- Haskell-Cafe - Haskell.org

sequencing data structures
by Martin Norbäck 21 Aug '03

21 Aug '03

I want to sequence data structures in an efficient manner, to store them to files and to send them over the network. Simply deriving Show and Read is not very good, because it's space inefficient and read cannot give any output until the whole data structure is parsed. So I thought I should store them in some space efficient format. First problem: how to make them derivable (so that I don't have to write boilerplate class instances for all my data structures). I read the "derivable type classes" paper, but it's not implemented in ghc (only Unit and :+: and :*: are, which is not enough). So how to go about it? Using DrIFT? Template Haskell? Second problem: how should such a format look like? Ideally, I want to be able to write an infinite data structure (at least one containing loops). If that is not possible I want to be able to read as lazily as possible, this means traversing the data structure in breadth first order, so that a cons form can be reached quickly. Does anyone have any thoughts/pointers on this subject? It would surprise me if nobody has had this problem before. Regards, Martin -- Martin Norbäck d95mback(a)dtek.chalmers.se Kapplandsgatan 40 +46 (0)708 26 33 60 S-414 78 GÖTEBORG http://www.dtek.chalmers.se/~d95mback/ SWEDEN OpenPGP ID: 3FA8580B

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RE: Database interface
by Bayley, Alistair 21 Aug '03

21 Aug '03

> From: Tim Docker [mailto:timd@macquarie.com.au] > > > It also has the benefit that the function you pass to doquery can > make > > other calls to doquery, without mucking up the 'current row' state. > > There would be one current row per Cursor, not one per Process. > > Is it normal or common to support multiple simultaneous queries on > a single DB connection? Depends on what you mean by "multiple simultaneous queries". A single query is usually synchronous w.r.t. returning a cursor i.e. you issue the query and get back a cursor in an atomic operation, and then you can do other things while you fetch rows from the cursor. If you need to have two SQL statements running at the same time (two big inserts or updates, say) then you usually need two sessions. It probably depends on the API, but some (Oracle, I believe) will allow you to open multiple sessions with the same connection. In a single session you could open a number of cursors (in any order), interleave fetches (in any order), and then close the cursors (in any order). So you could have: - a number of Connections (perhaps to different servers/databases), where - each Connection has one or more Sessions, and - each Session has zero or more Cursors (and one current Transaction). My session model and terminology are probably quite Oracle specific, but I hope it's reasonably clear. ***************************************************************** The information in this email and in any attachments is confidential and intended solely for the attention and use of the named addressee(s). This information may be subject to legal professional or other privilege or may otherwise be protected by work product immunity or other legal rules. It must not be disclosed to any person without our authority. If you are not the intended recipient, or a person responsible for delivering it to the intended recipient, you are not authorised to and must not disclose, copy, distribute, or retain this message or any part of it. *****************************************************************

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RE: Database interface
by Tim Docker 20 Aug '03

20 Aug '03

Tom Pledger writes: > By the way, how does the a in a -> b -> IO (b, Bool) work? > It looks like it has something to do with the current row. Does > doquery have to accommodate any possible substitution for a ? I fired this off without thinking about it too much, and looking at the prelude type signatures for fold. The list being folded over is implied by the DB query, is accessible through the IO monad. Hence a parameter is not required. It would really be: doquery :: Process -> String -> b -> (b -> IO (b,Bool)) -> IO b > I don't have a preference, but offer this view of the options: > > With an exception, "Stop, and return the last b you saw." > With a boolean, "Stop, and return this b." I think I like the behavior where, when the bool in the tuple is true, the b in the tuple is immediately returned from the query. Exceptions would be propagated to the caller of doquery without modification (but with appropriate cleanups). One thing that I am unsure about is whether the column value accessing functions that I specified before stringv :: Process -> CInt -> IO String doublev :: Process -> CInt -> IO Double intv :: Process -> CInt -> IO Int should return actions in the IO monad as above, or instead should be in some other DBQuery monad, that trivially extends the IO monad, but is only valid inside the doquery call. This would have the benefit of restricting the column access functions to inside a query via the type system. I'd also probably use a typeclass to specify a single colv function. ie: class DBCol a where colv :: DBQuery a instance DBCol String where... instance DBCol Double where... doquery :: Process -> String -> b -> (b -> DBQuery (b,Bool)) -> IO b Any comments? Tim

