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Numbers again
by Konrad Hinsen 22 Aug '03

22 Aug '03

I am getting a bit worried about the usability of Haskell for numerical work. The Haskell 98 report states that floating literals are represented as a conversion from Rational, which means that the literal is first converted to a Rational. I can't find anything in the Haskell report that states how this conversion should take place, and to what precision it should be correct. It could be made correct to any precision as Rationals are represented using Integers, but at least Hugs doesn't do that. By experimenting with some particular cases, I found that its internal Rational representation is even less accurate than the precision of Double permits, which means that it is impossible to specify literals to the full precision of Double. GHC behaved fine in my tests. But what can I safely assume from a Haskell implementation? Konrad.

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ML like pattern matching
by Cagdas Ozgenc 22 Aug '03

22 Aug '03

Greetings, How do I emulate the "when" clause in ML for pattern matching? In other words when a pattern is matched (from a list of patterns of a function) and to enforce additional predicates I use guards, but if the guard condition is not satisfied I want Haskell to get back to trying the remaining patterns. Thanks

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FFI Ptr CFile problem
by Christoph Flamm 22 Aug '03

22 Aug '03

hi, my problem is as follows: I have a c-function "bla()" which gets a stream for reading ( FILE *rx ). Within "bla()" alternating a c-function "getlineC()" and a haskell-function "getlineHS()" should read a line from stream rx and print it. void bla ( FILE *rx ) { char *line for ( ;; ) { line = getlineC( rx ); printf ( "%s\n", line ); free( line ); line = getlineHs( rx ); printf ( "%s\n", line ); } } How do i pass the stream FILE *rx to the haskell-function properly? I tried to pass FILE *rx as a Ptr Cfile and converting it back into something of type Handle in getlineHS, \begin{code} module Cfile ( getLineHs ) where import IO import Foreign ( Ptr ) import Foreign.C.Types ( CFile, CInt ) import Foreign.C.String ( CString, newCString ) import System.Posix.IO ( fdToHandle ) foreign export ccall "getLineHs" getLineHs :: Ptr CFile -> IO (CString) getLineHs :: Ptr CFile -> IO (CString) getLineHs file = do hdl <- getInStream file hGetLine hdl >>= newCString foreign import ccall "stdio.h fileno" cfileno :: Ptr CFile -> IO CInt getInStream :: Ptr CFile -> IO (Handle) getInStream f = cfileno f >>= fdToHandle . fromIntegral \end{code} but this doesn't work (i seem to lose the file position indicator) and i get the following error: Fail: end of file Action: hGetLine Handle: {loc=<file descriptor: 3>,type=readable, binary=True,buffering=block (8192)} File: <file descriptor: 3> If i pass the file position indicator explicitly as a double value between c and haskell and use the functions hTell/hSeek, ftell/fseek to get/set the file position indicator the whole thing works for files but doesn't work for pipes. Any help would be appreciated. Regards, Christoph

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Urgent business assistance
by Ola Abiola 22 Aug '03

