- Haskell-Cafe - Haskell.org

basic haskell question
by wenduan 22 May '05

22 May '05

hi, Anybody could please tell me that in the following two expressions what value does the [] take? foldl (/) 3 [] foldr (/) 3 [] when both of them are evaluated I got 3.0,but I thought I could get nothing out of there,cause its an empty list,does Haskell assume any default value for a empty list? -- ------------------------------------------------------------------------------------------------------- If you didn't succeed at the first attempt, you are just about average.

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Re: [Haskell-cafe] Python?
by karczma＠info.unicaen.fr 21 May '05

21 May '05

Tim Rowe writes: > On 5/11/05, Jerzy Karczmarczuk <karczma(a)info.unicaen.fr> wrote: > >> Give me one single language where [3-d arrays are] natural and immediate. > > I don't know how Matlab does it, but I find the C++ standard library > vector<vector<vector<float> > > > entirely intuitive (apart, perhaps, for the need for those two spaces)! Let's precise what I consider to be important in order to call it natural and immediate. And, in general, useful. 1. The definition of a concrete object, not just its type, but, say, the initialization with constants. Or/and, global initialization with zeros. 2. Easy synthesis of multi-dim matrices out of "planes", of submatrices of lesser dimensions; it can be an 'overlay', like, say, making a colour image out of three R/G/B planes, or making a 3D surfaces, or aking tensors through external products. 3. Easy indexing, and not only A[i][j][k], etc., but slicing, the extraction of sub-dimensional matrices, e.g., one column vector out of a 2D matrix in Matlab: A(3,:). Also, extracting parts (e.g. sub-images). Also, in mathematical context, "intelligent" indexing, e.g. treating a matrix as implicitly anti-symmetric. Here the CAS systems as Maple or Mathematica provide the adequate tools. C++ of course doesn't, unless you overload [] yourself. 4. Readable iterators, perhaps something more compact than insipid do-loops. 5. If those matrices are used as mathematical objects: tensors, etc., I want to have some simple notation for inner multiplications/ contractions, etc. This is not just the syntax problem, but the existence of good libraries as well... 6. Reshaping of those arrays. I thought that Matlab 'reshape' (or something similar in Numeric Python) is a baroque, rarely used construction. Now I use it quite often... So, plenty of things. That's why this is not so trivial... Jerzy Karczmarczuk

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forall in a lambda abstraction
by Scott J. 20 May '05

20 May '05

Hello, what's wrong with the code of test1 below. Ghci does complain with an error message that I don't understand. Thanks Jan id x = x test1 = \f ::(forall a. a -> a) -> (f True,f 'a') test = \f -> (f True,f False) h :: (forall a. a->a) -> (Bool,Char) --h :: (a.a -> a) -> (Bool,Char) h id = (id True, id 'a')

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RE: [Haskell-cafe] Tables in Haskell (data model /business app pr ototyping)
by Bayley, Alistair 20 May '05

20 May '05

> From: Rene de Visser [mailto:rene_de_visser@hotmail.com] > > I would rather use Haskell also for the persistency, global > constraint > maintanence, etc... rather than using an external database. > > The only thing I have found so far is HApps, but I haven't > figure out how > that works. > > Is there anything else? What do you mean by "external" database? If you just mean "on another server, accessed via a separately installed library or API" then you should consider Sqlite as a persistence storage solution. It's a doddle to install on common platforms, and consists of a single library which links to your program i.e. it runs in the same process-space. Both HSQL (and therefore HaskellDB, as it uses HSQL) and Takusen support Sqlite. http://haskelldb.sf.net/ http://cvs.sf.net/viewcvs.py/haskell-libs/libs/takusen/ Alistair. ----------------------------------------- ***************************************************************** Confidentiality Note: The information contained in this message, and any attachments, may contain confidential and/or privileged material. It is intended solely for the person(s) or entity to which it is addressed. Any review, retransmission, dissemination, or taking of any action in reliance upon this information by persons or entities other than the intended recipient(s) is prohibited. If you received this in error, please contact the sender and delete the material from any computer. *****************************************************************

