- Haskell-Cafe - Haskell.org

Fwd: [Haskell-cafe] Undecideable instances for one instance?
by Christophe Poucet 07 Jun '06

07 Jun '06

Hello Bulat, You are indeed correct. However I fail to see how there is any undecideability. If instead one specified it as follows, it would be fine: class Locatable a b | a -> b, b -> a where value :: a -> b wrap :: b -> a instance Locatatable (Located a) a where value (L a) = a wrap a = L a It compiles fine, yet this is semantically the same. On 6/7/06, Bulat Ziganshin <bulat.ziganshin(a)gmail.com> wrote: > > Hello Christophe, > > Wednesday, June 7, 2006, 12:27:22 PM, you wrote: > > > global instance of a typeclass (so there is no overlapping), GHC will > > still complain about overlapping. Is there any reason this is so or is > > GHC compains here about undecidability, not overlapping :) > > > -- Illegal instance declaration for `Locatable a' > > -- (There must be at least one non-type-variable in the instance head > > -- Use -fallow-undecidable-instances to permit this) > > -- In the instance declaration for `Locatable a' > > > Christophe(vincenz) > > > > > -- > Best regards, > Bulat mailto:Bulat.Ziganshin@gmail.com > >

1 0

FunDeps conflict
by Joel Björnson 06 Jun '06

06 Jun '06

Hi. I have a question regarding type classes and FunDeps. Consider the following code : > class Class2 a b | a -> b > class IsFoo a > data Bar a = Bar a > instance IsFoo a => Class2 a a > instance IsFoo a => Class2 (Bar a) a The last two instantiations will yield a 'Functional dependencies conflict error'. From what I understand, this is because (Bar a) *MAY* instantiate the type class isFoo, which would obviously lead to a conflict between the two instantiations. Is there anyway to get around this, knowing that (Bar a) won`t instantiate the isFoo class, or is the right answer simply to get rid of the FunDep ? Thanks /Joel

3 4

Problem in code
by Jenny678 06 Jun '06

06 Jun '06

hello, I can not find the problem, I use WinHugs and shows me(Syntax error in input (unexpected keyword "where"). here is my code: f :: (Float,Float) -> Float f (x,y) = (a(x,y) + 4) * (b(x,y) + 3) where a :: (Float,Float) -> Float a(x,y) = (x + y)*2 b :: (Float,Float) -> Float b(x,y) = (y - x)*5 How can I define this code with let in? Thanks for any help. -- View this message in context: http://www.nabble.com/Problem-in-code-t1742233.html#a4734684 Sent from the Haskell - Haskell-Cafe forum at Nabble.com.

2 1

Combinations
by developer＠imaginando.net 06 Jun '06

06 Jun '06

Hi, I need a functions which takes as argument a list of lists like this one: [[1,2],[3],[4]] and gives me a list of list with all the possible combinations like this one: [[1,3,4],[2,3,4]] In this case there are only 2 combinations but if there was more than one element in the other lists the results were much more. I'm a little stuck in this simple problem. First i was thinking in cartesian product but then i realized that i may not be what i really need. Any tip? Many thx, Nuno Santos

6 6

[ANNOUNCE] Hitchhikers guide to Haskell - chapter 5 is online
by Dmitry Astapov 05 Jun '06

05 Jun '06

It's bigger. It's better. It now comes with source code included. If you already know what it is - just go and read a new chapter. If you dont know what it is - just go and read it from the very beginning. http://www.haskell.org/haskellwiki/Hitchhikers_guide_to_Haskell Or "darcs get http://adept.linux.kiev.ua/repos/hhgtth" and drown me in patches. -- Dmitry Astapov //ADEpt GPG KeyID/fprint: F5D7639D/CA36 E6C4 815D 434D 0498 2B08 7867 4860 F5D7 639D

