- Haskell-Cafe - Haskell.org

Displaying steps in my interpreter
by Victor Nazarov 11 Jan '08

11 Jan '08

Hello, I have little practice in Haskell. And I look forward for suggestions on how to improve the code. Code is not working: some definitions are missed. The goal of the code is to implement the evaluator for untyped lambda-calculus. The main problem is how to display each step of reduction? More detailed questions follows in the end of the code. Terms of lambda-calculus is the following data-type: > > data (Binder bndr) => > Term bndr = > Var bndr > | Lam bndr (Term bndr) > | App (Term bndr) (Term bndr) > Binder class is used to generate new unique binders and to compare binders > > class (Eq bndr) => Binder bndr > where {- ... -} > Next we'll define the evaluator. Evaluator returns the WHNF for each well-formed term, or _|_ if there is no WHNF Straight forward version: whnf :: Term bndr -> Term bndr whnf = reduce [] where reduce (a:as) (Lam b i) = reduce as $ subst b a i reduce as (App f a) = reduce (a:as) f reduce as term = foldl term App as But our goal is to perform only one step of reduction. And to display term after each reduction We refactor the original definition to perform this: > > whnfGen :: (Maybe String -> Bool) -> Term bndr -> Term bndr > whnfGen isFinal = runSteps isFinal $ reduce' [] > > where reduce' (a:as) (Lam b i) = markStep "beta" >> reduce as $ subst b a i > reduce' as (App f a) = reduce (a:as) f > reduce' as term = unwind as term > > reduce as term = > do final <- checkFinal > if final > then unwind as term > else reduce' as term > > unwind as term = return $ foldl term App as > Steps is the monad: > > newtype Steps mark a = > Steps { unSteps :: StateT (Maybe mark) (Reader (Maybe mark -> Bool)) a } > > instance Monad (Steps mark) > where (Steps a) >>= f = Steps $ a >>= \x -> unSteps (f x) > return a = Steps $ return a > > runSteps :: (Maybe mark -> Bool) -> Steps a -> (Maybe mark, a) > runSteps isFinal act = runReader (runStateT (unSteps act) Nothing) isFinal > > checkFinal :: Steps mark Bool > checkFinal = > do st <- Steps $ get > isFinal <- Steps $ lift $ ask > return $ isFinal st > > markStep :: mark -> Steps mark () > markStep = Steps . put . Just > Normal whnf can be written as follows: > > whnf = whnfGen (const False) > To print the reduction steps we can use the following code > > printReductions t = > do t' <- whnfGen isJust t > print t' > printReductions t' > Is there any better ways to implement printReductions? Is there a way to abstract out this steps pattern? For example if I want to show the process of converting lambda-terms to combinatory logic terms (I, K, S basis). I'll have to implement the same pattern. I can reuse Steps monad, but, I'll have to use markStep, checkFinal and special version of recursion. Is there a way to abstract this? Is there a more effective way to perform this? In printReductions I have to rescan the syntax tree over and over again to find the redex. Is there a way to save the position in the tree, to print the tree and then to resume from saved position? Thanx for reading this :) -- vir http://vir.comtv.ru/

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The Monad.Reader (10) - Second call for copy
by Wouter Swierstra 11 Jan '08

11 Jan '08

Call for Copy The Monad.Reader - Issue 10 It is not too late to consider writing something for the next issue of The Monad.Reader. The deadline for Issue 10 is ** January 25, 2007 ** It doesn't matter if you're an established academic or if you have only just started learning Haskell, if you have something to say, please write article for The Monad.Reader. The Monad.Reader is a electronic magazine about all things Haskell. It is less-formal than journal, but somehow more enduring than a wiki- page. There have been a wide variety of articles, including: exciting code fragments, intriguing puzzles, book reviews, tutorials, and even half-baked research ideas. * Submission Details * Get in touch with me if you intend to submit something -- the sooner you let me know what you're up to, the better. Please submit articles for the next issue to me by e-mail (wss at cs.nott.ac.uk). Articles should be written according to the guidelines available from http://www.haskell.org/haskellwiki/The_Monad.Reader Please submit your article in PDF, together with any source files you used. The sources will be released together with the magazine under a BSD license. If you would like to submit an article, but have trouble with LaTeX please let me know and we'll sort something out. All the best, Wouter This message has been checked for viruses but the contents of an attachment may still contain software viruses, which could damage your computer system: you are advised to perform your own checks. Email communications with the University of Nottingham may be monitored as permitted by UK legislation.

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Re: [Haskell-cafe] Re: Why purely in haskell?
by Miguel Mitrofanov 11 Jan '08

11 Jan '08

> As GNU is not Unix, NaN is not a number, Since NaN /= NaN, I think, we should decipher "NaN" as "Not a NaN" instead.

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Subtypes and Co/Contra-variance
by Ashley Yakeley 11 Jan '08

11 Jan '08

Consider this: type Super = forall a. [a] type Sub = forall b. b cast0 :: Sub -> Super cast0 s = s cast1a :: forall p. (Super -> p) -> (Sub -> p) cast1a sp x = sp x cast1b :: forall p. (Super -> p) -> (Sub -> p) cast1b sp = sp This compiles except for cast1b (ghc -c -fglasgow-exts): Occurs check: cannot construct the infinite type: a = [a] When generalising the type(s) for `cast1b' I get a similar result when I try a different Super and Sub: type Super = (?x :: Bool) => Int type Sub = Int Couldn't match expected type `Sub' against inferred type `Super' When matching `(?x::Bool) => Int' and `Int' Expected type: Sub -> p Inferred type: Super -> p In the expression: sp In either case, if cast1a compiles, shouldn't cast1b compile? -- Ashley Yakeley

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Tim Sweeney (the gamer)
by Galchin Vasili 11 Jan '08

11 Jan '08

Hello, I have been reading with great interested Tim Sweeney's slides on the Next Generation Programming Language. Does anybody know his email address? Kind regards, Vasili

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RE: [Haskell-cafe] Is there anyone out there who can translate C# generics into Haskell?
by Nicholls, Mark 11 Jan '08