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Re: "let" vs. "where"
by Hamilton Richards 20 Aug '03

20 Aug '03

At 7:42 AM +0200 8/20/03, Jan Scheffczyk wrote: >Hi Andrew, > >> let x = expensiveComputation foo in x + x >> >> I would certainly hope that expensiveComputation wasn't called twice, >> and even though the language doesn't guarantee it, I have already >> written code that assumed it. > >I always thought that there is a tiny difference between "let" and "where": >Using "let" "expensiveComputation foo" might be computed twice (depending on >the compiler?). >But using: > > x + x > where x = expensiveComputation foo > >should compute the value for x only once. >Therefore, I always try to use "where" for common subexpressions. > >Please correct me if I'm wrong here. The reserved word "let" introduces an expression (a "let-expression"), but there's no such thing as a "where-expression". The reserved word "where" can be used only in definitions. For example, 3 + ((x + x) where x = expensiveComputation foo) is invalid syntax, whereas 3 + let x = expensiveComputation foo in x + x is OK. And here's a use of "where" in a definition: let z = x + x where x = 2 in z*5 That could have been written using only "let": let z = let x = 2 in x + x where x = 2 in z*5 Watch out for the Hugs 98 command line-- it allows z*5 where z = x + x where x = 2 but that's not a truly valid expression, because you can't embed it in a larger expression: 2 + (z*5 where z = x + x where x = 2) is properly identified as a syntax error. Regards, --Ham -- ------------------------------------------------------------------ Hamilton Richards, PhD Department of Computer Sciences Senior Lecturer The University of Texas at Austin 512-471-9525 1 University Station C0500 Taylor Hall 5.138 Austin, Texas 78712-1188 ham(a)cs.utexas.edu hrichrds(a)swbell.net ------------------------------------------------------------------

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More type design questions
by Konrad Hinsen 20 Aug '03

20 Aug '03

Continuing in my quest to understand type design in Haskell, here's another episode that leaves me scratching my head: module Foo where class Vect v a where (<+>) :: Floating a => v a -> v a -> v a data Vector a = Vector a a a instance Vect Vector a where (<+>) (Vector x1 y1 z1) (Vector x2 y2 z2) = Vector (x1+x2) (y1+y2) (z1+z2) instance Vect [Vector a] a where (<+>) l1 l2 = zipWith (<+>) l1 l2 The problem is the last instance declaration. Hugs says "Illegal type in class constraint", which is not very explicit (which class constraint?). GHCI is a bit more verbose: Kind error: Expecting kind `* -> *', but `[Vector a]' has kind `*' When checking kinds in `Vect [Vector a] a' In the instance declaration for `Vect [Vector a] a' I have vague memories of seeing mentioned the concept of "kind", but I can't remember where. Any help would be appreciated! Konrad.

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Tuples or Record
by Frederic BELLOC 18 Aug '03

18 Aug '03

Hello, When I have started my project, I use a Tuples but i would know if it is possible to create a record such C or Ocaml provide. I mean creating a structure where variables are accessible by a '.' or something like that. -- BELLOC Frederic EPITA 2005 STUDENT C, socks and sun !

