February 2007 - Haskell-Cafe

numeric minimization in Haskell
by Chad Scherrer 03 Mar '07

03 Mar '07

Does anyone know of any Haskell code for numeric minimization? I was thinking conjugate gradient would be good, but at this point I'd be happy with anything. I've found some code written by Tomasz Cholewo at http://ci.uofl.edu/tom/software/Haskell/ but it requires importing his "Arr.lhs" library, which is not publicly available. The only other thing I've been able to dig up is this www.st.cs.ru.nl/papers/1997/serp97-cgfunctional.ps.gz which suggests Haskell is slow for such problems. I suspect this was an implementation issue, so I don't think their code would be very helpful (though it would be nice to tidy it up and demonstrate the improvement - could it beat the Clean implementation they give?) The other possibility I was considering was using Alberto Ruiz's wrapper for the GSL library http://dis.um.es/~alberto/GSLHaskell/ The only problems with this are (1) requires having GSL available, so it's not as portable, and (2) does everything in terms of lists, which requires a lot of translations to and from lists (I'm using mutable arrays). If there's nothing already written that works together, one of these should give me a start, but I'd like to avoid reinventing the wheel if possible. Thanks! -- Chad Scherrer "Time flies like an arrow; fruit flies like a banana" -- Groucho Marx

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Using loop fusion for writing efficent high-level code Re[2]: [Haskell] [Fwd: Re: Computer Language Shootout]
by Bulat Ziganshin 02 Mar '07

02 Mar '07

Hello Duncan, Tuesday, February 27, 2007, 1:09:53 PM, you wrote: can you please provide examples of such code? the ultimate goal is to have tutorial on writing efficient code in high-level manner, but that is too ambitious. just some examples with a little explanation of how this is compiled? thanks in advance >> So I think we should do the same. It even shows in the Shootout - the >> programs that are simultaneously fastest and clearest are not pure >> Haskell, but delegate their innermost loops to tuned C libraries (FPS >> and GMP). > I should note that FPS is almost completely Haskell code, not C. We use > C for things like memcmp, memcpy, memset, reverse_copy, intersperse, > maximum, minimum and count. > Certainly some of the innards are low level style Haskell, though not > the kind that could be replicated in C because we use high level > transformations to fuse loop bodies together and wrap them in high > performance, low level loop code. > This is not the style where we just wrap well tuned C code, this is a > style where we generate high performance low level code from a high > level spec. This relies on GHC's excellent and programmable optimiser. > It's wrong to say that the shootout improvements were only down to > improved libraries. The performance of ByteString code improved very > significantly between GHC 6.4 and 6.6 and a large part of that was down > to optimiser improvements (not just the ForeignPtr rep change). Best regards, Bulat mailto:Bulat.Ziganshin@gmail.com

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Build failed - hidden package ?
by Dunric 02 Mar '07

02 Mar '07

Hi. I've got stuck with the following compilation error I probably can't fully understand: ---------------------------------------------------------------------- Building library... ... Using package config file: /usr/local/lib/ghc-6.6/package.conf wired-in package base mapped to base-2.0 wired-in package rts mapped to rts-1.0 wired-in package haskell98 mapped to haskell98-1.0 wired-in package template-haskell mapped to template-haskell-2.0 Hsc static flags: -static ... Graphics/UI/SDL/Rotozoomer.hs:15:7: Could not find module `Foreign.C': it is a member of package base, which is hidden ---------------------------------------------------------------------- What does such error mean ? Modules Foreign and Foreign.C are installed and available from the default path. Lines 12-15 of Rotozoomer.hs: ------------------------------------------ module Graphics.UI.SDL.Rotozoomer where import Foreign import Foreign.C ------------------------------------------ Any idea what's wrong, resp. some explanation why compilation fails ? Thanks in an advance. ------------------------------------------ www.icqsms.cz

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OO Design in Haskell Example (Draft)
by Steve Downey 02 Mar '07

