- Haskell-Cafe - Haskell.org

RE: Shooting yourself in the foot, a contribution
by Simon Marlow 23 May '03

23 May '03

> In Haskell, it's provably impossible to shoot yourself in the foot > without using unsafeShootSelfInFoot. Curiously, this doesn't > stop people > using unsafeShootSelfInFoot in released software. You can always use shootSelfInFootIO, but you need to be in the IO monad and use the MutableFoot type. The purely functional version, shootSelfInFoot, requires human cloning and won't be available for a couple of releases. Cheers, Simon

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Question about Function Efficiency
by Matthew Donadio 22 May '03

22 May '03

Hi all, I want to create a function that creates a cascade of function applications: cascade [ f1, f2, f3 ] x == (f3 . f2 . f1) x It's a bit trivial, but it makes implementing flowgraphs (which are usually drawn left-to-right) a bit easier. I plan on profiling this, but which of the following do you think is the most efficient implementation: > cascade1 :: Num a => [[a] -> [a]] -> [a] -> [a] > cascade1 [] = \x -> x > cascade1 (f:fs) = cascade1 fs . f > cascade2 :: Num a => [[a] -> [a]] -> [a] -> [a] > cascade2 cs = foldr (.) (\x -> x) (reverse cs) > cascade3 :: Num a => [[a] -> [a]] -> [a] -> [a] > cascade3 cs = foldl (.) (\x -> x) (reverse cs) Thanks. -- Matthew Donadio <m.p.donadio(a)ieee.org>

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RE: getpid() or something similar
by Simon Marlow 22 May '03

22 May '03

> Hal Daume writes: > > > In GHC in the Posix module, there's GetProcessID: > > Thanks for the pointer! > > Unfortunately, this is a GHC-only solution, which is really annoying > in my case because the software is completely compiler-independent, > and I don't want to restrict it to GHC because I need it to determine > a unique file name for a temporary file! > > Is there no other way? I cannot be the first person ever to need a > temporary file, can I? Surely there must be some portable way, too? Even using a ProcessID doesn't guarnatee uniqueness, it just gives you a good chance that your file won't conflict - you still have to cope with the situation that a file with that name already exists. The right way to do this is to try to open it for writing, and try a different name if the open fails. Using System.Random to get a starting point is probably just as good as using the ProcessID, and is more portable. Cheers, Simon

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Re: IO monad and lazy evaluation
by Hal Daume III 22 May '03

22 May '03

Moved to haskell-cafe... > Thanks for your comments. (I had overlooked "readFile", which certainly > looks safer. I'm not yet fully used to the distribution of functions > between standard libraries and the Prelude. As for my 'hSafeGetContents', > I realized exactly your point about using 'id' too late, after I'd sent my > email. :-( ) I've done this many many times...too often do I find myself in the state of "open mouth, insert foot." People tend to be pretty forgiving though. > The broader debate I was trying to evoke was: how, as a programmer using a > combination of monads and non-strict functions, can I be confident that > there aren't hidden interactions waiting to bite? Well, I think it's more the combination of *strict* monads and non-strict functions that causes trouble. In fact, as pointed out before, lazy IO uses unsafeInterleaveIO to work. I think the rationale for lazy IO is clear. Without lazy IO, the following program, when run on a 2gb file, will allocate 2gb of memory: > main = interact (map toUpper) whereas, with lazy IO, it will run in constant (small) space. I think the point of lazy IO (and people who designed it can probably speak better to this) is to allow people to continue to write pure functions. That is, without lazy IO, the above would have to be written something like: > main = do > b <- hIsEOF stdin > when (not b) $ do > c <- hGetChar stdin > putChar (toUpper c) > main or something like that. obviously in *this case*, we would prefer to use the pure function map. However, in order to use map, we need to only read in characters as they are demanded, which means we need lazy IO. I think that what it really boils down to is you need to ask yourself: am I benefiting by using lazy IO. In the 'interact (map toUpper)' case, clearly you are, so you should probably continue to you it. However, in a case where you know you're going to read the whole file and store it in some data structure and need to simultaneously use data from the beginning and end of the file, then you're probably not benefiting from lazy IO. It is somewhat unfortunate that the IO library doesn't contain strict versions of all the operations, but they're easy enough to write with DeepSeq in your pocket. > Using the hOpen ... hClose vs readFile case as an example, is the > difficulty here to do with the interleaving of multiple statements that > modify some state with non-strict functions that return results based on > some particular instance of that state? Is this really a fundamental mismatch? Yes, this is precisely the problem. Using hGetContents implicitly uses unsafeInterleaveIO, which basically only performs IO as demanded. When you then say 'hClose h', none of the input has been demanded, so nothing gets read. Ever. Someone (one of the Simons, perhaps) could comment on this, but it seems that the primary problem with using readFile is the open handles issue, which boils down to the definition of hGetContents: > hGetContents :: Handle -> IO String > hGetContents handle = > withHandle "hGetContents" handle $ \handle_ -> > case haType handle_ of > ClosedHandle -> ioe_closedHandle > SemiClosedHandle -> ioe_closedHandle > AppendHandle -> ioe_notReadable > WriteHandle -> ioe_notReadable > _ -> do xs <- lazyRead handle > return (handle_{ haType=SemiClosedHandle}, xs ) If this were changed to actually close the handle, rather than leaving it in a semi closed state (call this new function hGetContentsAndClose or something) and then readFileAndClose were implemented based on this, would this solve the problem? > I read recently that other functional languages (ML?) with strict > evaluation semantics can be used to mimic lazy evaluation, but saw no clue > how that would work. I'm wondering if any insight might be gained by > looking at how such mechanisms might interact with functional > state-transformer mechanisms? The basic idea is to do something like: > data Lazy a = > Value a (Lazy a) > Deferred (() -> Lazy a) or something, and then you provide a function: > force (Value a l) = Value a l > force (Deferred f) = force (f ()) You can then force values as you need them; presumably in strict evaluation semantics, 'f ()' would be evaluated only at the point that it's forced. This isn't really relevant, though, I don't think, since it's sort of the opposite of what we want to be doing... - Hal

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RE: getpid() or something similar
by Markus.Schnell＠infineon.com 22 May '03

22 May '03

You can't guarantee that another process didn't happen to write a file with just that name, pid or not. Markus > -----Original Message----- > From: Peter Simons [mailto:simons@cryp.to] > Sent: Thursday, May 22, 2003 12:06 PM > To: haskell-cafe(a)haskell.org > Subject: Re: getpid() or something similar > > > Simon Marlow writes: > > > Even using a ProcessID doesn't guarnatee uniqueness [...] > > Why not? If I use the file only temporarily (it is gone once the > process terminates), something like /tmp/foo.<pid> will be unique, for > all I can tell. > > > > The right way to do this is to try to open it for writing, and try > > a different name if the open fails. > > Unfortunately, this approach features a race condition because POSIX > has no notion of mandatory file locking. I'd really rather avoid this, > if I can. > > Is there no other way? And one that works across different compilers? > > Peter > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe >

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getpid() or something similar
by Peter Simons 22 May '03

