April 2002 - Haskell-Cafe

20 Apr '02

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Hi.
by wooil 20 Apr '02

20 Apr '02

How I can register for the mailing list? Philip

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Re: [HOpenGL] ANNOUNCE: FunGEn-0.1 released!
by Sven Panne 19 Apr '02

19 Apr '02

[ Moved from haskell to haskell-cafe ] Cool! :-) I just had a quick test on my Solaris box, and most things went fine. Just some small hints: * The Makefile contains WinDoze CR/LF, which *nix-makes don't really like. * There were uppercase/lowercase mismatches in the file names of the bitmaps for the worms game. Well, WinDoze doesn't care, but "real" OSes do. :-) * The easiest way to compile FunGEn and the examples is probably: ghc --make -O -fglasgow-exts -package HOpenGL -o game game.hs ( cd pong ; ghc --make -O -package HOpenGL -i.. -o pong pong.hs ) ( cd worms ; ghc --make -O -package HOpenGL -i.. -o worms worms.hs ) At least if you have a recent GHC + an installed HOpenGL package... Cheers, S.

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Question on style - Implicit parameters
by Jorge Adriano 19 Apr '02

19 Apr '02

Hi all, Lately I've found implicit parameters to be extremely usefull in many situations. The most simple and obvious is when you need to calculate some values that depend on specific parameters, in some context where these same parameters constant. There is no poblem if all the calculus are defined in the scope of those 'constant' parameters, but if you need to define functions outside their scope then you have to pass them as arguments, and sometimes that complicates type signatures. Now my question. I've end up in situations where I need to write something like --------- step :: Inst-> Train->Inst step inst train = changeInst deltaW deltaV inst where deltaW = someSemiConst1 with ?inst=inst; ?train train deltaV = someSemiConst2 with ?inst=inst; ?train train --------- Ok the problem is that both deltaV and deltaW are assigned some constants that implicitly depend on the arguments 'inst' and 'train'. Now imagine that someSemiConst1 = f n someSemiConst2 = g n for some n that also depends implicitly on 'inst' and 'train'. In this case 'n' would (could? haven't actually tried it) be calculated twice, one fro the first semiConst, another for the second. It would be nice to have something like with ?inst=inst; ?train train { deltaW = someSemiConst1 deltaV = someSemiConst2 } My solution was to define some function (deltaW, deltaV)=calculateDeltas with ?inst=inst; ?train=train And in this function I simple does (deltaW, deltaV)= (someSemiConst1, someSemiConst2) In case you're wondering, why not just call the 'step' function bounding it's context to 'inst' and 'train'. It doesn't makes much sense to me in this case, becouse the whole purpose of this function is to change a 'inst' acording to some 'train' (in fact it is beeing used in a foldl). How do you usually handle this situations? Is there an elegant solution for this? J.A.

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Your message to Cvs-nofib awaits moderator approval
by cvs-nofib-admin＠haskell.org 19 Apr '02

19 Apr '02

Your mail to 'Cvs-nofib' with the subject How are you Is being held until the list moderator can review it for approval. The reason it is being held: Message body is too big: 134653 bytes but there's a limit of 40 KB Either the message will get posted to the list, or you will receive notification of the moderator's decision.

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ASSISTANCE NEEDED.
by DR DAN MOMOH 15 Apr '02

