- Haskell-Cafe - Haskell.org

generic preprocessor for haskell
by Per Larsson 18 May '04

18 May '04

I have recently made a small detour into ocaml programming and I'm rather impressed with the generic preprocessor 'camlp4' for ocaml. Camlp4 allows the programmer to (i) interact with the ocaml parser and lexer to extend the concrete grammar with new syntax and (ii) to directly define the translation of such new syntax forms in terms of ocaml abstract syntax. Haskell programmers (at least in GHC) can also manipulate abstract syntax with the Template Haskell extension and the Language.Haskell.THSyntax library, but to my knowing there is no haskell compiler which supports the inclusion of new syntax forms. Wouldn't such a tool be cool? In the haskell community there are a number of handmade preprocesserors (DrIFT, Generic Haskell, HaRP, ...) which are both cumbersome to write, because the authors must write a (partial) haskell parser for every small extension, but also cumbersome to use because of increasing complexity to your makefiles and different standards for error reporting. Also, experimental extensions could quickly be written and evaluated by the haskell users, decreasing the dependency on the compiler maintainers. Per Larsson

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Re: [Haskell-cafe] Re: Lazy XML handling
by MR K P SCHUPKE 17 May '04

17 May '04

>I'd say it's still less easy (through clearly possible) for an application >to adopt alternative traversal patterns. Yes, thats true, but thats the order in which the tokenizer outputs tags... This is the order tags must be processed if you wish to stream - otherwise you need a heap of tags to keep the ones you are not ready to process from the tokenizer. In effect we can transform an algorithm that requires a different traversal into one that uses this traversal, at a possible slight increase in complexity. The gain is sequential stream based processing. Can you give me an example where processing HAS to be in a different order? In effect HaXml's representation actually simplifies to the above anyway... if you think about... [ A [B,C,D [E,F], G] H [I,J] ] => [(0,A),(1,B),(1,C),(1,D),(2,E),(2,F),(1,G),(0,H),(1,I),(1,J)] The nested lists does not allow any extra flexibility in traversal, at the cost of greater complexity. The one case it is simpler is if you want to look up a single node by its 'tree address' - This is more complex with the list-rep, as you have to count nodes at a given level, skipping irrelevent nodes... The list rep is very good at data extraction, if you want to extract a sub-tree of a given type you just ignore all tags until you find one of the correct type, then output all tags until the depth returns to the level of the initial tag. Regards, Keean.

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Re: [Haskell-cafe] Re: Lazy XML handling
by MR K P SCHUPKE 17 May '04

17 May '04

Please note... The list represents a tree because each node is: [(XmlTreeDepth,XmlElement)] This IS a tree, just a different representation. If what I am saying sounds strange check out the section in Knuth: Volume 1, on representing trees, you will find a whole load besides the C-type pointer based tree. I am not sure what the representation you just gave is??? [[ element ]] surely that simplifies to a list of elements... what am I missing? Regards, Keean.

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Programming style and XML processing in Haskell
by Graham Klyne 15 May '04

15 May '04

After an extended period of procrastination and nipping at the edges of the problem, I feel a need to tackle head on the requirement for a "usable" XML handling library in Haskell. As far back as 1999, Malcolm Wallace and Colin Runciman observed that "Haskell is a very suitable language for XML processing" [1], yet there still does not seem to be a generically useful XML handling library, suitable (say) for inclusion as part of a standard Haskell compiler library. Why do I say this? I have looked at three separate XML libraries, and each has problems which I perceive make them unsuitable for the purposes I have in mind. It may be that part of my problem here is a mistaken view of Haskell programming style, possible exposure of which is one of my reasons for posting this. My immediate goal is to create an RDF/XML parser, the output from which is a data structure representing an RDF graph. This involves parsing the XML to an XML-infoset-like form, then traversing this to extract information for the RDF graph. I want to create a function like this: parseRDFXML :: String -> RDFGraph My XML processing requirements: (1) basic XML parsing (2) predefined and character entity handling (< & &#n; etc) (3) general entity handling (DTD entity definitions and substitutions, per internal DTD subset) (4) easy access to values extracted from XML data (i.e. for XML-to-non-XML processing) (5) XML namespace handling (6) library usable outside the IO monad (i.e. by functions that return non-IO values) Non-requirements, but maybe nice to have: (7) parameter entities (8) External entities (9) XML/DTD validation (1)-(4) correspond roughly to the level of support required of XML parsers for handling "standalone" documents. Almost all modern usage of XML that I'm aware of depends to lesser or greater degree on on (5). (8) and (9) are, I think, in conflict with having library functions that can be used outside the IO monad, since they require that the parser be able to access external data. (7) is really a helper for DTD-based validation, and my own view is that validity checking is better performed using XML schema. Some of the facilities provided by (8) are now being addressed by alternative activities that build upon a basic XML (XInclude, Binary attachments for SOAP, etc.). Requirement (6) arises for me because I have adopted a style of programming in Haskell that is mostly consisting of pure functions, without recourse to monads. I use parser monads locally as required, and I use IO and state monads at the upper levels of my programs to deal with and record the program's interaction with the outside world. It seems to me that this approach leads to functions that are easier to pick up and use. I've found that, when using third party libraries, stand-alone functions present an easier learning curve compared with libraries that are based around a (sometimes complex) monadic state. Maybe I'm missing something here? ... Turning to the XML libraries, I've looked at three: (A) Joe English's HXML parser [2] (B) HaXml [3] (C) Haskell Xml Toolbox [4] (A) HXML is very easy to understand and use, but it does very little more than basic XML parsing. No level of DTD handling is provided, as far as I've been able to determine. (B) HaXML does a little more of what I want, to the extent that it can parse DTDs, and even perform some basic validity checking. I can't find anywhere in the code that seems to address substitution of entities defined in the DTD, and I'm not sure if it can parse a DTD and XML from the same XML file. There are references in the code to external DTD subsets, but I can't see any attempt to implement this. I have found that the HaXML's error handling is rather severe, in that there are a wide of input data errors that cause the library to 'error' rather than return a diagnostic value. (C) Haskell Xml Toolbox is the most functional package (being the only package with XML namespace support) and also the most difficult to use. Unfortunately, it seems that much of the DTD functionality (needed for expanding general entities) is performed I/O monad, as it is part of the code than performs validation, which, as noted above, needs access to external resources. My biggest problem with this package is that it seems to be very difficult and unwieldy to use as part of another library: much of the code seems to be oriented toward creating complete programs for XML-to-XML transformations of various kinds. I've a view that XML namespace support should be quite easy to graft onto either of the other packages, given an extension to the data type used to describe nodes and elements. Over the past couple of months, I've been wavering between pushing ahead with (B) or (C). Both have problems, and either would require significant effort on my part. If I use (C), it involves the least amount of new code, but I think I would find myself ripping out chunks of code to create functions that I can use outside the IO monad, which would effectively fork the codebase. If I use (B), I need to address the error handling problem, though I think I know roughly how to do that (I already made a start; details below). A previous problem I had was that the HaXML code needs CPP preprocessing, which was problematic for me, but since then a simple CPP-equivalent in Haskell has been implemented so I think I can work around that problem. I think I'd need to write new code to deal with entity substitution and namespaces, but I think both of those could be implemented as filters that layer on top of the basic package. So the pendulum swings again, and I now think that HaXML is looking like the most promising base for further development. ... What do I think an XML library for Haskell should look like? The component's I'd like to see would look something like this: XML parser :: String ---> (internal representation) | \ | -----> IO function to perform full validation | and external DTD handling [optional] v XML filter combinators --+--> entity substitution logic | +--> namespace handling | +--> XSLT processing [optional, for now **] v DOM-like read-only interface for access to data at level comparable to XML infoset (used to avoid dependency between applications that use infoset data and details of the internal representation used.) Does this seem reasonable? [**] my thought is that an XSLT document could be "compiled" into an XML filter function. #g -- [1] http://www.cs.york.ac.uk/fp/HaXml/icfp99.html#furtherwork [2] http://www.flightlab.com/~joe/hxml/ [3] http://www.cs.york.ac.uk/fp/HaXml/ [4] http://www.fh-wedel.de/~si/HXmlToolbox/ .... Work I've already done on HaXML: I made a start on a unit test program, and some modifications to the HMW combinator library to allow parse errors to be handled by the calling program. The initial test data has been stolen from the Hxml Toolbox software kit. The 4 test cases all run without errors under Hugs. Feel free to grab anything you think may be useful. The revisions are here: http://www.ninebynine.org/Software/HaskellUtils/HaXml-1.11/ The test program and data files are here: http://www.ninebynine.org/Software/HaskellUtils/HaXml-1.11/test/ (The test program has a commented-out feature to generate formatted versions of the input files which can be renamed for use as comparison test data.) The modified source code includes: + http://www.ninebynine.org/Software/HaskellUtils/HaXml-1.11/src/Text/ParserC… modified to include an option to return a diagnostic message or parser result, via an (Either String a) value. The original interface is (mostly) preserved, and new functions added to support the extended return values (e.g. papplydiag). + http://www.ninebynine.org/Software/HaskellUtils/HaXml-1.11/src/Text/XML/HaX… modified to work with the regised parser structure. A new function, xmlParseDiag, added to return an (Either String a) value. Also added an eof parser and documentOnly functions to perform some of the function of sanitycheck. (I also commented out the #if stuff so I could test under Hugs.) (Malcolm pointed out to me that the lexer also throws some errors, but I think that could be addressed by returning an error token and leaving the parser to deal with the resulting syntax error.) ------------ Graham Klyne For email: http://www.ninebynine.org/#Contact