22 May '03

Please pardon me if it turns out I'm just blind, but is there any function I can use to get some "unique" information to use for temporary file names and the like? A function that will tell me my process ID, for instance? I looked through Hugs and GHC, yet couldn't find anything but Data.Unique, and that looks like big-time overkill for the purpose. Any pointers are appreciated! Peter

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Socratic method and Imperativicles
by Steffen Mazanek 22 May '03

22 May '03

Hello, I just have read Platon's Euthyphron and was fascinated by the maieutics of Socrates. So I want to start the little project "Imperativicles", which should result in a dialog similar to http://web.mit.edu/kerberos/www/dialogue.html. Thereby Socrates confuses an impassionate imperative programmer which thinks in "lines of code" exclusively and points him to the beauty of functional programming. But there is one problem. I cannot express these important, well-known phrases of Socrates in English. So if anybody wants to contribute I would be happy. All ideas, e.g. argumentations, would be helpful and greatly appreciated. The "project" page is the HaWiKi: http://haskell.org/hawiki/SocraticMethod Best regards, Steffen -- Steffen Mazanek #Haskell,that's where I just curry until fail,unwords www.steffen-mazanek.de#any error,drop all undefined,maybe break,otherwise in #sequence span isControl and take max $,id: (d:[])

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Help: writing ffi bindings to a C library
by Bayley, Alistair 21 May '03

21 May '03

I want to write a set of bindings to Oracle's OCI library (a low-level C library for doing stuff to the Oracle database). Before I get started, has anyone done this already? (I don't think so, but I'd better ask.) My goal is to start with a simple set of low-level bindings, and build this up to higher-level database abstractions, and to hopefully port HaskellDB (if it's possible). But to start with I simply want to create a set of bindings for the OCI functions and get them working. Trouble is, I have no idea how to start writing a FFI binding. So far I've read: - Haskell 98 FFI Addendum (RC8) : a reference, but not a tutorial. - http://reid-consulting-uk.ltd.uk/docs/ffi.html : incomplete - http://haskell.org/ghc/docs/latest/html/users_guide/hsc2hs.html : is this for writing C that calls Haskell functions? Can anybody point me to a tutorial, or some illustrative examples? At the moment I'm trying to avoid the various ffi tools, and just use the basic ffi capabilities in GHC, so I can get an idea of how it all works. When I'm more comfortable with ffi then I'll think about using the various tools to speed things up. I'm not sure how to treat structures used by the C library. For example, a sample C program might have: int main () { sword rc; OCIEnv *envhp = NULL; OCIServer *srvhp = NULL; OCIError *errhp = NULL; OCISvcCtx *svchp = NULL; OCISession *usrhp = NULL; OCIStmt *stmtp = NULL; ... rc = OCIEnvCreate(&envhp, OCI_DEFAULT, 0, 0, 0, 0, 0, 0); rc = OCIHandleAlloc(envhp, (dvoid**) &errhp, OCI_HTYPE_ERROR, 0, 0); rc = OCIHandleAlloc(envhp, (dvoid**) &srvhp, OCI_HTYPE_SERVER, 0, 0); ... How do I handle values of OCIEnv, OCIServer, etc? I think I'm supposed to use Ptr, ForeignPtr, and Storable to reference them, but I don't see how to combine these types/classes (ForeignPtr takes values of type Ptr a; how do I construct a Ptr a? What type should "a" be?). ***************************************************************** 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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CC: Functional dependencies question
by oleg＠pobox.com 21 May '03

21 May '03

> hbar :: (Mult Energy Time t) => t > hbar = PhysicalUnit 6.62606876e-34 But what if we add to our library joule::Energy = PhysicalUnit 1.0 cal::Energy = PhysicalUnit 4.8 second::Time = PhysicalUnit 1.0 nanosec::Time = PhysicalUnit 1.0e-9 and scalarm:: Double -> (PhysicalUnit a1 b1 c1) -> (PhysicalUnit a1 b1 c1) x `scalarm` (PhysicalUnit y) = PhysicalUnit $ x * y then the user can write hbar = 6.62606876e-34 `scalarm` (joule `mult` second) or hbar = 6.62606876e-25 `scalarm` (joule `mult` nanosec) or hbar = (6.62606876e-25/4.8) `scalarm` (cal `mult` nanosec) Can't we? Or I'm missing something? GHC seems happy. We can probably define in the library transf:: (PhysicalUnit a1 b1 c1) -> (PhysicalUnit a1 b1 c1) -> Double transf (PhysicalUnit x) (PhysicalUnit y) = x/y and write hbar = (6.62606876e-25/(cal `transf` joule)) `scalarm` (cal `mult` nanosec) P.S. Just for the record: yet another way to introduce negative numbers is through an explicit constructor Neg. For example: http://pobox.com/~oleg/ftp/Computation/type-arithmetics.html (sorry, the code uses C++ templates. The idea is just the same however: a mere Prolog in a more verbose notation). The above example demonstrates division of signed Peano numbers. The Neg constructor becomes quite handy then.

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please help me
by John Jonas Savimbi 20 May '03

20 May '03

From: John Jonas Savimbi Johannesburg, South Africa Email: jjsavimbi(a)phantomemail.com May 19TH, 2003. Dear Sir, It is my humble pleasure to write you this letter irrespective of the fact that you do not know me. However, I got your name through your country business directory here in my search for a reliable and trustworthy person that can assist me confidently. My name is Mr.John Jonas Savimbi. I am the son of Late Dr. Jonas Savimbi from Angola. I am 29 years old and I have got two younger sisters of same blood. Our mother died almost a year ago. We are all fighting for political asylum in South Africa. We cannot find a home in Angola anymore because of the war that our late father fought with the government of Angola until he died in that war. The government of Angola is now mad at us because our late father also wanted to be president and caused the people to fight the war for him because he was the president of UNITA political movement of Angola. So now we are in South Africa and we are suffering too much because we don't have money and the government of South Africa will not give jobs to asylum seekers. I am much concerned about my younger sisters who are finding it very difficult to manage in the situation. But I know that we should not suffer this way. I know that I must not let my younger sisters get into trouble because of money. So after consideration I am hereby begging for your assistance to help me to recover the sum of USD $29 Million which is my inheritance from my late mother. My late mother had taken this money with her to Spain in a diplomatic metal box marked (Precious Stones). Subsequently she lodged the metal box (As precious Stones) with a Securities and Valuables Protection company in Spain. I am supposed to be the beneficiary in the event of her death. However, all effort I made to go Spain to claim was refused by the Spainish embassy. And this was while it was still possible for me to travel. Now I cannot travel anywhere anymore. So in the situation that we are facing now, I must find somebody who can go to the Spain on my behalf. I understand that I will have to transfer my beneficiary rights unto you. I am willing to do so in the hope that you are a God fearing person who will not abandon us after you have the money. So from the bottom of my heart I am giving you 20% of the all the money. Once we have enough to sustain us sufficiently, you may be come our fund manager and invest the rest wisely for us. If you will be kind to assist us, please inform me urgently. Kind regards to you and your family. Sincerely, John Jonas savimbi