1 0

Re: [Haskell-cafe] Features of Haskell]
by Bernard James POPE 05 Jun '06

05 Jun '06

On Sun, Jun 04, 2006 at 11:21:23AM +0200, Niels Van Och wrote: [snip] > Now, in a week I have to present this work formally, and I'm a bit > stumped as to how I'm going to do this. I've got about 15-20 minutes, > so I can only discuss the major features. Right now I'm thinking about: > > - Short introduction to functional programming, and Haskell in it > - Basic syntax in a few minutes > - Features like polymorphism, overloading, higher-order functions, > comprehensions, .. > - Lazy evaluation > - Monads: why and how? > > However, I'd love to know what you think. Furthermore, do you think I > should include an example on the usage of Haskell, and if so, which? Hi Niels, 15-20 mins isn't much time, so I would get straight to the point. I wouldn't bother introducing the syntax, just explain it as you go, to save time. I've always liked zipWith as an example for _short_ presentations. It is higher-order, polymorphic, and lazy. The classic fibonacci generator comes to mind as a "cute" example [1]. fibs = 0 : 1 : zipWith (+) fibs (tail fibs) If you want to show overloading and monads then you can extend it to zipWithM. You'll probably want to argue why all of the features you mentioned are important for real programming problems (not fibs). John Hughes' paper "Why Functional Programming Matters" is a good source of material. You might also like to argue that one of Haskell's strengths is that it is built up from a very small core language (the lambda calculus plus let). Higher-order functions, a good type system, and lazyness, allow many powerful features to be included in the programmer's toolkit without having to change the underlying core. For example, list comprehensions are simply syntax sugar, rather than some fundamentally new gadget. You might say Haskell takes abstraction and composition very seriously. To make your talk more even-handed, you might like to mention some of the outstanding problems faced by Haskell programmers. For instance, while we seem to have a good handle on the declarative side of programming, the operational side can be troublesome. Witness: space leaks and deepseq. The foldl leak is a possible example: sum = foldl (+) 0 Now some people might complain "Sure, Haskell looks nice in theory, but the proof is in the pudding; where's the pudding?". You might like to toss in a few examples of "real world" Haskell programs at the end, as a kind of pre-emptive strike against the non-believers. With some kind of note as to why Haskell was useful. Darcs and Pugs come to mind. As usual, all of this is qualified by the usual proviso: "It depends who your audience is." Add salt or sugar to taste. Hope that is of some help, Bernie. [1] Though, depending on your audience, this might be like playing "Stairway to Heaven" in a guitar shop.

1 0

ComputerWorld AU mention
by Manuel M T Chakravarty 04 Jun '06

04 Jun '06

This ComputerWorld (Australia) article, although about OSes, mentions Haskell favourably: http://www.computerworld.com.au/index.php?id=710118083 Technical detail on how Haskell helps to improve OS security is in a recent paper: http://www.cse.unsw.edu.au/~chak/papers/DEKC+06.html Manuel

1 0

Is there any url lib?
by Marc Weber 04 Jun '06

04 Jun '06

Hi. Did anyone implement something like pythons urllib yet? I wont to retrieve some files via http (I could use wget -O - for that ) and send some form information (post/get).. In other words: Something like expect but for downloading some documents from a website.. ;) Perhaps I should have another look at wash? I think I can find encoding/decoding parameters for method = get there.. Marc

2 1

Bug in Data.Graph
by Christophe Poucet 04 Jun '06

04 Jun '06

Dear, I think I have discovered a bug in Data.Graph. If one looks at the documentation it states: Vertex = (Node, Node) An edge from the first vertex to the second. Ergo (i,j) => j is reachable from i Then continuing, one reads that: topSort :: Graph -> [Vertex] A topological sort of the graph. The order is partially specified by the condition that a vertex /i/ precedes /j/ whenever /j/ is reachable from /i/ but not vice versa. However if one tries to evaluate it, one gets: > print . G.components $ G.buildG (1,4) [(1,2), (1,3), (2,4), (2,3)] > [1,3,2,4] According to specification we have the graph (with arrows pointing down) 1 / \ | 3 |/ 2 | 4 Therefore one would expect [4,2,3,1]. Is this a bug in the documentation or the implementation? With regards, Christophe -- Christophe Poucet Ph.D. Student Phone:+32 16 28 87 20 E-mail: Christophe (dot) Poucet (at) imec (dot) be Website: http://notvincenz.com/ IMEC vzw – Register of Legal Entities Leuven VAT BE 0425.260.668 – Kapeldreef 75, B-3001 Leuven, Belgium – www.imec.be *****DISCLAIMER***** This e-mail and/or its attachments may contain confidential information. It is intended solely for the intended addressee(s). Any use of the information contained herein by other persons is prohibited. IMEC vzw does not accept any liability for the contents of this e-mail and/or its attachments. **********

1 1

Library Repository?
by Bryan Burgers 03 Jun '06

03 Jun '06

Recently, I have seen a few messages to Haskell Cafe (Like Marc Weber's "Is there any url lib?" and Ferenc Wagner's "LDIF output library", as well as many replies like "oh, you can find that library at this link...") that have prodded me to ask what I've been wondering for a long time. What are the pros and cons of having a library repository for Haskell libraries. At current, I'm thinking of Perl's CPAN, which many Perl experts claim is the most important aspect of Perl ("Don't reinvent the wheel" is a common reply at Perlmonks.org) and I believe Ruby has something similar (RubyGems?). It seems to me that with the emergence of Cabal as a standard way to package Haskell libraries, this could be possible. Here's a short, non-exhaustive list of pros and cons I came up with. Pros: * Localized place to find libraries. If I want to know if a urllib exists, I can go to the server and search for 'url' and come up with relevant libraries. * Encourages programmers to share their libraries. If there's a place to put libraries, I would imagine that some people who have good libraries lying around might put them there. Or if they have something close, they might generalize it into a library and put it there. Cons: * It obviously needs a server, which costs money, and other things like that cost money. * Personel. It would take a bit of initiative to start a project like this, as well as continued upkeep. That would either require some knowledgable volunteers, or, once again, money. I'd love to hear the thoughts of the community on this. Bryan

2 1