11 Jan '08

[snip] >-- C#: interface IX1 { String foo1(int); } >class IX1 obj where > foo1 :: Int -> obj -> String Yep...I think that's what I'd do....though I would have done... "foo1 :: obj -> Int -> String" Does that matter? >-- C#: interface IX2<A> { String foo2(A); } >class IX2 obj a where > foo2 :: a -> obj -> String Ok same here >--C#: interface IX3<A> where A : IY { String foo3(A); } >class IY a where {- ... -} >class IY a => IX3 obj a where > foo3 :: a -> obj -> String Yep I think again I would have guessed at that >--C#: interface IX4<A> : IZ where A : IY >class IZ a where {- ... -} >class (IY a, IZ obj) => IX4 obj a where > foo4 :: a -> obj -> String Hmmmm...this would have been one of my guesses....but let me have a go... >This assumes your "objects" are immutable, otherwise you would have to >return (obj,String) instead of just String and then you most likely want to >use the state monad and "do notation" to make functional programming look >more like imperative programming. This is fine....my oop is largely immutable. >You really have to drop the OO way of thinking, >which I find the hardest :) >Haskell's type classes are more powerful in some sense than C# interfaces; >for example, in C# you can't attach an interface to any class (take for >example the Point struct), it has to be your own class, while in Haskell, >you can implement an instance of a type class for any datatype! OK but I was going to go onto Interface IX<A> where A : IX<A> { } And Interface IX<A,B> where A : B { } Where I cannot see a way to do the above in Haskell at all....as interfaces effectively operator on type classes not types....which seems inherently more powerful But if these could be done in Haskell the see what could be made of stuff like....which is obviously problematic in C# it obviously doesn't work....but potentially does make 'sense'. Interface IX<A> : A { } >Hope this helps a bit. As I'm new myself to Haskell, so take this with a >"grain of salt". It does...I will have a go with your sample answers. >Once you bite the bullet, I found Haskell a lot more fun than C#, although >C# of course comes with a gigantic .NET framework and tools... I'm looking at Haskell because of the formality of it's type system....but I'm actually not convinced it is as powerful as an OO one....i.e. OO ones operatate principally (in Haskell speak) on "type classes" not "types" >Peter -----Original Message----- From: haskell-cafe-bounces(a)haskell.org [mailto:haskell-cafe-bounces@haskell.org] On Behalf Of Nicholls, Mark Sent: Wednesday, January 02, 2008 5:41 PM To: haskell-cafe(a)haskell.org Subject: [Haskell-cafe] Is there anyone out there who can translate C# generics into Haskell? I'm trying to translate some standard C# constucts into Haskell... some of this seems easy.... Specifically 1) Interface IX { } 2) Interface IX<A> { } 3) Interface IX<A> Where A : IY { } 4) Interface IX<A> : IZ Where A : IY { } I can take a punt at the first 2....but then it all falls apart _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

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The Worker/Wrapper Transformation
by Achim Schneider 11 Jan '08

11 Jan '08

...is a paper about automatic specialisation of functions by unboxing arguments, one could say. I'm only on page 6, but already survived the first formalisms, which is bound to mean that the rest of the paper is likewise accessible, as hinted on at ltu. http://www.cs.nott.ac.uk/~gmh/wrapper.pdf The transformation itself is mindbogglingly easy, which makes this a good start: You only have to understand the formalisms, not so much what the formalisms are representing. To quote spj: It usually turns out to be more interesting and challenging than it seemed at first. I'm tempted to write that this is a paper for everyone trying to figure out what the heck Jonathan is talking about. -- (c) this sig last receiving data processing entity. Inspect headers for past copyright information. All rights reserved. Unauthorised copying, hiring, renting, public performance and/or broadcasting of this signature prohibited.

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viewing HS files in Firefox
by Isaac Dupree 11 Jan '08

11 Jan '08

When I try to go to one of the Module.hs files, e.g. on darcs.haskell.org, it now has type HS and Firefox refuses to display it (and only lets me download it). Does anyone know how to make Firefox treat certain file types as others (HS as plain text, in particular)? so that I can browse them with any convenience Thanks, Isaac

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The reason why I want Tim Sweeney's email address ..
by Galchin Vasili 11 Jan '08

11 Jan '08

Hello, http://www.coverity.com/html/library.php ....*Ensuring Code Quality in Multi-threaded Applications* ** *This "white paper" touches on Haskell's STM but also issues that Sweeney brought up in his slides(parallel programming in huge multi-cores and why the current thread-based paradigm is woefully inadequate. I already sent to Simon Peyton-Jones and Simon Marlowe.* ** *Regards, Vasili*

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GHC API?
by Galchin Vasili 11 Jan '08

11 Jan '08

Hello, I am reading http://hackage.haskell.org/trac/ghc/wiki/Commentary/Compiler/API. Is the GHC API a means of reflection Haskell? Or to put more simply what is its intent? Thanks, Vasili

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Re: [Haskell-cafe] confusion about 'instance'....
by Miguel Mitrofanov 11 Jan '08

11 Jan '08

> class A a > type T = (forall x.Num x=>x) > instance A T "type" declares a synonym, like #define in C - but working only on types. So, essentially, you wrote instance A (forall x.Num x => x) which is not very Haskelly. > I am simply trying to state that all members of typeclass Num are of > typeclass A.... You can't do that. But, if there would not be any other instances of A, then you don't need it at all, you can just use Num class. And if there are some, for example data D = D instance A D then it can happen (well, it's unlikely, but possible) that you or some other developer working on your code would declare instance Num D where ... After that, you would have two instances of A for the type D, one defined explicitly and one derived from the Num instance. That's not a problem for this empty class, but if class A is not empty, say class A x where a :: x then the compiler would be unable to decide which instance (and which "a") to choose. So allowing that leads to non-obvious bugs. However, if you trust yourself enough, you can do what you want to in this way: {-# OPTIONS_GHC -fglasgow-exts -fallow-undecidable-instances #-} class A a instance Num a => A a

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Re: [Haskell] Problems with Unicode Symbols as Infix Function Names in Propositional Calculus Haskell DSL
by Simon Marlow 10 Jan '08

10 Jan '08

Cetin Sert wrote: > I want to design a DSL in Haskell for propositional calculus. But > instead of using natural language names for functions like or, and, > implies etc. I want to use Unicode symbols as infix functions ¬, ˅, ˄, > →, ↔ But I keep getting error messages from the GHC parser. Is there a > way to make GHC parse my source files correctly? If it is not possible > yet, please consider this as a “feature request”. GHC supports unicode source files encoded using UTF-8 by default. It should be possible to use many unicode symbols for infix symbols. Note that when -XUnicodeSyntax is on, certain symbols have special meanings (e.g. → means ->). Without more information I can't tell exactly what problem you're encountering. If you supply the source code you're trying to compile, we might be able to help. Also, note that glasgow-haskell-users(a)haskell.org is a better forum for GHC-specific issues. Cheers, Simon

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Re: [Haskell-cafe] 0/0 > 1 == False
by Yitzchak Gale 10 Jan '08

10 Jan '08

Cristian Baboi wrote: > I think this should be put this way: > Bottom is a part of the semantic domain which is not Haskell. Rather, something outside Haskell that describes what Haskell programs mean. Yes. > In the semantic domain there is one bottom. > In Haskell there are many expressions that represent bottom. > One cannot test those for equality. Yes. > The result of a Haskell function applied to some arguments cannot be > bottom. I think you mean that they cannot be bottom if you want to compare them for equality. Yes. -Yitz

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Problem with own written monad
by Michael Roth 10 Jan '08

