- Haskell-Cafe - Haskell.org

Haskell 6.0 install
by Mark Espinoza 01 Sep '03

01 Sep '03

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Re: Haskell for non-Haskell's sake
by Nick Name 30 Aug '03

30 Aug '03

Alle 13:33, sabato 30 agosto 2003, Alastair Reid ha scritto: > - Knit (http://www.cs.utah.edu/flux/knit/) > > A component language for C Are there component languages wich work on multilanguage projects? Something like "my project has a module written in haskell, one in C and one in Ocaml, and I don't want an haskell 'list a' to pass through C nor corba to get into Ocaml". Is there any hope in this sense? V.

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Type classes question
by Eugene Nonko 30 Aug '03

30 Aug '03

Hello, I am trying to define class Dual and few instances as follows: === class Dual a where dual :: a -> a instance Dual Bool where dual = not instance (Dual a, Dual b) => Dual (a -> b) where dual f = dual . f . dual instance Dual a => Dual [a] where dual = reverse . map dual instance Num a => Dual a where dual = negate === For some reason Hugs does not lake the last definition saying 'ERROR dual.hs:13 - syntax error in instance head (constructor expected)'. What am I doing wrong? -- Eugene

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Monads and Maybe
by Konrad Hinsen 29 Aug '03

29 Aug '03

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? Konrad.

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Debugging
by Konrad Hinsen 29 Aug '03

29 Aug '03

My Haskell experiments have reached a size in which debugging tools would be more than welcome, so I looked around, and was very disappointed. I tried Hood, which is a pain to use (lots of editing of the code required), I looked at Buddha but didn't want to downgrade to GHC 5 for trying it (nor is my code Haskell 98, because of multi-parameter classes), and all that seems left to try is Hat, whose Web site I can't reach at the moment. So what are you Haskell programmers using for debugging in real life? Konrad.

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threaded balanced tree
by Lex Stein 28 Aug '03

28 Aug '03

Hello, Does anyone have a purely functional haskell implementation of a threaded, balanced tree? I modified Okasaki's SML red-black tree implementation, but my modifications mutate. I would be very interested in seeing an approach that doesn't use mutability. Thanks Lex -- Lex Stein http://www.eecs.harvard.edu/~stein/ stein(a)eecs.harvard.edu cell: 617-233-0246

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RE: New to Haskell
by Hal Daume 28 Aug '03

28 Aug '03

the first line says that 'fname' is a function which takes a list and returns a string. the list is of type '[([Char],a)]'. this means that it's a list of ([Char],a)s. these are pairs of [Char]s and as. a [Char] is a string (a string is a list of characters) and an a is a something -- anything you want. the second line tells you the value of fname when its argument list is empty ([]). the result in this case is the string "Result_Blank". the third line tells you what to do when the input is not empty. when you have a non-empty list, it's composed of a head and a tail, written (head:tail). since our list is of pairs, the head has the form (a,b). putting this together, we have that the list is of the form ((x,xs):ys). here, x is our a, xs is our b and ys is the tail. (x,xs) is our head. it then says that the result of this is just the value 'x'. this is simply the [Char] part (the first half) of the first pair in the list. HTH - Hal -- Hal Daume III | hdaume(a)isi.edu "Arrest this man, he talks in maths." | www.isi.edu/~hdaume > -----Original Message----- > From: haskell-cafe-admin(a)haskell.org > [mailto:haskell-cafe-admin@haskell.org] On Behalf Of Matthew Morvant > Sent: Thursday, August 28, 2003 8:44 AM > To: 'Haskell (haskell-cafe(a)haskell.org)' > Subject: New to Haskell > > > Hello, > I am very new to haskell. I was hoping someone could > explain something > to me > > fname :: [([Char],a)] -> String > fname [] = "Result_Blank" > fname ((x,xs):ys) = x > > What exactly [use little words :-)] does this do? > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe >

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New to Haskell
by Matthew Morvant 28 Aug '03

28 Aug '03

Hello, I am very new to haskell. I was hoping someone could explain something to me fname :: [([Char],a)] -> String fname [] = "Result_Blank" fname ((x,xs):ys) = x What exactly [use little words :-)] does this do?

