April 2007 - Haskell-Cafe

fftw bindings
by Fawzi Mohamed 20 Apr '07

20 Apr '07

Hi everybody, I was wondering if someone had fftw bindings for haskell, or if I should roll my own. I did a small search but found nothing. Fawzi

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Writing guards shorthand
by Joel Reymont 20 Apr '07

20 Apr '07

Support I want to infer the type given an Op that looks like this (incomplete): data Op = Minus | Plus | Mul | LT | GT Is there a shorthand way of bunching Minus, Plus and Mul in a function guard since they all result in TyNum whereas the rest in TyBool? I really don't want several function clauses and neither do I want separate guards for every constructor. Thanks, Joel -- http://wagerlabs.com/

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Vacancy for a PhD student
by Johan Jeuring 20 Apr '07

20 Apr '07

(Knowledge of Haskell is a big plus; implementation of most of the tools will be done in Haskell.) Vacancy: PhD student in the Strategy Feedback project 1+3 years, Open University the Netherlands, location: Heerlen (preference). The project: In many subjects students have to acquire procedural skills. Problems in mathematics are often solved using a standard procedure, such as for example solving a system of linear equations by subtracting equations from top to bottom, and then substituting variables from bottom to top. Problems in computer science, physics, chemistry, electrotechnics, and other subjects, often require procedural skills as well. Furthermore, procedural skills appear at any educational level. E-learning systems offer excellent possibilities for practicing procedural skills. The first explanations and motivation for a procedure that solves a particular kind of problems are probably best taught in a class room, or studied in a book, but the subsequent practice can often be done behind a computer. There exist many e-learning systems or intelligent tutoring systems that support practicing procedural skills. The tools vary widely in breadth, depth, user-interface, etc, but, unfortunately, almost all of them lack sophisticated techniques for providing immediate feedback. If feedback mechanisms are present, they are hard coded in the tools, often even with the exercises. This situation hampers the usage of e-learning systems for practicing procedural skills. This project aims to investigate techniques for providing flexible and immediate feedback in tools that support practicing procedural skills. We want to use advanced techniques from computer science, taken from fields such as term rewriting, strategies, error-correcting parsers, and generic programming to provide feedback at each intermediate step from the start towards the solution of an exercise. We want to further develop some of these techniques, and apply and experiment with them in the domain of e-learning systems. Our goal is to obtain e-learning systems that give immediate and useful feedback. We will cooperate on this project with the Software Technology group from Utrecht University. Requirements: University degree in Computer Science. Good knowledge of functional programming, and several advanced computer science techniques. Knowledge of parsing, rewriting, strategies, generic programming, etc. will be useful. More information about the project can be found on http://ideas.cs.uu.nl/wiki/index.php/IDEAS:StrategicFeedback or contact Johan Jeuring (johanj(a)cs.uu.nl, +31 6 40010053). To apply: send a letter and your curriculum vitae before May 5, 2007 to: Open Universiteit Nederland, afdeling Personele ondersteuning, Postbus 2960, 6401 DL Heerlen. U kunt ook or an e-mail to: martin.vanloo(a)ou.nl Mention vacancynr FAC/INF/07024 In both cases, please cc Johan Jeuring (johanj(a)cs.uu.nl)

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Morphing ASTs and scrapping boilerplate code
by Joel Reymont 19 Apr '07

19 Apr '07

Folks, I'm transforming ASTs as part of my compiler from one language into another. The source AST is a list of statements whereas the target AST is a "class definition". type Object a = State Obj a data Obj = Object { objSym :: Integer -- starting # for gensym , objVars :: M.Map String VarDecl , objProps :: M.Map String PropDecl , objMeths :: M.Map String MethodDecl } deriving (Show, Eq) I'm using a state monad and the transformations are supposed to update the state (object) as needed to add variables, methods, etc. class Morpher a b | a -> b where morph :: a -> Object b I have a lot of boilerplate code like this and wonder how I can scrape it. instance Morpher Type C.Type where morph TyInt = return C.TyInt morph TyFloat = return C.TyFloat morph TyStr = return C.TyStr morph TyBool = return C.TyBool morph TyColor = return C.TyColor morph TyStyle = return C.TyStyle morph (TyList ty) = liftM C.TyList (morph ty) morph (TyArray ty) = liftM C.TyArray (morph ty) morph (TySeries ty) = liftM C.TySeries (morph ty) morph (TyInput ty) = liftM C.TyProp (morph ty) morph (TyRef ty) = liftM C.TyRef (morph ty) morph TyUnit = return C.TyUnit morph TyPrintDest = return C.TyPrintDest Notice that I'm calling a constructor of the same name in a different module ... unless it's an exception. instance Morpher BackRef C.Expr where morph Now = C.Int 0 morph (BarsBack e) = morph e instance Morpher DataRef C.Expr where morph (DS x) = C.Int x How can I reduce the boilerplate here? Thanks, Joel -- http://wagerlabs.com/

