March 2003 - Haskell-Cafe

Re: African money
by Marcus Lindblom 26 Mar '03

26 Mar '03

From: John Peterson <peterson-john(a)cs.yale.edu> > > If the spam storm gets too heavy on haskell-cafe we can switch to > moderator approval as on the main Haskell list. We have the spam > assassin available at haskell.org but I've been reluctant to turn it > loose on the lists. Maybe the time has come ... Many lists use moderator approval on non-list members. Maybe spam assassin could be turned loose on just those non-member mails, and have those passing through be moderator reviewed? /Marcus

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26 Mar '03

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shortest path with Haskell
by tuyen pham 25 Mar '03

25 Mar '03

Hi every one ! I have a big problem with implementation the Dijkstra's shortest path with Haskell .the condition is it must be implemented with priority queue using list (or tree).Any of yo have the code for this ? I really really needs for help !!! thankssssssss a lot!!!!!! pham __________________________________________________ Do You Yahoo!? Everything you'll ever need on one web page from News and Sport to Email and Music Charts http://uk.my.yahoo.com

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(no subject)
by Mrs.Sese-seko 25 Mar '03

25 Mar '03

Dear friend, I am Mrs. Sese-Seko widow of late president Mobutu Sese-Seko of zaire? now known as democratic republic of congo (drc). I am moved to write you this letter, this was in confidence considering my present circumstance and situation. I escaped along with my husband and two of our sons Theophilus and Basher out of Democratic Republic of Congo (DRC) to Abidjan, cote d'ivoire where my family and I settled, while we later moved to settled in morroco where my husband later died of cancer disease. However due to this situation we decided to changed most of my husband's billions of dollars deposited in swiss bank and other countries into other forms of money coded for safe purpose because the new head of state of (dr) Mr, laurent kabila has made arrangement with the swiss government and other european countries to freeze all my late husband's treasures deposited in some european countries. hence my children and I decided laying low in Africa to study the situation till when things gets better. Now that President kabila is dead and the son taking over (Joseph kabila). one of my late husband's chateaux in southern france was confiscated by the french government, and as such I had to change my identity so that my investment will not be traced and confiscated. I have deposited the sum Thirty Million United State Dollars(US$30,000,000,00.) with a security company , for safekeeping. The funds are security coded to prevent them from knowing the content. What I want you to do is to indicate your interest that you will assist us by receiving the money on our behalf. Acknowledge this message, so that I can introduce you to my son (Theophilus) who has the out modalities for the claim of the said funds. I want you to assist in investing this money, but I will not want my identity revealed. I will also want to buy properties and stock in multi-national companies and to engage in other safe and non-speculative investments. May I at this point emphasise the high level of confidentiality, which this business demands, and hope you will not betray the trust and confidence, which I repose in you. in conclusion, if you want to assist us , my son shall put you in the true picture of the business, tell you where the funds are currently being maintained and also discuss other modalities including remuneration for your services. For this reason kindly furnish us your contact information, that is your personal telephone and fax number for confidential purpose. Best regards, Mrs,sese-seko

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Problem with hugs, concurrency and unsafeInterleaveIO (maybe a bug?)
by Nick Name 17 Mar '03

17 Mar '03

I am trying to lazily wait an MVar in hugs, in conjunction with concurrent haskell: ----- import Concurrent import IOExts f = do v <- newEmptyMVar c <- getContents forkIO (putMVar v (head c)) r <- unsafeInterleaveIO (takeMVar v) return v f2 = f >>= unsafeInterleaveIO . takeMVar main1 = f >>= takeMVar >>= print main2 = f2 >>= print main3 = f2 >>= (\ x -> yield >> print x) ----- main1 and main3 work flawlessly, but main2 aborts with "no more threads to run". I think that main2 should work, because in evaluating the result of f2, the main thread should suspend and yield. Is there somebody who wants to enlighten me? Vincenzo

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AW: How to read an initialization file.
by Markus.Schnell＠infineon.com 14 Mar '03

14 Mar '03

I would recommend reading the variables in the main and pack the rest of your program into a monad. But only if you really need to access the variables from everywhere and don't want to pass them around explicitely. Markus > I need to be able to access these values effieiently and from > virtually anywhere in the program. It would be nice to "at some > point" open the file and read all these variables, all at once.

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Redefining methods in a subCLASS
by Oleg 14 Mar '03

14 Mar '03

Hi Is it possible to redefine methods in a subclass (not an instance)? E.g. I get errors in class (Show a) => Employee a where speak :: a -> [Char] speak x = "Employee: " ++ (show x) class (Employee a) => Manager a where speak x = "Manager: " ++ (show x) Thanks Oleg

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Re: How to search for a string sequence in a file a rewrite it???
by Tom Pledger 14 Mar '03

14 Mar '03

(moving to haskell-cafe) Alexandre Weffort Thenorio writes: | Ooops a small error before but here is the right one. | | Great. I got almost everything. My problem now is: | | I got a function called findstr where | | findstr "aaaa" "zzzzz" ["xxxxaaaa","xxxaaaaxxx"] = | ["xxxxzzzzz","xxxzzzzzxxx"] with the inferred type findstr :: String -> String -> [String] -> [String] | and then I try something like | | fillIn lines = do | bkfilled <- (findstr str str2 lines) with the inferred type (findstr str str2 lines) :: [String] Note that [] (the list data constructor) is a monad. So, this 'do' expression is in the [] monad, where perhaps you intended the IO monad. Computations in the [] monad have the effect of iterating over the elements of lists. So, the bound variable gets the inferred type bkfilled :: String which you've noticed in the error message. | write bkfilled | | where write takes a [String] and concatenates it writing it to a file then. | But I get this error saying: | | Expected Type: [String] | Inferred Type: String | In the first argument of 'write' namelly 'bkfilled' | In a do expection pattern binding: write bkfilled | | Any idea?? I mean bkfilled is supposed to be [String] but it says it is a | String, any idea why??? The smallest code change to fix this is to add a 'return', which will wrap another monad (in this case, IO) around findstr's [String] result... > fillIn lines = do > bkfilled <- return (findstr str str2 lines) > write bkfilled ... but you should be able to simplify that code, using one of the monad laws in http://haskell.org/onlinereport/basic.html#sect6.3.6 Regards, Tom

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Learning Haskell, update
by Arjan van IJzendoorn 13 Mar '03

13 Mar '03

Hi all, I've removed the frequently asked questions box at the bottom. It was unused. Fritz Ruehr made a nice Learning Haskell logo. Thank you, Fritz! NHC is compilable under Windows now, so the remark "no windows version" has been removed. Is there someone who wants to make a more decent language comparison table? If not, I'm thinking of removing it. Many things are said about it already in the comp.lang.functional FAQ. Greetings, Arjan

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Explicit function call
by pavel＠joker.botik.ru 13 Mar '03

13 Mar '03

Hello, How can I make an explicit function call in a "do" sequence? Ex: ... do let a = myFunc ... b = myFunc ... c = "Something else" return c ... As I understand myFunc will not be executed, but I need it... Please, help. -- Pavel Zhbanov

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