November 2013 - Haskell-Cafe

10 PhD studentships in Nottingham
by Graham Hutton 29 Nov '13

29 Nov '13

Dear all, The School of Computer Science in Nottingham is advertising 10 fully-funded PhD studentships. Applicants in the area of the Functional Programming lab (fp.cs.nott.ac.uk) are encouraged! If you are interested in applying, please contact a potential academic supervisor in the FP lab prior to submitting your application. Best wishes, Graham +-----------------------------------------------------------+ 10 Fully-Funded PhD Studentships School of Computer Science University of Nottingham, UK Applications are invited for up to ten fully-funded PhD studentships in the School of Computer Science at the University of Nottingham, starting on 1st October 2014. The topics for the studentships are open, but should relate to the interests of one of the School¹s research groups: Agents Lab; Automated Scheduling, Optimisation and Planning; Computer Vision Lab; Functional Programming Lab; Intelligent Modelling and Analysis; Mixed Reality Lab; Networked Systems. The studentships are for three years and include a stipend of £13,726 per year and tuition fees, and are available to students of any nationality. Applicants are normally expected to have a first-class Undergraduate or Masters degree in Computer Science or a related discipline, and should discuss their interest and obtain the support of a potential supervisor in the School before applying. To apply, please submit the following items by email to <Christine.Fletcher(a)nottingham.ac.uk>: (1) a brief covering letter that describes your reasons for wishing to pursue a PhD, any ideas you have regarding possible areas or topics, and the name of a potential supervisor; (2) a copy of your CV, including your actual or expected degree class(es), and results of all University examinations; (3) an example of your technical writing, such as a project report or dissertation; (4) contact details for two academic referees. Closing date for applications: 10th January 2014 +-----------------------------------------------------------+ -- Prof Graham Hutton Functional Programming Lab School of Computer Science University of Nottingham, UK http://www.cs.nott.ac.uk/~gmh This message and any attachment are intended solely for the addressee and may contain confidential information. If you have received this message in error, please send it back to me, and immediately delete it. Please do not use, copy or disclose the information contained in this message or in any attachment. Any views or opinions expressed by the author of this email do not necessarily reflect the views of the University of Nottingham. This message has been checked for viruses but the contents of an attachment may still contain software viruses which could damage your computer system, you are advised to perform your own checks. Email communications with the University of Nottingham may be monitored as permitted by UK legislation.

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Fwd: vector fails to build with ghc on arm-linux-androideabi
by Herwig Hochleitner 29 Nov '13

29 Nov '13

Hello, I'm trying to build the openpgp libs for android. I can build and run HaskellActivity from the HelloWorld sample, but as when I try to install vector, I get the following build failure: ==================== $ arm-linux-androideabi-cabal install --reinstall --force-reinstall vector ... [ 5 of 19] Compiling Data.Vector.Fusion.Stream.Monadic ( Data/Vector/Fusion/Stream/Monadic.hs, dist/build/Data/Vector/Fusion/Stream/Monadic.o ) Loading package ghc-prim ... linking ... ghc: /home/androidbuilder/.ghc/android-14/arm-linux-androideabi-4.8/lib/arm-unknown-linux-androideabi-ghc-7.7.20130921/ghc-prim-0.3.1.0/HSghc-prim-0.3.1.0.o: unhandled ELF relocation(Rel) type 28 ghc: unable to load package `ghc-prim' Failed to install vector-0.10.9.1 ... ==================== It seems somewhat related to this closed bug: https://ghc.haskell.org/trac/ghc/ticket/7824 Am I doing something stupid? Can I fix this? cheers

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Query information about a haskell source file from the command-line
by Thiago Padilha 28 Nov '13

28 Nov '13

Is there any command-line tool that outputs information about a source haskell module? I'm looking for something like 'ghc-mod browse'(which displays all symbols exported by an installed module) but that works for plain haskell source files. Any ideas?