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by MR K P SCHUPKE 14 May '04

14 May '04

>> correcting parser >I would think this is a rather specialized requirement. I certainly don't >want a "correcting" parser for my work. But I can see that some >applications might... Especially if you are aiming for a genarally applicable library. What's really needed is a 'Strictness' switch for the parser, so you can select validating, or non-validating. The parser I wrote uses heuristsics that specify how to fix mistakes (like missing close tags) - this enables the XML parser to parse HTML with the correct set of rules. >This seems reasonable, and I'd expect a reasonable implementation (of a >filter) to stream via lazy evaluation where that matches the final usage >pattern. The outline I sketched (copied below) was intended to be built >upon something like HaXML's filter idea, so that streaming processing would >(in principle) be possible. If you don't use a list based representation, the only other way to get 'lazy' behaviour is to use the 'event' model - which is a lot more complex, but possible using one thread to do the parsing, and a Channel to pass the events through. I think you misunderstand what the parser and renderer do, the parser takes String input and outputs a stream of elements based on the BNF specification for XML... It looks like: data XmlElement = XMLDecl [XmlAttribute] | DocType XmlTagName XmlSystemLiteral XmlPubidLiteral | EmptyTag XmlTagName [XmlAttribute] | STag XmlTagName [XmlAttribute] | ETag XmlTagName | Text [XmlElement] | CharData String | CharRef Int | EntityRef XmlTagName | PERef XmlTagName | CDSect String | PI XmlTagName String | Comment String | Flush | Undefined | Unparsed String deriving (Show,Eq) The renderer takes a stream of these elements and converts to a String. All the filtering/reading/writing is done on streams of these elements. For example a filter could select only <Person ...> records from an XML data source, the person specific reader would then convert to a specific representation of a Person... myReadey :: [(XmlTreeDepth,XmlElement)] -> [Person] a writer does the opposite. Regards, Keean.

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Re: [Haskell-cafe] Programming style and XML processing in Haskell
by MR K P SCHUPKE 14 May '04

14 May '04

Just sticking in my two pence worth... I am not sure what application you intend this for, but I find most XML parsers completely useless. With my application programmers hat on, I do not want to validate against a DTD, I want to extract as much information as possible from bad XML... what I would like is a correcting parser - one which outputs XML in compliance, but will accept any old rubbish and make a best guess attempt to fix it up (based on a set of configurable heuristic rules)... Secondly I deal with very large documents, the tree form of which won't fit in memory, so I would see an XML parser doin the following... parser :: String -> [XmlElements] filter :: [XmlElements] -> [XmlElements] reader :: [XmlElements] -> ... output data types ... writer :: ... input data types ... -> [XmlElements] render :: [XmlElements] -> String In order to keep track of the tree structure the tree-depth of each element is encoded within the XmlElement type... thus allowing the data to be streamed through the filters/readers etc. This means the parser can output the first element as soon as it encounters the second element (lazy list == stream in Haskell) rather than having to wait until the last element as would happen with a DOM tree (it is a tree not a graph as XML elements can only contain sub-elements)... As I said the above is just my opinion, and as it happens I have written a parser that does the above... I guess that is why there are several parsers for XML available (different requirements) and there will probably be many more ... Regards, Keean.

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push/enter vs eval/apply question
by 7253146＠informatica.alumnos.uma.es 13 May '04

13 May '04

Hi. I have read part of the paper "Making a fast curry. Push/enter vs eval/apply for higher-order languages" (Simon Marlow ans Simon Peyton Jones, http://research.microsoft.com/Users/simonpj/papers/eval-apply/eval-apply.ps) where the tradeoffs of both currying models are discused. The paper takes the STG machine as design model. Just as an exercise, I tried to find the curying models for the G and TIM machines as described in "Implementing Functional Languages: a tutorial" (Simon Peyton Jones and David R. Lester, http://research.microsoft.com/Users/simonpj/Papers/pj-lester- book/student.pdf.gz). It was easy to find out that the fourth proposed implementation of the TIM machine was push/enter-oriented ("UpdateMarkers" instruction checks inside a function how many arguments it has been taken). But I have not find anything about for the G machine. It seems to not care about the number of arguments of a supercombinator: when one is evaluated, always have sufficient arguments, and partial applications are simple subgraphs with a mini spine of applications. I guess that only makes sense to talk about the two currying models push/enter and eval/apply when the considered machine is spineless, because on "spineful" or "spine-based" machines the graph of the lambda-calculus expression to evaluate is explicitly represented in memory, so we can see currying as lambda- application over generic lambda-variables. I am very interested on this matter. Please confirm my guess or explain why I am wrong, whoever knows it. Thanks in advance. Regards, Jose David --------------------------------------------- Este mensaje lo ha enviado un Alumno de la Universidad de Malaga. http://www.alumnos.uma.es/

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How to hold common data
by scott west 13 May '04

13 May '04

Hello all, I'm new to haskell and was just wondering if there was any efficient or standard way to store and modify some sort of state data. The functional nature of haskell has me confused in this respect! Basically what I want to achieve is an interactive program that allows you to edit some tables, and thus hold some data. I dont' need to be spoonfed an answer, but a general guideline would be very helpful :) Thanks for any help you could provide! Regards, Scott

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Re: [Haskell-cafe] How to hold common data
by MR K P SCHUPKE 13 May '04

13 May '04

simplest way... use a recursive function that feeds itself the state as one of its arguments...

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Surprising bugs
by Mark Carroll 12 May '04

12 May '04

I clearly don't understand Haskell very deeply yet because I dealt with a couple of interesting types of bug this week. One sort was where, if I have, f :: SomeType -> Stuff ... f = whatever g :: Stuff ... g = f someValue ...then I can get an error that suggests that maybe I'm violating the monomorphism restriction if I put in f's type but not g's. If I put both in, it becomes happy. Another sort was where, if I had an algebraic parametric type (is that the name? Ord a => Foo a b c, etc.) then I couldn't have, say, a Maybe (Foo a b c), where sometimes I called it with a (Just foo) and sometimes with a Nothing, because with the Nothings it would complain about, erm, some ambiguity to do with Ord a. However, it would be fixed if I made the Maybe X argument into two arguments, Bool -> X, the Bool indicating if the X was meant to have been a Nothing or a Just X. I hope both (or either!) of those made sense. I don't have the code immediately to hand to reproduce the details, but I just thought I'd try to recall what they were because I found these compiler complaints interesting. I don't fully understand what's happening, and I'm sure a newbie would be quite bewildered. (-: -- Mark

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Monadic Composition
by Jorge Adriano Aires 12 May '04

12 May '04

Hello, I needed to compose monadic functions, since I couldn't find anything in Control.Monad so I defined my own. I first came up with a left associative version: compLM [f1,..., fn] a = (f1 a >>= f2) >>= ... >>= fn > compLM :: Monad m => [a->m a] -> (a->m a) > compLM mfs a = foldl (>>=) (return a) mfs Later a right associative version, which I believe to be more efficient, particularly when some computation returns fail: compRM [f1,..., fn] a = \x->f1 x >>= (\x->f2 x >>= ... >>= \x->fn x) a > compRM :: Monad m => [a->m a] -> (a->m a) > compRM = foldr (\f g-> (\x ->f x >>= g)) return This higher order function seems quite useful to me and not trivial, so I expected it to be available. When this happens I wonder if I'm missing something. Am I complicating things? Is it actually available (or some other that does the trick) ? Or is it just me who finds this usefull enough to be in the libs? J.A.