22 Aug '03

Monsieur/Madam, Tout d�abord, je voudrais solliciter votre ultime confidence pour cette proposition. C�est par la vertu de la nature de la transaction qu�il est n�cessaire d��tre confidentiel et plus discret. J�ai obtenu votre adresse dans l�annuaire de votre pays . Je suis Mr. Ola ABIOLA, le fils du chef BASHORUN MKO ABIOLA qui �tait mort en d�tention politique le 7 juillet 1998. Mon p�re avait contest� l��lection pr�sidentielle au Nigeria du 12 juin 1993 qu�il avait gagn� mais annul�e. Il �tait emp�ch� du fauteuil pr�sidentiel par l�ancienne association du nord dans le pays connue populairement comme<<KADUNA MAFIA>>. Cette assoc iation regroupe les anciens chefs d�Etats militaire et la derni�re junte militaire G�n�ral SANI ABACHA qui est mort le 8 juin 1998. Ils ont jur� de ne pas abdiquer le fauteuil pr�sidentiel � un sudiste, revendiquant que le fauteuil est pour les natifs du nord. Pour se d�barrasser de mon p�re, ils raisonnaient que mon p�re avait d�pens� de l�argent pour gagner le mandat du public, d�o� l�annulation de l��lection . La situation s�est empyr�e la veille du 12 juin, Ier anniversaire(11 juin 1994) lorsque mon p�re s�est d�clar� pr�sident de la R�publique F�d�rale du Nigeria. Pour cette raison, la junte d�clare que mon p�re est recherch�. Il fut arr�t � et il n��tait pas ni jug� ni lib�r� jusqu�� sa mort. En outre, ma m�re ALHADJA KUDIRAT ABIOLA qui luttait pour la lib�ration de mon p�re fut assassin� par des inconnus arm�s le 4 juin 1996. Il faut noter qu�avant la mort de ma m�re, elle avait d�pos�e une somme de 35.300.000 US$(trente cinq million trois c ent mille) de dollars Am�ricains aupr�s d�une compagnie de s�curit� et ceci appartenait � mon p�re. Mon p�re �tait un polygame et depuis sa mort il y a des probl�mes � cause du partage de sa richesse et mes fr�res(de m�re diff�rente) faisaient tout pour s�asseoir sur les biens de mon p�re, d�o� ma d�cision de transf�rer ce fonds � un �tranger qui m�aidera pour son investissement dans une affaire lucrative � mon profit et � celui de mon petit fr�re, comme de m�me p�re et de m�me m�re et qui arrangera au ssi ma r�sidence dans son pays. Notez bien que ma famille n�est pas au courant de cette transaction, c�est uniquement vous et la compagnie de s�curit� et assurez-vous que cette transaction ne sera jamais sue par ma grande famille et les gens du Nigeria. J�ai d�cid� d�ex�cuter cette aff aire ici � Lom�-Togo d�o� ma r�sidence temporaire, pour des raisons de s�curit�s. Soyez inform� que toutes les modalit�s n�cessaire pour le transfert de ce fonds ont �t� parfaites et la transaction est � 100% de hors risque. D�ailleurs, les militaires sont rentr�s dans les casernes et le Nigeria a maintenant un gouvernement d�mocratiq ue. Vous �tes libres de m�informer dans votre r�ponse � cette lettre votre commission en pourcentage sur le montant total pour votre assistance. Notez tout de m�me que les 5% de la somme totale reste pour les d�penses pouvant �tre contract�es pour le pro cessus de la transaction. Contactez- moi et je vous donnerez l�image compl�te de ce projet. Dans l�attente de votre r�ponse rapide, veuillez agr�er l�__expression de mes sentiments les plus sincers. Ola ABIOLA

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Pascal Line in Haskell
by Job L. Napitupulu 22 Aug '03

22 Aug '03

Can anyone help me how to make a function which takes an integer n > 0 and returns the list of integers in Line of Pascal's Triangle. For examples, pascalLine 4 -> [1,4,6,4,1] pascalLine 7 -> [1,7,21,35,35,21,7,1]

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Re: No safety in numbers
by Jon Cast 21 Aug '03

21 Aug '03

Konrad Hinsen <hinsen(a)cnrs-orleans.fr> wrote: > On Thursday 21 August 2003 22:42, Jon Cast wrote: > > > > (or any other type I declare it to be). > > > > Right. But: you can declare it to have type Fractional alpha => > > alpha, which is the same type the constant has in the middle of an > > expression. > > I can make such a declaration, but it still gets converted to Double. How are you doing this? I'm not seeing the behavior you describe. > What I can do is turn my constant table into a function: > > constant :: Fractional a =3D> String -> a > constant "k_B" = 0.0083144708636327096 > > and have the return values be of type Fractional a =3D> a. But I > suppose there is some runtime cost associated with such a function > call, so I would rather avoid it. Jon Cast

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RE: Database interface
by Tim Docker 21 Aug '03