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Master thesis project - Haskell STM
by Johan Astborg 28 Nov '13

28 Nov '13

Dear Haskellers, I found Haskell and became enlightened for the first time in many years. I choose to continue on this path, and choose Haskell for my master thesis. I got some nice ideas from people on the #haskell channel . Altogether, I have elaborated a topic: "Evaluating Intel TSX in a MVCC based STM implementation for Haskell" The idea is to investigate whether the "new" Intel TSX can bring any value in terms of performance and saftey when implemented in a MVCC based STM to be used in the Haskell runtime system (GHC). The MVCC based STM using TSX will be evaluated and compared to the existing implementation, compiled with the two settings STM_CG_LOCK and STM_FG_LOCKS respectively. The biggest challange still, is to convince Lund University, LTH, to accept my own master thesis project. I have to present a detailed synopsis about the project, for them to choose an examinator and supervisor. As they put it, the examinator "must have time" and it's up to that person to decide whether or not this project can be reality. Feedback on any of these is highly appreciated: 1) Ideas and thoughts about this topic 2) Anyone interested in supervising (mainly formal) the project outside of LTH Thanks, Johan Astborg

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Return of the revenge of the revisit of the extensible records, reiterated
by oleg＠okmij.org 28 Nov '13

28 Nov '13

First of all, I should point out that faster extensible records have already been developed, within the HList framework: Just Do It While Compiling! Fast Extensible Records In Haskell http://www.fing.edu.uy/~mviera/papers/pepm13.pdf The authors did it without needing the total order on labels. However, one of the encoding of labels described in the HList paper had total order. I guess we weren't motivated enough to exploit it. > The real benefit of keeping the row sorted is that { x = 0 , y = 0 } and { > y = 0, x = 0 } have the same type. When a row is not sorted, as in HList, > then if we for example have an instance Eq for a row (because all elements > support Eq) then for using (==) both arguments would have to the same order > in the row or we need a manual call to a permutation function. When keeping > the row ordered, this is not necessary. The same kind of problem > occurs > > The same kind of problem occurs when we fix the type of a function to > a specific row: ( using whishful syntax ) > > f :: Rec [ x = Int , y = Int] -> Int > > If the row is not ordered, then f { y = 0 , x = 0 } will not typecheck and > will require a manual call to permute the row. This is all very true. However, if we wish to pass the function f above the record {y=0, x=0} (with permuted fields), we most likely wish to pass that function a record {x=0, y=0, z='a'} with more fields. Most of the time when we deal with extensible records, we really wish to explore their extensibility. Keeping fields sorted does not help at all in the latter problem -- we must manually insert the call to the subtyping coercion function. Once we do that, the problem with the order of the fields disappears. I also would like to point out that there are two sorts of record types. One sort is Rec [x: Int, y:Bool] in the imagine syntax. Current HList types are uglier versions of the above. But there is another sort: (HasField r x Int, HasField r y Bool) => r It represents an extensible record type. Extensibility is build in, and the order of the fields is immaterial. Quite a few functions on records can be given the above type. Furthermore, the second sort can be used not only with structural subtyping but also with nominal subtyping.

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Compiling to STG problem
by Maarten Dungen 27 Nov '13

27 Nov '13

Dear all, While trying to retrieve the STG using ghc I bumped into the following problem: I try to compile the source file in a more or less standard way, found in many examples on the web: ... setTargets targets _ <- load LoadAllTargets mgraph <- depanal [] False mapM_ (compileModSummary) mgraph Then I compile the individual files through the following pipeline (also more or less standard, I thought): compileModSummary = do ... stg <- parseModule theMod >>= typecheckModule >>= desugarModule >>= liftIO . hscSimplify env . coreModule >>= liftIO . tidyProgram env >>= prepPgm env . fst >>= liftIO . coreToStg dynflags … Then I do a dump to (just to check the results): writeFile “filename” (show . showOutputable $ stg) The problem is, if I do a stg dump from the command line (like <compiler> -ddump-stg --make Main.hs (where <compiler> is the intended compiler program) the dump is different from the stg dump I do at writefile, to the extent that functions are missing, which gives problems when linking the results. They are numbered functions, like ‘value1’, etc. I’m not very much into the ghcmake stuff, and I’m probably missing a lot, however I have been struggling with this for a couple of days now, and it is not getting any clearer. I guess my question would be, why is there a difference, and how could I avoid this? Any help or pointers appreciated. kind regards, Maarten p.s. This is just pseudo code. I necessary, I could probably strip my sources into a working example that displays the problem, if that would help.