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Cyclic Dependencies
by Georg Martius 12 May '04

12 May '04

Hi, I have some problems with cyclic dependencies. Module Foo: data Foo = Foo { bar :: Bar } Module Bar: class BarClass b where barfun :: b -> Foo -> Foo data Bar = forall b . (BarClass b) => Bar b Obviously I have to import both modules mutually. My workaround was to add a further type variable to BarClass. class BarClass' b f where barfun :: b -> f -> f data Bar' f = forall b . (BarClass' b f) => Bar b This seams to be fine, but in my case it is a bit more compicated. I need a Read instance of Foo. Therefor I have to implement the Read instance for the existential type Bar by my own and need to know about the instanced of BarClass. Unfortunately, the instances of BarClass need Foo and I have a cycle again. Thanks, Georg

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Visualising a type class hierarchy
by Andre Pang 12 May '04

12 May '04

Hey all, Does anyone know of a good way to visualise ("graph") a type class hierarchy, including one with multiple-parameter type classes? The traditional "inherits from" arrows used in graphs of object-oriented classes doesn't deal very well with type classes. A simple case, such as: class Super a class Super a => Sub a instance Super SuperType instance Super SubType instance Sub SubType works fine, but once you get into situations such as: class SuperMulti a b class SubMulti a b c | a b -> c instance SuperMulti a Foo => SubMulti a Bar c I haven't found elegant ways of visualising this at all: how do you draw arrows from SuperMulti to SubMulti, for instance? Has anyone else had a go at this? -- % Andre Pang : trust.in.love.to.save

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Re: [Haskell-cafe] Toy application advice wanted
by John Hughes 10 May '04

10 May '04

> But Scheme lets you change things if you want to! (The Haskell > afficianados gasp). Let's face it...in most long-running applications, > data changes a lot during the lifetime of a program. So somehow, you have > to manage the repercussions of state changes. Haskell does this with > monads. (I'm still wrapping my brain around them, but...) Apparently > monads plus some syntactic sugar are like functions that return a value > and the next function, which in turn returns a value and the next > function, etc. so that referential transparency is maintained. This is a > heckuva lot of work to do (set! foo 'bar). I hope I'm not far off base > with this. Gosh, this sounds complicated! I think there's a much simpler way to see it. In almost any language, some expressions have side-effects, and some don't. Use expressions with side-effects carelessly, in the wrong place, and you can screw up. In Haskell, expressions with side-effects have a type that says so (IO <type>), and that in turn lets the type-checker restrict where you use them (hence the many questions "How do I get an <alpha> from an IO <alpha>?" -- the point is, you can't). So you avoid screwing up. Not particularly complicated, not particularly costly. writeIORef foo Bar :: IO () -- empty result, with a side-effect. OK, there's more to it than this, once you start using other monads than IO or ST, and especially once you start overloading code on the underlying monad. But as an initial understanding this is enough. John Hughes

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Toy application advice wanted
by mikeb＠manor.org 09 May '04

09 May '04

Hi folks, I've got an interesting task this week for my job. (Note that this will undoubtably last for longer than a week). I'm evaluating several high-level languages as development vehicles for our next suite of applications. The languages I'm currently considering are Scheme, Erlang, and Haskell. Scheme and Haskell have fairly wide acceptance in their particular roles (Scheme as the "pretty" lisp, supporting higher-order functions and mostly strict [until you roll your own laziness with macros], and Haskell the absolute purest language with no side effects and fully lazy evaluation). Erlang has a really nice distributed computing model, and has a nice union of non- and side-effect qualities (although it's completely strict). The toy application I've "designed" for myself is a simple GUI-based app that can load a Sun .au format sound file, display its waveform in a window, perform some simple filtering, play the sound for the user, and then save the changed sound file back out to disk. If I get familiar enough with the respective environments I'd like to add zooming in/out and scrolling to the waveform display. This application should allow me to see all kinds of different characteristics of the language/implementation, such as performance (how fast can the filter run), I/O (how fast can I load/save data to disk), OS interaction (how easy can I "play" the audio), user acceptance (how easy is it to do GUI programming) and of course the software engineering goals of modularity, correctness, and simplicity. The end goal is for me to have three working programs, in three different languages, so that I can present to my boss the performance/software engineering characteristics of each system to choose our next development language. So I'm writing you to solicit help. I have an amortized four days (32 hours!!!) to implement this simple application in Haskell. Can anyone point me to example code, supporting libraries, tutorials, etc. that are in this area of development? If I have to dumb down some of my application in order to make a great discovery about a language, that's fine. I'm expecting some great differences between these three programs. Any advice/pointers/flames welcome. Thanks in advance. Mike -- Mike J. Bell This is all just my opinion. "Outside of a dog, a book is man's best friend. Inside it's too dark to read." mikeb(a)manor.org

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HDirect, [unique], troubles again!
by Vincenzo aka Nick Name 06 May '04

06 May '04

In my quest for a fuse binding for Haskell, which I really need at the moment, I have the following definition working: module HSFuse { interface stat{}; typedef int getattrT([string] char *,stat); typedef struct fuseOps { [ref] getattrT * getattr; } fuseOps; void fuse_main(int argc,[in,string,size_is(argc)]char** argv,[ref] fuseOps * op); }; but when I substitute "ref" with "unique", because some operations can be null, I get the errors below. The problem is that in the generated hs file there is data FuseOps = FuseOps {getattr :: GetattrT} which completely ignores my type declaration! Did I do something wrong? Commands invoked and errors follow, thanks for attention: ihc -fhs-to-c -fuse-ints-everywhere -c HSFuse.idl ghc -package hdirect -fglasgow-exts HSFuse.hs -c HSFuse.hs:136: Couldn't match `GHC.Base.String -> Stat -> GHC.IOBase.IO GHC.Base.Int' against `GHC.Ptr.Ptr (GHC.Base.String -> Stat -> GHC.IOBase.IO GHC.Base.Int)' Expected type: GHC.Ptr.Ptr (GHC.Ptr.Ptr (GHC.Base.String -> Stat -> GHC.IOBase.IO GHC.Base.Int)) -> GHC.Ptr.Ptr (GHC.Base.String -> Stat -> GHC.IOBase.IO GHC.Base.Int) -> GHC.IOBase.IO () Inferred type: GHC.Ptr.Ptr (GHC.Ptr.Ptr (GHC.Base.String -> Stat -> GHC.IOBase.IO GHC.Base.Int)) -> (GHC.Base.String -> Stat -> GHC.IOBase.IO GHC.Base.Int) -> GHC.IOBase.IO () In the second argument of `HDirect.writeunique', namely `writeGetattrT' In the definition of `writeFuseOps': writeFuseOps ptr (FuseOps getattr) = let pf0 = ptr pf1 = HDirect.addNCastPtr pf0 0 in HDirect.writeunique (HDirect.allocBytes (GHC.Real.fromIntegral sizeofGetattrT)) writeGetattrT pf1 getattr

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greencard and callbacks
by Vincenzo aka Nick Name 06 May '04

06 May '04

I realize that I myself might have already asked this question to some of you, but I am really confused now. Can I have some opinion on "what is the right way to implement callbacks from C to haskell"? Will I have to learn the FFI and stop it, or is there some tool to automate the process? I need to bind a single function which takes a record of callbacks, and that's all. Can greencard support callbacks? If yes, can someone provide a simple example? I thought to do: import Foreign.GreenCard %#include<stdio.h> %fun doit :: Int -> (Int -> Int) -> IO () %code printf("%d",arg2(arg1)); which does not work, as someone might already have expected :) Thanks all Vincenzo

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Re: [Haskell-cafe] Toy application advice wanted
by Jon Cast 05 May '04