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First order Haskell without Data
by Neil Mitchell 19 Apr '07

19 Apr '07

Hi, I've been wondering what is essential to Haskell and what isn't. Not from a point of view of what could be removed from the language, but what could be removed from a Core language. Given the features of higher-order, laziness and data types: Laziness can be converted to higher-order functions Data types can be converted to higher-order functions Higher-order functions can be converted to Data So as far as I can see it we have left: {data-types only} {higher-order functions only} But we don't have laziness only, so my question is if it is possible to represent all of Haskell in a first-order language without data types, but with laziness. If its possible to do so in a strict first-order language without data types, then I'd also be interested. Are any of these known results? Thanks Neil References: * data types -> higher order is in "Efficient Interpretation by Transforming Data Types and Patterns to Functions" http://www.cs.nott.ac.uk/~nhn/TFP2006/Papers/03-JansenKoopmanPlasmeijer-Eff… * laziness -> functions, functions -> data is in "Definitional Interpreters for Higher-Order Programming Languages" http://www.brics.dk/~hosc/local/HOSC-11-4-pp363-397.pdf

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Parallel executing of actions
by Mitar 19 Apr '07

19 Apr '07

Hi! Is there a parallel version of mapM_? Like it is for map (parMap)? I would like to execute actions in parallel as the computation of necessary data for actions is quite computational heavy. But it does not matter in which order those actions are executed. (I am rendering pixels with OpenGL and it does not matter in which order I draw them, but it matters that for one pixel it takes some time to calculate its color.) The example would be: main :: IO () main = do -- the order of printed characters is not important -- instead of putStrLn there will be a computationally big function -- so it would be great if those computations would be done in parallel -- and results printed out as they come mapM_ rwhnf (putStrLn) ["a","b","c","d"] Is this possible? Without unsafe functions? And without changing the semantics of the program. Mitar

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RE: [Haskell] haskell question
by Simon Peyton-Jones 19 Apr '07

19 Apr '07

[Redirecting to Haskell Cafe] Hi Greg I think it's very likely that you *can* do what you want. I have not taken long enough to understand just what you are trying to do, but I do know why your program is failing, and I think you'll agree it *should* fail. Look at these lines (slightly simplified) class CoreAgentSyntax x where offer :: (CoreProcessSyntax p) => [n] -> (p a x) -> x data MinimalAgentSyntax nn pp = Concretion [nn] pp instance CoreAgentSyntax (MinimalAgentSyntax nn pp) where offer = Concretion The class decl says that every instance of 'offer' must have type forall p, forall n, forall a. (CoreProcessSyntax p) => [n] -> p a x -> x where x is the instance type. Not the "forall n"! Even once x is fixed, we must have a function that is polymorphic in n,p,a But in your instance declaration, offer = Concretion has the type [nn] -> pp -> MinimalAgentSyntax nn pp No for-alls, because the whole instance decl binds the 'nn'. Hence it's just not polymorphic enough. I'm not sure how to fix your problem. Perhaps you need another argument for CoreAgentSyntax? class CoreAgentSyntax n x where offer :: (CoreProcessSyntax p) => [n] -> (p a x) -> x instance CoreAgentSyntax nn (MinimalAgentSyntax nn pp) where offer = Concretion now you are in better shape. There's another problem because you use 'p' with kind * in some places and (*->*->*) in others, but thats a separate issue. Oleg is good at this kind of stuff. Simon From: haskell-bounces(a)haskell.org [mailto:haskell-bounces@haskell.org] On Behalf Of Greg Meredith Sent: 18 April 2007 14:53 To: haskell(a)haskell.org Subject: [Haskell] haskell question Haskellians, In the attached file i have implemented the bare bones core of the syntax -- pre structural equivalence -- of Milner's presentation of \pi via abstractions and concretions. (There's a minor twist in that the notion of location affords more structure than in Milner's original presentation which i have added to support an ultimate probabilistic interpretation.) The file compiles with ghc as is. If you uncomment the last section, however, you see that to close the loop on the constraints for the agent syntax we hit a typing error. i think this is a hard constraint in Haskell that prevents this level of generality in the specification, but maybe you can see a way around it. i'm certainly no Haskell expert. Best wishes, --greg P.S. The reason for this level of generality is that with it you can close the loop and form a mutual recursion between the types for the syntax of processes and the type of names -- without having that choice predesigned into the structure of processes (or names). i can do this trick in OCaml with recursive modules, but it's definitely not pretty. i was hoping Haskell could make it pretty. -- L.G. Meredith Managing Partner Biosimilarity LLC 505 N 72nd St Seattle, WA 98103 +1 206.650.3740 http://biosimilarity.blogspot.com

