- Haskell-Cafe - Haskell.org

ANNOUNCE: HXML 0.2, XML parser for Haskell
by Joe English 06 Mar '02

06 Mar '02

Announcing HXML version 0.2, a non-validating XML parser written in Haskell. It is designed for space-efficiency, taking advantage of lazy evaluation to reduce memory requirements. HXML is available at: <URL: http://www.flightlab.com/~joe/hxml > The current version is 0.2, and is pre-beta quality. HXML has been tested with GHC 5.02, NHC 1.10, and various recent versions of Hugs 98. Changes in version 0.2: + New Arrow-based combinator library + Added support for CDATA sections + New function parseDocument recognizes (and ignores) the document prolog (XML and DOCTYPE declarations) + Several data structures and public functions have been renamed + Space fault in comment parsing fixed Please contact Joe English <jenglish(a)flightlab.com> with any questions, comments, or bug reports. --Joe English jenglish(a)flightlab.com

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Re: Haskell-Cafe digest, Vol 1 #342 - 1 msg
by Zhe Fu 06 Mar '02

06 Mar '02

I want to read a file's content into a string. content :: FilePath -> String content f = readFile f But when there is a type error: Type error in explicitly typed binding *** Term : content *** Type : FilePath -> IO String *** Does not match : FilePath -> String How can I solve it? Thanks. Zhe

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Re: how to read a file's content to a string?
by Ashley Yakeley 05 Mar '02

05 Mar '02

At 2002-03-05 14:43, Zhe Fu wrote: >content :: FilePath -> String >content f = readFile f You have the wrong type signature for 'content'. This should solve it: content :: FilePath -> IO String content f = readFile f Of course, you'll need to change whatever uses 'content'. -- Ashley Yakeley, Seattle WA

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Lazy Evaluation
by Nguyen Phan Dung 04 Mar '02

04 Mar '02

Hello, Sorry for asking such a silly question: Haskell is using lazy evaluation. So, it means we should be able to declare and use a list of billions elements without any trouble. So I declare my list as follow: mylist :: [Integer] mylist = [1..1000000000] In Hugs, I type mylist to print out all the elements inside. However, after printing about 22000 elements, the system crashs & outputs: "Garbage collection fails to reclaim sufficient memory" Would you please help me with this problem? Thank you very much, Phan Dung.

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RE: Ground Up
by Simon Peyton-Jones 03 Mar '02

03 Mar '02

| Further, Haskell, though _very_ nice, is probably not "the | best choice" for a "pragmatic programming". It is too alive. | Different compilers are not 100% compatible each to other, | and even different versions of the same compilers may be | incompatible. You should be ready either to stick yourself | and your co-wokers to a certain version of the compiler or to | spend some significal time to keep you programs working with | any compiler. It's true that GHC and Hugs and nhc don't accept exactly the same language but a) Haskell 98 is a very stable language that all three accept, and are committed to continuing to do so. b) If you stick to one compiler of the three, you can be pretty sure that programs that used to work will continue to work. (GHC, for example, always has a "stable branch", currently 5.02.2, that we maintain in parallel with introducing new features and bugs into the head. Bleeding-edge folk can and do compile the head from source, but you don't have to do that!) It's true that we occasionally make incompatible changes, but not often. | Maybe you will find that it's better to use Ocaml or | Scheme. Thay have own neats, and seem to be more stable. c) I have a very high opinion of Caml and Scheme, and their impls, but I think you are being a bit unfair on Haskell here. Caml has only one impl, and Scheme has many incompatible variants (eg PLT Scheme), so in practice you have to stick to one impl unless you use a the R3RS subset. I don't think any of them are more stable than any particular one of the Haskell impls. Simon

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Ground Up
by Jerry, JiJie 01 Mar '02

01 Mar '02

Good day everyone, I'm a haskell newbie trying to seeking advice to advance myself. My backgrounds are: * As a non-CS student, I have absolutely no knowledge of lambda calculus * And unfortunately I have to make painful decision on spending $30 on a book or three days' meal While my goals are: * Become a pragmatic haskell programmer in the shortest time * At the minimal expense What I have done for the past few weeks were: * I read almost all the free educational sources at www.haskell.org, * Subscribed to this mailing list and try to digest every mail * Read most of the "The Haskell School of Expression" (by Paul Hudak) and the non-theoretical chapters of the "An Introduction To Functional Programming Systems Using Haskell" (by AJT Davie), which are the only two introductory level haskell related books in our library However, my problems are: * I still don't understand most of the codes I found, like the various haskell libraries * I still have no clue of most (ok, almost all) of what is being discussed in this mailing list So I'm eager to know if there are cost-effective ways to achieve my goals or at least leverage myself from the beginner's level. Your advice would be greatly appreciated! Regards, Jerry

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Change a light bulb
by Shawn P. Garbett 01 Mar '02