05 May '04

Duncan Coutts <duncan.coutts(a)worcester.oxford.ac.uk> wrote: > On Wed, 2004-05-05 at 16:24, Andrei de A. Formiga wrote: > > I'm finding wxHaskell very nice, and a wxWidgets > > binding is something many other "advanced" languages > > don't have (even OCaml). The only downside is having a > > 'Hello World' GUI application with 7 Mb... but it runs > > quite well and smooth once it's loaded. > > We have the same problem with gtk2hs. A stripped simple(ish) prog weighs > in at about 2.5Mb. > > I'm not quite sure what's in there that makes it so big. > > Duncan ghc doesn't support dynamic linking for Haskell libraries (a combination of the amount of work involved and the platform-specific nature of the work and the size of modern hard disks and the frequency with which ghc's ABI changes and the number of implementation details which ghc sticks into the exported ABI of a library anyway (via cross-module inlining)). So any Haskell libraries you use will get statically linked. Jon Cast

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Re: [Haskell-cafe] Toy application advice wanted
by mikeb＠manor.org 05 May '04

05 May '04

On Tue, 4 May 2004, Bill Walsh wrote: > I am amazed that you can even think of doing anything in a week. > I have been at it for at least 15 years: one project; one dead set > proof-of-concept . Well I'm sure you're doing something much more useful/worthwhile than I! =) I understand that the example program I initially thought of is rather, well...typically imperative. That's sort of the reason I chose it. Not only does it have real-world ramifications to my customer (the problem domain is similar) but it will hopefully show me the worst of the best. I.e., Haskell is a beautiful language [disclaimer: this comes from me not really being a Haskell programmer, but admiring it from afar if you will] and can express some unbelievably advanced concepts elegantly, but is it really nasty doing "normal" software crap like loading a bunch of data, popping up GUI windows, etc. etc.? I was hoping that I can piece together something fairly quickly within two weeks in those three languages. They don't have to be robust, or even beautiful. In fact, I can guarantee they won't be. But the real point of the exercise is for me to figure out which environment is the easiest for me to get along with. > I will follow your progress with intense interest. > Your are obviously an experienced programmer, but it is your analytical > skills which interest me most. Well, I am an experienced programmer, but certainly not in the same class as most lambda-junkies as yourselves. I have a long sordid history of programming in imperative languages and I've been bitching about switching for long enough that it's time to put my money where my mouth is, so to speak. I've dabbled in Scheme and Erlang and ML. To be frank, I've all but discarded Erlang as a viable language. Don't get me wrong...it's got some great features; I just think that I could easily implement Erlang in Scheme or Haskell and be quite happy. (At some point in the future when I'm smarter =) So to me, the real question is: Haskell, the pure, or Scheme, the dirty? Haskell is lazy all the time. That's awfully nice...I'm not sure if there's a performance penalty somewhere, but assuming there isn't, kudos to it. Scheme lets you be selectively lazy with macros. At least it's highly customizable (for instance, pattern matching can be implemented in Scheme with macros, whereas other languages that have fixed special forms are quite limited). But Scheme lets you change things if you want to! (The Haskell afficianados gasp). Let's face it...in most long-running applications, data changes a lot during the lifetime of a program. So somehow, you have to manage the repercussions of state changes. Haskell does this with monads. (I'm still wrapping my brain around them, but...) Apparently monads plus some syntactic sugar are like functions that return a value and the next function, which in turn returns a value and the next function, etc. so that referential transparency is maintained. This is a heckuva lot of work to do (set! foo 'bar). I hope I'm not far off base with this. I guess it boils down to this: Haskell ensures that your bindings are never ever ever corrupted, and thus the sematic value of your program is never compromised from the outside. The cost of this is a sharp learning curve, and quite possibly a performance hit. Scheme lets you tromp all over stuff (well, you get the picture), in effect letting you still have control over when you really need to do something imperative for performance or simplicity reasons. I'm not sure which I *like* better. So far neither is visually appealing, mostly because I'm not familiar with them. Scheme seems to have a slightly bigger population than Haskell, which of course means squat because if I really cared about how many programmers of language X there are out there I'd just code in Java (which, sadly, I'm doing now). Anyway, I'm starting to ramble, but I talked to a friend who has similar feelings but is actually pretty good at Common Lisp. He suggested I refocus my energies, and I agree: instead of biting off more than I can chew, and having to learn a whole wad of APIs that aren't really about the language (read: wxHaskell or gtk2hs/the like, or audio packages etc.), just code some really simple problems. Like the Sieve of Eratosthenes, in all three languages. Or a simple publish/subscribe framework with a "master" state holder and many slaves. Or quicksort. Etc. etc. So I'm going to head down that path right now, and try to get a feel for the languages in a slightly more pure fashion. I'll still try to get performance metrics out of them, but I'm not going to bang my head against the wall learning new languages, GUI toolkits, and FFIs all in two weeks. =) Thanks for the replies so far, I really appreciate them. -- Mike J. Bell This is all just my opinion. "Outside of a dog, a book is man's best friend. Inside it's too dark to read." mikeb(a)manor.org

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[Haskell-cafe] REMINDER - Contributions to HC&A Report (May 2004 edition)
by Arthur van Leeuwen 03 May '04

03 May '04

Dear Haskellers, we would like to remind you of the opportunity to contribute to the May 2004 edition of the Haskell Communities & Activities Report. Why not take the few minutes necessary to type up a new or updated entry and send it in? We would love to see people write on the following subject(s): - The Haskell Mailing List - The Haskell Cafe - The Haskell 98 reports - Concurrent Haskell - hierarchical libraries - HaskellDB - HXml - QuickCheck - Yale Haskell group - Parallel and Distributed Functional Languages Research Group at Heriot-Watt University Ofcourse, if you have done other interesting things with Haskell we would very much like to know about those as well! If at all possible, could you please send in your contribution by friday *this* week, i.e. May 7th 2004? We will update the topics list on http://www.haskell.org/communities/topics.html with a nice tickmark as soon as we have received your contribution, to show that you too support the Haskell Communities and Activities Report! Thanks, Arthur (interim editor, May 2004 edition) -- Haskell Communities and Activities Report (May 2004 edition) All contributions are due in by the end of next week! http://www.haskell.org/communities/

1 0

Call for recommendation
by 无云 01 May '04

01 May '04

Hi, all I'm a newbie to Haskell and FP now, I need your help. Could anybody give me any advice on how to learn and master the Haskell language and thinking/programming in FP? I especially expect a list of some excellent books on Haskell. M.H.Fan 05/01/04 ======================================================== 是邮件你收有病毒我上！VIP邮箱全面保护！ http://vip.163.com 中国最大的免费邮箱在等你 25兆空间4兆附件！　　　　　 http://mail.163.com 点击网易泡泡惊喜无限全免费手机短信任你发!　　　　　　http://popo.163.com

3 2

Is it possible to read existential types?
by S. Alexander Jacobson 29 Apr '04

29 Apr '04

Is is possible to read/show an existentially typed list? Would it be possible if, instead of using read/show, I defined some variant that relies on the typeof/typeable infrastructure? Is there a way to read in a typename for use in coercing a read? -Alex- _________________________________________________________________ S. Alexander Jacobson mailto:me@alexjacobson.com tel:917-770-6565 http://alexjacobson.com

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Re: sizeFM type
by Tomasz Zielonka 28 Apr '04

28 Apr '04

On Mon, Apr 26, 2004 at 08:43:44PM -0400, ajb(a)spamcop.net wrote: > > And re sizeFM, I would note that Google has more > > than 2^31 pages indexed in memory. > > I would note, in addition, that they don't have that many pages indexed > on a single machine. Almost nobody has a database with that many records > on a single machine, even those who have clusters of 64 bit machines. It seems I qualify as almost nobody, cause I've had a database with more than 2^34 records on a single IA32 machine. However, I wouldn't ever think of loading the entire database to RAM. Best regards, Tom -- .signature: Too many levels of symbolic links

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Re: [Haskell-cafe] Re: sizeFM type
by S. Alexander Jacobson 27 Apr '04

