8.4. A detailed list of broken packages can be found at [4]. Keep in mind that those figures only approximate reality. For instance, we patch packages sometimes to make their builds succeed even though the pristine version would not, and so far much more effort has gone into fixing 7.8.4 builds than we spent on fixing 7.10.1 builds. Also, some packages may compile fine when you run "cabal install <foo>" on your workstation, but those builds may fail in the context of NixOS for whatever reason. If anyone were to repeat this experiment on Darwin, Windows, Ubuntu, or Red Hat, the results would be different from our numbers! Still, based on the information we have, it's probably fair to say that the GHC 7.10.1 update breaks approximately 30% of all Haskell builds that worked with 7.8.4. Best regards, Peter [1] https://github.com/nixos/nixpkgs [2] http://hydra.cryp.to/jobset/nixpkgs/haskell-updates [3] http://hydra.cryp.to/jobset/nixpkgs/haskell-updates-ghc7101 [4] http://hydra.cryp.to/eval/304489#tabs-still-fail

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GHC 7.10.1 on OS X
by Yongqian Li 31 Mar '15

31 Mar '15

Does anyone know how to resolve this issue? When I do a "cabal build", I get error: unknown directive .macosx_version_min 10, 0 ^ <no location info>: Error running clang! you need clang installed to use theLLVM backend (or GHC tried to execute clang incorrectly) <no location info>: ghc: phase `Clang (Assembler)' failed (exitcode = 1) However, my clang is fine, as is my LLVM: % clang --version Apple LLVM version 6.0 (clang-600.0.57) (based on LLVM 3.5svn) Target: x86_64-apple-darwin14.1.0 Thread model: posix % opt --version LLVM (http://llvm.org/) LLVM version 3.5.1 Optimized build with assertions. Built Jan 15 2015 (18:24:46). Default target: x86_64-apple-darwin14.1.0 Host CPU: core-avx2

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linear: instance Additive (Point f) ??
by Richard Eisenberg 31 Mar '15

31 Mar '15

Hi café, I'm in the middle of responding to https://github.com/goldfirere/units/pull/45 and trying to learn the `linear` package, which I have yet to use. I have what may be a basic question: why is there an `instance Additive f => Additive (Point f)` (in the Affine module)? It would seem that the whole point of Point is that it is *not* Additive. We don't want to add Points! Could someone enlighten me? Thanks! Richard PS: Of course, this instance is directly hurting my use of the package. But it is hurting my understanding of the package, because it disagrees with the mental model I've built up of the definitions.

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New Functional Programming Job Opportunities
by Functional Jobs 31 Mar '15

31 Mar '15

Here are some functional programming job opportunities that were posted recently: Full-stack Software Engineer at Capital Match http://functionaljobs.com/jobs/8802-full-stack-software-engineer-at-capital… Cheers, Sean Murphy FunctionalJobs.com

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preprocessing and Parsec
by Maurizio Vitale 30 Mar '15

30 Mar '15

G'day, I'm implementing a parser for a language that has a preprocessor. It has `include, `ifdef..`else..`endif, `define and uses of the `defined fragments, which are also prefixed by '`'. I can think of two ways of doing this: a preprocess :: Stream->Stream (for String, Text, ByteString) or processing preprocessor directives as part of the"whitespace skipping" at begin of file and after each token. The first one seems cleaner/easier (and also allow parsers not to require a (restricted) IO monad for reading files). The second one, seems to make easier to give nice error messages (in the separate preprocessor way, you can add something like #line to tell where inclusions came from, but you cannot easily do this for macro replacement). My eventual goal is to give clang-like error messages with macro expansion backtrace. Any recommendations? pointers to examples of parsec-based parsers that have a pre-processing step? Thanks, Maurizio

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Index for local documentation is incomplete
by Francesco Ariis 30 Mar '15

30 Mar '15

Dear Haskellers, I have a problem with local haddock documentation. Specifically, the documentation gets built but the index.html is not updated. As an example, here is the bottom part of the log of a package I installed a few days ago: > Haddock coverage: > 100% ( 6 / 6) in 'System.FilePath.Manip' > 100% ( 7 / 7) in 'System.FilePath.GlobPattern' > 100% ( 2 / 2) in 'System.FilePath.Glob' > 65% ( 41 / 63) in 'System.FilePath.Find' > Documentation created: dist/doc/html/filemanip/index.html > Creating package registration file: /tmp/pkgConf-filemanip-0.3.66815.3 > Installing library in > /home/user/.cabal/lib/i386-linux-ghc-7.8.3/filemanip-0.3.6.3 > Registering filemanip-0.3.6.3... The documentation files are present in ~/.cabal/share/doc/i386-linux-ghc-7.8.3/filemanip-0.3.6.3/html/index.html but when if I open `~/.cabal/share/doc/index.html`, there is no `System.FilePath.Find` module. cabal version is 1.22.0.0, GHC 7.8.3 Any idea on how to diagnose this?

