- Haskell-Cafe - Haskell.org

Quickcheck generator for arrays of integers
by Paulo J. Matos 11 Dec '07

11 Dec '07

Hello all, I using an IArray to represent a matrix. I'm trying to write some properties checks with Quickcheck. Quickcheck lacks instance generators for arrays of ints for example, is there anything I can use out there or should I define it myself? (I'm asking because it seems to be something that other people already did) Cheers, -- Paulo Jorge Matos - pocm at soton.ac.uk http://www.personal.soton.ac.uk/pocm PhD Student @ ECS University of Southampton, UK

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Literate HTML
by Neil Mitchell 11 Dec '07

11 Dec '07

Hi I want literate Haskell, but where the literate bit forming a document is actually HTML, not latex. Does anyone have any idea how to go about this? For a start, how do I persuade GHC to run the file: C:\Documents\Uni\tagsoup>runhaskell index.html Warning: ignoring unrecognised input `index.html' <interactive>:1:108: attempting to use module `Main' (Main.hs) which is not loaded And leading on to that, how Cabal might be used to compile the file. Secondly, what tricks can people think of for placing the code inside - I have come up with: <code> > foo = 1 </code> And: <code>  foo = 1 \end{code} </code> I can then hide the trailing \end{code} with a margin-bottm: -2em. Does anyone have any other ideas? Thanks Neil

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[OT] A nice organized collection of threads in Haskell-Cafe
by Vimal 11 Dec '07

11 Dec '07

Hi, I am working on a product to analyze posts made in Forums, Usenet and discussion mailing lists like Haskell-Cafe. For this, I require the messages to be accessible in this format: <forum> (* example: Haskell-cafe *) [ list of - <thread> [ list of - <post> </post> ] </thread> ] </forum> as XML. However, I find that that the messages (in haskell-cafe/usenet) themselves aren't organized in this fashion. I would like to know if there is any way in which I can get the archives in this fashion. Thanks, -- ~Vimal RLE :) encode = map (length &&& head) . group decode = concatMap (uncurry replicate)

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RE: [Haskell] Nested guards?
by Simon Peyton-Jones 11 Dec '07

11 Dec '07

[redirecting to Haskell Cafe] | > It is clear that this situation must not stay this way. Bit by bit, | > disciples of Perl and Python discover Haskell and demand that Haskell will | > be plastered with syntactic sugar until the simplicity of the functional | > approach isn’t visible anymore. Sadly, they seem to be successful with this | > as syntax extensions like parallel list comprehensions show. I think it is helpful to distinguish "superficial" complexity from "deep" complexity. All of Haskell's syntactic sugar is just that: it can be translated into a small purely-functional language using straightforward translation schemes. Even something relatively complicated like the "order by/group by" extension that Phil and I proposed at this year's Haskell workshop, has an easy translation that takes a dozen or two lines to give in full detail. In contrast, something like higher order functions, or mutable state, is deep complexity. Both have a pervasive effect on the language semantics and on its implementation. (The effect of mutable state is much, much worse, but they are both deep.) Concerning Haskell, I'm quite relaxed about superficial complexity, as you'll have seen from what happens in GHC. Section 3.6 of the History of Haskell paper addresses this point specifically [1]. You may disagree with the choice made by the Haskell committee at the time, and with subsequent developments, but it was quite a conscious choice, and one not without its advantages. Simon [1] http://research.microsoft.com/%7Esimonpj/papers/history-of-haskell/history.…

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What is the role of $!?
by PR Stanley 11 Dec '07

11 Dec '07

Hi What is the role of $! ? As far as I can gather it's something to do with strict application. Could someone explain what it is meant by the term strict application please? Thanks, Paul

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Software Tools in Haskell
by Tommy McGuire 10 Dec '07