27 Apr '04

On Tue, 27 Apr 2004, Tomasz Zielonka wrote: > The data you provided is interesting. However, if I had such a machine, > my work could be much less challenging :) I would note that people today are making strenuous efforts to retain compatibility with a Haskell98 standard that it only 6 years old! If we are making choices intended to have a lifetime of e.g. 10 years (see <http://www.joelonsoftware.com/articles/fog0000000017.html>), then I would observe that the 24gb machine I specced out will, assuming Moore's law lasts, cost only $2k in 5-6 years. The Sun Fire 15k with 288GB of memory and 72 Sparcs (think GPH!), that today costs $2.6m will cost only $155k. Note, since you don't need 72 CPUs, you'll probably be able to get this machine for <$50k. So, perhaps the question here is: What is the planned lifetime of sizeFM? Do we prefer the safety of knowing our code will work at arbitrary scale or the ecstacy of having it run fast today? Put differently, would you rather cope with hardware limitations today or software limitations tomorrow? -Alex- _________________________________________________________________ S. Alexander Jacobson mailto:me@alexjacobson.com tel:917-770-6565 http://alexjacobson.com

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Re: [Haskell] Re: sizeFM type
by Ketil Malde 27 Apr '04

27 Apr '04

[Moved to -cafe at SPJ's request] "Serge D. Mechveliani" <mechvel(a)botik.ru> writes: > On Mon, Apr 26, 2004 at 10:09:18PM +0200, Ketil Malde wrote: >> "Serge D. Mechveliani" <mechvel(a)botik.ru> writes: >>> length :: [a] -> Integer >>> smallLength :: [a] -> Int >> Hmm...isn't it possible to use a class here? Perhaps even Num? > I think, for many functions, like length, it is most natural to > return Integer only. Well, perhaps Integral is a better choice. I was perhaps thinking of functions like: average xs = sum xs / length xs I'd rather have a polymorphic "length", than different versions for each possible return type. One problem could be that with average xs = sum xs / fromIntegral (length xs) the return type of length becomes ambigous? -kzm -- If I haven't seen further, it is by standing in the footprints of giants

1 0

Data constructors
by Mark Carroll 26 Apr '04

26 Apr '04

I keep running into annoyance in having to name data constructors differently if they're for different types if they're in the same module or something. I wish that something like some Type.Constructor syntax worked in order to disambiguate. Or, better still, I have that problem with function names too (e.g. Data.List.union, Data.Set.union, IIRC) and it occurs to me that a lot of this can be resolved automatically because the types only make sense with one of the choices. I'm not really proposing any changes; more, I'm wondering what others' thinking is about this sort of thing - what annoys them, how they get around it, etc. -- Mark

4 4

Hierarchical Library
by haskell＠alias.spaceandtime.org 24 Apr '04

24 Apr '04

Hello, Why does this fail? ========================== import System.Posix test = Temp.mkstemp ========================== Are the libraries hierarchical in name only? Thanks, James

2 2

FiniteMap, Map, FiniteSet, Set
by haskell＠alias.spaceandtime.org 24 Apr '04

24 Apr '04

Here's a really elementary question: Why does the library have [FiniteMap and Set] instead of [FiniteMap and FiniteSet] or just [Map and Set]? Is there some reason for this inconsistency? Thanks, James

3 2

Syntax for modifying nested product types
by Mark Carroll 23 Apr '04

23 Apr '04

I have data objects where each component is a labelled field through which I access or modify it. I have a hierarchy of these - many of the fields are themselves such data objects, so I may need to apply a few selector functions to get down to what I want (call these "deep"). For fairly flat things, the updates using field labels are okay. However, when I have a function that needs to tweak various bits of various "deep" fields, based on various conditions, then it gets really ugly, not least because of the conditions: foo { bar = baz } becomes, let old = bar foo in foo { bar = if cond then f old else old } or whatever. Do I need such a let for each component and sub-component of the object that has a child that may be modified, or will foo { bar = if cond then f (bar foo) else bar foo } be suitably optimised? (The if's inside the {} because there may be multiple fields (or children thereof) being modified based on different conditions.) -- Mark

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Re: [Haskell-cafe] Splitting a list
by Steve Schafer 23 Apr '04

23 Apr '04

On Wed, 21 Apr 2004 14:57:57 +0100, you wrote: >How about implementing a directly recursive solution? Simply >accumulate the sum so far, together with the list elements you have >already peeled off. Once the sum plus the next element would exceed >the threshold, emit the accumulated elements, and reset the sum >to zero. > > splitlist threshold xs = split 0 [] xs > where > split n acc [] = reverse acc: [] > split n acc (x:xs) > | x >= threshold = error (show x++" exceeds threshold ") > | n+x > threshold = reverse acc : split 0 [] (x:xs) > | otherwise = split (n+x) (x:acc) xs Thanks. Apart from a small off-by-one problem (the "x >= threshold" test needs to be "x > threshold" instead), it works fine. I had actually started along those lines, but got bogged down in the details of passing the accumulator around, and ended up painting myself into a corner, so I abandoned that approach (prematurely, as it turns out). And thanks to everyone else who replied--I don't want to clutter the list with a lot of individual replies. As you can probably tell, I've only recently begun playing with Haskell, and the process of reconfiguring my neurons into recursive loops has not yet been completed. -Steve

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Re: [Haskell-cafe] Syntax for modifying nested product types
by MR K P SCHUPKE 23 Apr '04

23 Apr '04

I would just do: f' = if cond0 then f else id g' = if cond1 then g else id foo { bar = f' (bar foo) , wib = g' (wib foo) } I like seeing the record updated, rather than hiding it in a function. Regards, Keean.

1 0

Splitting a list
by Steve Schafer 22 Apr '04

22 Apr '04

I have a list of integers, e.g.: [1,5,3,17,8,9] I want to split it into a pair of lists, with the criterion being that the sum of the elements in the first list is as large as possible, but not exceeding a threshold value. For example, if the threshold is 10, the result should be: ([1,5,3],[17,8,9]) and then I want to recursively apply this process to the remainder of the list, with the end result being a list of lists of integers. Using the same list along with a threshold of 18, I would get: [[1,5,3],[17],[8,9]] I have devised a means of doing this: 1) Create an auxiliary list of integers, where the n'th element is equal to the sum of the first n elements of the original list. 2) Zip the auxiliary list with the original list. 3) Use span to break the list in two according to the threshold. 4) Unzip the two resulting lists and discard the auxiliary portions. 5) Repeat from step 1, operating on the tail of the list, until there's nothing left. Here's the code that implements this: runningSum :: (Ord a, Num a) => [a] -> [a] runningSum [] = [] runningSum (i:[]) = i : [] runningSum (i:j:js) = i : runningSum (i+j : js) zipWithSum :: (Ord a, Num a) => [a] -> [(a,a)] zipWithSum xs = zip (runningSum xs) xs threshold :: (Ord a, Num a) => [a] -> a -> ([(a,a)],[(a,a)]) threshold xs t = let test x = (t >= (fst x)) in span test (zipWithSum xs) splitFirst :: (Ord a, Num a) => [a] -> a -> ([a],[a]) splitFirst xs t = let (ys,zs) = threshold xs t in (snd (unzip ys), snd (unzip zs)) splitAll :: (Ord a, Num a) => [a] -> a -> [[a]] splitAll [] _ = [] splitAll xs t = let (ys, zs) = splitFirst xs t in ys : (splitAll zs t) (One thing that's missing from this code is a check to verify that no single element in the list is greater than the threshold, which should raise an error, rather than get stuck in an infinite loop.) The algorithm as implemented works fine, but it seems overly complicated and not very elegant. I get the feeling that I'm missing some obvious simplification, but I can't find it. Any ideas? Thanks, -Steve Schafer

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Re: [Haskell-cafe] Splitting a list (Modified by Doaitse Swierstra)
by Doaitse Swierstra 21 Apr '04