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Terra language: Lua+LLVM JIT+multi-stage metaprogramming
by oleg＠okmij.org 30 Mar '15

30 Mar '15

Bulat Ziganshin wrote: > It has predecessors like template haskell, metalua, julia, ocamlp4, > but all them lack either convenient multi-stage meta-programming, or > efficency of low-level code You have omitted MetaOCaml. One may argue about the convenience of MetaOCaml but one has to acknowledge that it is the typed and hygienic staged language. (The TExp subset of the new TH is typed and hygienic, but it does not permit any effects in the generator, which are often necessary). As to low level, it turns out relatively straightforward to interface MetaOCaml with LLVM, so that OCaml becomes a very powerful, typed and hygienic ``macro language'' for LLVM.

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RFC: rewrite-with-location proposal
by Michael Snoyman 30 Mar '15

30 Mar '15

Quite a while back, Simon Hengel and I put together a proposal[1] for a new feature in GHC. The basic idea is pretty simple: provide a new pragma that could be used like so: error :: String -> a errorLoc :: IO Location -> String -> a {-# REWRITE_WITH_LOCATION error errorLoc #-} Then all usages of `error` would be converted into calls to `errorLoc` by the compiler, passing in the location information of where the call originated from. Our three intended use cases are: * Locations for failing test cases in a test framework * Locations for log messages * assert/error/undefined Note that the current behavior of the assert function[2] already includes this kind of approach, but it is a special case hard-coded into the compiler. This proposal essentially generalizes that behavior and makes it available for all functions, whether included with GHC or user-defined. The proposal spells out some details of this approach, and contrasts with other methods being used today for the same purpose, such as TH and CPP. Michael [1] https://github.com/sol/rewrite-with-location [2] http://hackage.haskell.org/packages/archive/base/4.6.0.1/doc/html/Control-E…

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ANN: unification-fd 0.10.0
by wren romano 30 Mar '15