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AW: Poll: How to respond to homework questions
by Markus.Schnell＠infineon.com 28 Aug '03

28 Aug '03

OK: D E Not OK: A B C I have never seen anybody asking more than one "do my homework" question. I would encourage students to give it a try, even if it's not working, and give some advice on that. Pointing to a web page like "do your homework yourself - it will benefit you most that way" is a very good idea. Markus -- Markus Schnell > -----Ursprüngliche Nachricht----- > Von: Tom Pledger [mailto:Tom.Pledger@peace.com] > Gesendet: Mittwoch, 27. August 2003 23:43 > An: haskell-cafe(a)haskell.org > Betreff: Poll: How to respond to homework questions > > > Hi. > > I'm curious about what the people on this list consider appropriate, > as responses to homework questions. Even if there isn't a consensus, > it may be interesting to see how opinion is divided. > > Please consider the following. > > (A) Give a perfect answer. > (B) Give a subtly flawed answer. > (C) Give an obfuscated answer. > (D) Give a critique of what the questioner has tried so far. > (E) Give relevant general advice without answering the > specific question. > > I mean them to include giving references, e.g. a Wiki URL to relevant > general advice would count as (E). > > Which of those do you approve of? Which do you disapprove of? > > Please email me a concise vote, e.g. > > OK: B D E > Not OK: A > > and I'll post a collated result next week. > > - Tom > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe >

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Re: Debugging
by Markus.Schnell＠infineon.com 28 Aug '03

28 Aug '03

> Me too, of course. But it isn't always easy to test a function in isolation. > If it takes complex data structures as input, then the only reasonable way to > provide that input may be calling it from another piece of the code. Yes. Therefore the best thing is to codesign program and tests in parallel. Make your code testable from the beginning. That will lead to a more flexible design also. (In a perfect world I would follow this advice myself... ;-)) Cheers, Markus

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RE: Haskell indentation
by Hal Daume 27 Aug '03

27 Aug '03

Just my 2 cents... > The first is that I always put the where for local function > definitions > on a new line, e.g. > foo x = x + a + b > where a = 1 + x > b = 2 * x > I find this a lot clearer and prettier than, > foo x = x + a + b where > a = 1 + x > b = 2 * x I used to do it your style, but have switched to: > foo x = x + a + b > where > a = 1 + x > b = 2 * x because it makes cut&paste easier. instance/class declarations for me also don't obey this. > data AST > = Const Int > | Var String > | Lam String AST > | App AST AST > | Let String AST AST > | Add AST AST same here. or for records, I like: > data Rec = > Rec { > foo :: Int, > bar, > bluff :: String > } > Another difficult case that I haven't solidified a style for is large > type signatures. I tend to do something like: > myfoo :: (Context1, Context2, Context3) => > type1 -> -- maybe a description of type1 > type2 -> > type3 -> > result type it takes a lot more vertical space, but you also have easy haddockability and it makes it a lot easier to remember what all those parameters are... - hal

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ghc install redhat 7.3
by Mark Espinoza 27 Aug '03

27 Aug '03

Hello, I have been trying to install ghc on my redhat 7.3 system. I have about a 1.2 ghz processor. I got ghc-5.04.2 installed from an rpm. The ./configure for ghc-6.0.1 seemed to work fine. When I ran make it went for about an hour and a half, at which time it seemed to have been hung up for 15 or 20 minutes. I had to do other things so I stopped it and gave up for the day. Is it worth starting it before I go to bed, and see if it works by morning? What could I do to try to troubleshoot it? Is there an rpm for ghc-6.0.1? Thanks for your help. Sincerely, Mark Get your free e-mail address at http://felinemail.zzn.com __________________ http://www.zzn.com

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Haskell indentation
by Per Larsson 27 Aug '03

27 Aug '03

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

3 2

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

6 5

recursion
by Tn X-10n 24 Aug '03

24 Aug '03

6 5

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.

2 1

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

3 3

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 22 Aug '03

22 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 22 Aug '03

22 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 22 Aug '03

22 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 22 Aug '03

22 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 22 Aug '03

22 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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sequencing data structures
by Martin Norbäck 21 Aug '03

21 Aug '03

I want to sequence data structures in an efficient manner, to store them to files and to send them over the network. Simply deriving Show and Read is not very good, because it's space inefficient and read cannot give any output until the whole data structure is parsed. So I thought I should store them in some space efficient format. First problem: how to make them derivable (so that I don't have to write boilerplate class instances for all my data structures). I read the "derivable type classes" paper, but it's not implemented in ghc (only Unit and :+: and :*: are, which is not enough). So how to go about it? Using DrIFT? Template Haskell? Second problem: how should such a format look like? Ideally, I want to be able to write an infinite data structure (at least one containing loops). If that is not possible I want to be able to read as lazily as possible, this means traversing the data structure in breadth first order, so that a cons form can be reached quickly. Does anyone have any thoughts/pointers on this subject? It would surprise me if nobody has had this problem before. Regards, Martin -- Martin Norbäck d95mback(a)dtek.chalmers.se Kapplandsgatan 40 +46 (0)708 26 33 60 S-414 78 GÖTEBORG http://www.dtek.chalmers.se/~d95mback/ SWEDEN OpenPGP ID: 3FA8580B

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RE: Database interface
by Bayley, Alistair 21 Aug '03