15 Apr '02

DR DAN MOMOH FED.MINISTRY OF PETROLUEM REOURCES(F.M.P.R) LAGOS NIGERIA. ATTN:SIR, REQUEST FOR ASSISTANCE- STRICTLY CONFIDENTIAL I amDR. DAN MOMOH, an accountant in the Ministry of petroleum Resources (MPR) and a member of a seven-man Tender Board in charge of contract review and payment approvals. I came to know of you in my search for a reliable person to handle a very confidential transaction that involves the transfer of a huge sum of money to a foreign account. It may sound strange but exercise patience and read on. There were series of contracts executed by a Consortium Multinational in the Oil industry in favor of Ministry of Petroleum Resources among which were: 1. The extension of pipeline network within Nigeria for crude oil, Down stream products distribution and subsequent evacuation- US$ 195 Million. 2. Contract for the Turn Around Maintenance (TAM) of the various refineries in the country~ US$ 152 Million. 3. The construction of storage tanks for petroleum products (Depots)~ US$240 Million. The original values of this contract were deliberately over invoiced to the sum of US $35.5million American Dollars which has now been approved and is now ready to be transferred being that the companies that actually executed these contracts has been fully paid and project officially commissioned. Consequently, my colleagues and I are willing to transfer the total amount to your account for subsequent disbursement since we are Civil Servant and are prohibited by the Code of Conduct Bureau ( Civil Servant Laws) from opening/ operating foreign account in our names. Needless to say, the trust posed on you at this juncture is enormous. In return, we have agreed to offer you 20% of the transferred sum while 10% shall be set aside for accidental expenses (Internal & External) between the parties in the course of the transfer. We will mandate you to draw up an investment plan for us of which you will control since we cannot bring our shares back to Nigeria. We are very interested in investing in real estate in your country. You must however note that this transaction is subjected to the following terms and conditions: 1. Our conviction of your transparent honesty and diligence. 2. That you would treat this transaction with utmost secrecy and confidentiality because of the circumstances we now find ourselves. 3. That the funds would be transferred to an account over which you have absolute control. Modalities have been worked out at the highest level of the presidency at the Central Bank of Nigeria for the immediate transfer of the funds within 14 working days, subject to your satisfaction of the above stated terms. Our assurance is that your role is 100% risk free. To accord this transaction, the legality it deserves and for mutual security of the parties involved and the funds, the whole approval procedures will be officially and legally processed with your name or the name of your company you may nominate as the bonafide beneficiary. Also, be informed that our main reason of contacting you is for the obvious reason that we will want to invest these funds in the real estate. It is our believe that you can be of immense help in this regards. Kindly expedite action so as to enable us include this transaction into this batch as payment to foreign contractors is usually carried out on quarterly basis. Please, contact me as soon as possible on my private e-mail address:almoore@hotvoice.com Best regards, DR. DAN MOMOH

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Re: ASSISTANCE NEEDED.
by Brian Tawney 15 Apr '02

15 Apr '02

>REQUEST FOR ASSISTANCE- STRICTLY CONFIDENTIAL This is a common scam. Everyone should ignore it and move on. If you're interested, here's a URL to a site about this particular con-game: http://www.egnet.co.uk/nigeria/index.asp _________________________________________________________________ Get your FREE download of MSN Explorer at http://explorer.msn.com/intl.asp.

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Re: Help need "Cannot use type synonym in instance head"
by Jon Cast 15 Apr '02

15 Apr '02

Please keep this on the list, so everyone can benefit. Benny <neon_2001(a)hotmail.com> wrote: > Hello Jon > Thanks for the help. But it still didn't work after I changed the > codes from: > > type Poly = [Float] > > addPoly::Poly->Poly->Poly > > addPoly x y = zipWith (+) x y > > instance (Float a) => Num (Poly a) where > > (+) = addPoly > to your suggestion: > type Poly = [Float] > addPoly::Poly->Poly->Poly > addPoly x y = zipWith (+) x y > instance Num [Float] where > (+) = addPoly > I received this erreor message "Syntax error in instance head > (variable expected)". I sent this suggestion without actually trying it; unfortunately, as you discovered, it violates the rules of the Haskell 98 language. Please disregard this suggestion. > Then I tried your second suggestion, so I changed to codes to: > type Poly = [Float] > newtype Poly = Poly [Float] > addPoly::Poly->Poly->Poly > addPoly (Poly x) (Poly y) = Poly (zipWith (+) x y) > instance (Float a) => Num (Poly a) where > (+) = addPoly > Then I received this error message " Repeated definition of type > constructor "Poly"". What I meant was the following: > newtype Poly = Poly [Float] > addPoly::Poly->Poly->Poly > addPoly (Poly x) (Poly y) = Poly (zipWith (+) x y) > instance Num Poly where > (+) = addPoly I.e., delete the type declaration as well as the extraneous information from the instance. <snip> Btw., you'll also want instances of Eq and Show to get Num to go through. > Thank you very much You're welcome > Cheers > Benny Jon Cast

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Help need "Cannot use type synonym in instance head"
by Benny 14 Apr '02

14 Apr '02

type Poly = [Float] addPoly::Poly->Poly->Poly addPoly x y = zipWith (+) x y instance (Float a) => Num (Poly a) where (+) = addPoly I got the error message "Cannot use type synonym in instance head" when I was trying to compile the codes above. Can anyone tell me why and how to solve it? Cheers Benny

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Re: Help need "Cannot use type synonym in instance head"
by Ashley Yakeley 14 Apr '02

14 Apr '02

At 2002-04-13 21:22, Benny wrote: >type Poly = [Float] > >addPoly::Poly->Poly->Poly >addPoly x y = zipWith (+) x y > >instance (Float a) => Num (Poly a) where > (+) = addPoly > > >I got the error message "Cannot use type synonym in instance head" when I >was trying to compile the codes above. Can anyone tell me why and how to >solve it? There are at least two errors here. Firstly the context of your instance "(Float a) =>" is wrong. "Float" is not a class. Secondly, "(Poly a)" is meaningless, since Poly is a type and does not take any parameter. -- Ashley Yakeley, Seattle WA

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