March 2009 - Haskell-Cafe

hw do i solve this problems:
by THANDO NTUANE 18 Mar '09

18 Mar '09

kindly help me.. im stack but managed to solve exercise 1 and 2.

1 0

Any way to catch segmentation fault?
by Nick Rudnick 18 Mar '09

18 Mar '09

Hi all, doing some work with a regex library I stumbled over some segmentation faults due to illegal byte combinatations. Looking for a way to get this caught some way, I failed in finding any place at GHC (6.10.1) or Hackage libraries where this is covered -- a quick check with HUnit revealed it to be crashing by this phenomenon. So I would like to ask about the state of this issue, o Is there principally no way to (generally) catch segmentation faults?? (This would be hard to believe, as the described kind of noise is to be expected at production systems, especially with user generated content.) o Are segmentation faults «prohibited» in Haskell so the advice is just to change the used library and forget the whole stuff?? o Did I oversee the generic mechanism for catching of segmentation faults?? (If yes, do you please give me a hint??) o If no, is there a workaround which might be practicable?? Thanks a lot in advance, Dorinho

2 1

Google Summer of Code
by Malcolm Wallace 18 Mar '09

18 Mar '09

I am pleased to announce that haskell.org has once again been accepted as a mentoring organisation, for the 2009 Google Summer of Code. Student applications open on Monday (23rd March) at 1900 UTC, for a period of 12 days (until Fri 3rd April, also at 1900 UTC). Students applicants are encouraged to interact with the community via mailing lists, prior, during, and after the submission of their ideas for projects. In general terms, project ideas that benefit the whole community (e.g. infrastructure like ghc, cabal, libraries) will be preferred over things of more marginal interest (e.g. games). Unless you make a _really_ good case, of course! Because (sadly) the darcs community did not get accepted as a separate organisation this year, haskell.org will be willing to accept proposals relating to darcs (as it has done in previous years), but of course they will be competing in the general pool, alongside all the other ideas. Also, we are still accepting mentors. Regards, Malcolm

1 0

.hi inconsistency bug.
by Joe Fredette 18 Mar '09

18 Mar '09

List, I've got this project, source on patch-tag here[1] It's a nice little project, I've got the whole thing roughly working, it compiles okay, everything seems to work, until I try to run it, specifically when I run it in ghci, or when I run the main executable (which uses hint), and look at any type involving my "Email" type, it gives me the following error: Type syonym HackMail.Data.MainTypes.Filter: Can't find interface-file declaration for type constructor or class HackMail.Data.ParseEmail.Email Probable cause: bug in .hi-boot file, or inconsistent .hi file Use -ddump-if-trace to get an idea of which file caused the error As far as I understand, it wants to find the interface-file declaration for a specific type (Email) exported by the ParseEmail module, all of the exports (I think) are in order. I've tried mucking around with it a bit, but I don't fully understand what the error even means, much less how to fix it. Other relevant info, Email is exported in a roundabout way, namely by importing a module MainTypes, which exports a module Email, which exports a the ParseEmail Module, which exports the datatype Email. The "Filter" delcaration it _actually_ complains about (it's just the first place the email type is invoked) is: type Filter a = ReaderT (Config, Email) IO a nothing particularly special. Any help fixing this is greatly appreciated, I did find this bug report[2] which seems like it might be relevant. But trying to unregister -> cabal clean -> cabal install doesn't fix it. I've also tried manually removing the dist/ folder, and also unregistering the package. Thanks again. /Joe [1] http://patch-tag.com/repo/Hackmail/browse [2] http://hackage.haskell.org/trac/ghc/ticket/2057

2 6

Re: [Haskell-cafe] Who generates Haskell code and uses type information at runtime? (fwd)
by Chris Brown 18 Mar '09

18 Mar '09

I'm not sure if you got my previous message, as I was having some problems posting to the list... > Putting in a nutshell, I generalize an extensional defined function > definition into a recursive one. This is done in a number of steps by > modifying expressions and exploiting type information of > sub-expressions. For example: > > rev [] = [] > rev [a] = [a] > rev [a,b] = [b,a] > rev [a,b,c] = [c,b,a] > > ~~> > > rev x = y > > ~~> > > rev [] = [] > rev (x:xs) = (y:ys) > > ~~> > > rev [] = [] > rev (x:xs) = (last (x:xs)) : (reverse (x:xs)) > > > The initial set of rules is given by the user (in a file, via IO, ...). > The problem later is to infer the type of an intermediate variable, e.g. > 'y'. I'm still not entirely sure what you want to do here. But it sounds like HaRe could already do most of this for you via a sequence of folds, unfolds, introduce pattern matching and generative folding. HaRe already has built-in support for some symbolic evaluation, which is already used in the generative fold refactoring, and has type checking support too. http://www.cs.kent.ac.uk/projects/refactor-fp/hare.html If it doesn't do exactly what you want out of the tin, it does have a large API for designing transformations over Haskell code. http://www.cs.kent.ac.uk/projects/refactor-fp/hare/haddock/RefacUtils.html Chris. > > Thanks, > > Martin > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > -- Chris Brown Visualization Software Engineer, Peiriannydd Meddalwedd Delweddu. Cast Ltd., Technium CAST, Ffordd Penlan, Parc Menai, Bangor, Gwynedd UK. LL57 4HJ. Tel: +44 (0)1248 675038 Fax: +44 (0)1248 675012 Mobile: +44 (0)7917 763712 -- Centre for Advanced Software Technology Limited is a limited company registered in England and Wales. Registered Number: 04473521, Registered Office: Finance Office, Bangor University, College Road, Bangor, Gwynedd. LL57 2DG.