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RE: Functional dependencies question
by Simon Peyton-Jones 19 May '03

19 May '03

Think of it like this. Should this be acceptable? f :: a -> a -> a f x y = x && y No, because the type (forall a. a->a->a) is plainly more general than the actual function. The type tempts you to think that you could write f 3 4 but you can't. But I suppose you could argue that the function definition should be allowed, but if you ever called (f 3 4) you should get an error "Int found where Bool expected". Your signature is bar :: (Foo Char t) => t and we know (from the instance decl) that t must be Bool. Any other call to bar would be illegal, just as any cal to 'f' above is illegal, except at type Bool. In general, it seems to make sense to reject definitions as early as possible, hence GHC and Hugs reject it. It is undoubtedly odd that adding an instance declaration makes a legal program illegal, and that might be a reason for relaxing the rule... but it's not entirely easy to implement such a relaxation. Simon | -----Original Message----- | From: haskell-cafe-admin(a)haskell.org [mailto:haskell-cafe-admin@haskell.org] On Behalf Of Andrew | J Bromage | Sent: 12 May 2003 06:10 | To: haskell-cafe(a)haskell.org | Subject: Functional dependencies question | | G'day everyone. | | Suppose I have code like this: | | class Foo a b | a -> b where | foo :: a -> b | | bar :: (Foo Char t) => t | bar = foo 'a' | | All well and good so far. Let's add an instance of Foo... | | instance Foo Char Bool where | foo c = isAlpha c | | Now neither GHC nor Hugs will allow this to compile, as the declared | type of bar is "too general". | | This seems intuitively the wrong behaviour. Either the original | program was incorrect (as there was no visible instance of Foo for | type Char) or the amended program is correct (since the type of bar | is no more general than it was in the first version). | | Thoughts? | | Cheers, | Andrew Bromage | _______________________________________________ | Haskell-Cafe mailing list | Haskell-Cafe(a)haskell.org | http://www.haskell.org/mailman/listinfo/haskell-cafe

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n-term recurrences
by William Lee Irwin III 17 May '03

17 May '03

-- coefs starts with constant term, then coef of a(n-k) in k-term -- recurrence and goes up by indices -- vals starts with x0 and goes up by indices -- solves linear recurrences -- where xs !! (n+1) = head coefs + -- sum (zipWith (*) (tail coefs) [xs !! k | k <- [n - length coefs - 1..n]]) -- and the first few terms are given by xs !! n = vals !! n recurrence coefs vals | length coefs /= length vals + 1 = error "mismatched lengths of coefficients and values" | otherwise = vals ++ map coefSum ys where xs = recurrence coefs vals ys = transpose [drop k xs | k <- [0..length coefs - 1]] coefSum xs = head coefs + sum (zipWith (*) (tail coefs) xs) transpose xs = map head xs : transpose (map tail xs) -- This could be used, say, to generate the list of orthonormal -- polynomials generated by a three-term recurrence and then -- expand some function in a function series via Gaussian quadrature. -- e^(-x^2/2)H_n(x) or e^(-x^c/2)L^c_n(x) look especially attractive. -- OTOH that tactic may not be numerically useful, or useful only -- when the function series expansion is known a priori. -- It could also be a fancier way to generate Fibonacci numbers or -- Apery numbers or whatever. -- Not terribly deep, but it amused me for a few minutes to think of it. -- wli

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ELearnChina Conference 2003
by Edwin Jones 16 May '03

16 May '03

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Need help on Ptr
by Christian Buschmann 16 May '03

16 May '03

Hi! Can someone tell me, how i can check, if a Ptr is a nullPtr. Now I am using this function, but I think its not a very nice one: import Ptr isNullPtr :: (Ptr a) -> Bool isNullPtr ptr = (minusPtr ptr nullPtr) == 0 Maybe someone has got a better idea... christian

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Re: on termination
by oleg＠pobox.com 15 May '03

15 May '03

I'd like to clarify the termination decision procedure. I believe it can take the body of a function in our language and tell if the function terminates for all inputs. This implies that we can decide on termination without evaluating the function on all inputs (whose set is countably infinite). A small clarification: the 'branch-on-zero' statement should have the form "if-nonnegative exp then exp1 else exp2". Because the domain includes 'undefined' (aka NaN), the only functions that terminate for all inputs are those that leave their arguments alone and either propagate them or return constants. For example, the following functions terminate on all inputs f1(x) = x f2(x,y) = y f3(x) = 0 f4(x) = if-nonnegative power(5,125) then x else NaN The following functions will fail to terminate on at least one input: g1(x) = x + x g2(x) = if-non-negative x then x else NaN g3(x) = if-non-negative x*x then 0 else 1 Although it seems that g3 is equivalent to f3, that is not true: g3(NaN) diverges whereas f3(NaN) returns 0. These examples clarify a decision procedure, which can be stated as follows: if a partial evaluation of f(x...z) -- that is, unfolding plus constant propagation -- reduces f() to f1, f2, or f3 types, the function f() terminates on all inputs. Otherwise, there is one value (most likely NaN), that, when passed to f(), will cause it to diverge. Our partial evaluation procedure first unfolds all function calls. That is possible because the call graph does not have direct cycles (NPFIX must be treated as a special node in the call graph). Any operation (multiplication, addition, comparison, NPFIX) on a dynamic value can lead to a non-termination (if that value is NaN). Thus the only operations that can still guarantee termination are "if exp1 then exp2 else exp3" and "NPFIX exp1 exp2 exp3" where exp1 is a constant expression. We can evaluate it. Because it contains only primitive recursive functions, we provably can do that in some finite time. Therefore, we can decide which branch to take and continue to analyze exp2 or exp3. It seems the decision procedure is total and correct.

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RE: on termination
by Simon Peyton-Jones 15 May '03

15 May '03

| If you guys think haskell-cafe is improper for this conversation we can | continue on theory-edge on yahoo groups. But I don't think it's improper, | after all there is all kind of theoretical weirdness talked of in the main | haskell discussion list. Why should this be any less relevant??? Anyway this is Haskell Café, where the rules are pretty loose! (If it were on the main Haskell list I'd certainly suggest moving the thread to Haskell-café, but it seem ok there, to me.) S

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re: re: 你好
by 张丽 14 May '03

14 May '03

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Re: on termination
by Markus.Schnell＠infineon.com 14 May '03

14 May '03

Somebody asked for coffee? / / / \ \ \ / / / +++++++++++ + +--\ + +___/ \+++++/ ============= ========= -- Markus Schnell Infineon Technologies AG, CPR ET Tel +49 (89) 234-20875

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DUKAT KABILA(Jnr)
by Dukat Kabila 14 May '03