10 Jan '08

Hello list, while trying to learn the secrets of monads, I decided to write a simply monand for pure educational purpose. But it turned out that it isn't as easy as I thought... I circumnavigate quite a number of hurdles but now I reached a point where I'm at a loss. :-( The source: #! /usr/bin/env ghc data Stack a = Stack { run :: [a] -> (a, [a]) } push :: a -> Stack a push x = Stack f where f xs = ( x, x:xs ) pop :: Stack a pop = Stack f where f (x:xs) = ( x, xs ) top :: Stack a top = Stack f where f (x:xs) = ( x, x:xs ) instance Monad Stack where return x = Stack f where f xs = ( x, xs ) (>>=) stack g = Stack f where f s0 = (x2, s2) where (x1, s1) = run stack s0 (x2, s2) = run (g x1) s1 The errors: ./mymonad.hs:16:24: Couldn't match expected type `b' (a rigid variable) against inferred type `a' (a rigid variable) `b' is bound by the type signature for `>>=' at <no location info> `a' is bound by the type signature for `>>=' at <no location info> Expected type: [b] -> (b, [b]) Inferred type: [a] -> (b, [b]) In the first argument of `Stack', namely `f' In the expression: Stack f ./mymonad.hs:19:28: Couldn't match expected type `b' (a rigid variable) against inferred type `a' (a rigid variable) `b' is bound by the type signature for `>>=' at <no location info> `a' is bound by the type signature for `>>=' at <no location info> Expected type: [b] Inferred type: [a] In the second argument of `run', namely `s1' In the expression: run (g x1) s1 I think the problem is that my operator (>>=) is of type: Stack a -> (a -> Stack a) -> Stack a but should be: Stack a -> (a -> Stack b) -> Stack b But, I have simply no clue how to fix that. :-( Can anybody give my a hint? Thank you in advance. Michael Roth

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Problems with Unicode Symbols as Infix Function Names in Propositional Calculus Haskell DSL
by Cetin Sert 10 Jan '08

10 Jan '08

I want to design a DSL in Haskell for propositional calculus. But instead of using natural language names for functions like or, and, implies etc. I want to use Unicode symbols as infix functions ¬, ˅, ˄, →, ↔ But I keep getting error messages from the GHC parser. Is there a way to make GHC parse my source files correctly? If it is not possible yet, please consider this as a “feature request”. Best Regards, Cetin Sert INF 521, 4-6-2 69120 Heidelberg Germany http://www.corsis.de

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Refactoring status
by Peter Verswyvelen 10 Jan '08

10 Jan '08

Hi all, Is any work being done on Haskell refactoring support, like HaRe or others? Is anyone actively using refactoring? When using C#, I used Resharper a lot, and ever since, I'm really hooked to refactoring, so I miss it a lot when doing Haskelling. (I never seem to get a function name or signature right the first time. is it just me? J) I'm currently using Emacs with Haskell Mode (which does not offer refactoring support) but I think many of you use VIM (which does support it?) Can one use refactoring outside of an editor? This does not really sound practical, but maybe it works? Thank you, Peter PS: IMHO I don't think text should be the source format of our files. I think we should use a standarized decorated AST as the source, from which we can derive a textual (but also graphical) view and editor. Any comments on that? J

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Data.Binary and serialising doubles in IEEE format
by allan 10 Jan '08

10 Jan '08

Dear all I have what I think should be a relatively simple problem: I have a C program which reads in data from a binary file, it is a specific format so it knows that there are 3 integers followed by three doubles or whatever. So I'm trying to serialise the data from within a Haskell program and write it out to a file such that the C program can read this. I've gotten the integers to work no problem but I'm having difficulty with the doubles. My method is to use the Data.Binary library to write the data out as a bytestring and then simply write this out to disk. The problem is that the instance of Binary for Double does not serialise the data in the format which is expected by the C program (standard IEEE format using fread and a cast). Now there was a thread on (essentially) this very same topic on haskell-cafe back in April, see: http://www.haskell.org/pipermail/haskell-cafe/2007-April/024596.html There was some discussion about the 'right' thing to do and that the library intended to be split into two separate portions; one for users that wish to serialise their own data and have it readable by the same haskell program, and those that wish to communicate with programs written in other languages. From the above thread: > Perhaps we just don't care about ARM or other arches where GHC runs that > do not use IEEE formats, I don't know. If that were the case we'd say > something like: > > instance Binary Double where > put d = assert (isIEEE (undefined :: Double)) $ do > write (poke d) Now for my specific problem I'd be happy if I could write a function: putDouble :: Double -> Put and so obviously I tried: putDouble :: Double -> Put putDouble d = assert (isIEEE (undefined :: Double)) $ do write (poke d) However I seem to be stuck at this point, where does the 'write' function come from? Essentially then, is there anyone that can write my 'putDouble' function or give some hint as to how I might do it. For the moment assume that I'm not really concerned with portability across platforms at least for the time being (I certainly don't think that the C program I'm attempting to communicate with is particularly portable anyway). kind regards allan

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GHC 6.8.2 as a library on Windows and GHCi
by Peter Verswyvelen 10 Jan '08

10 Jan '08

I while I ago I sent an email regarding hs-plugins not working on windows. I now tried to directly use GHC API, but I also failed. The following program (which might be buggy, I copy/pasted from various sources) import DynFlags import GHC main = defaultErrorHandler defaultDynFlags $ do session <- newSession (Just "d:/app/ghc-6.8.2") flags <- getSessionDynFlags session (flags, _) <- parseDynamicFlags flags [] GHC.defaultCleanupHandler flags $ do setSessionDynFlags session flags{ hscTarget=HscInterpreted } prelude <- findModule session (mkModuleName "Prelude") Nothing setContext session [] [prelude] runStmt session "let n = 2 + 2" RunToCompletion -- n is bound runStmt session "n" RunToCompletion -- 4 is printed (note "it" is bound) gives the following error when run from GHCi *Main> :load "l:/test.hs" [1 of 1] Compiling Main ( l:/test.hs, interpreted ) Ok, modules loaded: Main. *Main> main GHCi runtime linker: fatal error: I found a duplicate definition for symbol _forkOS_entry whilst processing object file d:/app/ghc-6.8.2/lib\base-3.0.1.0/HSbase-3.0.1.0.o This could be caused by: * Loading two different object files which export the same symbol * Specifying the same object file twice on the GHCi command line * An incorrect `package.conf' entry, causing some object to be loaded twice. GHCi cannot safely continue in this situation. Exiting now. Sorry. Process haskell finished Is my code incorrect, or is this a (known?) bug in GHC 6.8.2 on Windows? I haven't tried the Linux version yet. NOTE: The program *does* run fine when compiling via GHC --make. Thanks, Peter

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Trouble with function with two clauses
by Fernando Rodriguez 10 Jan '08

10 Jan '08

Hi, I have the following type and function: data ConsCell a = Nil | Cons a (ConsCell a) deriving Show head' Nil = Nothing head' (Cons a _) = Just a Works fine, however, what's wrong with the following function? head'' | Nil = Nothing | Cons a _ = Just a Thanks!

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US Homeland Security program language security risks
by Galchin Vasili 10 Jan '08

10 Jan '08

Hello, https://buildsecurityin.us-cert.gov/daisy/bsi/articles/knowledge/coding/295… I stumbled across this page. It seems that Haskell and other strongly typed functional languages like Ml/OCaml will fare much, much better, e.g. buffer overrun. Thoughts . .... comments. Vasili

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Silly question: accessing a slot in a data type
by Fernando Rodriguez 10 Jan '08

10 Jan '08

Hi, I'm writing a very simple address book. I defined the follwoing types for Contact and AddressBook: type Name= String type PhoneNumber= Integer data Contact = Contact {name :: Name, phone:: PhoneNumber} deriving Show -- The addressBook is a lista of Contacts data AddressBook= AddressBook [Contact] deriving Show -- Create a test one ag = Agenda [Contact "BillG" 618965332, Contact "Linus" 5897458] I wan't to write a function to search for a given phone by name and return a Maybe PhoneNumber. I know how to do this recursively on a "raw" list, but how do I access the list inside AddressBook? Also, how do I represent an AddressBook with an empty list and one with data on its list for my pattern matching? Thanks!

