May 2005 - Haskell-Cafe

Array functions?
by Daniel Carrera 03 May '05

03 May '05

Hello, I hope these don't turn out to be RTFM questions, but I can't find them in my FM :-) 1) Is there a function to get the ith element from an array? 2) Is there a function to get the "index" of an entry in an array? I've implemented these two functions below: 1) find 0 (x:xs) = x find n (x:xs) = find (n-1) xs 2) index i (x:xs) = if i == x then 0 else 1 + index a xs This was a fun exercise, but I can't shack off the feeling that I just re-invented the wheel. I need these because I want to be able to swap any two elements from an array. This is my swap function: -- swap i j array => swaps the ith and jth elements of 'array'. -- swap i j arr = a_head ++ [item_j] ++ a_midd ++ [item_i] ++ a_tail where a_head = [a | a <- arr, index a arr < i] item_i = find i arr a_midd = [a | a <- arr,(index a arr > i) && (index a arr < j)] item_j = find j arr a_tail = [a | a <- arr, index a arr > j] I'm sure this was a poor way to accomplish this, but it was a learning experience. If anyone would like to show me a more elegant solution, I would be happy to see it. Cheers, Daniel. Haskell newbie.

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Clarification on proof section of HS: The Craft of FP
by Echo Nolan 03 May '05

03 May '05

Hello all, My copy of HS: The Craft Of FP just arrived and I was reading section 8.5 about induction. On page 141, Simon Thompson gives a method of proving properties of functions on lists: 1) Prove that the property holds for the null list 2) Prove that the property holds for (x:xs) under the assumption that the property holds for xs. My question is this: what is the proof of (P([]) and ( P(xs) => P(x:xs))) => P(xs)? In other words, how does proof of the property on empty lists and proof that the property holds on (x:xs) under the assumption that it holds on xs give proof that it holds on all lists? Isn't there a recursive dependency here? You are defining the proof for lists with at least one element based on the assumption that it holds for all lists, right? How does this get you a proof of the general property if it depends on the assumption of the general property? Regards and thanks, Echo Nolan.

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How to join two lists of lists?
by Khrystyna Mandziy 03 May '05

03 May '05

Hi all! I'm trying to join to lists of lists. The problem is, i would like to get a new list of lists of tuples and not list of tuples of lists (that what zip makes). list1 = [[1,2],[3,4],[5,6]] list2 = [[a,b],[c,d],[e,f]] desiredlist = [[(1,a),(2,b)],[(3,c),(4,d)],[(5,e),(6,f)]] Thanks for any help!

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Here's a curve ball for Haskell.
by Daniel Carrera 03 May '05

03 May '05

Alright, in Haskell there are no side-effects when you call a function twice on the same data you get the same result... I just decided I'll try to write a good pseudo random number generator in Haskell :-) I'm off to class now, but I'll try it tonight. Cheers, Daniel.

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Re: [Haskell-cafe] Speed comparison?
by Daniel Carrera 03 May '05

03 May '05

Greg Buchholz wrote: > You might find the "The Great Computer Language Shootout" > informative... > > http://shootout.alioth.debian.org/ Thanks! That's a great resource. At first glance, Haskell seems to do well over-all. Cheers, Daniel.

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RE: [Haskell-cafe] fptools in darcs now available
by Simon Marlow 03 May '05

03 May '05

On 01 May 2005 19:28, Sven Panne wrote: > Gour wrote: >> Simon Marlow (simonmar(a)microsoft.com) wrote: >>> Great news, thanks John. >>> >>> Is it possible to set up a two-way synch so we can move over to >>> darcs gradually? It's not really practical for us to move over in >>> one go, we've simply accumulated too many dependencies on CVS, and >>> there are lots of people using the repo with CVS. If we had a >>> two-way synch, we can experiment with darcs non-destructively. >> >> Great news, thanks Simon. >> >> Nice to hear you are considering to move to darcs. > > Hmmm, > > I don't want to spoil all the enthusiasm for a new tool, especially a > Haskell-based one, but being an old CVS veteran who recently fell into > love with subversion, I'd really like to know the benefits switching > to darcs. First of all, let me say that we haven't committed to darcs in any way - we're just planning to experiment with it. Besides, it's not my decision alone - lots of people (yourself included) have code in the CVS repository, so pulling the rug out is not an option. On to darcs vs. Subversion. I too was excited by Subversion until quite recently: it does a great job of being a "better CVS than CVS", but I don't think it goes far enough. The lack of distribution and good support for merging are the two main drawbacks, as I see it (I'm aware of SVK, though I haven't tried it). Fundamentally, darcs is built on a model that is far simpler than SVN: a repository is just a set of patches, a branch is just a repository. I think having a simpler model will have benefits in terms of accessibility - I've met several people who were just not comfortable with the CVS model, and couldn't get to grips with it. Distribution is useful, as others have mentioned. Having a local copy of the repo that I can commit to before pushing patches to the main repo is a big win, as is allowing other developers to have their own local branches, and the convenience of 'darcs send' is going to make it easier for people to contribute to GHC, and for us to accept contributions. Last I checked, SVN's branching support was lacking: making a branch is easy, but merging between branches is hard, just like CVS. Sure they plan to fix this (or maybe they've already improved things), but my guess is that darcs' support for merging will always be superior because It's What Darcs Does. Good support for merging just falls out of darcs' simpler model. And I aggree with Manuel's point: we should support darcs because it's written in Haskell! Cheers, Simon

