May 2003 - Haskell-Cafe

Socratic method and Imperativicles
by Steffen Mazanek 21 May '03

21 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 19 May '03

19 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 16 May '03

16 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 15 May '03

15 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 14 May '03

14 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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