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New Haskell and FPO merchandise available
by K. Fritz Ruehr 18 Aug '03

18 Aug '03

Newly-designed Haskell, FP and related merchandise (mostly T-shirts) are now available at the URLs below. I didn't receive many specific requests on logo/product combinations from my last post, so I have tried to pick and choose as best I can (I've included at least men's and women's white T-shirts, in two styles for the women, for all the new designs I used). It may be difficult to get items in time for ICFP in Uppsala, for which I apologize. On the other hand, the actual physical products are as yet untested (i.e., for print quality), so perhaps it is just as well if I manage to get some samples to wear to the conference: that way you may have a chance to see them "live" before you buy. Note that all items are provided at cost, i.e., I get no profit, although CafePress does. Almost all shirts have extra material on the back side: it turns out that no premium is charged for this, so I tried to give the best value for the money (exceptions include the "new logo" baby doll shirt, the "classy" logo golf shirt, all of the "dysfunctional programming" shirts and the Generic Haskell shirts; Pancito back-side art should be up soon). Certainly if there are any logos from the original set you would like turned into a product, or any other variations, let me know by return mail (see <http://www.willamette.edu/~fruehr/logos/intro.html>). -- Fritz Ruehr PS: Thanks to all who made such encouraging comments on the designs, and also to Andrew Cooke for the Pancito artwork. ---------------------- * Original haskell.org logo (Pan image on back): <http://www.cafeshops.com/haskell> * New Haskell logo (words on front, circle back): <http://www.cafeshops.com/haskellnewlogo> * Haskell "purity" logo (w/ "Pure Fun" on back): <http://www.cafeshops.com/purehaskell> * Alternate "classy" logo (feature list on back): <http://www.cafeshops.com/classyhaskell> * Dysfunctional programming (nothing on back): <http://www.cafeshops.com/dysfunc> * Strong, static types (typing judgement on back): <http://www.cafeshops.com/statictypes> * HOT [higher-order typed] (lambda on back): <http://www.cafeshops.com/highordertyped> * Do it in a Monad (monad laws on back): <http://www.cafeshops.com/doitmonad> * Generic Haskell logo (no back as yet): <http://www.cafeshops.com/generichaskell> * Pancito "bugs" artwork (no back *yet*): <http://www.cafeshops.com/pancito>

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Updated monad tutorial
by Jeff Newbern 18 Aug '03

18 Aug '03

Hello, I would like to thank everyone for their feedback on the monad tutorial I posted earlier. Based on that feedback, I have expanded and corrected the tutorial at www.nomaware.com/monads/html/index.html The primary changes are: - vastly expanded section on monad transformers - additional examples of monad transformers, including multiple transformers in a stack - added examples for liftM, guard, ap and msum - consistent use of "action" in IO monad "computation" in all others - made language more precise when discussing the IO type constructor - fixed typos, cleaned up some code in examples I am sufficiently pleased with this version to call it 1.0, but I still welcome any and all feedback. Thanks, Jeff Newbern -- Jeff Newbern <jnewbern(a)nomaware.com> Nomaware, Inc.

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Long Time
by Andrew Williams 15 Aug '03