14 May '03

> MR DUKAT KABILA 0031+615-421-228 > REQUEST FOR URGENT BUSINESS ASSISTANCE > >Your contact was availed to me by the chamber of commerce. It was given to me because of my diplomatic status as I did not disclose the actual reasons for which I sought your contact. But I was assured That you are reputable and trustworthy if you will be of assistance. I am Laurent DUKAT KABILA (Jnr) the second son of Late President LAURENT DESIRE KABILA the immediate Past president of the DEMOCRATIC REPUBLIC OF CONGO in Africa who was murdered by his opposition through his personal bodyguards in his bedroom on Tuesday 16th >January, 2001.I have the privilege of being mandated by my father colleagues to seek your immediate and urgent co-operation to receive into your bank account the sum of US$25m.(twenty-five million Dollars) and some thousands carats of Diamond. This money and treasures was lodged in a vault with a security firm in Europe and South-Africa. > >SOURCES OF DIAMONDS AND FUND >In August 2000, my father as a defence minister and president has a meeting with his cabinet and armychief about the defence budget for 2000 to 2001 which was US $700m. so he directed one of his best friend. Frederic Kibasa Maliba who was a minister of mines and a political party leader known as the Union Sacree de, I opposition radicale et ses allies (USORAL) to buy arms with US $200m on 5th January 2001; for him to finalized the arms deal, >my father was murdered. f.K. Maliba (FKM) and I have decided to keep the money with a foreigner after which he will use it to contest for the political election. Inspite of all this we have >resolved to present your or your company for the firm to pay it into your nominated account the above sum and diamonds. This transaction should be finalized within seven (7) working days and for your co-operation and partnership, we have unanimously agreed that you will be entitled to 5.5% of the money when successfully receive it in your account. The nature of your >business is not relevant to the successful execution of this transaction what we require is your total co-operation and commitment to ensure 100% risk-free transaction at both ends and to protect the persons involved in this transaction, strict confidence and utmost secrecy is >required even after the successful conclusion of this transaction. If this proposal is acceptable to you, kindly provide me with your personal telephone and fax through my E-mail box for immediate commencement of the transaction. >All correspondence is for the attention of my counsel: > >I count on your honour to keep my secret, SECRET. >Looking forward for your urgent reply >Thanks . >Best Regards > >DUKAT KABILA(Jnr) >

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How to create an unboxed array?
by Ron de Bruijn 13 May '03

13 May '03

I finally succeeded in creating the unsafeWrite and unsafeRead functions, but the newArray_ function will not work. I get the following error: Compiling Try ( try.hs, interpreted ) try.hs:37: Could not deduce (Num i) from the context (MArray IOUArray Foo IO, HasBounds IOUArray, Monad IO, Ix i) Probable fix: Add (Num i) to the class or instance method `newArray_' arising from use of `*' at try.hs:37 In the first argument of `newArray_', namely `(fst (bound), (snd (bound)) * 2)' In a 'do' expression pattern binding: newArray_ (fst (bound), (snd (bound)) * 2) Failed, modules loaded: none. Prelude> newArray_ bound= do newArray_ (fst(bound),snd(bound)*2) I don't really understand why I have to do something with the Numclass. I think it is to be sure of the fact that the the arguments given in the tuple are indextypes of type Num, so the values can always be multiplied using the * operator. How can I make this work and why will it work the way you would say it would work? And why can't I just put the types of the functions just above the definition, just like I do with any other function, is it just because of the fact this is a method of a class? newArray_::(MArray a e m, Ix i) => (i, i) -> m (a i e) newArray_ bound= do newArray_ (fst(bound),snd bound)) And then yet another question: if I had another datatype, like Foo String Int, then I just had to create two different arrays, one of Int and one of String, right? Greets Ron __________________________________ Do you Yahoo!? The New Yahoo! Search - Faster. Easier. Bingo. http://search.yahoo.com

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Web Server
by Shawn P. Garbett 13 May '03

13 May '03

I got into a discussion with folks in my local circle of computer heads. I was talking about what I liked about Haskell. Most of the local circle are sold on the idea that Perl is the best language ever written. They eventually challenged me: show us a web site that is being served by Haskell. Does anyone out there run a web site using Haskell? How much effort would be involved in creating a haskell_mod to plug into apache? Shawn

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Re: CC: Functional dependencies question
by oleg＠pobox.com 13 May '03

13 May '03

> > then the user can write > > hbar = 6.62606876e-34 `scalarm` (joule `mult` second) > That works, but IMO it's a bit unwieldy. It also doesn't help with > actually defining a reasonable type signature for hbar which is the > point of the exercise. Well, how about this? hbar = (PhysicalUnit 6.62606876e-34)::t where x::t = (undefined::Energy) `mult` (undefined::Time) the use of bottom might be a bit bizarre -- OTH, a physicist of a kind that cares about Planck constant should get used to the infinities -- take QED for example. A physicist might even appreciate that equation. It's interesting that in every thread I have participated over the last couple of months I mentioned 'undefined'. It must be my favorite Haskell value.

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CC: Functional dependencies question
by oleg＠pobox.com 13 May '03

13 May '03

Hello! Andrew J Bromage wrote: > class Foo a b | a -> b where > foo :: a -> b > > bar :: (Foo Char t) => t > bar = foo 'a' > > All well and good so far. Let's add an instance of Foo... > > instance Foo Char Bool where > foo c = isAlpha c > > Now neither GHC nor Hugs will allow this to compile, as the declared > type of bar is "too general". The type of bar is indeed too general. It should be bar:: forall t. (Foo Char t | Char -> t) => t or bar:: exists t. (Foo Char t) => t because the functional dependency implies that there can be only one such type that relates to Char. Yet such types are not expressible. Perhaps because of that, GHC and Hugs delay reporting an error. For example, if we omit the instance declaration, both GHC and Hugs seem happy. But they only seem. In Hugs, if we try to actually evaluate 'bar', we get Main> bar ERROR - Unresolved overloading *** Type : Foo Char a => a *** Expression : bar Indeed, an instance declaration is required. Incidentally, GHC can report the error even before running the code: if we just want to see the type of bar: *Main> :t bar No instance for (Foo Char t) arising from use of `bar' at <No locn> If we add an instance of Foo, such as "instance Foo Char Bool", then the signature of bar is clearly too polymorphic: there can be only one type t (namely, Bool) for which the constraint Foo Char t is true. Incidentally, local type variable again seem to help -- they can help express types that are normally inexpressible. If we define bar as bar :: (Foo Char t) => t bar ::tbar = (foo::(Char->tbar)) 'a' we get an error: /tmp/f.hs:10: Inferred type is less polymorphic than expected Quantified type variable `t' escapes It is mentioned in the environment: tbar = t is bound by the pattern type signature at /tmp/f.hs:1 When trying to generalize the type inferred for `bar' Signature type: forall t. (Foo Char t) => t Type to generalise: t When checking the type signature for `bar' When generalising the type(s) for `bar' which alerts us that the given type of bar is indeed too general. Now, we get the error right away. If we drop the explicit type declaration, bar ::tbar = (foo::(Char->tbar)) 'a' we get another error, /tmp/f.hs:12: No instance for (Foo Char tbar) arising from use of `foo' at /tmp/f.hs:12 When checking the type signature of the expression: foo :: Char -> tbar In the definition of `bar': (foo :: Char -> tbar) 'a' The compiler demands to see an instance of Foo Char t. If we add the instance now, the compiler is happy. bar ::tbar = (foo::(Char->tbar)) 'a' instance Foo Char Bool where foo c = c == 'a' *Main> bar True and the code even runs.