21 Apr '04

I have the impression that my posting did not make it, so I am trying again: How about: splitList weight l = munch l weight where munch [] _ = [[]] munch p@(x:xs) w = if w >= x then let (r:rr) = munch xs (w-x) in (x:r):rr else []:munch p weight main = print (splitList 18 [1,5,3,17,8,9]) Doaitse Swierstra PS: note that your problem is a bit underspecified: -- what to return for an empty list -- what to do if a number is larger than the weight On 2004 apr 21, at 15:42, Steve Schafer wrote: > I have a list of integers, e.g.: > > [1,5,3,17,8,9] > > I want to split it into a pair of lists, with the criterion being that > the sum of the elements in the first list is as large as possible, but > not exceeding a threshold value. For example, if the threshold is 10, > the result should be: > > ([1,5,3],[17,8,9]) > > and then I want to recursively apply this process to the remainder of > the list, with the end result being a list of lists of integers. Using > the same list along with a threshold of 18, I would get: > > [[1,5,3],[17],[8,9]] > > I have devised a means of doing this: > > 1) Create an auxiliary list of integers, where the n'th element is > equal > to the sum of the first n elements of the original list. > > 2) Zip the auxiliary list with the original list. > > 3) Use span to break the list in two according to the threshold. > > 4) Unzip the two resulting lists and discard the auxiliary portions. > > 5) Repeat from step 1, operating on the tail of the list, until there's > nothing left. > > Here's the code that implements this: > > runningSum :: (Ord a, Num a) => [a] -> [a] > runningSum [] = [] > runningSum (i:[]) = i : [] > runningSum (i:j:js) = i : runningSum (i+j : js) > > zipWithSum :: (Ord a, Num a) => [a] -> [(a,a)] > zipWithSum xs = zip (runningSum xs) xs > > threshold :: (Ord a, Num a) => [a] -> a -> > ([(a,a)],[(a,a)]) > threshold xs t = let test x = (t >= (fst x)) > in span test (zipWithSum xs) > > splitFirst :: (Ord a, Num a) => [a] -> a -> ([a],[a]) > splitFirst xs t = let (ys,zs) = threshold xs t > in (snd (unzip ys), snd (unzip zs)) > > splitAll :: (Ord a, Num a) => [a] -> a -> [[a]] > splitAll [] _ = [] > splitAll xs t = let (ys, zs) = splitFirst xs t > in ys : (splitAll zs t) > > (One thing that's missing from this code is a check to verify that no > single element in the list is greater than the threshold, which should > raise an error, rather than get stuck in an infinite loop.) > > The algorithm as implemented works fine, but it seems overly > complicated > and not very elegant. I get the feeling that I'm missing some obvious > simplification, but I can't find it. Any ideas? > > Thanks, > > -Steve Schafer > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

1 0

WildCard question
by Paul Cosby 14 Apr '04

14 Apr '04

Hi, I am very new to haskell and the program hugs and I am having problems using the wildcard operator _ Every time I try to use it in an definition it says something like the symbol /017 is not recognised. I am sure my defintions are correct as I tested it with an example defintion from Alan Thompsons craft of functional programming book. I am guessing that all the relevant modules have not been loaded properly. I have reinstalled hugs with full implementation and this has not helped. I am using a windows 98 PC. Any help suggestions? Or has anyone just got the wildcard definition which are could just paste into my scripts Many thanks --------------------------------- Yahoo! Messenger - Communicate instantly..."Ping" your friends today! Download Messenger Now

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[Haskell-cafe] Call for Contributions - HC&A Report (May 2004 edition)
by Arthur van Leeuwen 14 Apr '04

14 Apr '04

Dear Haskellers, two changes of season and one of editor later, we strongly suspect the Haskell community has also moved along admirably. Thus comes the need to refresh our list of all things Haskell, and give *you* the opportunity to tell all the world about your passion. We invite your contributions to the imminent sixth edition of the ================================================================ Haskell Communities & Activities Report http://www.haskell.org/communities/ Submission deadline: 7 May 2004 (please send your contributions to hcar(a)haskell.org, in plain ASCII or LaTeX format) ================================================================ The Haskell Communities & Activities Reports biannually report on the state of Haskell over the last 6 months, and sometimes on plans for the next 6 months. If you have only recently joined the Haskell world, browse the November 2003 edition -- you will find an overview of what's going on as well as starting points and links that may provide many answers. We would much like you to send your contributions in by the first week of May, and will then endeavor to publish the collective report roughly a month before the start of summer. Note that this is a great opportunity to gauge the state of your research, update your web pages, release new versions, announce entirely new projects, summarize developments in sub-communities for every Haskeller to see, etc. :) The way we like to do things is have you update all existing summaries, as the purpose of these reports is to show *recent/current* activities. We will drop any topics that haven't had any activity since May 2003, so if you haven't updated your topic since then, now is a good time! Ofcourse, if you have new things to share, so much the better. We would love to hear of it, and include it in the report. Please keep your contributions short though, a couple hundred words maximum, as we would like to keep the size of the report reasonable. It is intended as an overview after all. Looking forward to your contributions, Arthur (interim editor, May 2004 edition) ----------------------------------- topics New suggestions for current hot topics, activities, projects, etc. are welcome - especially with names and addresses of potential contacts, but here is a non-exclusive list of likely topics (see also http://www.haskell.org/communities/topics.html ): General Haskell developments; Haskell implementations; Haskell extensions; Standardization and documentation; Haskell tutorials, howtos and wikis; Organisation of Haskell tool and library development; Haskell-related projects and publications; new research, fancy tools, long-awaited libraries, cool applications; Feedback from specialist mailing lists to the Haskell community as a whole; Haskell announcements; all (recent) things Haskell Announcements: if you've announced anything new on the Haskell list over the last six months, you'll want to make sure that is reflected in this edition! Project pings: if you're maintaining a Haskell tool or library or somesuch, you'll want to let everyone know that it is still alive and actively maintained, even if there have been no new additions, but all the more if there have been new developments. Tutorials: if you've fought with some previously undocumented corner of Haskell, and have been kind enough to write down how you did manage to build that graphical user interface, or if you' ve written a tutorial about some useful programming techniques, this is your opportunity to spread the word (short, topic-specific, and hands-on tutorials that only show how to achieve a certain practical task would do a lot to make things easier for new Haskellers - please write some!) Applications: if you've been working quietly, using Haskell for some interesting project or application (commercial or otherwise), you might want to let others know about what you're using Haskell for, and about your experiences using the existing tools and libraries; are you using Haskell on your job? An interesting thread about using Haskell and more generally functional programming for non-Haskell things seems to recur with reasonable frequency - why not write a sentence or two about your use of Haskell for our report? Feedback: if you're on one of the many specialist Haskell mailing lists, you'll want to report on whatever progress has been made there (GUI API discussions, library organisation, etc.) If you're unsure whether a contact for your area of work has come forward yet, have a look at the report's potential topics page, or get in touch with me. I've contacted last time's contributors, hoping they will volunteer to provide updates of their reports, and will update the contacts on the topics page fairly regularly. But where you don't yet see contacts listed for your own subject of interest, you are very welcome to volunteer, or to remind your local community/project team/mailing list/research group/etc. that they really ought to get their act together and let the Haskell community as a whole know about what they've been doing! :-) -------------------------- what should I write? That depends on your topic, but as a general rule, it shouldn't take you long. A simple sentence or two about your use of Haskell could go into the "Individual Haskellers and their projects" section. If you're a company, or if you're working on a project using Haskell as the implementation language, a paragraph on that could go into the "Commercial Applications" and "Non-Commercial Applications" sections. A typical summary report about a tool/library/project/application/.. would be between 1 and 3 paragraphs of ASCII text (what's it about? major topics and results since the last report? current hot topics? major goals for the next six months?) plus pointers to material for further reading (typically to a home page, or to mailing list archives, specifications and drafts, implementations, meetings, minutes,..). For those who prefer templates to fill in, the report is edited in LaTeX, and an entry template might look something like this: \subsubsection{<MYSTUFF>} \report{<MY NAME}\\ <WHAT IS IT?> <WHAT IS ITS STATUS? / WHAT HAS HAPPENED SINCE LAST TIME?> <CAN OTHERS GET IT?> <WHAT ARE THE IMMEDIATE PLANS?> <FURTHER READING> \url{<PROJECT URL>} Browsing through previous editions should give you a good idea of the variety of possibilities, ranging from very brief to extensive. -- Haskell Communities and Activities Report (May 2004 edition) All contributions are due in by the 7th of May 2004! http://www.haskell.org/communities/

1 0

C Bindings?
by Russ Lewis 13 Apr '04

13 Apr '04

Does Haskell have some mechanism that allows it to link to C, or other imperative languages? I know, you could use the IO Monad to do it...using stdin and stdout as pipes to any other program. But is there a way to link Haskell into a C program? Thanks again for the help for a newbie... Russ