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Basic question concerning the category Hask (was: concerning data constructors)
by Yitzchak Gale 09 Jan '08

09 Jan '08

Hi Andrew, Andrew Bromage wrote: > I still say it "isn't a set" in the same way that a group "isn't a set". > Haskell data types have structure that is respected by Haskell > homomorphisms. Sets don't. Ah, that's certainly true. But what is that additional structure? In categories that have a forgetful functor to Set, the interesting part of their structure comes from the fact that their morphisms are only a proper subset of the morphisms in Set. So in what way are Set morphisms restricted from being Hask morphisms? In two ways, I think: 1. They must be computable. 2. They must satisfy certain strictness criteria involving undefined and seq. (1) can be rather boring, since computability is rarely is an issue in real-life programming problems. It seems that some people using concepts from categories in Haskell would prefer to ignore (2) as well. If so, we have been reduced to viewing Hask as more or less the same as a small subcategory of Set, with no additional structure that is of interest. In this view, there is nothing special about Haskell, except for the fact that category-theoretic concepts are a bit easier to express in Haskell than in many other languages. So perhaps a better name for this category would be Turing. I don't think that ignoring (2) is the right approach. I would like Hask to reflect the non-strict nature of Haskell, which is one of its most interesting features. Some people are worried that this version of Hask is missing certain nice properties that one would like to have. For example, it was recently claimed on this list that tuples are not products in that category. (Or some such. I would be interested to see a demonstration of that.) I am not impressed by those complaints. As usual in category theory, you define corresponding notions in Hask, and prove that they are preserved under the appropriate functors. That should always be easy. And if ever it is not, then you have discovered an interesting non-trivial consequence of laziness that deserves study. For example: I think that "monads" in Haskell should satisfy the monad laws in Hask, not just after applying the forgetful functor to Turing. That means that (>>=) must be strict in its second argument. That would very likely not break any existing programs, yet almost none of the existing monads satisfy this, not even IO. Regards, Yitz

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Trouble with indentation
by Fernando Rodriguez 09 Jan '08

09 Jan '08

Hi, I'm trying to write a function that finds out the week day using the Zeller congruence (http://en.wikipedia.org/wiki/Zeller's_congruence). However, ghc complains about: parse error on input `=' at the "if m <= 2 then " line. I believe this is some sort of layout error. Can someone point out what I am doing wrong? Here's the code: data DiaSemana = Lunes | Martes | Miercoles | Jueves | Viernes | Sabado | Domingo deriving (Show, Eq, Ord, Enum) diaDeSemana d m a = toEnum num :: DiaSemana where num = zeller x y z zeller x y z = (700 + (26 * x - 2) `div` 10 + d + y + y `div` 4 + z `div` 4 - 2 * z) `mod` 7 if m <= 2 then x = m + 10 y = (a - 1) `mod` 100 z = (a-1) 'div' 100 else x = m - 2 y = a `mod` 100 z = a `div` 100

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Currying and Partial Evaluation
by Fernando Rodriguez 09 Jan '08

09 Jan '08

Hi, Is currying in Haskell the same thing as Partial Evaluation (http://en.wikipedia.org/wiki/Partial_evaluation)? Am I getting partial evaluation for free just by using Haskell? Thanks!

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Re: PROPOSAL: Some more 'Applicative' combinators
by Sterling Clover 09 Jan '08

09 Jan '08

The more general question, which stems from a certain degree of ignorance on my part, is what uses folks have found for Alternative at all outside of parsing. --S On Jan 8, 2008, at 4:43 AM, apfelmus wrote: > Iavor Diatchki wrote: >> Bryan O'Sullivan wrote: >>> Why not make the task specificity clearer, with a separate >>> Control.Applicative.Parsing module? > > +1 > >> Now _this_ would definitely suggest that they are only good for >> parsing which would be misleading. > > Would it? I mean, I currently don't know a second use case for > them. Preferably, the names like skipMany , endBy , sepBy etc. > should still make sense in the new context. > > > Regards, > apfelmus > > _______________________________________________ > Libraries mailing list > Libraries(a)haskell.org > http://www.haskell.org/mailman/listinfo/libraries

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Please allow beginners to vocalize code. >>= :: -> &&& <- -<
by Richard Kelsall 09 Jan '08

09 Jan '08

A tip for people explaining Haskell to beginners: The acts of reading and speaking are closely integrated in the brain. When I am unable to 'say' something it is much more difficult to 'read' and absorb the text. There appears to be a misconception that it somehow helps beginners to understand things if they are not told how to 'say' these strange new symbols. Certainly for me, and I would guess for most people, this idea is completely wrong. When I 'read' a new operator such as >>= I want know how to 'say' it. I don't mean that posts on Haskell-Cafe should do this, but books and articles aimed at people who haven't used Haskell before should always vocalize the symbols as soon as they are introduced. On a related note, if there isn't already, it would be nice to have a page in the wiki that gives good ways of vocalizing the operators and vocalizing them in simple code snippets. I might get round to doing this sometime, maybe just a table something like this : Operator Formal Informal -------- ------ -------- :: has type -> maps to to example f :: Int -> Int f has type Int to Int Or, to descend into trivia, a subtle distinction might be usefully made between = and == f i = 54 * i f i is 54 times i x == 27 x equals 27 Richard.

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Hackage web page
by Neil Mitchell 09 Jan '08

09 Jan '08

Hi, The hackage web page confuses me: http://hackage.haskell.org/packages/hackage.html As a user, when I go to that page, I would be seeking one of four pieces of information: 1) How to install a cabal package 2) How to create and upload a cabal package 3) What packages are on hackage 4) What is hackage The first paragraph answers question 4, before pointing off at the trac pages, which contain similar and overlapping information, as well as completely other stuff. The second paragraph basically answers 3 and 1, but in rather roundabout ways, not including a link to the answer to 3, but including a link about CabalInstall which you can't use yet. The rest of the page is notes for the hackage developers. I just went to hackage looking for the answer to question 2. I can't find it. I have a rough idea where some of the information might be hiding, but we really need a walkthrough tutorial - and I'm sure I've seen one of these before. I would think something more like the style of this might work better: http://www-users.york.ac.uk/~socs235/welcome.htm Hackage has now graduated from being a nice idea to being a critial user-focused thingy, which is great. Perhaps the website needs a little bit of thought along those lines. This isn't intended to be a complaint at all - just a new perspective to the people who develop it and probably don't read the web page at all. Thanks Neil

