April 2007 - Haskell-Cafe

Soft references
by Benja Fallenstein 18 Apr '07

18 Apr '07

Hi all, Is there a way to achieve the effect of soft references in GHC? Or if not, is there any hope that they will be implemented in the future? (Soft references are like weak references, except that they are only reclaimed by the garbage collector if memory is short.) I'm building a memoization table of visualizations of a data set, but I can't store *all* visualizations of that set in memory at the same time. My current solution is to use a least-recently-used cache, but this is unsatisfying because I have to specify the maximum number of objects in the cache; if I make that too large, I run out of memory, so I have to make it so small that I can't possibly run out, meaning that I have to make it a lot smaller than it *could* be. With soft references, the program could memoize the "optimal" number of items, with the user being able to influence the behavior through the 'maximum heap size' RTS option. Any ideas? Thanks, - Benja

2 1

FW: [Haskell-cafe] Tutorial on Haskell
by Taillefer, Troy (EXP) 18 Apr '07

18 Apr '07

=======================LONG WINDED RANT WARNING ====================================== I am not try to hate on Haskell obviously I am a fan otherwise I wouldn't be on this list but criticizing other languages is not only a pointless waste of time it is logically flawed on many levels. A lot of the best software is written and still being written in C++ so I will have to disagree with you that C++ is unreasonable. C++ is dangerous and has its flaws especially in the hands of a noob but it is also has some merits as well. People have very logical reasons for choosing a programming language including 1. Familiarity 2. Available people to work with 3. Tools 4. Libraries 5. Legacy 6. Political 7. Personal Preference etc... In terms of reasoning it is easier to reason in a language/Paradigm that you are familiar with than one that might be better but you are totally unfamiliar with and must learn from scratch. Go on IRC #HASKELL and see how many people are struggling conceptual ideas like Monads, Arrows and Functors I don't go there a lot but every time I am there are a couple of people who are asking questions like just what is a Monad. For myself I want to learn Category Theory because it seems powerful just like Set Theory (which I have found extremely useful). I find manipulating pointers and mutable variables much easier to understand and saner. Maybe if I was a mathematician instead of an Engineer/Computer Scientist I would feel different. Saying Haskell is better and everyone should use it because it is better is about as futile as saying we should all be speaking Esperanto because it is better then English. You would be an awfully lonely person with almost nobody to talk to if all you spoke was Esperanto or Haskell for that matter. I have yet to meet face to face another person who has even written toy code in Haskell let alone any real significant app. I can't use Haskell at work because no one could maintain it. I can't use Haskell in my graduate studies since my supervisor can't read Haskell. So I am stuck writing toy code as a hobby and an educational experience. Programs are mostly for humans to read and modify and lets face it there isn't a lot of people fluent in Haskell. So if I want to work and interact with other Software professionals I better be able to speak the lingua franca which is a derivative of C aka C/C++/C#/Java ect. Obviously C++ isn't that bad because people can write good cool software in it. I like Haskell but I haven't seen a ton of Killer Haskell apps yet ( Or even one for that matter). DARCS is coming close if they fix their conflict problems then Haskell will have its first killer app that I know off. I heard they are working on it this summer looking forward to it DARCS is really cool. Sorry for the long rant but I get tired of people with their juvenile and unconstructive statements like "C++ sucks" Or Language X is better then Language Y even if it could ever be objectively true (which I am sorry it can't be) who cares. ============================END RANT=================================================== ________________________________ From: Sebastian Sylvan [mailto:sebastian.sylvan@gmail.com] Sent: Wednesday, April 18, 2007 1:50 PM To: Taillefer, Troy (EXP) Cc: haskell-cafe(a)haskell.org Subject: Re: [Haskell-cafe] Tutorial on Haskell On 4/18/07, Taillefer, Troy (EXP) <troy.taillefer(a)lmco.com> wrote: I have to strongly disagree with the statement that developers like to debug. Debugging is necessary because you can't reason about any "sizeable" piece of code just is not tractable even in Haskell. Now automated tools for reasoning about programs are very cool but lets face it no real world developer will sit down start to manually formally reason about large pieces of code. I think the emphasis when mentioning "reasoning" really shouldn't be "you can reason formally about your programs and prove that they don't go wrong", nor "when it has gone wrong, you can reason about the program to figure out why", it should be "since the language doesn't do batshit insane things behind your back, your programs will mostly work the first time". The "reasoning" isn't an active task that you schedule time for, at least for a casual user like me, it's part of the actual programming. You do "reasoning" when writing in C++ as well, but you often get it wrong (because the language is, shall we say, unreasonable?) and that causes bugs. -- Sebastian Sylvan +44(0)7857-300802 UIN: 44640862

2 1

SYB and opaque types
by Joel Reymont 18 Apr '07

18 Apr '07

Suppose I need to manually derive Data and Typeable for SourcePos from Parsec to make sure my code compiles. I won't actually be running the code I manually derive since the constructor that includes SourcePos will be skipped. With Neil Mitchell's (and #haskell) help I'm doing this to strip token locations from my AST: import Data.Generics import qualified Text.ParserCombinators.Parsec as P instance Data SourcePos where gfoldl r k x = k x typename_SourcePos = mkTyCon "SourcePos" instance Typeable SourcePos where typeOf _ = mkTyConApp typename_SourcePos ([]) strip = everywhere (mkT f) where f (TokenPos a _) = a f x = x I know that the warnings about gunfold, toConstr and dataTypeOf are harmless but how would I define them to avoid the warnings? Also, the definition of strip above requires -fno-monomorphism- restriction. Should I not worry about it? The code runs just fine, locations are being stripped and tests pass. Thanks, Joel -- http://wagerlabs.com/

