August 2010 - Haskell-Cafe

A question about State Monad and Monad in general
by C K Kashyap 12 Aug '10

12 Aug '10

Hi, I looked at State Monad yesterday and this question popped into my mind. >From what I gather State Monad essentially allows the use of Haskell's do notation to "invisibly" pass around a state. So, does the use of Monadic style fetch us more than syntactic convenience? Again, if I understand correctly, in Mutable Arrays also, is anything getting modified in place really? If not, what is the real reason for better efficiency? -- Regards, Kashyap

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Accepting and returning polyvariadic functions
by Will Jones 12 Aug '10

12 Aug '10

Hi all, I'm trying to write a function (I'll call it `vtuple' for lack of a better name) that returns a function that itself returns multiple arguments in the form of a tuple. For example: > {-# LANGUAGE FlexibleInstances #-} > {-# LANGUAGE FunctionalDependencies #-} > {-# LANGUAGE MultiParamTypeClasses #-} > f :: Int -> IO () > f = undefined > g :: Int -> Int -> IO () > g = undefined > h :: Int -> Int -> Int -> IO () > h = undefined vtuple f :: IO (Int -> (Int, ())) vtuple g :: IO (Int -> Int -> (Int, (Int, ()))) I've tried to type vtuple using a type class; my current effort is something like: > class VTuple ia ir a r | r -> a, a -> ia where > vtuple :: (ia -> ir) -> IO (a -> r) > instance VTuple Int (IO ()) Int (Int, ()) where > --vtuple :: (Int -> IO ()) -> IO (Int -> (Int, ())) > vtuple = undefined > instance VTuple ia ir a r > => VTuple Int (ia -> ir) Int (a -> (Int, r)) where > --vtuple :: (Int -> ia -> ir) -> IO (Int -> a -> (Int, r)) > vtuple = undefined But this is problematic, since arrows creep in: For one argument (fine): vtuple :: (Int -> IO ()) -> IO (Int -> (Int, ())) > vf :: IO (Int -> (Int, ())) > vf = vtuple f For two arguments (also fine): vtuple :: (Int -> Int -> IO ()) -> IO (Int -> Int -> (Int, (Int, ()))) > vg :: IO (Int -> Int -> (Int, (Int, ()))) > vg = vtuple g For three (noooo!): vtuple :: (Int -> Int -> IO ()) -> IO (Int -> Int -> (Int, (Int -> (Int32, (Int32, ()))))) And so on. I've thought about it and it seems impossible to solve this problem -- you keep needing to ``split'' the function type one arrow further on. Is this a job for Template Haskell or is there a solution I'm missing here? Note that I'd also like to use types other than Int, but I don't think this is the primary complication here (touch wood). Any help much appreciated, thanks, Will

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Re: Haskell in Industry
by Tom Hawkins 12 Aug '10

12 Aug '10

Hi Eil, Sorry, your email got lost in my inbox. I hope you don't mind me copying haskell-cafe. > I saw a video of a presentation you gave at CUFP awhile back and was > hoping to ask you a couple of questions. > I'm currently a junior at UT Dallas and trying to figure out what I'm > going to do after finishing undergrad. For the past two years or so one > of my main interests has been language implementations. It's something > that I'd like to continue to work with. For a long time I thought I > wanted to stay in academia and do research, but recently I've realized > that I'm more interested in applying these advanced programming > languages to real-world problems, such as what you have done with the > Atom DSL. Good, we need more functional programmers actually solving real problems. But please put your skills to work in an industry other than investment banking. > > So, I'm curious about your experiences with functional programming in > industry. > Have you found it hard to get companies interested in Haskell? Yes, it is very hard. But it can also be very easy. The best approach is to find a job or industry that does not already have a large software component, but desperately needs one. This was the reason Haskell was successful at Eaton. Though Eaton is a huge company, this particular division had surprisingly few software developers, and even fewer managers who understood software development. In short, there was no one to question the move to Haskell. IMO, the biggest obstacle to Haskell are programmers -- especially shear numbers of them. > Is it difficult to hire good programmers for a less mainstream language? No. Every time I put a call out for resumes, I've gotten several candidates that would have fit the bill. The problem has been convincing my management to interview these leads. > Have you had any trouble training people to use Haskell? Yes. I find that out of 10 people I train, only about 2 pick it up and run with it. I'm starting to believe you are either wired for functional programming, or you're not. > Do many of the engineers using Haskell at Eaton have advanced degrees? Actually most of Eaton's Haskell programmers do not have advanced degrees; myself included. In fact one of our most successful programmers was an automotive technician in his previous job. This supports my hypothesis that your functional programming aptitude is determined at birth. Best of luck with the job hunt. If by chance Eaton starts hiring again, I'll give you a ping. -Tom

