- Haskell-Cafe - Haskell.org

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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Re: Yet Another Monad Tutorial
by Bernard James POPE 15 Aug '03

15 Aug '03

Hi all, I've been following this discussion with some interest. Last time I looked, hugs, ghc and nhc98 all implemented I/O differently. No doubt there are even more alternatives in other Haskell implementations (real or imaginary). In itself this is probably not too surprising. They are, afterall, different systems with different goals etc. However, I have often wondered what it is that constitutes a correct implementation of the I/O system in Haskell. I might be wrong, but I don't think the Haskell Report helps much in this regard. The only source of semantics for Haskell's I/O that I know of is "Tackling the Awkward Squad" by Simon Peyton Jones. If there are others I'd love to have them pointed out. However, this paper deals with many extensions that are not in Haskell 98. I'm not sure if there is enough in it to decide if a given implementation is correct (I use the word "correct" loosely). I would really love to see a discussion of the design space for I/O. Including what the current compilers use, what other options there might be, and even what other languages use (like Clean's and Mercury's uniqueness systems). If I were to build a Haskell compiler tomorrow this is one thing I would like to know - what do I have to do to ensure that my implementation of I/O is correct and what design options do I have? No doubt the I/O systems of current compilers have grown with those systems, but from the perspective of someone who is outside of the implementations it would be interesting to know why things are done the way they are, and what other options were considered, rejected, etc. For example, the state-of-the-world threading approach might be chosen because it is economical with respect to implementation cost in an existing compiler (such as GHC). But if we were to write a compiler from scratch would/should we follow the same route? Also, the "Keep the monad right through" method that Simon mentions in the "awkward squad" paper is interesting, but I wonder if that is too hard to implement in Haskell due to extensive overloading of the monad operators. I'll stop rambling now. Cheers, Bernie.

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RE: IO Bool -> Bool
by Kevin S. Millikin 15 Aug '03

15 Aug '03

On Wednesday, August 13, 2003 11:20 PM, Tn X-10n [SMTP:kawairashii@hotmail.com] wrote: > is it possible to convert IO Bool to Bool? Sure. Which Bool do you want? True? > toTrue :: IO Bool -> Bool > toTrue x = True Or False? > toFalse :: IO Bool -> Bool > toFalse x = False Maybe that's not what you had in mind.

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RE: Database interface
by Tim Docker 14 Aug '03

14 Aug '03

oleg(a)pobox.com wrote: > If I may interject, that's precisely how a Scheme DB interface is > designed. The main function is a left-fold. Not quite though: it > provides for a premature termination: > > A major procedure: DB1:fold-left PROC INITIAL-SEED QUERY-OBJECT Premature termination sounds like a useful property. I can see two ways this could be done: keep the previous signature, and use an exception to exit early, or add a boolean return value like the scheme version: doquery :: Process -> String -> (a -> b -> IO (b,Bool)) -> b -> IO b Any opinions on which alternative wouuld be preferable? Tim

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Re: Yet Another Monad Tutorial
by Jeff Newbern 14 Aug '03

14 Aug '03

Alistair, Thanks for your message regarding the debate about monads and purity. What do you think about these additions as a discussion of the issue? Does this address everything you think is needed? Is there a clearer way to explain it? Thanks, Jeff In the section "No Way Out": ---------- The IO monad is a familiar example of a one-way monad in Haskell. Because you can't escape from the IO monad, it is impossible to write a function that does a computation in the IO monad but returns a non-monadic value. Not only are functions of the type IO a -> a impossible to create, but any function whose result type does not contain the IO type constructor is guaranteed not to use the IO monad. Other monads, such as List and Maybe, do allow values out of the monad. So it is possible to write non-monadic functions that internally do computations in those monads. The one-way nature of the IO monad also has consequences when combining monads, a topic that is discussed in part III. ---------- and a little farther down: ---------- Some people argue that using monads to introduce non-pure features into Haskell disqualifies it from claiming to be a pure functional language. This subtle question — not particularly relevant to the practical programmer — is revisited in the context of the I/O monad later in the tutorial. ---------- Later, in the section on the I/O monad: ---------- In Haskell, the top-level main function must have type IO (), so that programs are typically structured at the top level as an imperative-style sequence of I/O actions and calls to functional-style code. Revisiting the debate about the purity of Haskell (in a functional sense), it is important to note that the IO monad only simulates imperative-style I/O. The functions exported from the IO module do not perform I/O themselves. They return I/O actions, which describe an I/O operation to be performed. The I/O actions are combined within the IO monad (in a purely functional manner) to create more complex I/O actions, resulting in the final I/O action that is the main value of the program. The result of the Haskell compiler is an executable function incorporating the main I/O action. Executing the program "applies" this ultimate I/O action to the outside world to produce a new state of the world. This occurs only once per execution of the program, and since the state of the world changes for each execution, the issue of purity is neatly side-stepped. ---------- -- Jeff Newbern <jnewbern(a)nomaware.com> Nomaware, Inc.

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