21 Aug '03

> From: Tim Docker [mailto:timd@macquarie.com.au] > > > 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? Depends on what you mean by "multiple simultaneous queries". A single query is usually synchronous w.r.t. returning a cursor i.e. you issue the query and get back a cursor in an atomic operation, and then you can do other things while you fetch rows from the cursor. If you need to have two SQL statements running at the same time (two big inserts or updates, say) then you usually need two sessions. It probably depends on the API, but some (Oracle, I believe) will allow you to open multiple sessions with the same connection. In a single session you could open a number of cursors (in any order), interleave fetches (in any order), and then close the cursors (in any order). So you could have: - a number of Connections (perhaps to different servers/databases), where - each Connection has one or more Sessions, and - each Session has zero or more Cursors (and one current Transaction). My session model and terminology are probably quite Oracle specific, but I hope it's reasonably clear. ***************************************************************** 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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RE: Database interface
by Tim Docker 21 Aug '03

21 Aug '03

Tom Pledger writes: > By the way, how does the a in a -> b -> IO (b, Bool) work? > It looks like it has something to do with the current row. Does > doquery have to accommodate any possible substitution for a ? I fired this off without thinking about it too much, and looking at the prelude type signatures for fold. The list being folded over is implied by the DB query, is accessible through the IO monad. Hence a parameter is not required. It would really be: doquery :: Process -> String -> b -> (b -> IO (b,Bool)) -> IO b > I don't have a preference, but offer this view of the options: > > With an exception, "Stop, and return the last b you saw." > With a boolean, "Stop, and return this b." I think I like the behavior where, when the bool in the tuple is true, the b in the tuple is immediately returned from the query. Exceptions would be propagated to the caller of doquery without modification (but with appropriate cleanups). One thing that I am unsure about is whether the column value accessing functions that I specified before stringv :: Process -> CInt -> IO String doublev :: Process -> CInt -> IO Double intv :: Process -> CInt -> IO Int should return actions in the IO monad as above, or instead should be in some other DBQuery monad, that trivially extends the IO monad, but is only valid inside the doquery call. This would have the benefit of restricting the column access functions to inside a query via the type system. I'd also probably use a typeclass to specify a single colv function. ie: class DBCol a where colv :: DBQuery a instance DBCol String where... instance DBCol Double where... doquery :: Process -> String -> b -> (b -> DBQuery (b,Bool)) -> IO b Any comments? Tim

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Re: "let" vs. "where"
by Hamilton Richards 20 Aug '03

20 Aug '03

At 7:42 AM +0200 8/20/03, Jan Scheffczyk wrote: >Hi Andrew, > >> let x = expensiveComputation foo in x + x >> >> I would certainly hope that expensiveComputation wasn't called twice, >> and even though the language doesn't guarantee it, I have already >> written code that assumed it. > >I always thought that there is a tiny difference between "let" and "where": >Using "let" "expensiveComputation foo" might be computed twice (depending on >the compiler?). >But using: > > x + x > where x = expensiveComputation foo > >should compute the value for x only once. >Therefore, I always try to use "where" for common subexpressions. > >Please correct me if I'm wrong here. The reserved word "let" introduces an expression (a "let-expression"), but there's no such thing as a "where-expression". The reserved word "where" can be used only in definitions. For example, 3 + ((x + x) where x = expensiveComputation foo) is invalid syntax, whereas 3 + let x = expensiveComputation foo in x + x is OK. And here's a use of "where" in a definition: let z = x + x where x = 2 in z*5 That could have been written using only "let": let z = let x = 2 in x + x where x = 2 in z*5 Watch out for the Hugs 98 command line-- it allows z*5 where z = x + x where x = 2 but that's not a truly valid expression, because you can't embed it in a larger expression: 2 + (z*5 where z = x + x where x = 2) is properly identified as a syntax error. Regards, --Ham -- ------------------------------------------------------------------ Hamilton Richards, PhD Department of Computer Sciences Senior Lecturer The University of Texas at Austin 512-471-9525 1 University Station C0500 Taylor Hall 5.138 Austin, Texas 78712-1188 ham(a)cs.utexas.edu hrichrds(a)swbell.net ------------------------------------------------------------------

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More type design questions
by Konrad Hinsen 20 Aug '03

20 Aug '03

Continuing in my quest to understand type design in Haskell, here's another episode that leaves me scratching my head: module Foo where class Vect v a where (<+>) :: Floating a => v a -> v a -> v a data Vector a = Vector a a a instance Vect Vector a where (<+>) (Vector x1 y1 z1) (Vector x2 y2 z2) = Vector (x1+x2) (y1+y2) (z1+z2) instance Vect [Vector a] a where (<+>) l1 l2 = zipWith (<+>) l1 l2 The problem is the last instance declaration. Hugs says "Illegal type in class constraint", which is not very explicit (which class constraint?). GHCI is a bit more verbose: Kind error: Expecting kind `* -> *', but `[Vector a]' has kind `*' When checking kinds in `Vect [Vector a] a' In the instance declaration for `Vect [Vector a] a' I have vague memories of seeing mentioned the concept of "kind", but I can't remember where. Any help would be appreciated! Konrad.