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Re: Type classes and type equality
by oleg＠pobox.com 19 Apr '07

19 Apr '07

Neil Mitchell wrote: > I'm looking for a type class which checks whether two types are the > same or not. This problem is more complex than appears. It has been solved, however. IncoherentInstances are not required, as IncoherentInstances are generally unsafe. For the full discussion of various solutions, please see Section 9 and Appendix D of the HList paper: http://homepages.cwi.nl/~ralf/HList/paper.pdf The HList code is available http://darcs.haskell.org/HList/ It includes the examples from the paper.

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Fwd: [Haskell-cafe] Transpose using array comprehension
by Chris Witte 19 Apr '07

19 Apr '07

I was trying to use the array comprehension of the Data Parallel Haskell package. http://haskell.org/haskellwiki/GHC/Data_Parallel_Haskell for instance I can define {-# OPTIONS -fparr -fglasgow-exts #-} module MatTest where import GHC.PArr ident = [:[:1,0:],[:0,1:]:] aVect = [:4,5:] dotP :: Num a => [:a:] -> [:a:] -> a dotP xs ys = sumP [:x * y | x <- xs | y <- ys :] matVecMul :: Num a => [:[:a:]:] -> [:a:] -> [:a:] matVecMul xs ys = [:dotP x ys | x <- xs:] but when I tried to define a matrix matrix multiplication matMel :: Num a => [:[:a:]:] -> [:[:a:]:] -> [:a:] I was at a loss for how to do it because i couldn't define transpose. On 4/18/07, Henning Thielemann <lemming(a)henning-thielemann.de> wrote: > > On Wed, 18 Apr 2007, Chris Witte wrote: > > > I just started playing around with GHC.PArr and array comprehension > > and I was wondering if there is a way to define the transpose of a > > matrix using array comprehension? > > Why not > > let swap :: (i,j) -> (j,i) > swap (a,b) = (b,a) > in ixmap (let (lower,upper) = bounds arr in (swap lower, swap upper)) > swap arr > > ? >

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Re: [Haskell-cafe] question about Data.Binary and Double instance
by Duncan Coutts 18 Apr '07

18 Apr '07

On Wed, 2007-04-18 at 08:34 -0700, Bryan O'Sullivan wrote: > Duncan Coutts wrote: > > > I'm currently exploring more design ideas for Data.Binary including how > > to deal with alignment. Eliminating unnecessary bounds checks and using > > aligned memory operations also significantly improves performance. I can > > get up to ~750Mb/s serialisation out of a peak memory bandwidth of > > ~1750Mb/s, though a Haskell word-writing loop can only get ~850Mb/s. > > What are you using to measure the peak number? It seems very low to me. > Even Opterons of a few years of vintage can manage more than 5GB/s. I was using a C word writing loop with no unrolling. On x86 that turned into a 4 asm instruction loop. Reading words is indeed a good deal faster, and on my x86-64 machine reading 8-byte words is faster still. > It's also quite normal to get about half of your peak bandwidth unless > you're going out of your way to use non-temporal loads and stores (e.g. > via SSE2), which is something that e.g. gcc is not good at at all, if > you're using -fvia-c. I'm not interested so much in the peak throughput of the machine as what could be realistically achievable for binary serialisation. So comparing to a non-unrolled C loop seems fair to me. With sufficient improvements in the GHC backend we should be able to approach that. Then any difference is overhead in the Binary library, and that's what I'm really trying to measure. Duncan

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