- Haskell-Cafe - Haskell.org

Long Time
by Andrew Williams 16 Aug '03

16 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 15 Aug '03

15 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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RE: Database interface
by Bayley, Alistair 14 Aug '03

14 Aug '03

> -----Original Message----- > From: Thomas L. Bevan [mailto:thomas_bevan@toll.com.au] > > This type of interface is fine as far as it goes, but there is only > ever a loose coupling between the database and application. > > The HaskellDB work implemented a relational calculus that ensured > that all queries were checked at compile time for validity against > a particular database. HaskellDB doesn't actually type check against the database; it checks against types defined in a module, which is generated from the database. If you change the database, then you need to regenerate the module. I prefer the approach of passing an extraction function/IO action to the interface. This is the approach taken by HaSql, HaskellDB (I think, although it's so indirect I find it hard to follow), and Tim Docker's Sybase library, and it's the design I intend to use for my Oracle library (when I get around to it). I still want to use HaskellDB (or at least the relational calculus part of it), so I was thinking of splitting it into two pieces: a library that submits SQL queries and returns the results (database interface), and a library that constructs queries with the relational calculus and generates SQL from them. Obviously the database interface must be written for each DBMS product, while the relational calculus library ought to be independent of DBMS (although it might be wise to have SQL compatibility flags for the SQL it generates. For example: Oracle 8 (what I'm using now) only supports SQL-92). ***************************************************************** 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 Bayley, Alistair 14 Aug '03

14 Aug '03

> From: Thomas L. Bevan [mailto:thomas_bevan@toll.com.au] > > Ideally, we would have the following. > > 1/ A high level combinator library for relational calculus built on, > > 2/ A standard low-level Haskell API > i.e. functions like Connection -> String -> ( a -> b -> IO b) -> IO b > > 3/ Database specific bindings. > > The problem is that projections and cartesian products > generate new types, at > least in the way it was done in HaskellDB. The form the > calculus takes would > need to be substantially reworked. I don't see why the generation of new types would cause a problem. Can't we get the calculus library to generate the extraction function at the same time, so that it returns rows of the appropriate type? The generated extraction function would have to be built from standard extraction functions exposed by the low-level API. Although, would this limit the set of types you could use to just those in the low-level API? ***************************************************************** 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 14 Aug '03

14 Aug '03

Tom Pledger writes: > | > doquery :: (DeepSeq v) => > | > Process -> String -> IO v -> IO [v] > | > | Can you explain what the constraint does here? > > Yes. It soothes my superstitious fears about unsafeInterleaveIO. > > I imagined that: > > - doquery (like getContents) uses unsafeInterleaveIO to make the > resulting list lazy, i.e. avoid grabbing all the rows at the > outset, Actually no. I'm not sure of the correct terminology here, but my implementation was strict wrt to reading from the database - all records are read during the IO action, and not secretly later on. Whilst I can see some benefits of reading the records lazily, wouldn't this result in problems with database connection management, locks, transactions, etc. From reading the list, it sounds like equivalent issues crop up pretty regularly with getContents. If you used the fold variant, doquery :: Process -> String -> (a -> b -> IO b) -> b -> IO b where the folding function returns an IO action, then you could process rows as you go in any case, without requiring unsafeInterleaveIO. Tim

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RE: Database interface
by oleg＠pobox.com 14 Aug '03

14 Aug '03

> I'll probably generalise the query function to do a fold, rathen than > always accumulate a list: > doquery :: Process -> String -> (a -> b -> IO b) -> b -> IO b 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 A QUERY-OBJECT (which in this implementation is a list of fragments that make a SQL statement, in the reverse order -- without the terminating semi-colon) is submitted to the database, using the default database connection. PROC is a procedure: SEED COL COL ... The procedure PROC takes 1+n arguments where n is the number of columns in the the table returned by the query. The procedure PROC must return two values: CONTINUE? NEW-SEED The query is executed, and the PROC is applied to each returned row in order. The first invocation of PROC receives INITIAL-SEED as its first argument. Each following invocation of PROC receives as the first argument the NEW-SEED result of the previous invocation of PROC. The CONTINUE? result of PROC is an early termination flag. If that flag is returned as #f, any further applications of PROC are skipped and DB1:fold-left finishes. The function DB1:fold-left returns NEW-SEED produced by the last invocation of PROC. If the query yielded no rows, DB1:fold-left returns the INITIAL-SEED. Thus DB1:fold-left is identical to the left fold over a sequence, modulo the early termination. There are a few minor variants of the above procedure, optimized for common particular cases: a query that is expected to return at most one row, and a query that expects to return at most one row with exactly one column. The latter query has the same interface as a lookup in a finite map. The QUERY-OBJECT is of coursed not built by hand. There is a domain-specific language (which greatly resembles the source language and makes some use of quasi-quotation) whose result is a query object. Therefore, I can write both the query and the handlers of the query result using the same syntax. The premature termination is important. Database connections and cursors are too precious resources to leave them for the garbage collector to finalize. More discussion and pointers can be found at http://srfi.schemers.org/srfi-44/mail-archive/msg00023.html The interface has been used in an industrial application.

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