02 Mar '07

In the last OO design in Haskell thread (and probably in every one preceeding it), it was suggested that having some examples might be a good idea. Since most people with existing designs will have some familiarity with Design Patterns, and those are typical building blocks for OO designs, it occured to me that implementing some of them might be a useful excersize. If for nothing other than learning some more Haskell. Now, some of them are probably bad ideas for implementing in Haskell. There's a better, or more natural, way than what is suggested by the design pattern. Visitor is probably not a good pattern to follow, for example. On the other hand, others may still be useful, even in a functional language. So, I've been working on a Composite example. I've used existential types to have a generic proxy to the base type, rather than a simple algebraic type, since adding new leaves to the algebraic type means modifying the whole type, a violation of the Open-Closed principle (open for extension, closed for modification) The interface of the composite. Two methods, add and draw. > class IComponent e where > draw ::e -> String > add :: (IComponent e') => e -> e' -> Component A proxy type which can hold any kind of component, and provides the 'virtual' dispatch implementation. That is, it forwards to the add or draw implementation of the instance it is proxying for. > data Component = > forall e.(IComponent e) => Component e > componentDraw :: Component -> String > componentDraw (Component c) = draw c > componentAdd :: (IComponent e) => Component -> e -> Component > componentAdd (Component e) a = Component (add e a) > instance IComponent Component where > draw = componentDraw > add = componentAdd The Single type, which is the leaf node in this composite, add is a no-op, except for wrapping the value in a Component. Since there isn't an implicit down cast from the 'derived' Single to the 'base' Component. > data Leaf = > Text String > deriving(Show, Eq) > leafDraw :: Leaf -> String > leafDraw (Text s) = show s > leafAdd :: (IComponent e) => Leaf -> e -> Component > leafAdd s _ = Component s > instance IComponent Leaf where > draw = leafDraw > add = leafAdd The Composite type, which holds a list of Components through the composite proxy. I was tempted to make the list a state variable, so that add could modify rather than produce a new Many, but I wanted to get the basics working. > data Composite = > Many [Component] > compositeDraw :: Composite -> String > compositeDraw (Many []) = "()" > compositeDraw (Many leaves) = "(" ++ (foldr1 (++) $ map draw leaves) ++ ")" > compositeAdd :: (IComponent e) => Composite -> e -> Component > compositeAdd (Many leaves) c = > Component $ Many ((Component c) : leaves) > instance IComponent Composite where > draw = compositeDraw > add = compositeAdd >

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Hi can u explain me how drop works in Haskell
by iliali16 01 Mar '07

01 Mar '07

Hi I am trying to implement the function drop in haskell the thing is that I I have been trying for some time and I came up with this code where I am trying to do recursion: drop :: Integer -> [Integer] -> [Integer] drop 0 (x:xs) = (x:xs) drop n (x:xs) |n < lList (x:xs) = dropN (n-1) xs : |otherwise = [] So I want to understand how would this work and what exacttly should I put as an answer on line 4 couse that is where I am lost. I know I might got the base case wrong as well but I don't know what to think for it. I have done the lList as a function before writing this one. Thanks to those who can help me understand this. Thanks alot in advance! Have a nice day! -- View this message in context: http://www.nabble.com/Hi-can-u-explain-me-how-drop-works-in-Haskell-tf32904… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Logic programming using lazy evaluation
by Henning Thielemann 01 Mar '07

01 Mar '07

I suspect that someone has already done this: A Haskell library which solves a system of simple equations, where it is only necessary to derive a value from an equation where all but one variables are determined. Say 1+2=x -- add 1 2 x y*z=20 -- times y z 20 x+y=5 -- add x y 5 should be solved as x=3 y=2 z=10 Of course, it is easy to do this for a finite number of variables and equations by assigning integer identifiers to the variables, then scanning the equations repeatedly until all determinable variables are determined. However, imagine I have an infinite number of equations and an infinite number of variables, say 1=x0 -- equal 1 x0 2*x0=x1 -- times 2 x0 x1 2*x1=x2 -- times 2 x1 x2 2*x2=x3 -- times 2 x2 x3 ... Accessing variable values by integer identifiers means that the garbage collector cannot free values that are no longer needed. Thus I thought about how to solve the equations by lazy evaluation. Maybe it is possible to ty the knot this way let (_,_,x0) = add 1 2 x (y0,z0,_) = times y z 20 (x1,y1,_) = add x y 5 x = alternatives [x0,x1] y = alternatives [y0,y1] z = alternatives [z0] in (solve x, solve y, solve z) Independent from the question of how to implement 'alternatives' and 'solve' I wonder if there is a less cumbersome way to do this kind of logic programming in Haskell.