4 4

Cabal and DB packages
by manu 09 Jan '08

09 Jan '08

Hello, I'm trying to do something that should be fairly simple, installing some DB package so I can use MySQL or SQLite. However I've had troubles building HSQL, HaskellDB and Takusen before giving up (I am using ghc 6.8.1 and Cabal-1.2.2.0). ------------------ with HSQL : Database/HSQL.hsc:66:7: Could not find module `System.Time': it is a member of package old-time-1.0.0.0, which is hidden so I added 'old-time' to the 'build-depends' line in the .cabal file but then it fails building with Building hsql-1.7... [1 of 2] Compiling Database.HSQL.Types ( Database/HSQL/Types.hs, dist/ build/Database/HSQL/Types.o ) Database/HSQL/Types.hs:134:18: Not in scope: type variable `forall' Database/HSQL/Types.hs:134:25: Not in scope: type variable `a' Database/HSQL/Types.hs:134:27: Illegal operator `.' in type `forall a . (Int -> FieldDef -> (FieldDef -> CString -> Int -> IO a) -> IO a)' (Use -XTypeOperators to allow operators in types) etc... At this point, it goes over my head... --------------- What the hell, I'll use HaskellDB and FlatDB, but then I get this message while building HaskellDB : src/Database/HaskellDB/HDBRec.hs:106:11: Illegal signature in pattern: l f a Use -XPatternSignatures to permit it when I add 'PatternSignatures' to the 'Extensions:' line in .cabal, it fails configuring with : Setup.hs: haskelldb.cabal:8: Parse of field 'extensions' failed: Jeeeesus, nevermind, I'll try Takusen --------------- with Takusen : $ runhaskell Setup.hs configure Setup.hs:26:7: Could not find module `Distribution.Program': Use -v to see a list of the files searched for. I give up... ---------------- Is there any simple way to do it though ? Is there any problems with Cabal that I need to work around ? Help !! Manu

4 3

Newbie question: the need for parentheses
by Fernando Rodriguez 09 Jan '08

09 Jan '08

Hi, I have this function that sums up a list of numbers: suma [] = 0 suma (h:t) = h + suma t However, why do I need the parenthes in the second clause? What does the compiler think I'm trying to do when I type suma [] = 0 suma h:t = h + suma t Thanks! :-)

4 3

Compiler backend question
by Peter Verswyvelen 09 Jan '08

09 Jan '08

If I understand it correctly, the GHC compiler either directly generates machinecode, or it uses C as an intermediate language. I also read somewhere that C is not the most efficient intermediate representation for functional languages, and that one gets better performance when generating native machine code. However, from the discussions in this list, I could conclude that the low level machine code optimizer of GHC is far from state-of-the-art compared to industry standard C/C++ compilers. I was wondering, why doesn't GHC use the GCC (or any other standard compiler) backend intermediate code? The backend of GCC generates highly optimized code no? Or is the intermediate code format of GCC (or other compilers) not suitable for Haskell? Another question regarding the backend: a cool feature of the Microsoft Visual C++ (MVC) compiler is its ability to perform "LTCG" (link-time-code-generation), performing whole program optimization. It something like this possible with Haskell / (or the GCC backend?). Would it be possible to feed all the generated C code of the GHC compiler into the MVC compiler, to generate one big MVC / LTCG generated executable? It would be interesting to see how much the whole program optimization approach (which can do analysis on the program as if it was a single module) would improve performance... Cheers, Peter

8 12

Concurrency questions
by Andrew Coppin 08 Jan '08

08 Jan '08

Just a couple of things I was wondering about... 1. Is there some way to assign a "priority" to Haskell threads? (The behaviour I'd like is that high priority threads always run first, and low priority threads potentially never run at all unless there's an available processor which is completely idle.) 2. I have a situation where I have a thread generating some data and putting it into a mutable array, and another thread trying to read that data. Is there a way I can make the reader thread block if it tries to read a cell that hasn't been computed yet, but not introduce too much overhead for cells that have been filled in? 3. Would it be hard to make it so that the number of real threads (the RTS +N flag) could be adjusted at runtime?

8 12

Implicit parameters and Arrows/Yampa?
by Peter Verswyvelen 08 Jan '08

08 Jan '08

If I understand it correctly, implicit parameters in Haskell allow you to pass values to functions with explicitly adding a parameter to each of the functions being "called" (I appologize for my imperative terminology here. How would I say this correctly? Being "evaluated"?) The arrows always use tuples to group the input and output parameters, like: foo :: SF (Int,Int,Int) (Int,Int) foo = proc (x,y,z) -> do p <- cat -< (x,y) q <- dog -< z returnA -< (p,q) where cat = proc (x,y) -> returnA -< (x+y) dog = proc z -> returnA -< 10*z Suppose I don't want to explicitly pass the x and y parameters to the cat (and deeper) arrows, but make them implicit. I guess that would be impossible? I mean, I can't use implicit parameters language extension to make the arrow input parameters implicit? Thanks, Peter

5 8

Wikipedia on first-class object
by Cristian Baboi 08 Jan '08

08 Jan '08

http://en.wikipedia.org/wiki/First-class_object The term was coined by Christopher Strachey in the context of “functions as first-class citizens” in the mid-1960's.[1] Depending on the language, this can imply: 1. being expressible as an anonymous literal value 2. being storable in variables 3. being storable in data structures 4. having an intrinsic identity (independent of any given name) 5. being comparable for equality with other entities 6. being passable as a parameter to a procedure/function 7. being returnable as the result of a procedure/function 8. being constructable at runtime 9. being printable 10. being readable 11. being transmissible among distributed processes 12. being storable outside running processes I'll guess that 5,9,12 does not apply to Haskell functions. ________ Information from NOD32 ________ This message was checked by NOD32 Antivirus System for Linux Mail Servers. part000.txt - is OK http://www.eset.com

32 184

ANN: Build fixed for regex-base,posix,compat,pcre
by ChrisK 08 Jan '08

08 Jan '08

ANNOUNCEMENT: Build fixed for regex-base, regex-posix, regex-compat, regex-pcre The changes are mainly to the Cabal build files to support ghc-6.8 and ghc-6.6 simultaneously. They definitely work with cabal version 1.2.3.0 (required for regex-pcre). The regex-base, regex-posix, and regex-compat packages also seem to work with cabal-1.2.2.0 (but not regex-pcre). The darcs repositories under http://darcs.haskell.org/packages/regex-unstable/ hold the new code. I have uploaded new versions of the most popular regex-* packages to hackage: Mon Jan 7 13:18:00 PST 2008 ChrisKuklewicz regex-pcre-0.94.1 Mon Jan 7 13:05:57 PST 2008 ChrisKuklewicz regex-posix-0.93.1 Mon Jan 7 13:04:49 PST 2008 ChrisKuklewicz regex-base-0.93.1 Mon Jan 7 06:50:54 PST 2008 ChrisKuklewicz regex-compat-0.91 I would like to thank everyone who sent me updated, and particularly Olivier Boudry who sent patches. These build fixes have only been tested with ghc-6.6.1 on Mac OS X 10.5.1 (powerpc) and with ghc-6.8.2 on Linux (x86). Happy New Year, Chris Kuklewicz