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Monad classes and FreeT
by Alejandro Serrano Mena 27 Nov '13

27 Nov '13

Dear Café, I'm continuing my travel through operational and free, and was wondering: which is the reason why there's no instance such as (Functor f, MonadReader e m) => MonadReader e (FreeT f m) included in the 'free' package? It seems reasonable to me to have those instances. At least, there's the corresponding one for MonadIO. Should Free be considered as some monad to have 'at the bottom of the stack' as IO is? Thanks in advance.

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btc-e interfacing + bot
by Miro Karpis 26 Nov '13

26 Nov '13

Am thinking to inteface btc-e + make a bot. Anybody willing to join forces? m.

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Re: [Haskell-cafe] using external instance declarations
by Kata 26 Nov '13

26 Nov '13

The problem is that your definition of arbitrary for the Building instance isn’t indented under the ‘instance Arbitrary Building’, so it’s getting interpreted as two top-level statements: one that Building is an Arbitrary of Instance, and one that defines a value named ‘arbitrary’. Hence the error message. To fix this, just indent the definition of arbitrary like you did in the Glass definition. On Tuesday, November 26, 2013 at 11:06 AM, Kees Bleijenberg wrote: > I have a Building with a field glass: Glass. > I want to use quickcheck. Therefor I wrote a TestGlass.hs with the Arbitrary instance declaration for Glass. I also want to make Building an instance of Arbitrary. For the field glass in the Building I need the arbitrary :: Gen Glass function out of TestGlass.hs. > How do I export and use the instance declarations from TestGlass.hs? > > Glass.hs > module Glass( > Glass(..)) > where > data Glass = Glass Int > > TestGlass.hs > module TestGlass() > where > import Test.QuickCheck > import Test.QuickCheck.Gen > import Glass > > instance Arbitrary Glass where > arbitrary = do > n <- choose (1,8) :: Gen Int > return $ Glass n > > > Building.hs > module Building () > where > import Glass > import TestGlass > import Test.QuickCheck > import Test.QuickCheck.Gen > > data Building = Building { > glass :: Glass > } > instance Arbitrary Building where > arbitrary = do > g <- arbitrary :: (Gen Glass) > return Building { glass = g} > > > When I compile Building I get: > Building.hs:13:23: > Ambiguous occurrence `arbitrary' > It could refer to either `Building.arbitrary', > defined at Building.hs:12:1 > or `Test.QuickCheck.arbitrary', > imported from `Test.QuickCheck' at Building.hs:5:1-22 > (and originally defined in `Test.QuickCheck.Arbitrary') > > It looks like the compiler doesn’t look at the instance declarations in the TestGlass module. > What is wrong in my code? > I want to split the testcode as much as possible from the non-test code and I don’t want to merge the modules Building and Glass (they are both already big). > > Kees > > > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org (mailto:Haskell-Cafe@haskell.org) > http://www.haskell.org/mailman/listinfo/haskell-cafe > >

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using external instance declarations
by Kees Bleijenberg 26 Nov '13

26 Nov '13

I have a Building with a field glass: Glass. I want to use quickcheck. Therefor I wrote a TestGlass.hs with the Arbitrary instance declaration for Glass. I also want to make Building an instance of Arbitrary. For the field glass in the Building I need the arbitrary :: Gen Glass function out of TestGlass.hs. How do I export and use the instance declarations from TestGlass.hs? Glass.hs module Glass( Glass(..)) where data Glass = Glass Int TestGlass.hs module TestGlass() where import Test.QuickCheck import Test.QuickCheck.Gen import Glass instance Arbitrary Glass where arbitrary = do n <- choose (1,8) :: Gen Int return $ Glass n Building.hs module Building () where import Glass import TestGlass import Test.QuickCheck import Test.QuickCheck.Gen data Building = Building { glass :: Glass } instance Arbitrary Building where arbitrary = do g <- arbitrary :: (Gen Glass) return Building { glass = g} When I compile Building I get: Building.hs:13:23: Ambiguous occurrence `arbitrary' It could refer to either `Building.arbitrary', defined at Building.hs:12:1 or `Test.QuickCheck.arbitrary', imported from `Test.QuickCheck' at Building.hs:5:1-22 (and originally defined in `Test.QuickCheck.Arbitrary') It looks like the compiler doesn't look at the instance declarations in the TestGlass module. What is wrong in my code? I want to split the testcode as much as possible from the non-test code and I don't want to merge the modules Building and Glass (they are both already big). Kees

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