- Haskell-Cafe - Haskell.org

Constraint-folding
by Andras Slemmer 01 Dec '13

01 Dec '13

Hi, I stumbled upon something pretty neat, and although I'm 95% sure Oleg did this already 10 years ago in Haskell98 I thought I'd share it because I find it gorgeous! The basic issue I was having is that whenever I wrote class instances for a lifted type (we'll use Nat): class Class (n :: Nat) instance Class Zero instance (Class m) => Class (Succ m) I always had to litter my code with "(Class n) =>" restrictions even in places where they just shouldn't belong. However my gut feeling was that the generic instance "should" be implied, as we covered all cases. A while ago I proposed a new syntax to do just this ( https://ghc.haskell.org/trac/ghc/ticket/6150) which failed miserably, and for good reason! However there is a way to do it anyway:) What we need is basically a way to "construct" an instance for the fully polymorpic case: "instance Class n". We cannot do this directly as it would overlap with our original instances (and we couldn't implement it anyway), we need another way of representing class instances: type W p = forall a. (p => a) -> a W p simply "wraps" the constraint p into a function that eliminates the constraint on a passed in value. Now we can treat a constraint as a function. We will also need a way to "pattern match" on our lifted types, we will do this with an indexed GADT: data WNat n where -- W for Witness WZero :: WNat Zero WSucc :: WNat n -> WNat (Succ n) And finally the neat part; one can write a general typeclass-polymorphic induction principle on these wrapped constraints (or a "constraint-fold"): natFold :: WNat n -> W (p Zero) -> (forall m. W (p m) -> W (p (Succ m))) -> W (p n) W (p Zero) in the p ~ Class case corresponds to "instance Class Zero", and (forall m. W (p m) -> W (p (Succ m))) corresponds to "instance (Class m) => Class (Succ m)" This function is precisely what we need in order to construct the generic instance! (Note: the implementation of natFold is NOT trivial as we need to wrestle with the type checker in the inductive case, but it is a nice excercise. The solution is here: http://lpaste.net/96429) Now we can construct the generic instance: genericClass :: WNat n -> (Class n => a) -> a genericClass n = natFold n (\x -> x) (\f x -> f x) -- Cannot use id as that would try to unify away our constraints Or equivalently: class WNatClass n where witness :: WNat n instance WNatClass Zero where witness = WZero instance (WNatClass m) => WNatClass (Succ m) where witness = WSucc witness genericClass' :: (WNatClass n) => (Class n => a) -> a genericClass' = genericClass witness Now we can decouple all our "(Class n) =>" and similar constraints, all we need is a WNatClass restriction, which I think is a good enough trade-off (and it's not really a restriction anyway). What do you think? ex

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free vs. operational vs. free-operational
by Alejandro Serrano Mena 01 Dec '13

01 Dec '13

Dear Café, I've been reading lately about the relation between the 'free' package and the 'operational' package for rolling your own monads [1] [2]. Furthermore, I've discovered that the 'free-operational' package, which is some sort of bridge between the two worlds, and provides not only Monad but also Applicative and Alternative instances for ProgramT. The problem is that right now everything is a little confused in my head. In particular, I have the following questions: - I've read that free allows you to 'bake algebraic laws' in the resulting monad. How does this work? Why doesn't operational offer that feature? - One of the things I really like from the free package is that it provides support for many monad transformer stacks, whereas operational does not? Is there any special restriction why operational cannot do this? Would it be possible to provide similar instances for free-operational? - It seems that free gives more features (Alternative, Applicative) with the same work. In which situations should I prefer operational to free? I really like the separation between providing a data type and then a interpretation that operational embodies... - Should I replace my usage of operational with free-operational altogether? Thanks for the help :) [1] http://www.reddit.com/r/haskell/comments/17a33g/free_functors_the_reason_fr… [2] http://stackoverflow.com/questions/14263363/is-operational-really-isomorphi…

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Re: [Haskell-cafe] options for the RTS in a dll
by Nicolas Wu 01 Dec '13

01 Dec '13

On Sun, Dec 1, 2013 at 10:10 AM, Kees Bleijenberg <k.bleijenberg(a)lijbrandt.nl> wrote: > But when I call hs_init_ghc with rtsoptions –h I get 3 lines with on each > line: Can’t open profiling report file <unknown>.hp. > > With –p I get missing profiling report <unknown>.prof > > Any idea what I’am doing wrong? Sounds odd. Have you ensured you have write permissions where the profiling files will be created?

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options for the RTS in a dll
by Kees Bleijenberg 01 Dec '13

01 Dec '13

I've written a windows dll in Haskell. The calling program is written in Delphi (pascal). I want to profile this dll. Because you don't have a command line in a dll for +RTS. -RTS, I call the dll with hs_init_ghc (a exported function in the dll). I had to translate some parts of rtsopts.h to Delphi. Everyting seems okay. The program does not hang, no catastrophic failures. When I start with the parameter for rtsoptions in hs_init_ghc I get: with -t --machine-readable ->machine readable overview on the screen. with -B sounds the bell with each garbage collection, and also --info works as expected. But when I call hs_init_ghc with rtsoptions -h I get 3 lines with on each line: Can't open profiling report file <unknown>.hp. With -p I get missing profiling report <unknown>.prof Any idea what I'am doing wrong? Kees

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[ANN] XSLT2 for C/C++: alpha realease of Saxon-HE 9.5 (and soon Xpath2, Xquery)
by Olivier JEULIN 29 Nov '13

29 Nov '13

Hello, for those working with XML and XSLT/XQuery/XPath, you may be interested to know that Saxonica is releasing an alpha version of its free (and open source) XSLT2/Xquery engine Saxon-HE 9.5. (I'm not affiliated with Saxon, I'm just spreading the news :-) ) The good news is: this finally brings XSLT 2 to C/C++, instead of XSLT 1 (which is already supported by libxml2). XSLT 2 is a MUCH more mature version of XSLT, with cleaner definition of function and a nice functional approach. XSLT 3 will be even better (higher order functions) but may not be supported by the free version of Saxon. http://www.saxonica.com/saxon-c/index.xml I hope that someone will make a binding for haskell ^^,

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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 29 Nov '13

29 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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