10 Dec '07

In the "if anyone is interested,..." department.... For reasons that remain unclear, early this fall I started translating Brian W. Kernighan and P.J. Plaugher's classic _Software Tools in Pascal_ into Haskell. I have completed most of it, up to the second part of chapter 8 which presents a proto-m4 preprocessor. I have the code online including notes, comments, descriptions, and a few alternate approaches. Attractions include: * A fair gamut of the usual Unix suspects: proto-cat, proto-wc, proto-tr, proto-compress, proto-ar, proto-grep, etc. * A usable editor, if you consider a de-featured ed-alike to be usable. * A simple monadic regular expression engine. * Zippers, Parsec, the State monad, the StateT monad transformer, and other attempts to sully Computing Science's brightest jewels. * Lots and lots of really bad Haskell, including a fair bit that is a direct translation of 30-year old Pascal (see xindex in translit, Ch. 2, if you need to skip lunch). Programming really has advanced, you know. Anyway, the URL is: http://www.crsr.net/Programming_Languages/SoftwareTools Questions and comments would be appreciated, especially suggestions for how to make the code cleaner and more understandable. Flames and mockery are welcome, too, but only if they're funny---remember, I've been staring at Haskell, Pascal (plus my job-related Perl, CORBA, and C++) for a while; there's no telling what my mental state is like. [I had intended to wait until I had the whole thing done to make this announcement, but I recently moved and have not made much forward progress since, other than putting what I had done online.] -- Tommy M. McGuire mcguire(a)crsr.net

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Re: type class question
by Peter Padawitz 10 Dec '07

10 Dec '07

Jules Bean wrote: > Peter Padawitz wrote: > >>>> So the fundep would solve the problem. >>> >> But, actually, it doesn't :-( > > But actually, it does! Indeed... Sorry, I think I left intE out of the cycle. This might be the reason why it did not work before. > Ben Franksen's answer from yesterday compiles fine for me if I add the > missing fundep, block -> command. > > Your original code compiles without error, given the fundep. Exact > code I compiled attached at the bottom of this document. You may have > to repair long lines! > > Incidentally, I question why the "compFoo" are methods. Why not just > make them polymorphic functions? They don't look like you expect > instances to change them. The code continues to compile if I make them > functions and amend their signatures as required. I put compFoo into the class for the same reason why /= is part of the class Eq: both functions are unique as soon as the others have been instantiated. > > {-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies #-} > > type Block = [Command] > data Command = Skip | Assign String IntE | Cond BoolE Block Block | > Loop BoolE Block > data IntE = IntE Int | Var String | Sub IntE IntE | Sum [IntE] | > Prod [IntE] > data BoolE = BoolE Bool | Greater IntE IntE | Not BoolE > > class Java block command intE boolE | block -> command, command -> > intE, intE -> boolE, boolE -> block > where block_ :: [command] -> block > skip :: command > assign :: String -> intE -> command > cond :: boolE -> block -> block -> command > loop :: boolE -> block -> command > intE_ :: Int -> intE > var :: String -> intE > sub :: intE -> intE -> intE > sum_ :: [intE] -> intE > prod :: [intE] -> intE > boolE_ :: Bool -> boolE > greater :: intE -> intE -> boolE > not_ :: boolE -> boolE > compBlock :: Block -> block > compBlock = block_ . map compCommand > compCommand :: Command -> command > compCommand Skip = skip > compCommand (Assign x e) = assign x (compIntE e) > compCommand (Cond be cs cs') = cond (compBoolE be) (compBlock > cs) (compBlock cs') > compCommand (Loop be cs) = loop (compBoolE be) (compBlock cs) > compIntE :: IntE -> intE > compIntE (IntE i) = intE_ i > compIntE (Var x) = var x > compIntE (Sub e e') = sub (compIntE e) (compIntE e') > compIntE (Sum es) = sum_ (map compIntE es) > compIntE (Prod es) = prod (map compIntE es) > compBoolE :: BoolE -> boolE > compBoolE (BoolE b) = boolE_ b > compBoolE (Greater e e') = greater (compIntE e) (compIntE e') > compBoolE (Not be) = not_ (compBoolE be)

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help
by Ryan Bloor 10 Dec '07

10 Dec '07

hi I am writing a basic Parser from scratch. So far I have functions;# removeSpaces# match - which checks if a string is a substring of another# orParser which combines two parser's abilities# Basic pasrers like... parseInt, parseTrue, parseFalse, parseBoolusing the orParser on True and False.What I want to do now is have a parseBinaryOp that recognises:parseBinaryOp "+" "(5 + 2) if" >>>gives>> [(EInt 5, EInt 2, "if")]So I think that I have to split the initial string into four parts."+" becomes op'(' becomes tokenF')' becomes tokenB"5" becomes e1"2" becomes e2parseBinaryOp :: String -> String -> [(Expr, Expr, String)]parseBinaryOp op str = let (tokenF,e1,op,e2,tokenB) =I am not sure how to go about separating the string for how I need itusing my other functiuons. Ryan _________________________________________________________________ The next generation of MSN Hotmail has arrived - Windows Live Hotmail http://www.newhotmail.co.uk