1 0

GHC's dynamic linker and Windows
by Peter Verswyvelen 07 Jan '08

07 Jan '08

I recently tried the latest version of HS-PLUGINS, and it gave an error on Windows. After a bit of Googling it seemed Conal Elliot had the same problem. I reported this problem to the author. This is also (one of) the reason why I could not get YI running on Windows. Now, according to the documentation, HS-PLUGINS seems to be mainly a handy wrapper around GHC's dynamic linker and compiler. Suppose I would use GHC's dynamic linker, interpreter and compiler directly (aha "GHC as a library"); does this work on Windows, or are there known problems with these? Thanks, Peter

3 2

Type Mismatch
by Cetin Sert 07 Jan '08

07 Jan '08

Hi, I'm new to Haskell programming and have the following problem. ----------------------------------------------------- (|>) f g = g f data Stream a where S :: (s -> Step s a) -> s -> Stream a data Step s a = Done | Yield a s | Skip a s toStream :: [a] -> Stream a toStream ax = S step ax where step [] = Done step (a:ax) = Yield a ax fromStream :: Stream a -> [a] fromStream (S step s) = loop s where loop s = case step s of Done -> [] Skip a s' -> loop s' Yield a s' -> a : loop s' filterStream :: (a -> Bool) -> Stream a -> Stream a filterStream p (S step s) = S filter s where filter s = case step s of Done -> Done Skip a s' -> Skip a s' Yield a s' -> if p a then Yield a s' else Skip a s' mapStream :: (a -> b) -> Stream a -> Stream b mapStream f (S step s) = S map s where map s = case step s of Done -> Done Skip a s' -> Skip (f a) s' Yield a s' -> Yield (f a) s' class Streamable a where to :: a -> Stream a instance Streamable [a] where to = toStream -- ERROR: see below s f x = x |> toStream |> f |> fromStream smap x = s (mapStream x) sfilter x = s (filterStream x) (%) a b = mod a b main = do print ([0..20] |> sfilter (\x -> x % 2 == 0)) --------------------------------------------------------- Error 1 Couldn't match expected type `a' (a rigid variable) against inferred type `[a]' `a' is bound by the instance declaration at C:\Users\Sert\Lab\Haskell\HaskellApp1\HaskellApp1\src/Main.hs:63:0 Expected type: [a] -> Stream [a] Inferred type: [[a]] -> Stream [a] In the expression: toStream In the definition of `to': to = toStream C:\Users\Sert\Lab\Haskell\HaskellApp1\HaskellApp1\src\Main.hs 64 8 How can I make the types match so that I can declare lists streamable? Is there something like in-place type annotations as in ML/OCaml/F#? Best Regards, Cetin Sert corsis.de/blog

2 1

haskell package installation problem
by Brian Park 07 Jan '08

07 Jan '08

Hi, I was installing various haskell packages from hackage. When I was installing HaXml, I think it was complaining about Text.PrettyPrint.HughesPJ not installed or something. (can't remember the specific message and I can't reproduce now...) So I installed "pretty-1.0.0.0" package as well. Ever since then, when I try to install other haskell packages, I get the following error message: "brian@bp-desktop:~/Download/mtl-1.1.0.0$ runghc Setup.hs configure <interactive>: /usr/local/lib/ghc-6.8.2/lib/Cabal-1.2.3.0/HSCabal-1.2.3.0.o: unknown symbol `prettyzm1zi0zi0zi0_TextziPrettyPrintziHughesPJ_lvl18_closure' ghc-6.8.2: unable to load package `Cabal-1.2.3.0'" Does anyone know what the problem is? Currently installed packages are: ========= /usr/local/lib/ghc-6.8.2/package.conf: Cabal-1.2.3.0, HTTP-3001.0.4, HUnit-1.2.0.0, X11-1.4.1, array-0.1.0.0, base-3.0.1.0, bytestring-0.9.0.1, containers-0.1.0.1, directory-1.0.0.0, filepath-1.1.0.0, (ghc-6.8.2), haskell98-1.0.1.0, hpc-0.5.0.0, hxt-7.4, mtl-1.1.0.0, network-2.1.0.0, old-locale-1.0.0.0, old-time-1.0.0.0, packedstring-0.1.0.0, parsec-2.1.0.0, polyparse-1.1, pretty-1.0.0.0, process-1.0.0.0, random-1.0.0.0, readline-1.0.1.0, rts-1.0, template-haskell-2.2.0.0, unix-2.3.0.0, xmonad-0.5, xmonad-contrib-0.5 ========= Thank you, - Brian

2 1

PHP/code generation libraries?
by Justin Bailey 07 Jan '08

07 Jan '08

I'm working on a project which would generate a PHP data-access layer from a Haskell model. I'm wondering what libraries might be already be available for generating PHP or other types of code. The pretty-printing library is one option. Any other suggestions? Thanks in advance. Justin

2 1

reifying is-a
by Jorge Marques Pelizzoni 07 Jan '08

07 Jan '08

Hi, all! I guess what I am about to ask is currently impossible, but as you haskellers always manage to amaze me here it goes. Given two type classes A t and B t, I'd like to derive (two) different A t instances depending exactly on whether t is an instance of B. In other words, is it possible to define a type-level function/class IsB t f such that: IsB t HTrue <=> (B t) exists IsB t HFalse <=> otherwise? If not, is this wish intrisically pointless? Thanks in advance. Cheers, Jorge M. Pelizzoni ICMC - Universidade de São Paulo

1 0

map/reduce example
by Bulat Ziganshin 07 Jan '08

07 Jan '08

Hello Haskell-Cafe, i've written small program which demonstrates how map/reduce may be implemented in Haskell. it counts amount of words in file, splitting it into 64kb blocks processed by two threads. their results are combined by another two threads. how it may be made better? in particular, is it strict in values send over channels? {-# LANGUAGE BangPatterns #-} import Control.Concurrent import Control.Concurrent.Chan import Control.Monad import Data.IORef import Data.ByteString.Char8 as B hiding (length) import System.Environment import System.IO main = do (file:_) <- getArgs h <- openBinaryFile file ReadMode map <- newChan reduce <- newChan result <- newChan replicateM_ 2 (forkIO$ mapThread map reduce) replicateM_ 2 (forkIO$ reduceThread reduce result) jobs <- new 0 untilM (hIsEOF h) $ do str <- B.hGet h 65536 writeChan map str jobs += 1 jobs' <- val jobs writeChan reduce (0,-jobs') res <- readChan result print res mapThread map reduce = forever $ do str <- readChan map let !sum = length (B.words str) writeChan reduce (sum,1) reduceThread reduce result = forever $ do (sum1,n1) <- readChan reduce (sum2,n2) <- readChan reduce let (!sum,!n) = (sum1+sum2,n1+n2) case n of 0 -> writeChan result sum _ -> writeChan reduce (sum,n) untilM cond action = do deny <- cond unless deny $ do action untilM cond action forever action = action >> forever action infixl 0 =:, += new = newIORef val = readIORef a=:b = writeIORef a b a+=b = modifyIORef a (\a->a+b) -- Best regards, Bulat mailto:Bulat.Ziganshin@gmail.com