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Tuples or Record
by Frederic BELLOC 19 Aug '03

19 Aug '03

Hello, When I have started my project, I use a Tuples but i would know if it is possible to create a record such C or Ocaml provide. I mean creating a structure where variables are accessible by a '.' or something like that. -- BELLOC Frederic EPITA 2005 STUDENT C, socks and sun !

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New Haskell and FPO merchandise available
by K. Fritz Ruehr 18 Aug '03

18 Aug '03

Newly-designed Haskell, FP and related merchandise (mostly T-shirts) are now available at the URLs below. I didn't receive many specific requests on logo/product combinations from my last post, so I have tried to pick and choose as best I can (I've included at least men's and women's white T-shirts, in two styles for the women, for all the new designs I used). It may be difficult to get items in time for ICFP in Uppsala, for which I apologize. On the other hand, the actual physical products are as yet untested (i.e., for print quality), so perhaps it is just as well if I manage to get some samples to wear to the conference: that way you may have a chance to see them "live" before you buy. Note that all items are provided at cost, i.e., I get no profit, although CafePress does. Almost all shirts have extra material on the back side: it turns out that no premium is charged for this, so I tried to give the best value for the money (exceptions include the "new logo" baby doll shirt, the "classy" logo golf shirt, all of the "dysfunctional programming" shirts and the Generic Haskell shirts; Pancito back-side art should be up soon). Certainly if there are any logos from the original set you would like turned into a product, or any other variations, let me know by return mail (see <http://www.willamette.edu/~fruehr/logos/intro.html>). -- Fritz Ruehr PS: Thanks to all who made such encouraging comments on the designs, and also to Andrew Cooke for the Pancito artwork. ---------------------- * Original haskell.org logo (Pan image on back): <http://www.cafeshops.com/haskell> * New Haskell logo (words on front, circle back): <http://www.cafeshops.com/haskellnewlogo> * Haskell "purity" logo (w/ "Pure Fun" on back): <http://www.cafeshops.com/purehaskell> * Alternate "classy" logo (feature list on back): <http://www.cafeshops.com/classyhaskell> * Dysfunctional programming (nothing on back): <http://www.cafeshops.com/dysfunc> * Strong, static types (typing judgement on back): <http://www.cafeshops.com/statictypes> * HOT [higher-order typed] (lambda on back): <http://www.cafeshops.com/highordertyped> * Do it in a Monad (monad laws on back): <http://www.cafeshops.com/doitmonad> * Generic Haskell logo (no back as yet): <http://www.cafeshops.com/generichaskell> * Pancito "bugs" artwork (no back *yet*): <http://www.cafeshops.com/pancito>

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Updated monad tutorial
by Jeff Newbern 18 Aug '03

18 Aug '03

Hello, I would like to thank everyone for their feedback on the monad tutorial I posted earlier. Based on that feedback, I have expanded and corrected the tutorial at www.nomaware.com/monads/html/index.html The primary changes are: - vastly expanded section on monad transformers - additional examples of monad transformers, including multiple transformers in a stack - added examples for liftM, guard, ap and msum - consistent use of "action" in IO monad "computation" in all others - made language more precise when discussing the IO type constructor - fixed typos, cleaned up some code in examples I am sufficiently pleased with this version to call it 1.0, but I still welcome any and all feedback. Thanks, Jeff Newbern -- Jeff Newbern <jnewbern(a)nomaware.com> Nomaware, Inc.

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Long Time
by Andrew Williams 16 Aug '03