21 Aug '03

Tom Pledger wrote: > How about introducing a Cursor abstract data type? > > doquery :: Process -> String -> b -> (Cursor -> b -> IO (b, Bool)) > -> IO b > stringv :: Cursor -> CInt -> IO String > doublev :: Cursor -> CInt -> IO Double > intv :: Cursor -> CInt -> IO Int > > This achieves the restriction you're after, because doquery is the > only exported producer of Cursor, and stringv etc. are the only > exported consumers of Cursor. I like this. Not sure about whether I'd call it Cursor or just Row. Code something like results <- doquery dbconn sqltext [] \row results -> do name <- colv row 1 address <- colv row 2 return (name,address):results seems quite tidy. > It also has the benefit that the function you pass to doquery can make > other calls to doquery, without mucking up the 'current row' state. > There would be one current row per Cursor, not one per Process. Is it normal or common to support multiple simultaneous queries on a single DB connection? Tim

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Re: Monads and Maybe
by Jon Cast 21 Aug '03

21 Aug '03

> tor 2003-08-21 klockan 22.26 skrev Jon Cast: > > Yes. Many complicated proposals have been made, but there's a > > straightforward, general mechanism: > > > > > addMaybe :: Num alpha => Maybe alpha -> Maybe alpha -> Maybe alpha > > > addMaybe a b = a >>= \x -> > > > b >>= \y -> > > > return (x + y) > > > > or > > > > > addMaybe a b = do > > > x <- a > > > y <- b > > > return (x + y) > > or > > addMaybe = Monad.liftM2 (+) > > I personally use those monadic lifting functions a lot. Monad.sequence > combined with list comprehension is another favorite. So do I. But that style doesn't work as well with functions that are already in the monad. So, I mentioned the more general style. Jon Cast

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Re: No safety in numbers
by Jon Cast 21 Aug '03

21 Aug '03

Konrad Hinsen <hinsen(a)cnrs-orleans.fr> wrote: > I am trying to write a larger piece of code using only type > constraints for all the numbers, not specific types, in order to be > able to choose the precision as late as possible. Good for you! (I say this in all seriousness.) > This works rather well (something I can't say of many other > languages), but one problem I keep running into is constants. > > Something I need frequently is, for example, the Boltzman constant, > 0.0083144708636327096 in my unit system. I can certainly type > 0.0083144708636327096 everywhere in the code and make things work, > literals have the nice property of being overloaded. But for the sake > of readibility, I prefer to give this beast a name and have the > explicit value only once in my code. So I create a module "Constants" > with something like > > k_B = 0.0083144708636327096 > > The trouble is that k_B then becomes "Double" by default Right. The good old monomorphism rule, I assume. Query: does this rule ever make a programmer's job easier? Beats me. > (or any other type I declare it to be). Right. But: you can declare it to have type Fractional alpha => alpha, which is the same type the constant has in the middle of an expression. <snip> Jon Cast

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Re: Monads and Maybe
by Jon Cast 21 Aug '03

21 Aug '03

Konrad Hinsen <hinsen(a)cnrs-orleans.fr> wrote: > I have been following the recent "Monad tutorial" discussion with > interest, and even read the tutorial, which is a useful addition to > the existing Haskell documentation. So useful in fact that it raises a > question... > > The whole monad mechanism seems to geared towards functions of one > argument, plus eventually state, that get chained together. How about > functions with several arguments? > > As an example, I'll use the Maybe monad. Suppose I want to write code > to handle experimental data, in which there might be missing values. I > might then decide to represent measurements by data of type "Maybe > Double", with missing values represented by "Nothing". I could then go > on to define functions on missing values, which would return "Nothing" > when their argument is "Nothing", and I could string these functions > together via the monad mechanism. Fine. But how would I handle > e.g. addition of two such values? The result should be "Nothing" when > either of its arguments is "Nothing". Is there any mechanism to handle > that? Yes. Many complicated proposals have been made, but there's a straightforward, general mechanism: > addMaybe :: Num alpha => Maybe alpha -> Maybe alpha -> Maybe alpha > addMaybe a b = a >>= \x -> > b >>= \y -> > return (x + y) or > addMaybe a b = do > x <- a > y <- b > return (x + y) (Incidentally, monads in Haskell are specified slightly differently than the Categorical version to enable precisely this sort of pattern.) Jon Cast

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