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Homework
by thomas_bevan＠aardvark.net.au 26 Aug '03

26 Aug '03

Seeing as its thst time of year again and everyone is posting their homework, has anyone got any good puzzles to do? I wouldn't mind having a go at something a bit tricky. Tom

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Re: Yet Another Monad Tutorial
by blaat blaat 26 Aug '03

26 Aug '03

To many questions, not enough mail. First, thanks for all your replies. Second, I stand totally corrected on the fact that we cannot break down monads. Functions of type m a->b are called impure, see also unsafePerformIO. I am questioning (a) the exact relation between monads, monadic IO, IO in general and (b) whether the impure restriction is actually a warranted restriction, which relates to, what I find, strange evaluation behavior of the IO monad. What is the difference between putStr "a", (putStr "a", putStr "a"), putStr (putStr "a"), putStr (show (putStr "a"))? At the moment objects of type IO a _only_ get evaluted when they are the _sole_ result of a Haskell program. What kind of strange dichotomy is that? Is not the _only_ interpretation of this dichotomy that objects of type IO describe impure programs (composed of actions which are only evaluated when...)? And, if so, why can't we decompose these programs (before they get evaluated)? And, if so, is not the only relation between a monad and doing functional IO that there is a monadic manner to construct descriptions of impure programs? If so, are there also non-monadic ways which might be equally interesting? As stated by Keith, deriving show for IO is possible. In such a setting putStr (putStr "a") I imagine returns "putStr \"a\"", which I find far more orthogonal than what we have now. (Have to find the reference still). Does this boil down to an impure or a pure decomposition? >From: Derek Elkins <ddarius(a)hotpop.com> >To: "blaat blaat" <l4t3r(a)hotmail.com>, haskell-cafe(a)haskell.org >pragmatic short one. > > > What did we lose? Well, introspection comes to mind. A monad is > > strange in the sense that, actually, there does not seem to be any > > reason to compute its value since we cannot possibly get anything out > > of it. In case of IO, only the haskell interpreter can. And, > > introspection is a nice thing to have, especially for compiler > > writers. Although I think the benefits in Haskell would be limited, an > > application might be that I could write my own promotion functions on > > my own programs. > >Lot's of issues with this paragraph, but here are the questions. What >do you mean by "promotion function"? Example of promotion: map f . map g = map (f . g) A promotion function would transform objects of type IO a into objects of type IO a while performing promotions. How would you write it in your >hypothetical Haskell? How is it related to IO? How do monads and/or the >IO monad in particular cause problems? Why couldn't the same be done >using monads or why would it be significantly harder if it can be done? In a monadic approach, I imagine functions like "unbind" which now would break purity. [..] > > I believe that central in the > > IO-issue is not the fact that we are using monads for IO - but that we > > can purely create impure programs which do the IO for us ;-), and that > > those do not have to be of the monadic kind. I still stand here (but am totally willing to be corrected on this account too). Hmpf, maybe someone more enlightened should write a paper on it. Cheers, l4t3r _________________________________________________________________ MSN 8 with e-mail virus protection service: 2 months FREE* http://join.msn.com/?page=features/virus

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AW: Haskell indentation
by Markus.Schnell＠infineon.com 26 Aug '03

26 Aug '03

If your code goes out to far right, it's a good idea to make a function out of it. > main = do > args <- getArgs > case args of > (fname:_) -> normalOp > _ -> helpScreen > > normalOp = do > ... > > helpScreen = putStrLn helpStr Code should be formatted in a way easy to scan and overview. You have to SEE the structure of the code. That depends very much on what your program does. No rules of thumb. Experience and experiments will do. (That was taken *directly* out of my convoluted brain and may be totally dumb.) Markus -- Markus Schnell Infineon Technologies AG, CPR ET Tel +49 (89) 234-20875 > -----Ursprüngliche Nachricht----- > Von: Per Larsson [mailto:per@L4i.se] > Gesendet: Dienstag, 26. August 2003 14:01 > An: haskell-cafe(a)haskell.org > Betreff: Haskell indentation > > > Hi, > > I have problems finding a pleasing indentation style for haskell code. > Especially nested do-blocks have a tendency to run away to > the right margin. > When looking on source code from experienced haskell > programmers, there > seems not to be any consensus at all, everyone uses their own > convention > and in many cases one changes style in the same module. > > Also, the automatic tools are problematic: the emacs mode I'm using > bails out in certain contexts and there are few user > customizations available. > The haskell-src module in the GHC library offers a parser > and pretty-printer for haskell code with nice options for customizing > the indentation, but it can't handle comments which is a problem if > you want to use it as a basis for implementing a indentation tool. > > Is there anyone who have given this some thought and have some > suggestions for a consistent indentation style and/or desktop tools > which I'm not aware of? > > Per > > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe >

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Re: Homework
by oleg＠pobox.com 26 Aug '03

26 Aug '03

> Here's one: figure out what the following does :-) > puzzle = (!!) $ map (1:) $ iterate (s (lzw (+)) (1:)) [] where > s f g x = f x (g x) > lzw op xs [] = xs > lzw op [] ys = ys > lzw op (x:xs) (y:ys) = op x y : lzw op xs ys Can be written simpler puzzle = (!!) $ iterate (s (lzw (+)) (0:)) [1] where s f g x = f x (g x) lzw op [] ys = ys lzw op (x:xs) (y:ys) = op x y : lzw op xs ys Incidentally, a small change gives a different series: puzzle1 = (!!) $ iterate (s ((lzw (+)).(0:)) (1:)) [] where s f g x = f x (g x) lzw op [] ys = ys lzw op (x:xs) (y:ys) = op x y : lzw op xs ys can you tell which? Without loading into GHCi? Finally, how can we possibly live without the following: puzzle2 = (!!) $ iterate (s ((lzw (+)).(1:).(0:)) (0:)) [1,1] where s f g x = f x (g x) lzw op xs [] = [] lzw op (x:xs) (y:ys) = op x y : lzw op xs ys

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IO Bool -> Bool
by Tn X-10n 24 Aug '03

24 Aug '03

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recursion
by Tn X-10n 23 Aug '03

23 Aug '03

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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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