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WGP Call for Participation
by Bruno Oliveira 11 Aug '10

11 Aug '10

====================================================================== CALL FOR PARTICIPATION WGP 2010 6th ACM SIGPLAN Workshop on Generic Programming Baltimore, Maryland, US Sunday, September 26th, 2010 http://osl.iu.edu/wgp2010 Collocated with the International Conference on Functional Programming (ICFP 2010) ====================================================================== Goals of the workshop --------------------- Generic programming is about making programs more adaptable by making them more general. Generic programs often embody non-traditional kinds of polymorphism; ordinary programs are obtained from them by suitably instantiating their parameters. In contrast with normal programs, the parameters of a generic program are often quite rich in structure; for example they may be other programs, types or type constructors, class hierarchies, or even programming paradigms. Generic programming techniques have always been of interest, both to practitioners and to theoreticians, and, for at least 20 years, generic programming techniques have been a specific focus of research in the functional and object-oriented programming communities. Generic programming has gradually spread to more and more mainstream languages, and today is widely used in industry. This workshop brings together leading researchers and practitioners in generic programming from around the world, and features papers capturing the state of the art in this important area. Program ------- * 09.00-10.00: Session 1 Chair: Marcin Zalewski * Welcome + PC chair report Bruno C. d. S. Oliveira and Marcin Zalewski * Outrageous but Meaningful Coincidences (Dependent type-safe syntax and evaluation) Conor McBride * 10.00-10.30: Tea/coffee * 10.30-12.30: Session 2 Chair: Shin-Cheng Mu * Scrap Your Zippers: A Generic Zipper for Heterogeneous Types Michael D. Adams * Generic Storage in Haskell Sebastiaan Visser and Andres Loeh * Generic Selections of Subexpressions Martijn van Steenbergen, José Pedro Magalhães and Johan Jeuring * 12.30-14.00: Lunch * 14.00-16.00: Session 3 Chair: Bruno C. d. S. Oliveira * Generic Multiset Programming for Language-Integrated Querying Fritz Henglein and Ken Friis Larsen * Algorithms for Traversal-Based Generic Programming Bryan Chadwick and Karl Lieberherr * Ad-hoc Polymorphism and Dynamic Typing in a Statically Typed Functional Language Thomas van Noort, Peter Achten and Rinus Plasmeijer * 16.00-16.30: Tea/coffee * 16.30-18.00: Session 4 Chair: Conor McBride * Reason Isomorphically! Ralf Hinze and Daniel James * Constructing Datatype-Generic Fully Polynomial-Time Approximation Schemes Using Generalised Thinning Shin-Cheng Mu, Yu-Han Lyu and Akimasa Morihata Workshop homepage: http://osl.iu.edu/wgp2010/ Registration link: https://regmaster3.com/2010conf/ICFP10/register.php Local arrangements: http://www.icfpconference.org/icfp2010/local.html

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Can we come out of a monad?
by C K Kashyap 11 Aug '10

11 Aug '10

Hi, In the code here - http://hpaste.org/fastcgi/hpaste.fcgi/view?id=28393#a28393 If I look at the type of modifiedImage, its simply ByteString - but isn't it actually getting into and back out of the state monad? I am of the understanding that once you into a monad, you cant get out of it? Is this breaking the "monad" scheme? -- Regards, Kashyap

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cabal haddock doesn't work for me on win7
by Ryan Ingram 11 Aug '10

11 Aug '10

I'm not sure what's going on, here's the error I get: C:\haskell\ListZipper-1.1.1.0>cabal haddock Running Haddock for ListZipper-1.1.1.0... Preprocessing library ListZipper-1.1.1.0... Warning: The documentation for the following packages are not installed. No links will be generated to these packages: Cabal-1.8.0.2, QuickCheck-2.1.0.3, Win32-2.2.0.1, array-0.3.0.0, base-3.0.3.2, base-4.2.0.0, bin-package-db-0.0.0.0, ffi-1.0, rts-1.0, bytestring-0.9.1.5, containers-0.3.0.0, directory-1.0.1.0, extensible-exceptions-0.1.1.1, filepath-1.1.0.3, ghc-6.12.1, ghc-binary-0.5.0.2, ghc-prim-0.2.0.0, hpc-0.5.0.4, integer-gmp-0.2.0.0, mtl-1.1.0.2, old-locale-1.0.0.2, old-time-1.0.0.3, pretty-1.0.1.1, process-1.0.1.2, random-1.0.0.2, syb-0.1.0.2, template-haskell-2.4.0.0, time-1.1.4 Warning: Data.List.Zipper: could not find link destinations for: GHC.Classes.Eq GHC.Show.Show Test.QuickCheck.Arbitrary.Arbitrary Test.QuickCheck.Arbitrary.CoArbitrary GHC.Bool.Bool Data.Maybe.Maybe haddock: internal Haddock or GHC error: E:\Program Files (x86)\Haskell\haddock-2.7.2\html\haddock.css: openFile: does not exist (No such file or directory) Haskell platform is installed on "C:", I have no idea where it's getting the "E:" from. I can specify the correct css location with --css, but then I get the same error looking for haskell_icon.gif. -- ryan