3 2

Re: [Haskell] ANN: Data.Proposition 0.1
by Jan-Willem Maessen 18 Apr '07

18 Apr '07

Hmm, your BDD implementation claims (in the comment at the top) that "Equality is fast and accurate." But you don't do sharing optimizations, and use derived Eq (rather than object identity), so it's exponential in the number of nodes. Consider: A / \ B | \ / C / \ D | \ / E Here there are five nodes, but we will do two calls to see if C==C, four calls to see if E==E. If I continue the diagram downwards I add two nodes and double the number of equality calls on the leaf. The sharing optimizations are rather important to making an efficient BDD implementation. I haven't yet seen a Haskell-only BDD implementation that didn't have one or more of the following flaws: 1) IO in all the result types, even though the underlying abstraction is pure. 2) Inability to garbage collect unused nodes. 3) Loss of sharing leading to loss of fast equality. Does anyone have a Haskell-only BDD library which avoids all these problems? I wrote one with flaw #2, but was unable to make weak pointers and finalization behave in anything like a sensible fashion and gave up as it was a weekend project. The concurrency + unsafePerformIO mix was trickier than I initially expected, too. That said, this BDD implementation is pretty similar to the performance & behavior you'd get from Data.IntSet (where the bits of your int correspond to the True/False values of your variables). -Jan-Willem Maessen On Apr 16, 2007, at 9:07 AM, Neil Mitchell wrote: > Hi, > > I am now releasing Data.Proposition. This library handles > propositions, logical formulae consisting of literals without > quantification. It automatically simplifies a proposition as it is > constructed using simple rules provided by the programmer. > Implementations of propositions in terms of an abstract syntax tree > and as a Binary Decision Diagram (BDD) are provided. A standard > interface is provided for all propositions. > > Website: http://www-users.cs.york.ac.uk/~ndm/proposition/ > Darcs: darcs get --partial http://www.cs.york.ac.uk/fp/darcs/ > proposition/ > Haddock: http://www.cs.york.ac.uk/fp/haddock/proposition/Data- > Proposition.html > > The Haddock documentation also has a short example in it. > > This library is used substantially in the tool I have developed for my > PhD, and has been extensively tested. > > Thanks > > Neil > _______________________________________________ > Haskell mailing list > Haskell(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell

3 2

Re: [Haskell-cafe] Re: Translating perl -> haskell, string "fill ins" with an error on invalid inputseems awfullycomplex. Is there a way to simplify?
by Thomas Hartman 18 Apr '07

18 Apr '07

Claus and Evan ++; that was very helpful. FWIW, my gut feeling is that Claus's first version was easier to understand than the revision with printf, which seems to me to involve a lot more monadic wizardry (Functor, MonadError, fmap, mapm). The first version, which just used maybe, was clear to me within seconds. But again, I learned a lot. Thanks.

3 6

Transpose using array comprehension
by Chris Witte 18 Apr '07

18 Apr '07

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? Cheers Chris

2 1

Tutorial on Haskell
by dfeustel＠mindspring.com 18 Apr '07

18 Apr '07

What would be really useful is a Haskell Cookbook that shows how to do in Haskell things that are so easily done in imperative languages. How to solve simultaneous equations using Gaussian elimination comes to mind. Lots of examples would be great. Dave Feustel

2 1

Haskell job
by Lennart Augustsson 18 Apr '07

18 Apr '07

Hi all, Credit Suisse has started using Haskell within GMAG, the group that provides financial analytics for the bank. The main use of Haskell is for a Domain Specific Embedded Language that is being used by the people writing the financial analytics (these people typically have a PhD in mathematics or physics). We are looking to extend our Haskell group (which currently is me and Ganesh Sittampalam), so if you're an experienced Haskell person take the chance! Here's a URL to the Credit Suisse job ad: https://creditsuisse.taleo.net/servlets/CareerSection? art_ip_action=FlowDispatcher&flowTypeNo=13&pageSeq=2&reqNo=53820&art_ser vlet_language=en&selected_language=en&csNo=10020#topOfCsPage If you have any questions email Howard Mansell, howard.mansell@credit- suisse.com, or me. -- Lennart

1 0

Re: Release plans
by Brandon Michael Moore 18 Apr '07

18 Apr '07

On Tue, Apr 17, 2007 at 12:50:48PM +0200, Doaitse Swierstra wrote: > Just to show what kind of problems we are currently facing. The > following type checks in our EHC compiler and in Hugs, but not in the > GHC: > > module Test where > > data T s = forall x. T (s -> (x -> s) -> (x, s, Int)) > > run :: (forall s . T s) -> Int > run ts = case ts of > T g -> let (x,_, b) = g x id > in b Consider this additional code which also typechecks in Hugs: v :: forall s . T s v = T f f :: s -> ([s] -> s) -> ([s], s, Int) f v g = let x = [v] in (x, g x, 0) due to parametricity, run v can't depend on x or g x. Apparently id has type [x] -> x. Are EHC and Hugs supposed to support equirecursive types? Brandon

1 1

unsafeInerleaveIO and randomIO
by Marc Weber 17 Apr '07

17 Apr '07

stefan has pointed me a nice version: ============= ======================================================= randomInts :: IO [Int] randomInts = randoms `fmap` newStdGen main = do ints <- randomInts print $ take 5 ints =========== ========================================================= Anyway I'm curious why ============= ======================================================= module Main where import Data.Char import Control.Monad import Random import System.IO.Unsafe randomInts :: IO [Int] randomInts = unsafeInterleaveIO $ sequence $ cycle [unsafeInterleaveIO randomIO] main = do ints <- unsafeInterleaveIO randomInts print $ take 5 ints ============= ======================================================= doesn't return. Where did I miss another unsafeInerleaveIO to make it lazy enough? I still need a hint. Marc

3 3