3 2

RE: [Haskell-cafe] Building Haddock on Windows
by Bayley, Alistair 13 Apr '04

13 Apr '04

> Try upgrading GHC. Thanks, I'll try that. Would it be too much to ask that Haddock be distributed with GHC? Reasons for: - GHC library docs are produced with Haddock - GHC library docs already include Haddock interface files - would ensure version of Haddock matches that used to produce interface files - would save lazy developers like myself from downloading and compiling. Think of it as one less barrier to overcome; a small step closer to more widespread adoption. Reasons against: - would favour one documentation tool over others - would add an unnecessary item to GHC build/packaging procedure (is this much of an overhead?) Mind you, if you include Haddock, why stop there? Why not include other developers tools, like (say) Hmake, Happy, Hat, ...? Alistair. > -----Original Message----- > From: Simon Marlow [mailto:simonmar@microsoft.com] > Sent: 13 April 2004 10:59 > To: Bayley, Alistair > Cc: haskell-cafe(a)haskell.org > Subject: RE: [Haskell-cafe] Building Haddock on Windows > > > Yes, I suspect you have a mismatch between the version of Haddock on > your system and the version of Haddock used to produce the interfaces. > Try upgrading GHC. > > Cheers, > Simon > > > -----Original Message----- > > From: Bayley, Alistair [mailto:Alistair_Bayley@ldn.invesco.com] > > Sent: 05 April 2004 16:19 > > To: Simon Marlow > > Cc: haskell-cafe(a)haskell.org > > Subject: RE: [Haskell-cafe] Building Haddock on Windows > > > > Trying to invoke Haddock with > > --read-interface=c:\ghc\ghc-6.0\doc\html\base\base.haddock > > > > Initially I get a stack overflow, so I added: +RTS -K2M -RTS ... > > > > and now I get: > > Fail: end of file > > Action: Data.Binary.getWord8 > > > > Is the --read-interface option specified correctly? This is > > where my GHC > > installation is, and the base.haddock file exists in there. I > > assume this is > > the interface file. > > > > Is this a compatibility problem? i.e. haddock file > > distributed with GHC 6.0 > > too old for Haddock 0.6? ----------------------------------------- ***************************************************************** Confidentiality Note: The information contained in this message, and any attachments, may contain confidential and/or privileged material. It is intended solely for the person(s) or entity to which it is addressed. Any review, retransmission, dissemination, or taking of any action in reliance upon this information by persons or entities other than the intended recipient(s) is prohibited. If you received this in error, please contact the sender and delete the material from any computer. *****************************************************************

1 0

What's wrong with my Haskell code?
by Meihui Fan 13 Apr '04

13 Apr '04

when loading the following code, hugs escaped and reported that ERROR "cal24.hs":10 - Instance of Integral (Ratio Integer) required for definition of Main.eval I don't know why and how to solve it, anyone help me? data ETree = Add ETree ETree | Sub ETree ETree | Mul ETree ETree | Div ETree ETree | Node Integer deriving Show eval :: ETree->Maybe Rational eval (Node x) = Just (fromInteger x) eval (Add t1 t2) = do { x<-eval t1; y<-eval t2; if x>=y then return (x+y) else Nothing } eval (Sub t1 t2) = do { x<-eval t1; y<-eval t2; return (x-y) } eval (Mul t1 t2) = do { x<-eval t1; y<-eval t2; if x>=y then return (x*y) else Nothing } eval (Div t1 t2) = do { x<-eval t1; y<-eval t2; if y/=0 then return (x `div` y) else Nothing }

2 1

RE: [Haskell-cafe] Building Haddock on Windows
by Simon Marlow 13 Apr '04

13 Apr '04

Yes, I suspect you have a mismatch between the version of Haddock on your system and the version of Haddock used to produce the interfaces. Try upgrading GHC. Cheers, Simon > -----Original Message----- > From: Bayley, Alistair [mailto:Alistair_Bayley@ldn.invesco.com] > Sent: 05 April 2004 16:19 > To: Simon Marlow > Cc: haskell-cafe(a)haskell.org > Subject: RE: [Haskell-cafe] Building Haddock on Windows > > Trying to invoke Haddock with > --read-interface=c:\ghc\ghc-6.0\doc\html\base\base.haddock > > Initially I get a stack overflow, so I added: +RTS -K2M -RTS ... > > and now I get: > Fail: end of file > Action: Data.Binary.getWord8 > > Is the --read-interface option specified correctly? This is > where my GHC > installation is, and the base.haddock file exists in there. I > assume this is > the interface file. > > Is this a compatibility problem? i.e. haddock file > distributed with GHC 6.0 > too old for Haddock 0.6? > > > > -----Original Message----- > > From: Simon Marlow [mailto:simonmar@microsoft.com] > > Sent: 17 March 2004 15:29 > > To: Bayley, Alistair > > Cc: haskell-cafe(a)haskell.org > > Subject: RE: [Haskell-cafe] Building Haddock on Windows > > > > > > > - why doesn't it know what Int, String, Float, IO, Monad, > > > Show, etc are? > > > What invocation option do I need to ensure it links to > > > Prelude/hierarchical-library stuff? (I think it's > > > --read-interface; must I > > > generate interface files for the Prelude and libraries though?) > > > > You get the interfaces for the libraries with a GHC > > installation. Just > > use the --read-interface flag to tell Haddock about them. > > > > Cheers, > > Simon > > ----------------------------------------- > ***************************************************************** > Confidentiality Note: The information contained in this > message, and any attachments, may contain confidential > and/or privileged material. It is intended solely for the > person(s) or entity to which it is addressed. Any review, > retransmission, dissemination, or taking of any action in > reliance upon this information by persons or entities other > than the intended recipient(s) is prohibited. If you received > this in error, please contact the sender and delete the > material from any computer. > ***************************************************************** > >

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handling read exceptions
by S. Alexander Jacobson 13 Apr '04

13 Apr '04

I want to read strings that look like "2" or "hello" into values of type Integer or String. The problem is that read requires that strings be read as "\"hello\"". Is there a way either to convince read to not require wrapping quotation marks or, alternetively, to catch a read exception, and do something sane? -Alex- _________________________________________________________________ S. Alexander Jacobson mailto:me@alexjacobson.com tel:917-770-6565 http://alexjacobson.com

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[Haskell-cafe] A useful trick for inclusion and exclusion of commented code
by Sunil KOTHARI 12 Apr '04

12 Apr '04

Here's a little trick that I often use for inclusion and exclusion of a chunk of code. Imagine we have a function foo defined below. The three lines of code can be included or excluded just by a changing a single character --- that's the trick. So, the idea is to comment our code in a slightly different way like this to exclude the code from executing: 1 {- 2 myand :: Bool -> Bool -> Bool 3 myand a b = if a == b then True 4 else False 5 --} Notice how line 5 is commented '--}' instead of '-}'. Now, if we change the first character in line 11 from '{' to '-' the entire chunk of code from line 12-14 no longer remains comment. 11 -- 12 myand :: Bool -> Bool -> Bool 13 myand a b = if a == b then True 14 else False 15 --} This trick can also be applied to C++ comments. Best wishes, Sunil

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Re: [Haskell-cafe] Newbie: Is it possible to catch _|_ ?
by Jon Cast 10 Apr '04

10 Apr '04

Russ Lewis <spamhole-2001-07-16(a)deming-os.org> wrote: > Another newbie question: Is it possible to catch _|_ - that is, to > encounter it, handle it and continue? I'm assuming that you all are > going to say no, since I can't figure out any way to do this and retain > the functional nature of Haskell. This isn't possible in a deterministic language, for several reasons. The IO monad, however, is non-deterministic, and its `catch' function can be used to catch any _|_ value that can be caught, specifically those arising from calls to throw, error, and certain infinite loops. It is non-deterministic, though, so it won't catch all _|_s, nor will it give any guarantee as to which _|_ it will catch (for example, in error 3 + throw (DynException (toDynamic False)) it's indeterminate whether error's return value or throw's return value will ultimately be caught. Jon Cast

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Re: [Haskell-cafe] How to implement `amb' operator?
by Andy Moran 07 Apr '04