01 Mar '02

-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 I finally got a clue. I got the Haskell program working. Now to build upon it. Jay's comment really made the IO monad concept sink in for me. Also reading through Manuael M T Chakravarty's Boolean editor was mind expanding as well. My biggest question is how to simplify, make more efficient, make more lazy what I have here. There's so much more to do now..... - -- State model of a light bulb with IO import Monad import IO main :: IO () main = do -- Turn off buffering so response is immediate hSetBuffering stdin NoBuffering hSetBuffering stdout NoBuffering -- Process from the intialState process initialState return () - -- A generic state-machine processor process :: State -> IO () process st = do -- retrieve current stimulus s <- getStimulus putChar '\n' -- compute new state and response (r, st') <- do { return (s st) } -- do the specified response r -- if the state is lambda then exit if( st' /= Lambda) then process st' else return () - -- Now define state for our humble lightbulb - -- State definition data State = Dark | Light | Lambda deriving (Show, Eq, Enum) - -- The inital state initialState :: State initialState = Dark - -- Response definitions type Response = IO () activate :: Response activate = putStr "Haskell said, let there be light.\n" deactivate :: Response deactivate = putStr "And darkness was upon the void.\n" - -- Here is the state transforms (i.e. stimuli) type Stimulus = State -> (Response, State) on :: Stimulus on Dark = (activate, Light) on Light = (return (), Light) off :: Stimulus off Dark = (return (), Dark) off Light = (deactivate, Dark) exit :: Stimulus exit x = (return (), Lambda) - -- Create a stimulus source from stdin charToStimulus :: Char -> Stimulus charToStimulus '1' = on charToStimulus '0' = off charToStimulus c = exit getStimulus :: IO Stimulus getStimulus = liftM charToStimulus getChar Shawn - -- You're in a maze of twisty little statements, all alike. Public Key available from http://www.garbett.org/public-key -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.0.6 (GNU/Linux) Comment: For info see http://www.gnupg.org iD8DBQE8f8hxDtpPjAQxZ6ARAtWFAJ9fQ7rURyKebnSlBvrEklBSqnnb1QCdHHtM I4nSaTg0Sl2vIz+Yx1Z7ab8= =FKTr -----END PGP SIGNATURE-----

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All Segments Problem
by Saswat Anand 01 Mar '02

01 Mar '02

Hi All, This is not a haskell problem. I need some pointers which some of you might provide. In program calculation community, segement problems e.g. maximum sum problem have been studied extensively. The problems involve finding a segment of the list that maximizes some objective function and satisfies some predicate. But I am wondering if is there similar study on the family of problems where we need to find all the segments of a list that satisfy some predicate. I am interested in predicates involving forall and existential quantifiers. Thanks for any suggestion. Cheers, Saswat

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Changing a light bulb.
by Shawn P. Garbett 28 Feb '02

28 Feb '02

-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 Now after my earlier soap box tirade I'm trying to put my code where my mouth is. >>blush<< It's harder than I thought. I've broken it down into a State Model of a light bulb. The goal, to find out just how much Haskell code does it take to change the state of a light bulb? Here's a very crippled version. Is there an easy way to introduce state without changing the type of main??? I've written a much more complex example, but I got into things like main :: STT(IO) using a monad transformer. So really, the big question is, must I use a monad transformer? - ------------------------------------------------------------------------------------------------ - -- State model of a light bulb import Monad import IO main :: IO () main = do hSetBuffering stdin NoBuffering process process :: IO () process = do i <- assign initialState putChar '\n' print(s) if (s /= Exit) then process else return () data State = Dark | Light initialState :: State initialState = Dark data Stimulus = On | Off | Exit deriving (Show, Eq, Enum) charToStimulus :: Char -> Stimulus charToStimulus '1' = On charToStimulus '0' = Off charToStimulus c = Exit getStimulus :: IO Stimulus getStimulus = liftM charToStimulus getChar - -- State transformer newtype St a = MkSt (State -> (a, State)) - -- State transformer applied to state apply :: St a -> State -> (a, State) apply (MkSt f) s = f s - -- State monad instance Monad St where return x = MkSt f where f s = (x,s) p >>= q = MkSt f where f s = apply (q x) s' where (x, s') = apply p s - -- Useful State operations fetch :: St State fetch = MkSt f where f s = (s,s) assign :: State -> St () assign s' = MkSt f where f s = ((),s') done :: St () done = return () - ----------------------------------------------------------------------------------------------------------------------- - -- You're in a maze of twisty little statements, all alike. Public Key available from http://www.garbett.org/public-key -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.0.6 (GNU/Linux) Comment: For info see http://www.gnupg.org iD8DBQE8fqdpDtpPjAQxZ6ARAkCWAJ9pGcS/hQpoK12AdcEmt8Y/0rEv9ACfWGtO 1FL9VwopmgsAyUAv9RL++T8= =a9xu -----END PGP SIGNATURE-----

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Re: Ground Up
by Rijk J. C. van Haaften 28 Feb '02

28 Feb '02

Jerry wrote >However, my problems are: >* I still don't understand most of the codes I found, like the various > haskell libraries Practice is the only answer to this problem, as Keith Wansbrough says. >* I still have no clue of most (ok, almost all) of what is being > discussed in this mailing list Though I am a CS student, being highly interested in Haskell and practising a lot, I didn't understand most of the discussions on this mailing list. I started understanding them only after I got involved in implementing a Haskell compiler. Therefore, don't worry about this point. A haskell-user doesn't need to know the details of the haskell compiler ins and outs. Rijk-Jan van Haaften

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