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Re: Why the instance declaration , passing on Hug but failing on GHC 6.2.1?
by Daniel Fischer 19 May '05

19 May '05

Am Donnerstag, 19. Mai 2005 10:39 schrieb Stefan Holdermans: > Wukaichen, > > > data People = A | B | C | D | E | F deriving (Show, Eq, Ord, Enum) > > data Pair a = Pair a a deriving Eq > > > > instance Show (Pair People) where > > show (Pair x y) = "<" ++ show x ++ ", " ++ show y ++ ">" > > GHC is right, actually. Haskell 98 requires instance declarations to > have the form > > instance (...) => C (T a1 ... an) > > where a1 are type variables, not type constructors as in your example. > > However, you can use the Glasgow extensions to get rid of this > restrictions. For example by adding > > {-# OPTIONS -fglasgow-exts#-} > > to the top of your file. > > HTH, > > Stefan > Alternatively, one might define instance Show a => Show (Pair a) where ... By the way, the original code also needs the '-98' option for hugs, so the behaviour isn't really different. Cheers, Daniel

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www.haskell.org
by Colin Paul Adams 19 May '05

19 May '05

I'm trying to download a darcs client, but I get: "The connection was refused when trying to connect to www.haskell.org" from Firefox on Linux. -- Colin Adams Preston Lancashire

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Bit fiddling
by Florian Weimer 19 May '05

19 May '05

I'm toying a bit with Haskell and wondering what's the best way to implement bit fiddling. Most of my applications involve serializing and deserializing small blobs (IP packets, for instance), and after browsing the GHC library documentation, I'm not sure which appraoch I should use. That's why I'd appreciate pointers to sample code. Portability beyond GHC is not required. It's not necessary to update blobs, only to parse them, and serialize new ones. And in the beginning, I'd like to write the boilerplate manually. 8-)

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Data types and Haskell classes
by Jens Blanck 18 May '05

18 May '05

How would I introduce number classes that are extended with plus and minus infinity? I'd like to have polymorphism over these new classes, something like a signature f :: (Real a, Extended a b) => b -> b which clearly is not part of the current syntax, but I hope you get the picture. What are the elegant ways of doing this? Somewhat related (I think), I have a class of dyadic numbers, it is clearly in Num and Real, but not in Fractional. Now, I would like to have intervals, both dyadic and rational, providing operations like lower and upper end point, centre, diameter, the partial order of interval inclusion, and membership for a dyadic/rational number. All these operations can be computed on dyadic numbers (only division by 2 is needed). Could I make the rational numbers belong to a class of "averageable numbers" or something like that so that I can get actual dyadic numbers returned when they happen to be dyadic? Jens

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Haskell-related Job Posting, Portland OR area
by Isaac Jones 17 May '05

17 May '05

Galois Connections does most of its development in Haskell, and this job may involve some Haskell development, so I felt it was on topic for this list. Senior Test Engineer Galois Connections, Inc., located near Portland, Oregon, designs and develops high confidence software for critical applications using cutting edge methods and technology. Innovative and highly creative problem-solving in the demanding application areas of security, information assurance and cryptography has earned Galois a reputation of excellence in both private and government sectors. Are you seeking an intellectually challenging position in which you'll own a mission-critical piece of a mission-critical product? Do you aspire to work with a team that shares your level of commitment and enthusiasm to develop tomorrow's high-assurance technology today? If so, you might be just the person we're looking for. Galois has recently opened a position for a Senior Test Engineer who will have end-to-end responsibility for the QA process, including planning, QA infrastructure, and assessments. This person will work in a highly cooperative and collaborative team setting and will be responsible for ensuring that our high-assurance projects deliver robust and fully functional systems according to client expectations and potential certifying agencies. For more details about this exciting opportunity please go to: http://www.galois.com/job_testengineer.php To learn more about Galois visit our website at http://www.galois.com

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Text search
by Gracjan Polak 17 May '05

17 May '05

Hi, Simple question: I need a function that matches string in another string. Something like: find (isSuffixOf "needle") (inits "haystack") This one is beautiful, but not very practical. Could anybody point me to some haskell library that does some searching, using KMP for example? -- Gracjan

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