07 Apr '04

[ Was sent only to participants, but I meant to send it to the list ] On Wednesday 07 April 2004 06:04 am, Brandon Michael Moore wrote: > Keith is talking about a "comitted choice" style of nondeterminism, where > one of the arguments is picked and the computation continues from there. > > If you want a computation with backtracking, or a list of all possibly > results then you should use the list monad, or another monad that > supports nondetermanism. An important feature of amb is known as "bottom-avoidance", and you can get that with Keith's scheme (although it won't be terribly fair; see mailing list discussions about the fairness or lack thereof of the GHC RTS scheduler), but you can't get it with backtracking (indeed, backtracking is entirely deterministic). Of course, Keith's scheme forces you to work within the IO monad, as well it should. If we allowed amb to have the type amb :: a -> a -> a then all of the transformations carried out within GHC's mid-stage would need to be re-examined for their correctness (in the context of a semantics that understood sharing, as non-determinism makes sharing observable). It's possible that many would still be valid, but it's by no means clear. To my knowledge, no-one has developed a correct semantics for sharing and amb that enables proof of simple transformations like let-floating or lambda-floating or even call-by-need beta reduction, let alone some of the more complex ones like deforestation (which requires a theory that can handle recursion well). Cheers, Andy -- Andy Moran Ph. (503) 626 6616, x113 Galois Connections Inc. Fax. (503) 350 0833 12725 SW Millikan Way, Suite #290 http://www.galois.com Beaverton, OR 97005 moran(a)galois.com

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RE: [Haskell-cafe] How to implement `amb' operator?
by Bayley, Alistair 07 Apr '04

07 Apr '04

Not wanting to appear glib, but... SICP 4.3.3 Implementing the Amb Evaluator http://mitpress.mit.edu/sicp/full-text/book/book-Z-H-28.html#%_sec_4.3.3 Ralf Hinze (2000): Deriving Backtracking Monad Transformers http://www.informatik.uni-bonn.de/~ralf/publications.html#P12 > -----Original Message----- > From: Meihui Fan [mailto:mhfan@ustc.edu] > Sent: 07 April 2004 21:20 > To: haskell-cafe(a)haskell.org > Subject: [Haskell-cafe] How to implement `amb' operator? > > > Hi, > I think it's good idea to compute non-deterministic > problems with the `amb' > operator, just as in LISP/scheme. But how to implement it in haskell? > > noclouds ----------------------------------------- ***************************************************************** Confidentiality Note: The information contained in this message, and any attachments, may contain confidential and/or privileged material. It is intended solely for the person(s) or entity to which it is addressed. Any review, retransmission, dissemination, or taking of any action in reliance upon this information by persons or entities other than the intended recipient(s) is prohibited. If you received this in error, please contact the sender and delete the material from any computer. *****************************************************************

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How to implement `amb' operator?
by Meihui Fan 07 Apr '04

07 Apr '04

Hi, I think it's good idea to compute non-deterministic problems with the `amb' operator, just as in LISP/scheme. But how to implement it in haskell? noclouds

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Newbie: Is it possible to catch _|_ ?
by Russ Lewis 07 Apr '04

07 Apr '04

Another newbie question: Is it possible to catch _|_ - that is, to encounter it, handle it and continue? I'm assuming that you all are going to say no, since I can't figure out any way to do this and retain the functional nature of Haskell.

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Data structure for filesystem representation
by Gerd M 06 Apr '04

06 Apr '04

Hello! For an university project i need to design a data structure (in haskell) that's capable of representing a unix filesystem and therefore fast insertion/deletion. Support for persistent storage is also required. Principally i was thinking to use a Binary Search Tree or an AVL Tree but since this is my first real world application in Haskell I'm not so sure if this is a good (performant) solution. Any advice is welcome! Regards, Gerd. _________________________________________________________________ MSN 8 with e-mail virus protection service: 2 months FREE* http://join.msn.com/?page=features/virus

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RE: [Haskell-cafe] Using -fcontext-stackXX in an OPTIONS pragma
by Simon Peyton-Jones 06 Apr '04

06 Apr '04

Contrary to the manual, -fcontext-stackN is a static flag, so you have to set it on the command line, I'm afraid. I'll fix the manual. Simon | -----Original Message----- | From: haskell-cafe-bounces(a)haskell.org [mailto:haskell-cafe-bounces@haskell.org] On Behalf Of | Anders Höckersten | Sent: 05 April 2004 13:43 | To: haskell-cafe(a)haskell.org | Subject: [Haskell-cafe] Using -fcontext-stackXX in an OPTIONS pragma | | Hi! | I am currently trying to use GHC 6.2 together with the OPTIONS pragma. | My program is generating files and needs to set the context stack | differently for different files. I try to do this using the OPTIONS | pragma, by having this line at the top of my files: | {-# OPTIONS -fglasgow-exts -fcontext-stackXX #-} | (where XX is some generated number) | However, compiling this gives me an error: | Test.hs: unknown flags in {-# OPTIONS #-} pragma: -fcontext-stack60 | | Is it supposed to be possible to set the context stack using the OPTIONS | pragma, and if so, what am I missing? | | /Anders | _______________________________________________ | Haskell-Cafe mailing list | Haskell-Cafe(a)haskell.org | http://www.haskell.org/mailman/listinfo/haskell-cafe

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[Haskell-cafe] Fuse functions given to map and filter without RULES pragma
by Roman Beslik 05 Apr '04

05 Apr '04

I've found a funny way to fuse functions given to map and filter without RULES pragma: toSelect = (,) Just fromSelect = uncurry mapMaybe transformFirst f (x, y) = (f x, y) maybe f = transformFirst (\g x -> g x >>= f) map f = maybe (Just . f) filter f = maybe (\x -> if f x then Just x else Nothing) test :: [Int] test = fromSelect (map (*2) (filter (<10) (map (+3) (toSelect [1,3,4,6,8])))) Everything looks like obvious application of filter and map, but GHC inline (*2), (<10), (+3) (with -O flag) :^). -- Best regards, RB.

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Re: [Haskell-cafe] Context for type parameters of type constructors
by Henning Thielemann 05 Apr '04

05 Apr '04

On 3 Apr 2004, Dylan Thurston wrote: > You might want to add a functional dependency, if you only have one > type of scalars per vertor space: > > > class VectorSpace v a | v -> awhere > > zero :: v > > add :: v -> v -> v > > scale :: a -> v -> v > > But then again, you might not. > > > > instance Num a => VectorSpace a a where > > > zero = 0 > > > add = (+) > > > scale = (*) I see ... functional dependency means "for each 'v' there is only one 'a' that forms a VectorSpace with 'v'". Thus the "VectorSpace a a" instance would exclude any other instance. Since this instance causes 'undecidable instance' and 'overlapping instance' errors in each constellation I tried I finally decided to content myself with special instances instance VectorSpace Double Double instance VectorSpace Float Float These will be the anchors for each type construction recursion needed for more complex VectorSpaces. But e.g. instance VectorSpace (Complex Double) (Complex Double) is still rejected as overlapping with instance (RealFloat v, VectorSpace a v) => VectorSpace a (Complex v) of course ... unfortunately. In addition I separated a class VectorSpacePure from the VectorSpace class that contains the operations that don't need the scalar type 'a', i.e. 'zero' and 'add'. Further I swapped consistently 'a' and 'v' in the class definition and in the quantity data type, i.e. "class VectorSpace a v" and "data Quantity a v". I suppose that it will more oftenly be necessary to say something about the vectors that can built from a certain scalar (i.e. Quantity a) rather than about the scalar types that can be the base of a vector type. > By the way, depending how you resolve the issue above, you might want > instead > > > instance (RealFloat a, VectorSpace b a) => VectorSpace [b] a where > > ... That's really a nice generalization! With the specialised instances for Float and Double as shown above this works. Now I can built VectorSpace over several levels, e.g. a list of complex numbers as vectors with respect to real numbers. But it prohibits functional dependency, right? Functional dependency would probably allow the compiler to detect which scalar type is associated with an vector and thus the compiler wouldn't ask me to tell it explicitly all the time. > Alternatively, if you want to consider varying the scalars, you can > add 'a' as a dummy type variable to 'Quantity': > > > data Quantity v a = Quantity v > > > > instance (Show v, Fractional a, Normed v a) => > > Show (Quantity v a) where > > show (Quantity v) = > > let nv::a = norm v > > in (show (scale (1/nv) v)) ++ "*" ++ > > (show nv) Since I can't use functional dependency I chose this way. Maybe a type class like the VectorSpace finds the way to a revised version of Prelude? Thanks a lot for the help!

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