2 2

Functional dependencies question
by Andrew J Bromage 13 May '03

13 May '03

G'day everyone. Suppose I have code like this: class Foo a b | a -> b where foo :: a -> b bar :: (Foo Char t) => t bar = foo 'a' All well and good so far. Let's add an instance of Foo... instance Foo Char Bool where foo c = isAlpha c Now neither GHC nor Hugs will allow this to compile, as the declared type of bar is "too general". This seems intuitively the wrong behaviour. Either the original program was incorrect (as there was no visible instance of Foo for type Char) or the amended program is correct (since the type of bar is no more general than it was in the first version). Thoughts? Cheers, Andrew Bromage

2 2

RE: Web Server
by Markus.Schnell＠infineon.com 12 May '03

12 May '03

You could ask the guys at http://www.informatik.uni-freiburg.de/~thiemann/haskell/WASH/ Markus > -----Original Message----- > From: Shawn P. Garbett [mailto:listman@garbett.org] > Sent: Monday, May 12, 2003 3:44 PM > To: haskell-cafe(a)haskell.org > Subject: Web Server > > > I got into a discussion with folks in my local circle of > computer heads. I was > talking about what I liked about Haskell. Most of the local > circle are sold > on the idea that Perl is the best language ever written. They > eventually > challenged me: show us a web site that is being served by Haskell. > > Does anyone out there run a web site using Haskell? > How much effort would be involved in creating a haskell_mod > to plug into > apache? > > Shawn > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe >

1 0

haskell-cafe
by sms 12 May '03

12 May '03

你要的网页给你找到了 http://www.creatonly.com/sohusms.htm

1 0

Re: Random access times
by Ron de Bruijn 10 May '03

10 May '03

I finally succeeded in doing your little exercise :), except for the newArray_. I get the following error: Compiling Try ( try.hs, interpreted ) try.hs:37: Could not deduce (Num i) from the context (MArray IOUArray Foo IO, HasBounds IOUArray, Monad IO, Ix i) Probable fix: Add (Num i) to the class or instance method `newArray_' arising from use of `*' at try.hs:37 In the first argument of `newArray_', namely `(fst (bound), (snd (bound)) * 2)' In a 'do' expression pattern binding: newArray_ (fst (bound), (snd (bound)) * 2) Failed, modules loaded: none. Prelude> newArray_ bound= do newArray_ (fst(bound),snd(bound)*2) I don't really understand why I have to do something with the Numclass. I think it is to be sure of the fact that the the arguments given in the tuple are indextypes of type Num, so the values can always be multiplied using the * operator. How can I make this work and why will it work the way you would say it would work? And why can't I just put the types of the functions just above the definition, just like I do with any other function, is it just because of the fact this is a method of a class? newArray_::(MArray a e m, Ix i) => (i, i) -> m (a i e) newArray_ bound= do newArray_ (fst(bound),snd(bound)) Greets Ron __________________________________ Do you Yahoo!? The New Yahoo! Search - Faster. Easier. Bingo. http://search.yahoo.com

1 0

searching for a substring
by sashan 09 May '03

09 May '03

Hi I've written a function that checks for a substring in a given string. This seems like a common thing to do and was wondering if there is a function that does this in one of the standard modules. stringContains :: String -> String -> Bool stringContains _ [] = True stringContains [] _ = False stringContains (h:t) (h1:t1) = if h == h1 then stringContains t t1 else stringContains t (h1:t1) -- sashan http://sashan.netfirms.com -------------------------------------------------- Brain: Here we are, Pinky--at the dawn of time! Pinky: Narf, Brain. Wake me at the noon of time.

5 5

Re: on termination
by oleg＠pobox.com 09 May '03

09 May '03

Peter Gammie wrote: > What I want: a class of functions that don't all terminate, don't all not > terminate, and furthermore I can tell how a given element of the class > will behave without running it. It must have at least some expressiveness; > this is up for grabs, but it'll have to encompass at least primitive > recursion to be interesting. Let us consider a first-order language of functions defined with the help of addition, multiplication, branching-on-zero, and NPFIX. Functional applications are allowed provided they use the names of previously defined functions. (NPFIX n) is the n-th Church numeral (aka fold) NPFIX n f seed --> f (f ... (f seed)) n times (n>=0) seed otherwise The second argument of NPFIX is the name of a previously defined function (a known name, a functional constant). The domain of our functions consists of integers and a special value 'undefined' (to be called NaN for short). The reduction rules are as usual, with an addition NaN + x -> NaN + x for any x, etc. That is, any operation with NaN leads to non-termination. The language obviously includes every primitive recursive function. There are terminating expressions (any expression that never directly or indirectly mentions NaN always terminates). There are obviously non-terminating functions, such as f(x) = x + NaN. It is also clear that the halting predicate for our language is decidable. Given an expressing, we examine it and the body of all involved functions. If we don't find any NaN, we yield TRUE. If we find a NaN, we "abstractly" interpret our expression. That is, we execute it step by step until we come across a step that tries to add, compare, multiply NaN. At this point we yield FALSE. If our abstract interpreter halts, we yield TRUE. Obviously our abstract interpreter is just the concrete interpreter with one small change. Because any sequence of NaN-free reductions in our language is a sequence of reductions for some primitive recursive function, such sequence must terminate. We're a bit running afoul of the requirement: > I can tell how a given element of the class will behave without > running it but then, a decision procedure must take some 'deciding', so to speak -- if it is non-trivial. We must have a deciding machine. In our example, the deciding machine is the original machine with a slight modification.

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on termination
by Cagdas Ozgenc 09 May '03

09 May '03

Greetings. This may be a little off topic, but I could not get much response from comp.theory, so here it goes: Is it possible to have a programming language that can be used to produce only the programs that terminate, but at the same time decide all recursive languages? Or does such limitation automatically reduce the class of languages that such a programming language decide to primitive-recursive? Perhaps, I couldn't explain it very clearly. I suppose another way of saying this is: Is there a strongly normalizing system that can decide the class of recursive languages? Perhaps something less strong than a Turing Machine, for example deciding all recursive languages but not recognizing any recursively enumerable languages. Thanks

5 12

Re: [Arrays] Random Access Times ?
by Ron de Bruijn 08 May '03

08 May '03

--- Hal Daume III <hdaume(a)ISI.EDU> wrote: > > The one is use is a unboxed and you mean I should > > import the Hugslibrary IOExts, just like in my > code , > > right? > > if you're using arrays of complex values like your > data type, i seriously > doubt you're using unboxed arrays. you will have to > write special > instance methods for them. and yes, you should just > be able to use it as > is. > I really don't have any idea about how to create an instance of that IOArray. The only thing I know about this instance is: instance IOArray bounds myDataType where, but that's nog enough. Can you (again) help me? __________________________________ Do you Yahoo!? The New Yahoo! Search - Faster. Easier. Bingo. http://search.yahoo.com

2 4

Annuity Payment
by sdGrego99＠email.com 08 May '03

08 May '03

http://www.helpu-web.com/annuities/ Here is that great annuity site Frank told me about! http://www.helpu-web.com/annuities/ Thanks, Alan you want out: http://1st-helpu.com/rm.html

1 0

writing a function to build a list of ints from user input
by sashan 06 May '03

06 May '03

I'm trying to write a function that builds a list from user input. This is what I've come up with so far. --a program that makes a list of numbers from user input makeList :: IO [Int] makeList = do s <- getLine if read s /= 0 then return (read s:makeList) else return [] However it fails with the following error: Couldn't match `[a]' against `IO [Int]' Expected type: [a] Inferred type: IO [Int] In the second argument of `(:)', namely `makeList' In the first argument of `return', namely `((read s) : makeList)' It's trying to match a list, [a], with type IO [Int]. This causes it to fail. How do I get around this? -- sashan http://sashan.netfirms.com --------------------------- Brain: Here we are, Pinky--at the dawn of time! Pinky: Narf, Brain. Wake me at the noon of time.