16 Aug '03

Dear Sir, I know you may be suprised to receive this mail, especially as we have not met in person before but Please do not be embarrassed for I mean good. I was opportuned to have your contact from the South Africa Information Exchange (S.A.I.E.), in my search for a reliable person to handle a very confidential transaction which involve a transfer of fund and its investment in your country. My name is ANDREW WILLIAMS, a South African expatriate staff of one of the reputable financial institutions in Zurich, Switzerland. We act as financial advisors and on special arrangements, accept Deposits of valuables from our customers for safekeeping. Few months ago, we received information about the sudden death of one of our prominent customers from Angola in Africa. Our enquiry at the Embassy confirmed that he died with the leader of UNITA, Dr. Jonas Savimbi in an ambush laid by the enemy Government forces. The issue now is that the customer has a Deposit of Fifty Million US Dollars ( USD50M) in our custody, which he declared as a consignment of GOLD & DIAMONDS. I am the only person with the accurate information about the Deposit because I acted as his adviser and proxy when he was alive. I knew the money actually came from Diamond Mines in Angola where he was the Chairman of the States' Mining Corporation during the Government of now late Dr. Savimbi. It may also interest you to know that, for obvious reasons, the man did not provide any next-of-kin at the time of making the Deposit, and more so, he lodged the entire deposit under the name of LONG TIME INVESTMENTS LTD, with a non-existing address. The Deposit is now due for clearing and all efforts by our company to contact any of his relatives through the address he provided proved futile. By the regulations of the company, if more 30 days passes without any person showing up claiming to be the next of kin or relative of the customer, the entire Deposit will be recovered and if found to be cash, the money will be given away to Charities or secretly be converted into trading in favor of the company, and can never be accounted for again. With my position as his proxy, and the knowledge I have about the Deposit, I can successfully organize the claim of the Deposit without any problem provided I have a credible and capable person to receive the Deposit. I cannot function in this regard because of my status in the company, therefore, I am proposing to you to receive the money in your care for our mutual personal use. Being an insider, I know what to do to pull the deal through without any problem whatsoever. The Deposit is being secured in our central Warehouse in Europe and all I need from you now are your PARTICULARS, including, TELEPHONE and FAX NUMBERS to set up the process required to release the Deposit to you. Once that is done, the Deposit will be released to you through a friendly Security/Safe Deposit Company in Europe where you have to move in to collect . I will complete the Deposit releasing process within 2-3 days and furnish you with particulars and contact person of the security company in Europe so that you can arrage for the clearing and collection. My dear Friend, no doubt, this is an opportunity for us to excel in life. Therefore, if you are interested kindly inform me by email or you can phone at +41-795-328250, and please do not forget to furnish your particulars including, Address, Telephone and Fax numbers to enable me commence the operation immediately. I am willing to share 40% of the total funds to you, 10% would be given out to Charities while 50% would remain for me. I am interested to invest my share of the money in Real Estates, Capital & Money markets in your country. With your support and co-operation, this transaction is set to be completed within one week. Be rest assured that the business is 100% risk-free, safe and secure but for security reasons however, you are advised to treat it with all privacies and strictest of confidentiality. Best regards, ANDREW WILLIAMS.

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PLS REPLY
by alisesay＠phantomemail.com 15 Aug '03

15 Aug '03

Dear Sir This letter may come to you as a surprise because we have not met. The decision to write you is out of a sincere desire to establish a business/mutual relationship with you. I got your contact through the international trade and commerce chamber (ITCC) in Madrid. My name is Mr. Ali Sesay Karimu, the son of John Karimu (the former Secretary of state for finance) who was killed and mutilated by the military junta led by Major Paul Koroma after over-throwing the elected government of President Tijan Kabba. Though, I do not know to what extent you are familiar with events and disturbances in Sierra-Leone but it has consistent features in the CNN, BBC news bulletins. The pressure of war drove the rest of my family out of Sierra-Leone into exile in Spain where we have been living under political asylum for the past 3 years. Sadly, my mother died of cancer 3 months ago and was buried in Madrid. Prior to her death, she gave me a certificate for a secret deposit that my father made in a security company in Europe (destination will be disclosed later for security reason). The deposit which is worth $32,000.000 (Thirty-two Million U.S. Dollars) was money set apart for government propaganda against the rebel forces. He made the deposit in his name with the hope of converting it to his personal use at the end of the war but was killed as a result of his opposition to the rebel forces when the conflict intensified. I have contacted the Security Company to confirm the deposit and establish ownership. The money was however deposited as family treasure. Due to the death of my mother and the continued hostility in Sierra-Leone, I have decided to solicit the assistance of an honest and trustworthy person or company that will assist in investing the money. I cannot do it alone due to my present political status and total ignorance of the business world. You will be given a negotiable percentage of the money at the end of the transaction. If you are honest, trustworthy and interested in the above proposal, contact me immediately through the number/email address indicated above for more details. You must maintain absolute confidentiality to ensure success. Please, indicate your personal Tel./Fax nos. when replying. Regards, Ali Sesay Karimu.

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Re: Yet Another Monad Tutorial
by Bernard James POPE 15 Aug '03

