February 2002 - Haskell

Re: (no subject)
by bracaman 07 Feb '02

07 Feb '02

If you can use elem :: Eq a => a -> [a] -> Bool function, you can do: f x l= or (map (elem x) l On Thursday 07 February 2002 20:17, you wrote: > hi,how do I define a function to test whether a number > is an element of a list of lists of numbers. > > elem :: Int -> [[Int]] -> Bool > > __________________________________________________ > Do You Yahoo!? > Send FREE Valentine eCards with Yahoo! Greetings! > http://greetings.yahoo.com > _______________________________________________ > Haskell mailing list > Haskell(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell

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RE: String manipulation.
by Chris Angus 07 Feb '02

07 Feb '02

You may want to use reads as read will call error if the string is not an integer or use something akin to the example below readMaybeInt :: String -> Maybe Int readMaybeInt s = case reads s of [(x,_)] -> Just x _ -> Nothing -----Original Message----- From: Hal Daume III [mailto:hdaume@ISI.EDU] Sent: 07 February 2002 17:54 To: DK Cc: haskell(a)haskell.org Subject: Re: String manipulation. You should have a look at the Read class. A member a of the Read class has a function read :: String -> a. so, for example, you can say: (read "5.0") :: Double to read this as a double. You could also read it as an integer or something. If you have a list of strings that you want to convert to a list of integers, you could do: map (\x -> (read x)::Double) [list of strings] Hope that helps. - Hal -- Hal Daume III "Computer science is no more about computers | hdaume(a)isi.edu than astronomy is about telescopes." -Dijkstra | www.isi.edu/~hdaume On Thu, 7 Feb 2002, DK wrote: > Hello. First of all I am a beginner in Haskell, and I must confess I do not yet fully understand it. I need to write a program in Haskell, though, and I am having some difficulties... > > What I would like to ask, is how can I take a string from a list, and manipulate it, in order to convert it to an integer. Any help would be very appreciated. > > Thank you very much, > Dimitris > _______________________________________________ Haskell mailing list Haskell(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell

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RE: (no subject)
by Konst Sushenko 07 Feb '02

07 Feb '02

One way to do it would be to look at the prelude's elem function. Otherwise, just scan each list until you find a match. > -----Original Message----- > From: Phil Haymen [mailto:phaymen@yahoo.com] > Sent: Thursday, February 07, 2002 12:18 PM > To: haskell(a)haskell.org > Subject: (no subject) > > > hi,how do I define a function to test whether a number > is an element of a list of lists of numbers. > > elem :: Int -> [[Int]] -> Bool > > __________________________________________________ > Do You Yahoo!? > Send FREE Valentine eCards with Yahoo! Greetings! > http://greetings.yahoo.com > _______________________________________________ > Haskell mailing list > Haskell(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell >

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(no subject)
by Phil Haymen 07 Feb '02

07 Feb '02

hi,how do I define a function to test whether a number is an element of a list of lists of numbers. elem :: Int -> [[Int]] -> Bool __________________________________________________ Do You Yahoo!? Send FREE Valentine eCards with Yahoo! Greetings! http://greetings.yahoo.com

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layout rules for "where"
by Zhanyong Wan 07 Feb '02

07 Feb '02

Hi, I have been a big fan of Haskell's layout rules, but was got by them lately. Here's what I was doing. I wrote something like: > foo = > ... > where > bar = ... > > ... > -- 600 lines of Haskell code suppressed > > baz = ... Unsurprisingly, when I load this module in Hugs, I can use baz as it is in the top level. Later, I had a better implementation for foo, so I no longer needed the local definition for bar. I made the change, but unfortunately forgot to remove the "where" keyword, so the code became: > foo = > ... > where > > ... > -- 600 lines of Haskell code suppressed > > baz = ... and it compiled! However, when I tried to use baz, Hugs complained that it's undefined. Why? Because the compiler thinks that *all the code* (that is, 600+ lines) after the dangling "where" is just local definitions to foo. I was lucky to spot the source of the problem quickly because I knew I recently changed that line, but in general, this kind of error can be very hard to locate. (In my case, the offending code is 600 lines away from where the bug is manifested, and the number can be arbitrarily high.) I imagine this would be particularly scary to Haskell beginners and may even turn them away. Can't we require that a local definition be more indented than the enclosing definition? That way my code would've been rejected by the compiler, and the error message (something like "Empty where-clause on line xxx") can actually be helpful. Alternatively, if the compiler warns about all unused local bindings by default (See also the thread "Why is this legal" on the same mailing list), it would avoid the vast confusion. -- # Zhanyong Wan http://pantheon.yale.edu/~zw23/ ____ # Yale University, Dept of Computer Science /\___\ # P.O.Box 208285, New Haven, CT 06520-8285 ||___|

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WAAAPL 2002, call for papers
by Okasaki, C. DR EECS 07 Feb '02