3 4

RE: writing a function to build a list of ints from user input
by Simon Marlow 06 May '03

06 May '03

> On Tue, May 06, 2003 at 05:02:56AM -0500, Galen Menzel wrote: > > > > makeList :: IO [Int] > > makeList = do s <- getLine > > case read s of > > 0 -> return [] > > x -> do xs <- makeList > > return (x:xs) > > > > I changed the if to a case so that (read s) doesn't get > called twice. > > Does the compiler not optimize away identical calls? Or is it > not allowed to, for some reason? Yes, GHC will common up the repeated expression when -O is turned on. > On the other hand, if I write a function like: > > long_list = [1..100000] ++ [1..100000] ++ [1..100000] ++ [1..100000] > > And then run a > > putStr $ unlines $ map show long_list > > I would expect this to take very little memory, which would > mean that the > [1..100000] would need to be calculated four separate times > rather than > being calculated once and stored... Well, this is one of the disadvantages of common sub-expression elimination: you have to store the result of the sub-expression between uses. It is a space/time tradeoff, and in cases like the above, a pretty poor one. But GHC does it anyway, because in most (probably all) of the cases we've seen it is a win. Cheers, Simon

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do expression
by sashan 05 May '03

05 May '03

Hi I'm trying to write a simple program to get user input and execute a function based on the input. The program follows: import IO main = do putStrLn "1) f1" putStrLn "2) f2" choice <- getLine case choice of 1 -> do putStrLn "f1" 2 -> do putStrLn "f2" _ -> do putStrLn "" When compiled with ghc I get the following error: test.hs:8: No instance for (Num String) arising from the pattern `2' at test.hs:8 In a case alternative: 2 -> do putStrLn "f2" In the case expression: case choice of 1 -> do putStrLn "f1" 2 -> do putStrLn "f2" _ -> do putStrLn "" Please can someone explain. Thanks

3 2

Maximum value in a list
by sashan 05 May '03

05 May '03

Hi I was reading through the tutorial Yet Another Haskell Tutorial and doing chapter 3 ex 5, write a function using foldr and max to return the maximum value in a list. I came up with the following that will work for positive numbers. maxInList :: [Int]->Int maxInList [] = 0 maxInList l = foldr max 0 l Is there an identity for the max function? Because currently if the list [-1,-3,-4] is passed to maxInList it will return 0.

4 4

Re: Fast Mutable arrays and a general question about IO
by Jon Cast 05 May '03

05 May '03

Ron de Bruijn <rondebruijn(a)yahoo.com> wrote: > > Why? It's not always necessary to go for maximum speed; do you have > > a reason for needing it in this case? > > Almost any operation in my program works on array's and before the > program terminates, there have been an awful lot of operations on > it. If there is an other kind of array that mutates and doesn't create > a copy of it when changing a value and isn't more than 1,5 times as > slow as an array in C++ then that should also be ok. The program would > run in a scale of hours or days. And if it's twice as fast, than I > could calculate two times as much. Alright. <snip> > > I think you mean `above'. In any case, could you show us some code > > you have that's not working? That'll make it much easier for us to > > answer your questions. > > > > Well I don't have really code, because I want to begin programming it. > > I will restate my goal: I want to be able to read, update and create > arrays in Haskell of type myDataType (suppose my constructor function > is Con Int String Int) Alright. You seem to have the concepts down here, so I would recommend you try to start on your own, and mail us back when you have a problem. > on a way that's fast, preferabily the fastest method. I don't know what the fastest method is, although I doubt you'll get anything (natively) faster than IOArray, or IOUArray if you only need arrays of primitive types. <snip> > > > So to put it more concrete: In any Haskell program that does IO > > > there is always some function that has this form: > > > > > > do x<-someSource > > > putBoundVariableToSomeOtherFunctionThatDoesMonadicOperations > > > (restOfPureFunctionalProgramForExample x) > > > > No. > > Why not? Sorry; I put that reply in the wrong place; I meant it to refer to the item below it (meaning it's not possible to do IO within a value of type Int). You are correct that there must be a function in any program with /at least/ that form (i.e., there could be more statements, but if you want to do processing within pure Haskell, you need to wrap that processing ultimately roughly as shown above). Jon Cast

4 5

popen bug?
by Mark Preston 05 May '03

05 May '03

Is there a bug in the GHC implementation of the popen function that manifests itself when more than 4k is written to the pipe? I'm trying to reproduce in Haskell the standard UNIX shell pipe idiom: cat 4097 | cat where 4097 is a file containing 4097 bytes. My Haskell code looks as follows: -- snip -- import Posix import IO import System main :: IO () main = do sequence_ $ map printChunk chunks printChunk :: String -> IO () printChunk str = do (find_output, _, _) <- popen "cat" [] (Just str) (cat_output, _, _) <- popen "cat" [] (Just find_output) putStrLn cat_output putStrLn $ show (length str) lengths = [4095,4096..] str = repeat 'a' chunks = zipWith take lengths (repeat str) -- end snip -- I get two pieces of output, one for each of the 4095 and 4096 bytes, but nothing for 4097 bytes, the program just hangs. Is this some bizarre feature of lazy evaluation, or a mistake in the way that popen is implemented (or both)? I'm using ghc-5.04.3 on linux 2.4.20-8 (Red Hat 9). Thanks, Mark. _________________________________________________________________ On the move? Get Hotmail on your mobile phone http://www.msn.co.uk/mobile

2 1

RE: Maximum value in a list
by Garner, Robin 05 May '03

05 May '03

You'll find in the Prelude that Int is an instance of Bounded, so minBound and maxBound are defined, and are the identities you need. cheers -----Original Message----- From: sashan [mailto:sashang@ihug.co.nz] Sent: Monday, May 05, 2003 12:30 PM To: haskell-cafe(a)haskell.org Subject: Maximum value in a list Hi I was reading through the tutorial Yet Another Haskell Tutorial and doing chapter 3 ex 5, write a function using foldr and max to return the maximum value in a list. I came up with the following that will work for positive numbers. maxInList :: [Int]->Int maxInList [] = 0 maxInList l = foldr max 0 l Is there an identity for the max function? Because currently if the list [-1,-3,-4] is passed to maxInList it will return 0. _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