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A little bit OT: Google as a global directory/resolver for cabalized packages?
by Dimitry Golubovsky 02 May '05

02 May '05

If one tries to search Google: http://www.google.com/search?hl=en&lr=&c2coff=1&q=exposed-modules+version+f… it returns URLs of all Google-indexed .cabal files (currently 7- just this few?) So if one makes such a request from a program, then parses the resulting HTML extracting <a> tags containing links ending with ".cabal", it would result in a directory of .cabal files locations suitable for furter processing. Similarly, if all locations of a particular package need to be found, package name needs to be added to the query string. Using Google API might be an alternative, but it returns only up to 10 results (per their docs) which might be enough to find all mirrors of a single package, but not the whole list of available packages. Publishing in such a directory might be simple: just place one's .cabal file in a place where Google can index it. One weakness: too easy to spam. Could this be a (cheap) way to set up package name -> URL resolution? -- Dimitry Golubovsky Anywhere on the Web

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Random State Monad and Stochastics
by Dominic Steinitz 02 May '05

02 May '05

I don't think they are in the standard libraries but there was some discussion about them a few months ago but I couldn't find a reference. Peter, Can you supply one? I think you were a participant in the discussion. Did you put a library of this sort of thing together? Here's my tuppenceworth which I used to send a sequence of ip packets (as actions) and to stop when I got a destination unreachable: sequenceWhile_ :: Monad m => (a -> Bool) -> [m a] -> m () sequenceWhile_ p [] = return () sequenceWhile_ p (x:xs) = x >>= \c -> if (p c) then sequenceWhile_ p xs else return () Dominic.

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RE: [Haskell-cafe] Typeclasses and instances
by Simon Peyton-Jones 02 May '05

02 May '05

I've finally gotten around to fixing this. The fix won't be in 6.4 (ever) as mentioned below, but it's done in the HEAD. Simon | -----Original Message----- | From: haskell-cafe-bounces(a)haskell.org [mailto:haskell-cafe-bounces@haskell.org] On Behalf Of | Simon Peyton-Jones | Sent: 18 March 2005 13:01 | To: Mark Carroll; haskell-cafe(a)haskell.org | Subject: RE: [Haskell-cafe] Typeclasses and instances | | Strange. This is a bug in GHC that crept in when I implemented GADTs. | It turns out to be slightly structural; fixing it will change the | interface-file format. So it won't get fixed in 6.4; but will in the | HEAD. That doesn't bother me; the context on a data/newtype decl is a | strange thing anyway; only useful for generating extra errors... but | yes, it's a bug thanks for pointing it out. | | Simon | | | -----Original Message----- | | From: haskell-cafe-bounces(a)haskell.org | [mailto:haskell-cafe-bounces@haskell.org] On Behalf Of Mark | | Carroll | | Sent: 17 March 2005 19:58 | | To: haskell-cafe(a)haskell.org | | Subject: Re: [Haskell-cafe] Typeclasses and instances | | | | Another note, with more help from friends: | | | | It turns out that GHC 6.4 will let me do, | | | | newtype Floating a => Test a = Test [a] deriving Show | | | | x = Test [False, True] | | | | but, if I change "newtype" to "data", it then says, | | | | No instance for (Floating Bool) | | | | I'm not sure I quite understand what's going on. | | | | -- Mark | | _______________________________________________ | | Haskell-Cafe mailing list | | Haskell-Cafe(a)haskell.org | | http://www.haskell.org/mailman/listinfo/haskell-cafe | _______________________________________________ | Haskell-Cafe mailing list | Haskell-Cafe(a)haskell.org | http://www.haskell.org/mailman/listinfo/haskell-cafe

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HGL documentation
by Peter Padawitz 02 May '05

02 May '05

Is there a HGL documentation in one volume (e.g. pdf file)? Printing all pieces of http://www.haskell.org/ghc/docs/latest/html/libraries/HGL is a little tedious... Peter

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