15 Aug '03

Hi all, I've been following this discussion with some interest. Last time I looked, hugs, ghc and nhc98 all implemented I/O differently. No doubt there are even more alternatives in other Haskell implementations (real or imaginary). In itself this is probably not too surprising. They are, afterall, different systems with different goals etc. However, I have often wondered what it is that constitutes a correct implementation of the I/O system in Haskell. I might be wrong, but I don't think the Haskell Report helps much in this regard. The only source of semantics for Haskell's I/O that I know of is "Tackling the Awkward Squad" by Simon Peyton Jones. If there are others I'd love to have them pointed out. However, this paper deals with many extensions that are not in Haskell 98. I'm not sure if there is enough in it to decide if a given implementation is correct (I use the word "correct" loosely). I would really love to see a discussion of the design space for I/O. Including what the current compilers use, what other options there might be, and even what other languages use (like Clean's and Mercury's uniqueness systems). If I were to build a Haskell compiler tomorrow this is one thing I would like to know - what do I have to do to ensure that my implementation of I/O is correct and what design options do I have? No doubt the I/O systems of current compilers have grown with those systems, but from the perspective of someone who is outside of the implementations it would be interesting to know why things are done the way they are, and what other options were considered, rejected, etc. For example, the state-of-the-world threading approach might be chosen because it is economical with respect to implementation cost in an existing compiler (such as GHC). But if we were to write a compiler from scratch would/should we follow the same route? Also, the "Keep the monad right through" method that Simon mentions in the "awkward squad" paper is interesting, but I wonder if that is too hard to implement in Haskell due to extensive overloading of the monad operators. I'll stop rambling now. Cheers, Bernie.

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RE: IO Bool -> Bool
by Kevin S. Millikin 15 Aug '03

15 Aug '03

On Wednesday, August 13, 2003 11:20 PM, Tn X-10n [SMTP:kawairashii@hotmail.com] wrote: > is it possible to convert IO Bool to Bool? Sure. Which Bool do you want? True? > toTrue :: IO Bool -> Bool > toTrue x = True Or False? > toFalse :: IO Bool -> Bool > toFalse x = False Maybe that's not what you had in mind.

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RE: Database interface
by Tim Docker 15 Aug '03

15 Aug '03

oleg(a)pobox.com wrote: > If I may interject, that's precisely how a Scheme DB interface is > designed. The main function is a left-fold. Not quite though: it > provides for a premature termination: > > A major procedure: DB1:fold-left PROC INITIAL-SEED QUERY-OBJECT Premature termination sounds like a useful property. I can see two ways this could be done: keep the previous signature, and use an exception to exit early, or add a boolean return value like the scheme version: doquery :: Process -> String -> (a -> b -> IO (b,Bool)) -> b -> IO b Any opinions on which alternative wouuld be preferable? Tim

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Re: Yet Another Monad Tutorial
by Jeff Newbern 14 Aug '03

14 Aug '03

Alistair, Thanks for your message regarding the debate about monads and purity. What do you think about these additions as a discussion of the issue? Does this address everything you think is needed? Is there a clearer way to explain it? Thanks, Jeff In the section "No Way Out": ---------- The IO monad is a familiar example of a one-way monad in Haskell. Because you can't escape from the IO monad, it is impossible to write a function that does a computation in the IO monad but returns a non-monadic value. Not only are functions of the type IO a -> a impossible to create, but any function whose result type does not contain the IO type constructor is guaranteed not to use the IO monad. Other monads, such as List and Maybe, do allow values out of the monad. So it is possible to write non-monadic functions that internally do computations in those monads. The one-way nature of the IO monad also has consequences when combining monads, a topic that is discussed in part III. ---------- and a little farther down: ---------- Some people argue that using monads to introduce non-pure features into Haskell disqualifies it from claiming to be a pure functional language. This subtle question — not particularly relevant to the practical programmer — is revisited in the context of the I/O monad later in the tutorial. ---------- Later, in the section on the I/O monad: ---------- In Haskell, the top-level main function must have type IO (), so that programs are typically structured at the top level as an imperative-style sequence of I/O actions and calls to functional-style code. Revisiting the debate about the purity of Haskell (in a functional sense), it is important to note that the IO monad only simulates imperative-style I/O. The functions exported from the IO module do not perform I/O themselves. They return I/O actions, which describe an I/O operation to be performed. The I/O actions are combined within the IO monad (in a purely functional manner) to create more complex I/O actions, resulting in the final I/O action that is the main value of the program. The result of the Haskell compiler is an executable function incorporating the main I/O action. Executing the program "applies" this ultimate I/O action to the outside world to produce a new state of the world. This occurs only once per execution of the program, and since the state of the world changes for each execution, the issue of purity is neatly side-stepped. ---------- -- Jeff Newbern <jnewbern(a)nomaware.com> Nomaware, Inc.

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RE: Database interface
by Bayley, Alistair 14 Aug '03

14 Aug '03

> -----Original Message----- > From: Thomas L. Bevan [mailto:thomas_bevan@toll.com.au] > > This type of interface is fine as far as it goes, but there is only > ever a loose coupling between the database and application. > > The HaskellDB work implemented a relational calculus that ensured > that all queries were checked at compile time for validity against > a particular database. HaskellDB doesn't actually type check against the database; it checks against types defined in a module, which is generated from the database. If you change the database, then you need to regenerate the module. I prefer the approach of passing an extraction function/IO action to the interface. This is the approach taken by HaSql, HaskellDB (I think, although it's so indirect I find it hard to follow), and Tim Docker's Sybase library, and it's the design I intend to use for my Oracle library (when I get around to it). I still want to use HaskellDB (or at least the relational calculus part of it), so I was thinking of splitting it into two pieces: a library that submits SQL queries and returns the results (database interface), and a library that constructs queries with the relational calculus and generates SQL from them. Obviously the database interface must be written for each DBMS product, while the relational calculus library ought to be independent of DBMS (although it might be wise to have SQL compatibility flags for the SQL it generates. For example: Oracle 8 (what I'm using now) only supports SQL-92). ***************************************************************** 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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RE: Database interface
by Bayley, Alistair 14 Aug '03