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Do real programs need IO? (was IO is a bad example for Monads)
by Conal Elliott 10 Dec '07

10 Dec '07

On Dec 9, 2007 10:07 AM, Daniel Fischer <daniel.is.fischer(a)web.de> wrote: > Interactive programmes without using IO? Cool :) And how! > I think you misunderstood Lennart. Thanks for checking. In this case, I think I understood Lennart fine and that he was saying what you're saying. > Would you deny that any useful programme has to do at least some of the following: > -accept programme arguments at invocation > -get input, be it from a keyboard, mouse, reading files, pipes... > -output a result or state info, to the monitor, a file, a pipe... === If by "programme", you mean the code I write, then I'm happy to deny that my programme has to do these things. Examples below. If you include a stateful RTS, then no I don't deny it. > I think Lennart was referring to that, you HAVE to know a little IO to write > programmes, at least getArgs, getLine, putStr(Ln), readFile, writeFile, > appendFile. And therefore some use of the IO monad has to be taught > relatively early. Explicit imperative programming is just one way to deal with input & output, not the only way. As proof, see FRP, Pan, or TV programs, which contain uses of none of these functions. (Nor could they, as these libraries are functional, having IO-free types and semantics.) Moreover, use of imperative programming sacrifices some of the semantic simplicity & composability that makes FP so appealing. That's why I'd like to see this belief in its necessity dispelled. That said, I don't think the existing functional (non-IO) approaches to interaction are quite there yet with the flexibility of imperative programming. It will take more work to get them there, and that work is mostly likely to be pursued by people who doubt the necessity of IO for writing "real programs". In that sense, Lennart's and your statements are self-fulfilling prophechies, as are mine. BTW, if you haven't seen it already, please check out http://haskell.org/haskellwiki/TV . The TV (tangible values) approach includes a simple algebra of interfaces (input/output) and keeps separable from the core computation. The separability allows the interface parts to be composed in parallel with the core part. For instance, when two function-valued TVs are composed, the interfaces are peeled off, so that the core functions can be composed directly. The output half of one interface and the matching input half of the other are discarded. The remaining input and output halves are recombined into a new interface, which is used as the interface of the composed TV. The core interface algebra can be used for text stream i/o, GUIs, and many other possible styles of information passing. I mention TV, because it's an example of combining the purity & composability I love about FP with the usability a "real" app. For more about this combination, please see my Google tech talk "Tangible Functional Programming: a modern marriage of usability and composability" ( http://conal-elliott.blogspot.com/2007/11/tangible-functional-programming-m…) That talk focus on end-user composability, but the essential points apply as well to explicit programming. As I mentioned before, TV (a) is currently less flexible than imperative/IO programming, and (b) has the composability, guaranteed safety, and amenability to reasoning of pure functional programming. Cheers, - Conal Am Sonntag, 9. Dezember 2007 18:31 schrieb Conal Elliott: > > > IO is important because you can't write any real program without using > > > it. > > > > Ouch! I get awfully discouraged when I read statements like this one. > The > > more people who believe it, the more true it becomes. If you want to do > > functional programming, instead of imperative programming in a > functional > > language, you can. For instance, write real, interactive programs in > FRP, > > phooey, or TV. And if you do, you'll get semantic simplicity, powerful > & > > simpler reasoning, safety and composability. > > > > - Conal > > > On Dec 8, 2007 1:26 AM, Lennart Augustsson <lennart(a)augustsson.net> wrote: > > [...] > > IO is important because you can't write any real program without using it. > > So why not teach enough of it to get people off the ground straight away? > > People who hang around long enough to do some more Haskell programming > > will run into the other monads sooner or later. But IO is an unavoidable step to > > writing Haskell programs.

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I'm translating <All about Monads> to Chinese
by Albert Lee 10 Dec '07

10 Dec '07

I have been confussed by monad for a long time. and I can't stand for it any more. so I start to translate the tutorial <All About Monads> to my mother language Chinese. My English is not good enough, so this work is only for my own study~ I know there are some other Chinese fellow in this list, wish it would be helpful. I will work for one chapter everyday. the address is: http://www.kamang.net

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