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ANN improve-0.0.4
by Tom Hawkins 11 Aug '10

11 Aug '10

Hi, ImProve [1] is a little imperative DSL that compiles to C code. Intended for high assurance embedded applications, ImProve is also an infinite state, unbounded model checker. Meaning ImProve can verify assertions in a program will always be true. Here's an example: module Main where import Language.ImProve launchControl :: Stmt () launchControl = do presidentialAuthority <- input bool "presidentialAuthority" buttonPressed <- input bool "buttonPressed" launchMissiles <- bool' "launchMissiles" $ (presidentialAuthority ||. buttonPressed) assert "conditionsToLaunch" $ launchMissiles ==. (presidentialAuthority &&. buttonPressed) main :: IO () main = verify "yices" 20 launchControl Running this program yields: verifying conditionsToLaunch FAILED: disproved basis in k = 1 (see trace) On verification failures, ImProve will emit a counter example showing the violation, which is essentially a program trace showing how things could go wrong. In this case, the trace looks like this: initialize launchMissiles := false cycle 0 input presidentialAuthority <== true input buttonPressed <== false launchMissiles <== true assertion FAILED: conditionsToLaunch This basically means Barack was somehow able to launch on his own. (Man! I even voted for the guy!) Under the hood, ImProve uses Yices [2] for SMT solving, and a method of unbounded model checking known as k-induction. Don't expect too much from ImProve; it's a very restrictive language. It's basically three data types (bool, int, float), variables assignments, and 'if' statements. No procedures, no loops, no arrays, no pointers, etc. However, it can still do interesting things -- especially with Haskell combinators in play. Hope you find it useful. And as always, questions, comments, and feedback welcome. -Tom [1] http://hackage.haskell.org/package/improve [2] http://yices.csl.sri.com/

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Is there any experience using Software Transactional Memory in substantial applications?
by Serguey Zefirov 11 Aug '10

11 Aug '10

Recently we discussed Haskell and especially types in Russian part of LiveJournal and of course we talk about STM. My opponent gave me that link: http://logicaloptimizer.blogspot.com/2010/06/so-microsofts-experiments-with… It says that performance with STM in Microsoft Research was more than horrible. I failed to find convincing counter-evidence on the web. Not for Haskell, even for Java/C#/C++. So I asking Haskell-cafe for clarification. Do anyone here have an experience with STM in computing-intensive tasks? Did it help there? What are the possible reasons for STM failure in MR?

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Re: [Haskell-cafe] Can we come out of a monad?
by John Lato 11 Aug '10

11 Aug '10

> From: Alexey Khudyakov <alexey.skladnoy(a)gmail.com> > > On Fri, 30 Jul 2010 09:29:59 +0200 > Stefan Holdermans <stefan(a)vectorfabrics.com> wrote: > >> > No I think here we breaking out from _arbitrary_ monad. If monadic > function works for every monad then it must work for identity monad > too. Here is simplest form of purify function: > >> purify2 :: (forall m . Monad m => m a) -> a >> purify2 m = runIdentity m > > I wonder could this function be written without resorting to concrete > monad I don't find purify2 particularly helpful because I almost never want to break out of any arbitrary monad; I want to be able to break out of a specific monad without knowing which monad it is, that is: purify3 :: Monad m => m a -> a purify3 = undefined --the only possible definition... However, I just realized that something else is almost as good: evalCont :: Cont r a -> r evalCont k = runCont k id As Cont is the "Mother of all monads", it can be used to emulate the behavior of any other monad. If you had a library with instances of MonadReader, MonadWriter, MonadState, etc. (or operational equivalents) defined for the Cont monad, then you would have a purify function that works for all interesting monads (except IO, STM, and maybe ST), which is almost as good. John

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Re: [web-devel] statically compiled css
by Tim Matthews 11 Aug '10

11 Aug '10

On Thu, Aug 5, 2010 at 11:33 PM, Mark Bradley <barkmadley(a)gmail.com> wrote: but CSS type checking might be possible within hamlet. > I have often wondered "OK haml implemented now what about sass". Michael Snoyman what is your opinions on sass? Would a sass inspired syntax like you did with the haml->hamlet fit in well and if so, as it often best to keep styles separate, could a quasi quoted language live in in a separate haskell module and then at run time it recreates the separate css files on first launch? Thanks Tim Matthews

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