1 0

[Arrays] Random Access Times ?
by Ron de Bruijn 04 May '03

04 May '03

Hi there, I tested below program with for x filled in 1 and 50000. And I saw that when I used 50000 it took more than ten times more time than when I used 1, to complete the expression. So much for randow access memory(RAM). Isn't there somekind of other array that really works with random access? module Test where import IOExts data Lesson = Lesson String Int Int String String deriving Show main = do testing <- newIOArray (0,60000) (Lesson "Hallo" 0 0 "" "") sequence(map(writeIOArray testing x) (test)) a<-readIOArray testing 0 putStr (decompose a) test::[Lesson] test=(replicate 100000 (Lesson "" 1 2 "" "")) decompose (Lesson s1 _ _ _ _) = s1 Greets Ron __________________________________ Do you Yahoo!? The New Yahoo! Search - Faster. Easier. Bingo. http://search.yahoo.com

4 8

RE: Fast Mutable arrays and a general question about IO
by Garner, Robin 04 May '03

04 May '03

Here's an example of mine, that uses a hash table to construct a word frequency table. By no means is this great code - for example the hash table filling up will lead to an infinite loop ... This function produces a word frequency list using an intermediate hash table to accumulate the counts. > makemap :: [String] -> [(String,Int)] > makemap l = filter ((> 0) . snd) $ > runST (emptyHT >>= flip (foldM insertHT) l >>= getElems) The hash table structure, an array.ST > maxHash = 10000 ::Int > > type HT s = STArray s Int (String,Int) > > emptyHT :: ST s (HT s) > emptyHT = newArray (0,maxHash) ("",0) Update the hash table with a single entry > insertHT :: HT s -> String -> ST s (HT s) > insertHT h s = findSlot s h (hash s) >>= updateSlot h s >> return h Finds the correct slot in the hash table, returning the slot index and the old count. > findSlot :: String -> HT s -> Int -> ST s (Int,Int) > findSlot s h i = do (s',c) <- readArray h i > if c == 0 || s == s' > then return (i,c) > else findSlot s h ((i+1) `mod` maxHash) > > updateSlot :: HT s -> String -> (Int,Int) -> ST s () > updateSlot h s (i,n) = writeArray h i (s,n+1) Simple (simplistic ?) hash function > hash :: String -> Int > hash = (`mod` maxHash) . foldl' hash' 0 > where hash' acc c = acc * 2 + ord c

1 0

Re: Fast Mutable arrays and a general question about IO
by Jon Cast 03 May '03

03 May '03

Ron de Bruijn <rondebruijn(a)yahoo.com> wrote: > Hi there, > > I have been busy for a couple of days now, to just get a bit more > feeling on Haskell and I have read a bunch of tutorials, also about > Monads and experimented of course. And now I want to use it for my > real world problem. I think I understand the languageprinciples good > enough, but I yet don't understand the syntax of all of the > functioninterfaces. And that's my problem. > > I want to create the fastest array as possible in Haskell Why? It's not always necessary to go for maximum speed; do you have a reason for needing it in this case? > and I understood that I should use this function(the function below > that creates a mutable array). The only problem is that I don't kwow > how to use it. > > Constructing mutable arrays > > newArray :: (MArray a e m, Ix i) => (i, i) -> e -> m (a i e) > Builds a new array, with every element initialised to undefined. No; that's newArray_. (newArray bounds x) builds an array with every element initialized to x. > Reading and writing mutable arrays > > readArray :: (MArray a e m, Ix i) => a i e -> i -> m e > > Read an element from a mutable array > > writeArray :: (MArray a e m, Ix i) => a i e -> i -> e -> m () > Write an element in a mutable array > > I see it takes two arguments: a tuple of an index type (I use Int, so > for example (0,1) 0 for the lower bound, 1 for the upper bound.), I assume you're talking about newArray here, right? If so, you're correct. > but the e (according to "GHC documentation" it says the "element > type"). Well the type of my elements should be my own datatype > myDataType, but I can't even get it done to create simple mutable > array's of Int or Bool. > How can I make it work with my own datatype myDataType (could you give > some examples for the use of the below functions?)? I think you mean `above'. In any case, could you show us some code you have that's not working? That'll make it much easier for us to answer your questions. > Is it OK to say that in Haskell any value that is put from an extern > source of the program (reading a file, reading stdin etc.) to a really > pure function, must be in a do notation (essentially being a Monadic > operation)? Yes. More specifically, they don't need to use do-notation (although if you don't know why not, don't worry about it), but they do have to be monadic. Specifically, they have to use the IO monad. > So to put it more concrete: In any Haskell program that does IO there > is always some function that has this form: > > do x<-someSource > putBoundVariableToSomeOtherFunctionThatDoesMonadicOperations > (restOfPureFunctionalProgramForExample x) No. > So it's not possible to write some function that does the following: > > f::Int > f do x<-getChar --suppose the Char is a number > someSpecialReturnFunction x > > and than the result of the IO is only Int after some alterations of > above code. Correct. (I don't think this is a big issue; a Haskell program is required to execute in terms of a value `main' in the IO monad.) <snip> Jon Cast

4 5

Re: How to Check whether a file exists and IO[String] vs [String]
by Jon Cast 03 May '03

03 May '03

Alexandre Weffort Thenorio <alethenorio(a)home.se> wrote: > Hello. I am having two problems. I have a program that reads two texts > file and writes a new text with its info. So I have done so that if > main file doesn't exist I just output an error string but my problem > is when it comes to the second file. This file is not exactly needed > so if it is not there, the program should just ignore the file. I > tried doing that but if the second file is there I also get an error > saying that it couldn;t find the file. So is it possible to check > whether a file exists Yes. See Directory.doesFileExist. > or not outputting and empty string if not or the actual data from it > if so without giving an error and making the program not run?? > > For example > > main = do > mainfile <- readFile "xxxxx.txt" > secondfile <- readMyFile "yyyy.txt" > writeFile "test.txt" mainfile++secondfile > > > readMyFile file = do > list <- readFile file > if list == "" then return "" else return list > > Apparently this doesn't work and if file doesn't exist it still gives > me an error. Correct. readFile always assumes the file exists (not to mention, if list == "", then list = "", so we can calculate: readMyFile file = do list <- readFile file if list == "" then return "" else return list = do list <- readFile file if list == "" then return list else return list = do list <- readFile file return list = readFile file so your readMyFile function as written is unnecessary.) If you want to take a different action if the file doesn't exists you have to use doesFileExist as above. > Another problem I ran into is with IO[String] and [String]. > > I have a function like > > foo = do > nf <- readFile "xxx.txt" > return (lines nf) > > Then when I try using foo like a [String] in other programs I run into > type error. True. IO a /= a; they're not even isomorphic. > How can I turn this IO[String] into a [String] so I don't actually > have to open this file in the main and then use it as an argument of > another function running "lines' on main method???? If you want to pass the result into f, you can say: do x <- foo return (f x) or fmap f foo. Jon Cast