2 1

encoding for least fixpoint
by ben 18 Mar '09

18 Mar '09

Hello, I am trying to understand the definition of (co)inductive types in Haskell. After reading the beginning of Vene's thesis [1] I was happy, because I understood the definition of the least fixpoint: newtype Mu f = In (f (Mu f)). But this definition coincides with his definition of the greatest fixpoint newtype Nu f = Wrap (f (Nu f)), which in reality is not true (e. g. for f x = a * x -- the 'stream functor' the type Mu f should be empty, isn't it?). Then I stumbled over a blog entry of Shin-Cheng Mu [2] and from there over an article of Wadler [3], where the least fixpoint is encoded as Lfix X. F X = All X. (F X -> X) -> X. and the greatest fixpoint as Gfix X. F X = Exists X. (X -> F X) * X. I would like to understand these definitions, or get an intuition about their meaning. Could somebody give some explanation or a pointer to a place where I can find one? Thanks a lot in advance, ben [1] http://www.cs.ut.ee/~varmo/papers/thesis.pdf [2] http://www.iis.sinica.edu.tw/~scm/2007/encoding-inductive-and-coinductive-t… [3]http://homepages.inf.ed.ac.uk/wadler/papers/free-rectypes/free-rectypes.txt

7 11

Need an overview of FP-related research topics
by Yitzchak Gale 18 Mar '09

18 Mar '09

I spoke to a faculty member in a decent Computer Science department in which no one has ever done anything related to FP. (You may say that is an inherent contradiction, but what can I do, the department does have a good reputation. I am withholding names to protect the innocent.) This faculty member happens to be the one who was forced to teach the single programming languages course offered by the department, due to his having the least seniority in the department. As such, he is now actually starting to become a bit interested in FP and Haskell. I gave him a few links to Haskell resources, which he appreciated. But coming from more of a pragmatic non-research point of view, I didn't really know what to say. The best I could do was to point him to the Haskell Wikibook, and to the "Why FP Matters" paper. I would like some links that would give such a person a nice overview of the various active areas of FP-related research these days, leaning towards Haskell. I want to give him a fairly broad view of what is interesting and exciting, why various topics are important, where to find ideas for collaboration and applications to other areas, etc. I actually know about a few departments like that. This could be a good strategy for drumming up more research interest in Haskell. In some cases, the person in question has already been influenced somewhat by the lisp fanatics, so I would like some help in how to deal with that also. Thanks, Yitz

4 4

libgmp for GHC 6.10.1 on Mac OS X 10.5
by Dean Herington 18 Mar '09

18 Mar '09

I'm trying to install GHC 6.10.1 on Mac OS X 10.5 (PowerPC). I installed Xcode 3.1.2. I built libgmp 4.2.4 and installed it in /usr/local/lib. When I do "./configure" in GHC's dist directory, however, I get: bash-3.2$ ./configure checking build system type... powerpc-apple-darwin9.6.0 checking host system type... powerpc-apple-darwin9.6.0 checking target system type... powerpc-apple-darwin9.6.0 Which we'll further canonicalise into: powerpc-apple-darwin checking for path to top of build tree... dyld: Library not loaded: /usr/local/lib/libgmp.3.dylib Referenced from: /Users/family/Desktop/Downloads/ghc-6.10.1/dist/utils/pwd/pwd Reason: no suitable image found. Did find: /usr/local/lib/libgmp.3.dylib: mach-o, but wrong architecture /usr/local/lib/libgmp.3.dylib: mach-o, but wrong architecture configure: error: cannot determine current directory Any ideas what I'm doing wrong? Thanks.

3 4

UNPACK multiple constructors
by Louis Wasserman 18 Mar '09

18 Mar '09

{-# UNPACK #-} is specified to work on single-constructor types, but would there be advantages to, for instance, compiling data BinTree e = Node e {-# UNPACK #-} !(Maybe (BinTree e)) {-# UNPACK #-} !(Maybe (BinTree e)) into four constructors, one for each combination of Maybe constructors? Thoughts? Louis Wasserman wasserman.louis(a)gmail.com

1 0

symbolic evaluator for Haskell?
by Tim Newsham 18 Mar '09

18 Mar '09

Is there a symbolic evaluator for Haskell that will perform all applications except on specified functions? Ie. I would love something that would take foldr (+) (6 `div` 5) [1,2,3*4] and "(+) (*)" and return 1 + (2 + (3*4 + 1)) by performing all the applications except for (+) and (*). (Something that supports ghc extensions is preferred :) Tim Newsham http://www.thenewsh.com/~newsham/

4 4