14 Aug '03

> From: Thomas L. Bevan [mailto:thomas_bevan@toll.com.au] > > Ideally, we would have the following. > > 1/ A high level combinator library for relational calculus built on, > > 2/ A standard low-level Haskell API > i.e. functions like Connection -> String -> ( a -> b -> IO b) -> IO b > > 3/ Database specific bindings. > > The problem is that projections and cartesian products > generate new types, at > least in the way it was done in HaskellDB. The form the > calculus takes would > need to be substantially reworked. I don't see why the generation of new types would cause a problem. Can't we get the calculus library to generate the extraction function at the same time, so that it returns rows of the appropriate type? The generated extraction function would have to be built from standard extraction functions exposed by the low-level API. Although, would this limit the set of types you could use to just those in the low-level API? ***************************************************************** 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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RE: Database interface
by Tim Docker 14 Aug '03

14 Aug '03

Tom Pledger writes: > | > doquery :: (DeepSeq v) => > | > Process -> String -> IO v -> IO [v] > | > | Can you explain what the constraint does here? > > Yes. It soothes my superstitious fears about unsafeInterleaveIO. > > I imagined that: > > - doquery (like getContents) uses unsafeInterleaveIO to make the > resulting list lazy, i.e. avoid grabbing all the rows at the > outset, Actually no. I'm not sure of the correct terminology here, but my implementation was strict wrt to reading from the database - all records are read during the IO action, and not secretly later on. Whilst I can see some benefits of reading the records lazily, wouldn't this result in problems with database connection management, locks, transactions, etc. From reading the list, it sounds like equivalent issues crop up pretty regularly with getContents. If you used the fold variant, doquery :: Process -> String -> (a -> b -> IO b) -> b -> IO b where the folding function returns an IO action, then you could process rows as you go in any case, without requiring unsafeInterleaveIO. Tim

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RE: Database interface
by oleg＠pobox.com 14 Aug '03

14 Aug '03

> I'll probably generalise the query function to do a fold, rathen than > always accumulate a list: > doquery :: Process -> String -> (a -> b -> IO b) -> b -> IO b If I may interject, that's precisely how a Scheme DB interface is designed. The main function is a left-fold. Not quite though: it provides for a premature termination: A major procedure: DB1:fold-left PROC INITIAL-SEED QUERY-OBJECT A QUERY-OBJECT (which in this implementation is a list of fragments that make a SQL statement, in the reverse order -- without the terminating semi-colon) is submitted to the database, using the default database connection. PROC is a procedure: SEED COL COL ... The procedure PROC takes 1+n arguments where n is the number of columns in the the table returned by the query. The procedure PROC must return two values: CONTINUE? NEW-SEED The query is executed, and the PROC is applied to each returned row in order. The first invocation of PROC receives INITIAL-SEED as its first argument. Each following invocation of PROC receives as the first argument the NEW-SEED result of the previous invocation of PROC. The CONTINUE? result of PROC is an early termination flag. If that flag is returned as #f, any further applications of PROC are skipped and DB1:fold-left finishes. The function DB1:fold-left returns NEW-SEED produced by the last invocation of PROC. If the query yielded no rows, DB1:fold-left returns the INITIAL-SEED. Thus DB1:fold-left is identical to the left fold over a sequence, modulo the early termination. There are a few minor variants of the above procedure, optimized for common particular cases: a query that is expected to return at most one row, and a query that expects to return at most one row with exactly one column. The latter query has the same interface as a lookup in a finite map. The QUERY-OBJECT is of coursed not built by hand. There is a domain-specific language (which greatly resembles the source language and makes some use of quasi-quotation) whose result is a query object. Therefore, I can write both the query and the handlers of the query result using the same syntax. The premature termination is important. Database connections and cursors are too precious resources to leave them for the garbage collector to finalize. More discussion and pointers can be found at http://srfi.schemers.org/srfi-44/mail-archive/msg00023.html The interface has been used in an industrial application.