30 Mar '15

-------------------------------------------- -- unification-fd 0.10.0 -------------------------------------------- The unification-fd package offers generic functions for single-sorted first-order structural unification (think of programming in Prolog, or of the metavariables in type inference)[1][2]. The library *is* sufficient for implementing higher-rank type systems a la [Peyton Jones, Vytiniotis, Weirich, Shields], but bear in mind that unification variables are the metavariables of type inference--- not the type-variables. As of this version, the library is also sufficient for implementing (non-recursive) feature structure unification. An effort has been made to make the package as portable as possible. However, because it uses the ST monad and the mtl-2 package it can't be H98 nor H2010. However, it only uses the following common extensions which should be well supported[3]: Rank2Types MultiParamTypeClasses FunctionalDependencies -- Alas, necessary for type inference FlexibleContexts -- Necessary for practical use of MPTCs FlexibleInstances -- Necessary for practical use of MPTCs UndecidableInstances -- For Show instances due to two-level types -------------------------------------------- -- Changes since 0.8.0 (2012-07-11) -------------------------------------------- * (0.8.1) Added Functor, Foldable, and Traversable instances for COntrol.Unification.Types.UnificationFailure. Thanks Graham Rogers. * (0.9.0) Changed the fundeps on BindingMonad and RankedBindingMonad so that things compule under GHC 7.8.2 * (0.9.0) eta-expanded the rewrite rules in Data.Functor.Fixedpoint so that GHC 7.8 no longer gives warnings about not firing due to (.) possibly being inlined first * Cleaned things up to compile cleanly for GHC 7.10 * For EitherK and MaybeK, liberalized the instances to only require Applicative instead of Monad wherever possible. * Completely revamped the old UnificationFailure data type as the new UFailure data type and Fallible type class. This was necessary to clean up some deprecation warnings about using Control.Monad.Error, to remove the hack of needing an UnknownError constructor, and to make the data type for representing failures more extensible. -------------------------------------------- -- Description -------------------------------------------- The unification API is generic in the type of the structures being unified and in the implementation of unification variables, following the two-level types pearl of Sheard (2001). This style mixes well with Swierstra (2008), though an implementation of the latter is not included in this package. That is, all you have to do is define the functor whose fixed-point is the recursive type you're interested in: -- The non-recursive structure of terms data S a = ... -- The recursive term type type Term = Fix S And then provide an instance for Unifiable, where zipMatch performs one level of equality testing for terms and returns the one-level spine filled with pairs of subterms to be recursively checked, or Nothing if this level doesn't match. Each subterm can be separately marked as being resolved, Left xy, or as requiring further unification, Right(x,y). class (Traversable t) => Unifiable t where zipMatch :: t a -> t a -> Maybe (t (Either a (a, a))) The choice of which variable implementation to use is defined by similarly simple classes Variable and BindingMonad. We store the variable bindings in a monad, for obvious reasons. In case it's not obvious, see Dijkstra et al. (2008) for benchmarks demonstrating the cost of naively applying bindings eagerly. There are currently two implementations of variables provided: one based on STRefs, and another based on a state monad carrying an IntMap. The former has the benefit of O(1) access time, but the latter is plenty fast and has the benefit of supporting backtracking. Backtracking itself is provided by the logict package and is described in Kiselyov et al. (2005). In addition to this modularity, unification-fd implements a number of optimizations over the algorithm presented in Sheard (2001)--- which is also the algorithm presented in Cardelli (1987). * Their implementation uses path compression, which we retain. Though we modify the compression algorithm in order to make sharing observable. * In addition, we perform aggressive opportunistic observable sharing, a potentially novel method of introducing even more sharing than is provided by the monadic bindings. Basically, we make it so that we can use the observable sharing provided by the modified path compression as much as possible (without introducing any new variables). * And we remove the notoriously expensive occurs-check, replacing it with visited-sets (which detect cyclic terms more lazily and without the asymptotic overhead of the occurs-check). A variant of unification which retains the occurs-check is also provided, in case you really need to fail fast. * Finally, a highly experimental branch of the API performs *weighted* path compression, which is asymptotically optimal. Unfortunately, the current implementation is quite a bit uglier than the unweighted version, and I haven't had a chance to perform benchmarks to see how the constant factors compare. Hence moving it to an experimental branch. These optimizations pass a test suite for detecting obvious errors. If you find any bugs, do be sure to let me know. Also, if you happen to have a test suite or benchmark suite for unification on hand, I'd love to get a copy. -------------------------------------------- -- Notes and limitations -------------------------------------------- [1] At present the library does not appear amenable for implementing higher-rank unification itself; i.e., for higher-ranked metavariables, or higher-ranked logic programming. To be fully general we'd have to abstract over which structural positions are co/contravariant, whether the unification variables should be predicative or impredicative, as well as the isomorphisms of moving quantifiers around. It's on my todo list, but it's certainly non-trivial. If you have any suggestions, feel free to contact me. [2] At present it is only suitable for single-sorted (aka untyped) unification, a la Prolog. In the future I aim to support multi-sorted (aka typed) unification, however doing so is complicated by the fact that it can lead to the loss of MGUs; so it will likely be offered as an alternative to the single-sorted variant, similar to how the weighted path-compression is currently offered as an alternative. [3] With the exception of fundeps which are notoriously difficult to implement. However, they are supported by Hugs and GHC 6.6, so I don't feel bad about requiring them. Once the API stabilizes a bit more I plan to release a unification-tf package which uses type families instead, for those who feel type families are easier to implement or use. There have been a couple requests for unification-tf, so I've bumped it up on my todo list. -------------------------------------------- -- References -------------------------------------------- Luca Cardelli (1987) /Basic polymorphic typechecking/. Science of Computer Programming, 8(2):147--172. Atze Dijkstra, Arie Middelkoop, S. Doaitse Swierstra (2008) /Efficient Functional Unification and Substitution/, Technical Report UU-CS-2008-027, Utrecht University. <http://www.cs.uu.nl/research/techreps/repo/CS-2008/2008-027.pdf> Simon Peyton Jones, Dimitrios Vytiniotis, Stephanie Weirich, Mark Shields /Practical type inference for arbitrary-rank types/, to appear in the Journal of Functional Programming. (Draft of 31 July 2007.) Oleg Kiselyov, Chung-chieh Shan, Daniel P. Friedman, and Amr Sabry (2005) /Backtracking, Interleaving, and/ /Terminating Monad Transformers/, ICFP. <http://www.cs.rutgers.edu/~ccshan/logicprog/LogicT-icfp2005.pdf> Tim Sheard (2001) /Generic Unification via Two-Level Types/ /and Paramterized Modules/, Functional Pearl, ICFP. <http://web.cecs.pdx.edu/~sheard/papers/generic.ps> Tim Sheard & Emir Pasalic (2004) /Two-Level Types and/ /Parameterized Modules/. JFP 14(5): 547--587. This is an expanded version of Sheard (2001) with new examples. <http://web.cecs.pdx.edu/~sheard/papers/JfpPearl.ps> Wouter Swierstra (2008) /Data types a la carte/, Functional Pearl. JFP 18: 423--436. <http://www.cs.ru.nl/~wouters/Publications/DataTypesALaCarte.pdf> -------------------------------------------- -- Links -------------------------------------------- Homepage: http://code.haskell.org/~wren/ Hackage: http://hackage.haskell.org/package/unification-fd Darcs: http://community.haskell.org/~wren/unification-fd Haddock (Darcs version): http://community.haskell.org/~wren/unification-fd/dist/doc/html/unification… -- Live well, ~wren

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