15 Aug '03

Dear Sir, I know you may be suprised to receive this mail, especially as we have not met in person before but Please do not be embarrassed for I mean good. I was opportuned to have your contact from the South Africa Information Exchange (S.A.I.E.), in my search for a reliable person to handle a very confidential transaction which involve a transfer of fund and its investment in your country. My name is ANDREW WILLIAMS, a South African expatriate staff of one of the reputable financial institutions in Zurich, Switzerland. We act as financial advisors and on special arrangements, accept Deposits of valuables from our customers for safekeeping. Few months ago, we received information about the sudden death of one of our prominent customers from Angola in Africa. Our enquiry at the Embassy confirmed that he died with the leader of UNITA, Dr. Jonas Savimbi in an ambush laid by the enemy Government forces. The issue now is that the customer has a Deposit of Fifty Million US Dollars ( USD50M) in our custody, which he declared as a consignment of GOLD & DIAMONDS. I am the only person with the accurate information about the Deposit because I acted as his adviser and proxy when he was alive. I knew the money actually came from Diamond Mines in Angola where he was the Chairman of the States' Mining Corporation during the Government of now late Dr. Savimbi. It may also interest you to know that, for obvious reasons, the man did not provide any next-of-kin at the time of making the Deposit, and more so, he lodged the entire deposit under the name of LONG TIME INVESTMENTS LTD, with a non-existing address. The Deposit is now due for clearing and all efforts by our company to contact any of his relatives through the address he provided proved futile. By the regulations of the company, if more 30 days passes without any person showing up claiming to be the next of kin or relative of the customer, the entire Deposit will be recovered and if found to be cash, the money will be given away to Charities or secretly be converted into trading in favor of the company, and can never be accounted for again. With my position as his proxy, and the knowledge I have about the Deposit, I can successfully organize the claim of the Deposit without any problem provided I have a credible and capable person to receive the Deposit. I cannot function in this regard because of my status in the company, therefore, I am proposing to you to receive the money in your care for our mutual personal use. Being an insider, I know what to do to pull the deal through without any problem whatsoever. The Deposit is being secured in our central Warehouse in Europe and all I need from you now are your PARTICULARS, including, TELEPHONE and FAX NUMBERS to set up the process required to release the Deposit to you. Once that is done, the Deposit will be released to you through a friendly Security/Safe Deposit Company in Europe where you have to move in to collect . I will complete the Deposit releasing process within 2-3 days and furnish you with particulars and contact person of the security company in Europe so that you can arrage for the clearing and collection. My dear Friend, no doubt, this is an opportunity for us to excel in life. Therefore, if you are interested kindly inform me by email or you can phone at +41-795-328250, and please do not forget to furnish your particulars including, Address, Telephone and Fax numbers to enable me commence the operation immediately. I am willing to share 40% of the total funds to you, 10% would be given out to Charities while 50% would remain for me. I am interested to invest my share of the money in Real Estates, Capital & Money markets in your country. With your support and co-operation, this transaction is set to be completed within one week. Be rest assured that the business is 100% risk-free, safe and secure but for security reasons however, you are advised to treat it with all privacies and strictest of confidentiality. Best regards, ANDREW WILLIAMS.

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PLS REPLY
by alisesay＠phantomemail.com 15 Aug '03

15 Aug '03

Dear Sir This letter may come to you as a surprise because we have not met. The decision to write you is out of a sincere desire to establish a business/mutual relationship with you. I got your contact through the international trade and commerce chamber (ITCC) in Madrid. My name is Mr. Ali Sesay Karimu, the son of John Karimu (the former Secretary of state for finance) who was killed and mutilated by the military junta led by Major Paul Koroma after over-throwing the elected government of President Tijan Kabba. Though, I do not know to what extent you are familiar with events and disturbances in Sierra-Leone but it has consistent features in the CNN, BBC news bulletins. The pressure of war drove the rest of my family out of Sierra-Leone into exile in Spain where we have been living under political asylum for the past 3 years. Sadly, my mother died of cancer 3 months ago and was buried in Madrid. Prior to her death, she gave me a certificate for a secret deposit that my father made in a security company in Europe (destination will be disclosed later for security reason). The deposit which is worth $32,000.000 (Thirty-two Million U.S. Dollars) was money set apart for government propaganda against the rebel forces. He made the deposit in his name with the hope of converting it to his personal use at the end of the war but was killed as a result of his opposition to the rebel forces when the conflict intensified. I have contacted the Security Company to confirm the deposit and establish ownership. The money was however deposited as family treasure. Due to the death of my mother and the continued hostility in Sierra-Leone, I have decided to solicit the assistance of an honest and trustworthy person or company that will assist in investing the money. I cannot do it alone due to my present political status and total ignorance of the business world. You will be given a negotiable percentage of the money at the end of the transaction. If you are honest, trustworthy and interested in the above proposal, contact me immediately through the number/email address indicated above for more details. You must maintain absolute confidentiality to ensure success. Please, indicate your personal Tel./Fax nos. when replying. Regards, Ali Sesay Karimu.

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