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TFP 2007: Last Call for Participation
by TFP 2007 28 Feb '07

28 Feb '07

Dear Colleagues, The deadline to register for TFP 2007 is quickly approaching. You may register at: http://cs.shu.edu/tfp2007/registration.html . Also, please be advised that the deadline to make hotel reservations at the guaranteed rate for TFP 2007 participants is also quickly approaching. Hotel information can be found at: http://cs.shu.edu/tfp2007/accomodations.html . You may browse the program and schedule for TFP 2007 at: http://cs.shu.edu/tfp2007/schedule.html . We look forward to seeing you in NYC!!! Cheers, Marco ************************************************************************************ Dr. Marco T. Morazan TFP 2007 Program Committee Chair http://cs.shu.edu/tfp2007/

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Plotting in Haskell
by Chris Witte 28 Feb '07

28 Feb '07

Are there any good libraries for drawing plots (2D and 3D) in Haskell (under Windows using GHC)? Cheers Chris Witte

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Re: overlapping instances, selecting if type a does not belong to class?
by oleg＠pobox.com 28 Feb '07

28 Feb '07

The problem you report can be fixed with some trickery and local functional dependencies. I'd like to show a different solution, which follows a useful general pattern, of isolating overlapping instances to one small part of the program that analyzes the type. The rest of the type program just uses the results of the analysis. > I wasn't able to find the definition of AllOf(But): It is in the complete code http://pobox.com/~oleg/ftp/Haskell/poly2.hs It isn't that interesting: > data AllOfBut x y {-# OPTIONS -fglasgow-exts #-} {-# OPTIONS -fallow-undecidable-instances #-} {-# OPTIONS -fallow-overlapping-instances #-} module HAA where import HListPrelude data HTrue data HFalse -- If two arguments are HList's, concatenate them. If only one of them -- is HList, add the other to the head. We assume at least one is an HList. class IsHList a b | a -> b instance IsHList HNil HTrue instance IsHList (HCons a b) HTrue instance TypeCast HFalse y => IsHList x y -- The are no overlapping instances below class (HList c) => HListAppendArbitrary a b c | a b -> c where hAppendArbitrary :: a -> b -> c instance (IsHList a af, IsHList b bf, HAA af bf a b c) => HListAppendArbitrary a b c where hAppendArbitrary = haa (undefined::af) (undefined::bf) class (HList c) => HAA af bf a b c | af bf a b -> c where haa :: af -> bf -> a -> b -> c -- Both are HLists instance (HList c, HAppend a b c) => HAA HTrue HTrue a b c where haa _ _ = hAppend instance HList a => HAA HTrue HFalse a b (HCons b a) where haa _ _ a b = HCons b a instance HList b => HAA HFalse HTrue a b (HCons a b) where haa _ _ a b = HCons a b -- deliberately unimplemented: at least one must be an HList... -- instance HList b => HAA HFalse HFalse a b c where test1 = hAppendArbitrary HNil True test2 = hAppendArbitrary True (HCons () HNil) test3 = hAppendArbitrary (HCons () HNil) (HCons False HNil) test4 = hAppendArbitrary (HCons () HNil) 'a' -- testf = hAppendArbitrary 'n' () -- TypeCast is skipped

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What inhabits this type: (forall a. a -> b) -> (forall a. m a -> m b)
by Jacques Carette 28 Feb '07

28 Feb '07

Since my last query was answered so quickly, let's try another. I have looked on Hoogle. I would have asked Djinn, but I don't have it around. So, can someone find a term that inhabits (forall a. a -> b) -> (forall a. m a -> m b) ? I think of this as the type of functions that, given a function from any boxed-up a to a b, will give me a function from a boxed-up ma to a m b -- m does not have to be a Monad!. Jacques

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