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haskell newbie seeking for advice
by Jose A.Ortega Ruiz 14 Aug '03

14 Aug '03

hi all, as stated in the subject, i'm a newcomer to haskell programming: i've read some tutorials and (portions of) a couple of books and am really fascinated with the language. but my haskell coding experience is limited to toy programs and short exercises. so i decided to try my hand at a small project to really learn haskell and the functional programming mindset[1], and i would appreciate to hear your opinions and comments on some issues before i start coding: - build toolchain. i'm used to the autoconfig/automake/make tools for C/C++ projects, and to ant for Java stuff. what do haskellers use? my initial thought was using a plain Makefile with ghc, but there is also hmake and maybe other tools i am missing. what would you use? [2] - graphics toolkit. one of my programs will be a GUI. there seems to be a lot of choice here: gtk2hs looks nice (as used in hircules) and fudgets seems to have a pretty interesting and well-documented architecture (which covers also client/server programming: i'll be using a client/server architecture too) but its widgets are uglier and look alien in current linux desktops. i'd like to have a fudgets-gtk framework of sorts :) and then, there is htoolkit, Object I/O and whatnot... any recommendations? - TCP/IP stuff. i will have a client/server architecture with TCP/IP as the transport protocol. other than fudgets, i'm not aware of any haskell framework for IPC: am i missing something? - XML. i'll be handling XML documents, and HaXml seems to me an excellent candidate to cover this area, but, of course, i'm very interested in hearing of alternatives... thanks a lot for your help! cheers, jao Footnotes: [1] if that matters, the project will consist on a set of utilities for handling a document metadata database, based upon the OMF standard. [2] my project will consist of several executables, sharing a set of common libraries and, therefore, i plan to use (if possible) a non-flat directory layout. -- Come to think of it, there are already a million monkeys on a million typewriters, and Usenet is NOTHING like Shakespeare. - Blair Houghton.

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RE: Database interface
by Tim Docker 14 Aug '03

14 Aug '03

Tom Pledger writes: > This is a pretty good way to stop those nasty vague SQL row types at > the Haskell border and turn them into something respectable. Perhaps > it would even be worth constraining the extracted type to be in > DeepSeq > > doquery :: (DeepSeq v) => > Process -> String -> IO v -> IO [v] Can you explain what the constraint does here? Although I haven't touched the db interface in the meantime, when/if I do, I'll probably generalise the query function to do a fold, rathen than always accumulate a list: doquery :: Process -> String -> (a -> b -> IO b) -> b -> IO b Tim

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Fwd: Re: Yet Another Monad Tutorial
by blaat blaat 13 Aug '03

13 Aug '03

>From: Derek Elkins <ddarius(a)hotpop.com> >"blaat blaat" <l4t3r(a)hotmail.com> wrote: [...] > > However, given that observation, the fact that IO is a monad I find to > > be a _rather arbitrary design decision_; why not define a small term > > language which may be passed to the OS? > >That would be confusing to use or restrictive (unless done exceedingly >well perhaps), it would be awkward to define, and it would pretty much >require arbitrary design decisions (what should be included in this >language? what are it's semantics? etc). > > Why not have (some limited) > > introspection on programs which we build? > >It would be round-about in both implementation and use to do this. >Nevertheless, you can easily view the IO monad as an imperative language >-embedded- in Haskell. Even when writing mostly imperative code >Haskell gives you much more flexibility and protection than many >imperative languages, especially the more popular ones. > >[...] > > > Are we missing out on usefull stuff? > >No, but if we were, it'd be trivial to test/get it with an appropriate >top-level runTheProgram IO computation definable in Haskell today. Trivial to test I fully agree with; however, to the missing out on usefull stuff I am not convinced. Please allow me to continue my trolling. To use the IO monad for impure calculations is, of course, a design decision. It is arbitrary to the point that I have the freedom not to agree with it; but hey, even I know the thing works ;-). The key point to my email was that doing impure IO possibly has little to do with a monadic style of programming but with the fact that we can functionally construct a description of an impure program which is evaluated _only_ when delivered as a sole result of a Haskell computation. I find that somewhat odd; but not to the point I cannot live with it ;-) About the indirectness issue. The benefit of choosing a monad is clear - the evaluation/rewrite of an action corresponds directly with an impure subroutine call. Moreover, the action can be discarded and, as you pointed out, the description of these impure subroutine calls are trivially embedded directly into Haskell - giving typesafety as a reward. It is both effective and efficient. Great.... Hail to the monad! Hmmm, uhm, but let's think further... 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. Think of the following though experiment. What if I would have a function "unparse:IO a->String" which gives the textual representation of the specification of the object of type IO a? Clearly, IO is not a monad anymore. Moreover, I don't think it would break the purity of Haskell. And surely, in case of the IO monad, we can think of better manners to break a program down. Hmm, I am rambling. The point is, I think there is no reason to discard all alternatives to monads; 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 (with all respect to the baffling ingenuity of the invention). Cheers, l4t3r _________________________________________________________________ Protect your PC - get McAfee.com VirusScan Online http://clinic.mcafee.com/clinic/ibuy/campaign.asp?cid=3963

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