07 Feb '02

Apologies if you receive multiple copies... ============================================================================ CALL FOR PAPERS ACM SIGPLAN WAAAPL 2002 [Deadline for submission: 3rd June 2002] Workshop on Algorithmic Aspects of Advanced Programming Languages Part of PLI'02 Pittsburgh, PA, USA date to be announced (probably Oct 7 or Oct 8, 2002) http://www.cs.uni-bonn.de/~ralf/waaapl02.{html,pdf,ps,dvi,txt} ============================================================================ Scope ----- WAAAPL (pronounced "wapple") seeks papers on all aspects of the design, analysis, evaluation, or synthesis of algorithms or data structures in the context of advanced programming languages, such as functional or logic languages, where traditional algorithms or data structures may be awkward or impossible to apply. Possible topics include (but are not limited to): o new algorithms or data structures, o empirical studies of existing algorithms or data structures, o new techniques or frameworks for the design, analysis, evaluation, or synthesis of algorithms or data structures, o applications or case studies, o pedagogical issues (language aspects of teaching algorithms or algorithmic aspects of teaching languages). A previous WAAAPL workshop has been held in Paris (1999). Submission details ------------------ Deadline for submission: 3rd June 2002 Notification of acceptance: 1st July 2002 Final submission due: 1st August 2002 WAAAPL Workshop: to be announced (probably Oct 7 or Oct 8, 2002) Authors should submit papers of at most 12 pages, in postscript format, formatted for A4 paper, to Ralf Hinze (ralf(a)cs.uni-bonn.de) or Chris Okasaki (Christopher.Okasaki(a)usma.edu) by 3rd June 2002. The proceedings will be published by ACM and will appear in the ACM digital library. Programme committee ------------------- Richard Bird Oxford University Michael Hanus University of Kiel Ralf Hinze University of Bonn (co-chair) Zhenjiang Hu University of Tokyo Haim Kaplan Tel Aviv University Chris Okasaki United States Military Academy (co-chair) Melissa O'Neill Harvey Mudd College ============================================================================

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ST and IO. Two problems. Some old some new?
by Amr Sabry 07 Feb '02

07 Feb '02

Background. We have two notions of state (ST and IO) with two mediating operations: unsafeIOtoST :: IO a -> ST s a stToIO :: ST s a -> IO a unsafeIOtoST is documented as unsafe because exceptions that would have been caught in the IO monad are not caught in the ST monad but this is irrelevant to the rest of the message. There are instead two other problems that I'd like to discuss. --------------------------------------------------------------------------- 1. Sequencing. Consider: m0 f = do x <- newIORef 10 y <- return (f x) v <- readIORef x print v m1 f = do x <- newIORef 10 y <- return (f x) v <- seq y $ readIORef x print v f1 x = runST (do v <- unsafeIOtoST $ readIORef x unsafeIOtoST $ writeIORef x (v+v)) f2 x = runST (do v <- unsafeIOtoST $ readIORef x seq v $ unsafeIOtoST $ writeIORef x (v+v)) f3 x = runST (unsafeIOtoST (do v <- readIORef x writeIORef x (v+v))) On Hugs (Dec. 2001): m0 f1 = 10 m0 f2 = 10 m0 f3 = 10 m1 f1 = segmentation fault m1 f2 = 20 m1 f3 = 20 Is it possible to combine ST-state and IO-state in a reasonable way without hacking with evaluation order? --------------------------------------------------------------------------- 2. Soundness of stToIO A year ago there was a discussion about the soundness of stToIO. Intuitively in the type of stToIO, the type variable s that is critical for enforcing soundness of encapsulation is ignored. This enables one to encapsulate regions even if they export local locations as follows: data Var a = Var { get :: () -> IO a, set :: a -> IO ()} nloc :: Int -> Var Int nloc a = runST (do x <- newSTRef a return (Var {get = \() -> stToIO $ readSTRef x, set = \a -> stToIO $ writeSTRef x a})) Instead of returning the location directly (which would not typecheck), one can instead return a pair of a getter and setter that enable reading and writing of the location. Going one step further, we can encapsulate the operations for reading and writing the locations and then write gensym of type () -> Int! rloc :: Var Int -> Int rloc (Var {get,set}) = runST (do v <- unsafeIOtoST $ get () seq v $ return v) wloc :: Var Int -> Int -> () wloc (Var {get,set}) a = runST (do s <- unsafeIOtoST $ set a seq s $ return s) gensym :: () -> Int gensym = let x = nloc 0 in \() -> let y = rloc x z = wloc x (y+1) in seq y $ seq z $ y test = (gensym(), gensym(), gensym()) -- evalutes to (0,1,2) It is clear that this whole thing should be completely rethought. I am working on a proposal but thought I'd get some comments from the community. Thanks. --Amr

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code.box.sk and haskell
by bracaman 07 Feb '02

07 Feb '02

Attention all Haskellers! The programming site code.box.sk (http://code.box.sk) always had a little section about haskell created by me. But now, it's much better! All the haskellers may post news, submit links, and we even got a forum!!!! If you need any information, or if you want to contribute, mail me :)) My best regards, João Ferreira aka bracaman

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Position of arguments in function definition and performance
by José Romildo Malaquias 07 Feb '02

07 Feb '02

Hello. Please, tell me which set of definitions below should I expected to be more efficient: the reverse1 or the reverse2 functions. reverse1 [] ys = ys reverse1 (x:xs) ys = reverse2 (x:ys) xs reverse2 ys [] = ys reverse2 ys (x:xs) = reverse2 (x:ys) xs The difference rely on the position of the argument in which the pattern matching is done in the function definition. Regards. Romildo -- Prof. José Romildo Malaquias Departamento de Computação http://iceb.ufop.br/~romildo Universidade Federal de Ouro Preto romildo(a)iceb.ufop.br Brasil romildo(a)uber.com.br

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RE: Reference types
by Ashley Yakeley 07 Feb '02

07 Feb '02

At 2002-02-07 02:09, I wrote: >The kind of generalisation you are proposing is, in my opinion, best done >explicitly by Haskell. Primitive functions and types should be as simple, >concrete and primitive as possible. Let Haskell do the clever >generalisation stuff. As a rule, I'm opposed to any generalisation in the standard libraries over IO and (ST s) that cannot be made to work for user-defined monads. People will use them in their code, and those monads will become "privileged". I admit I have a stake in this, my JVM-Bridge makes extensive use of lifted monads, and so I'd like porting code between monads to be as easy as possible. The way forward for this is classes and types in the standard libraries that generalise over any monad which has the necessary properties. -- Ashley Yakeley, Seattle WA

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