3 2

Re: [Haskell-cafe] what does @ mean?.....
by Serge LE HUITOUZE 07 Jan '08

07 Jan '08

Jedaï wrote: > [...] > Yes, but in the second version, it has to reconstruct (S a) before > comparing it to "one" where in the first it could do the comparison > directly. In this cas there may be some optimisation involved that > negate this difference but in many case it can do a real performance > difference. > The "as-pattern" (@ means as) is both practical and performant in most cases. Chris wrote: > The first example takes apart the (Just n) and later reconstructs (Just n). > Unless the compiler is fairly clever, this will cause the new (Just n) to be a > new allocation instead of reusing the input value. The second form uses an I tend to believe that the '@' notation is mere syntactic sugar. Indeed, it seems to me that Haskell compilers need not to be very clever to share the identical sub-expressions, for one very simple reason implied by Haskell semantics: referential transparency. Am I right, or am I missing something? And, of course, I'm not saying that syntactic sugar is bad. As Alfonso noted, it saves some typing and avoids screwing up while trying to modify multiple occurrences of the same sub-expression. And, an even more important I think, a software engineering benefit: It emphasizes for the reader that we really are talking about the very same thing at two (or more) different places. --Serge No virus found in this outgoing message. Checked by AVG Free Edition. Version: 7.5.516 / Virus Database: 269.17.9/1198 - Release Date: 26/12/2007 17:26