2 1

I want to hear from you today my good friend.
by Farnades Clark 02 May '03

02 May '03

Mr. Farnades Clark Amsterdam, The Netherlands Dear Sir, I am an accountant and a member of the Contract Tender Board (CTB) of the Petroleum, Oil & Gas Corporation of South Africa (Pty) Ltd also known as PetroSA. I am presently on a special diplomatic duty in the South Africa foreign office in Amsterdam, the Netherlands. I got your contact through discreet inquiries in my search for a reliable individual or company who can handle a strictly confidential transaction that involves the transfer of a reasonable sum of money to a foreign account. I have no doubt in your ability to handle a financial business of considerable amount, which will form the bedrock of an extensive business partnership in due course. In unfolding this proposal, I want to count on your status that you will handle this transaction with all maturity and confidentiality it deserves. The money we want to transfer to you amount to a total of US$15.500,000.00 (Fifteen Million, Five Hundred Thousand United States Dollars) only. It was extracted from various over-invoiced contracts, masterminded by myself and some of my colleagues for our mutual benefit when the contracts was awarded by the Contracts Tender Board (CTB) of the Petroleum, Oil & Gas Corporation of South Africa (PTY) Ltd. The above over-invoiced sum has been drafted and short-listed for final payment to a foreign bank account and it has been reflected as an amount to be paid to a foreign contractor for contract that has been executed for the Petroleum, Oil & Gas Corporation of South Africa (PTY) Ltd. The money is currently floating in the suspense account of the South Africa Reserve Bank (SARB) awaiting claim by a foreign partner, which status we absolutely want you to assume. The code of conduct bureau in South Africa has made it an offence for civil servants like us to(A) operate a foreign account (B) own a foreign company and (C) take up contract while still in active service. This is why we need your assistance. Now is the optimum period to realize this fund following the recent directive given to all-federal parastatals and corporations to pay off all foreign debts. I therefore seek your assistance to allow the remittance of this fund to be made to your personal or companys bank account. We have agreed that 30% of the total sum will be given to you and 70% will be for me and my colleagues after the successful transference of the fund to your bank account. I want to assure you that all modalities for the successful transfer of this money to your account has been perfectly worked out and as such, there is no fear of failure. However, I would want to believe that you are honest enough and will not raise any misgiving attitude In any aspect of the transaction. Most importantly, you will keep this transaction very confidential so as not to tarnish the confidence reposed on these officials by the nations financial house. If this business proposal interest you, kindly indicate your willingness to me strictly on email Address: farnades_2003(a)rediffmail.com) and telephone No: +31-630-298-657 for immediate commencement of the transaction. To enable this fund to be successfully transferred to your account, you will also have to provide the following information: 1.Your company name and address that you wish to use as the beneficiary to claim the fund. 2.Your private telephone,fax Numbers. 3.Your Bank name, address and account number where you would want the money remitted. More information about the remittance of the fund to your account will be sent to you when I receive your response. Meanwhile, your swift response will help us to meet the ultimatum given to all foreign contractors in this payment quarter to ensure that funds are claimed. We also wish to discuss investment strategy with you. We have the plan of investing part of our fund in real estate with your assistance in your country. Please, treat this information as confidential. Yours faithfully, farnades clark

1 0

(no subject)
by Thomas F. Kochmann 02 May '03

02 May '03

Hi, Does anybody know how to transfer arguments such as through C's args and argv[]? Thomas

2 1

Fast Mutable arrays and a general question about IO
by Ron de Bruijn 01 May '03

01 May '03

Hi there, I have been busy for a couple of days now, to just get a bit more feeling on Haskell and I have read a bunch of tutorials, also about Monads and experimented of course. And now I want to use it for my real world problem. I think I understand the languageprinciples good enough, but I yet don't understand the syntax of all of the functioninterfaces. And that's my problem. I want to create the fastest array as possible in Haskell and I understood that I should use this function(the function below that creates a mutable array). The only problem is that I don't kwow how to use it. Constructing mutable arrays newArray :: (MArray a e m, Ix i) => (i, i) -> e -> m (a i e) Builds a new array, with every element initialised to undefined. Reading and writing mutable arrays readArray :: (MArray a e m, Ix i) => a i e -> i -> m e Read an element from a mutable array writeArray :: (MArray a e m, Ix i) => a i e -> i -> e -> m () Write an element in a mutable array I see it takes two arguments: a tuple of an index type(I use Int, so for example (0,1) 0 for the lower bound, 1 for the upper bound.) , but the e (according to "GHC documentation" it says the "element type"). Well the type of my elements should be my own datatype myDataType, but I can't even get it done to create simple mutable array's of Int or Bool. How can I make it work with my own datatype myDataType(could you give some examples for the use of the below functions?)? Is it OK to say that in Haskell any value that is put from an extern source of the program(reading a file, reading stdin etc.) to a really pure function, must be in a do notation(essentially being a Monadic operation)? So to put it more concrete: In any Haskell program that does IO there is always some function that has this form: do x<-someSource putBoundVariableToSomeOtherFunctionThatDoesMonadicOperations(restOfPureFunctionalProgramForExample x) So it's not possible to write some function that does the following: f::Int f do x<-getChar --suppose the Char is a number someSpecialReturnFunction x and than the result of the IO is only Int after some alterations of above code. P.S. Will it give any significant loss in speed if myDataType is lazy (no use of the !-sign in the constructors))? P.P.S. If there are array's that are even faster in Haskell, please let me know? Thanks in advance, Ron __________________________________ Do you Yahoo!? The New Yahoo! Search - Faster. Easier. Bingo. http://search.yahoo.com

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01 May '03

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30 Apr '03

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Is Haskell usefull for me?
by Ron de Bruijn 29 Apr '03

29 Apr '03

Hi there, I have developed a new timetablingalgorithm, at least I think it is.. I already tested it in C++, but it's not quite complete and before it's a really great application the program will be tens of times bigger. I used Helium(subsetlanguage of Haskell) in the past few months and I think it's working better dan C++(especially because there's a lot to know about C++, before you really "know" C++). I want to use Haskell for creating a GUI. Furtermore I need some speed(code can't be more than 1.5 times slower than C++). And I want to use large array's(>60000 elements) where each element had an type in it with another 5 elements. data Thing=ConFunction Int Int Int Int Int Thus an array of Thing. I use Windows 98 Se (It seems that some libs. are only for UNIX.) Is it usefull for me to use Haskell, are there enough libraries so I can easily recreate and improve my program? Could you give some examples of possible functions or libraries I should use? Regards, Ron __________________________________ Do you Yahoo!? The New Yahoo! Search - Faster. Easier. Bingo. http://search.yahoo.com

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Re: seeking lore of the QuickCheck masters
by Lauri Alanko 29 Apr '03

29 Apr '03

On Tue, Apr 15, 2003 at 11:40:25AM +0100, Malcolm Wallace wrote: > You want a generator that can produce an arbitrary code fragment, > *given* what type you want it to have. So first, generate an arbitrary > type, then generate an arbitrary expression for it. "Generate an arbitrary proposition, then generate an arbitrary proof for it." Boy, it's _that_ simple? :) Lauri Alanko la(a)iki.fi

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