4 4

Software Tools in Haskell
by Gwern Branwen 07 Jan '08

07 Jan '08

On 2007.12.10 13:52:41 -0600, Tommy McGuire <mcguire(a)crsr.net> scribbled 1.7K characters: > 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 Some of those really look like they could be simpler, like 'copy' - couldn't that simply be 'main = interact (id)'? Have you seen <http://haskell.org/haskellwiki/Simple_Unix_tools>? For example, 'charcount' could be a lot simpler - 'charcount = showln . length' would work, wouldn't it, for the core logic, and the whole thing might look like: > main = do (print . showln . length) =<< getContents Similarly wordcount could be a lot shorter, like 'wc_l = showln . length . lines' (showln is a convenience function: showln a = show a ++ "\n") I... I want to provide a one-liner for 'detab', but it looks impressively monstrous and I'm not sure I understand it. One final comment: as regards run-length encoding, there's a really neat way to do it. I wrote a little article on how to do it a while ago, so I guess I'll just paste it in here. :) --- Recently I was playing with and working on a clone of the old Gradius arcade games which was written in Haskell, Monadius <http://hackage.haskell.org/cgi-bin/hackage-scripts/package/Monadius-0.9>. Most of my changes were not particularly interesting (cleaning up, Cabalizing, fixing warnings, switching all Integers to Ints and so on), but in its Demo.hs, I found an interesting solution to an interesting problem which seems to be a good example of how Haskell's abstractions can really shine. So, suppose we have these data items, which are levels which are specified by a pair of numbers and then a long list of numbers, often very repetitious. Perhaps a particular level might be represented this way: > level1 = ((Int,Int),[Int]) > level1 = ((2,1),[0,0,0,0,0,0,0,0,0,0,0,0,8,8,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,73,73,73,65,65,65,65,65,65,65,65,65,65,65,65,65,65,65,65,65,65,65,65,65,65,65,65,65,65,65,65,65,65,65,65,65,65,65,65,65,65,69,69,69,69,69,69,65,17,17,17,17,17,17,17,17,17,17,17,17,17,25,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,1,1,1,1,1,1,1,1,1,1,9,9,9,1,1,1,1,1,1,1,1,1,1,1,1,1,65,65,65,65,65,65,1,1,1,1,1,1,1,1,33,33,1,1,1,1,33,33,33,33,33,33,33,33,33,33,33,33,33,33,33,33,33,33,33,33,33,33,33,33,49,49,49,49,33,33,33,1,1,1,1,1,1,1,9,9,9,1,1,1,1,1,1,1,1,1,33,33,33,33,1,1,1,1,1,1,1,1,1,9,9,1,1,1,1,1,33,33,33,33,33,33,33,33,33,33,33,33,33,33,33,33,33,33,33,33,33,33,33,33,1,17,17,17,17,17,17,17,17,17,17,17,17,17,17,17,17,17,17,17,17,17,17,17,17,17,17,17,17,17,17,17,17,17,81,81,81,81,81,81,81,81,81,81,17,17,17,17,17,17,17,17,17,17,17,17,17,17,17,17,17,17,17,17,17,17,17,17,17,17,17,17,17,17,17,17,17,17,17,1,1,1,1,1,1,1,17,17,17,17,17,17,17,17,17,17,17,1,1,1,1,1,1,1,1,1,1,1,1,17,17,17,1,1,1,1,1,1,1,1,1,1,1,65,65,65,65,65,65,65,65,65,65,65,65,65,65,65,65,65,65,65,65,65,65,65,65,65,65,65,65,65,65,65,65,65,97,33,33,33,33,37,5,69,69,65,65,65,65,65,65,67,67,67,67,67,67,67,67,67,75,75,75,75,75,75,75,75,75,75,75,75,11,11,11,11,11,11,11,11,11,11,11,11,11,11,11,11,11,11,11,11,11,11,11,11,11,11,11,11,11,11,11,11,11,11,11,11,11,11,11,11,11,11,11,11,11,11,11,11,11,11,11,11,11,11,11,11,11,11,11,11,11,11,11,11,11,11,11,11,11,11,11,11,11,11,11,3,3,3,3,3,3,3,3,3,3,3,3,3,11,11,3,3,3,3,3,3,3,3,3,3,11,3,3,3,3,3,3,3,3,3,3,35,35,35,35,35,35,35,35,35,35,35,35,35,35,35,35,35,35,35,3,3,3,3,3,3,3,3,67,67,67,67,67,67,67,67,67,67,67,67,67,67,67,67,3,3,3,3,3,3,3,67,67,67,67,67,67,3,3,3,67,67,3,3,3,3,3,67,67,67,67,67,67,67,67,67,67,3,3,3,3,3,3,67,67,67,67,67,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,11,11,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,35,35,35,35,35,3,3,3,35,35,35,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,35,35,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,19,19,19,19,19,19,19,51,51,51,51,51,51,51,51,51,51,51,51,51,51,51,51,51,51,51,51,51,35,35,35,35,35,35,35,35,35,35,35,35,35,35,35,35,35,35,35,35,35,35,35,35,35,35,35,35,35,35,35,35,35,35,35,35,35,35,35,35,35,35,35,35,35,35,35,35,35,35,35,35,35,35,35,35,35,35,35,35,35,35,35,43,43,43,43,43,43,43,43,43,43,43,43,43,43,43,11,11,11,11,11,43,43,43,43,43,43,43,43,35,35,35,35,35,35,35,35,3,3,3,35,35,35,35,35,35,35,35,35,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,67,67,67,67,67,67,67,67,67,67,67,67,67,67,67,67,67,67,67,67,67,67,67,67,67,67,67,67,67,67,67,67,67,67,67,67,67,67,67,67,67,67,67,67,67,67,67,67,67,67,67,67,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,35,35,35,35,35,35,35,35,35,35,35,35,35,3,3,3,35,35,35,35,35,35,35,35,35,35,35,3,3,3,3,3,35,35,35,35,35,35,35,35,35,3,3,3,3,35,35,35,35,35,35,3,3,3,3,35,35,35,35,35,3,3,3,3,3,3,3,3,3,3,3,3,3,19,19,19,19,19,19,19,19,19,19,19,19,19,19,3,3,3,3,19,19,19,19,3,3,3,3,3,3,19,19,19,19,3,3,3,3,3,3,3,19,19,19,19,19,3,3,3,3,3,3,3,3,19,19,19,19,19,19,19,19,83,83,83,83,83,67,67,67,3,3,19,19,19,19,19,19,19,19,19,19,19,19,83,83,83,83,83,19,19,19,19,19,19,19,19,19,19,19,19,19,19,19,19,19,83,83,83,83,83,83,83,83,83,19,19,3,3,3,3,3,67,67,67,67,67,67,67,67,67,67,67,67,67,67,67,67,67,67,67,67,75,11,11,11,11,11,11,11,11,11,11,11,11,11,11,11,11,11,11,11,11,11,11,11,11,11,11,11,11,11,11,11,11,11,11,11,11,11,11,43,35,35,35,35,3,3,3,3,3,3,3,67,67,67,67,67,67,67,67,67,67,67,67,67,67,67,67,67,67,67,67,67,67,67,67,67,67,67,67,67,67,67,67,67,67,67,67,67,67,67,67,67,67,67,67,67,67,67,67,67,67,67,67,67,67,67,67,67,3,3,19,19,19,19,19,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,19,19,19,19,19,19,3,3,3,3,3,3,3,3,67,67,67,67,67,67,83,83,83,83,83,83,83,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,19,19,19,19,19,19,19,19,19,51,35,35,35,35,35,35,35,35,35,35,35,35,35,35,35,35,35,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,35,35,35,35,35,35,35,35,35,35,35,35,35,35,3,3,3,3,3,3,3,35,35,43,43,43,43,43,43,43,43,43,43,43,11,11,11,11,11,11,3,3,3,3,3,3,3,67,67,67,67,67,67,67,67,67,67,67,67,67,67,67,67,67,67,67,67,67,67,67,67,67,67,3,3,3,3,3,3,3,35,35,35,35,35,35,35,35,35,35,35,35,35,43,43,43,43,43,11,11,11,11,11,11,11,3,3,67,67,67,67,67,83,19,19,3,3,67,67,67,67,67,67,67,67,67,67,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0]) This is clearly a bad way of representing things. We could just scrap this representation as Ints completely and perhaps define it as > -- Assume that these datatypes are already defined and everything > level1 :: ((Geometry,Geometry),[Enemies]) > level1 = ((Tall,Narrow), > > [FlyEnemy,FlyEnemy,FlyEnemy,FlyEnemy,FlyEnemy,FlyEnemy,Shooter,PowerUp,Boss..]) > -- and so on But this representation, while certainly more symbolic, is still very repetitious. We need some way of expressing this more concisely, of, in short, *compressing* it. In this vein of thought, our first observation should be that we do not need to resort to Gzipping it or adding dependencies on fancy compression libraries or anything; there is a very obvious way to compress it already - the entire thing is practically just a series of repeated numbers. We should be able to replace the length enumeration of [0,0,0,0,0,0,0,0,0,0,0,0] with something simpler, say the number of repetition and what is to be repeated so that our entry would look like (12,0). This representation is definitely shorter and more importantly, easier to modify and not a constant. It is possible that there may be a performance benefit here, as we've gotten rid of a large constant that would have to be defined in the program itself and instead replaced it with a shorter function which evaluates to the same thing. So, what is the type of our decompressing function? Well, we need to turn a (Int,Int) into a [Int]; even better, we want to turn a whole list of (Int,Int)s into a single list of [Int]. Thus, our end goal is going to be a function of this type: > rleDecode :: [(Int,Int)] -> [Int] Let us tackle the single tuple example first. The second entry defines what we need, and the first entry defines how many we need. We could write a recursive function that takes the parameter, decreases the first entry by one, and cons on one example of the second entry. It could look like this: > rleRecursiveDecode :: (Int,Int) -> [Int] > rleRecursiveDecode (0,_) = [] > rleRecursiveDecode (n,b) = b : (rleRecursiveDecode (n-1,b)) But this really is not the best way to go. It is not necessarily easy to follow, and if there is one thing I have learned about Haskell programming, it is that the most obvious approach (in this case, primitive recursion) may not be the best way to go. This is code that could have as well been written in Scheme or something. It is complicated because we are trying to ensure that we generate an item for the list only at the exact moment we need to add it into the list; we are programming as if our language is strict, in other words. So, what is the lazy way of doing things? Infinite lists, of course. We create an infinite list containing only the second entry, and we merely take as many as the first entry says we need. Simple enough! > -- Infinite list of our item of interest. > reduplicate a = a:a > rleLazyDecode :: (Int,Int) -> [Int] > rleLazyDecode (n,b) = take n (reduplicate b) Now, reduplicate is a simple enough function to define, but it already has a definition in the standard libraries - 'cycle'. (I assume you know what 'take' is, but that is also fairly easy to define once you seen the need for it.) So: > rleLazyDecode (n,b) = take n (cycle b) Might as well remove the parentheses: > rleLazyDecode (n,b) = take n $ cycle b A satisfying, short, functional, and lazy one-liner. From here the definition of rleDecode is almost trivial: we extend it to a list of tuples by throwing in a map, and we turn the resulting list of lists into a single list by way of 'concat': > rleDecode ns = concat $ map rleLazyDecode ns We can tweak this further, as 'concat . map' is a common enough idiom that there is a shortcut: > rleDecode ns = concatMap rleLazyDecode ns Aw heck - make it points-free: > rleDecode = concatMap rleLazyDecode And substitute in the rleLazyDecode definition: > rleDecode = concatMap (\(n,b) -> take n $ cycle b) We could also write a version that omits the explicit lambda and naming of parameters by use of the helpful but somewhat esoteric 'uncurry' function; Uncurry takes apart the tuple. Tts type is: > uncurry :: (a -> b -> c) -> (a, b) -> c We can actually go even further into the realms of incomprehensibility. It turns out that lists are counter-intuitively a monad! This means we can use bind and all the rest of the operations defined by the Monad typeclass to operate on lists and other things. So we can write the bizarre (but short and effective) version of rleDecode that follows: > rleDecode = (uncurry replicate =<<) And we are done! We can now represent the first list like this: > d1 = ((2,1),d) > where d = rleDecode [(5, 3), (10, 67), (6, 3), (5, 67), (29, 3), (2, 11), (29, 3), (5, 35), (3, 3), (3, 35), (24, 3), (2, 35), (19, 3), (7, 19), (21, 51), (63, 35), (15, 43), (5, 11), (8, 43), (8, 35), (3, 3), (9, 35), (20, 3), (52, 67), (32, 3), (13, 35), (3, 3), (11, 35), (5, 3), (9, 35), (4, 3), (6, 35), (4, 3), (5, 35), (13, 3), (14, 19), (4, 3), (4, 19), (6, 3), (4, 19), (7, 3), (5, 19), (8, 3), (8, 19), (5, 83), (3, 67), (2, 3), (12, 19), (5, 83), (17, 19), (9, 83), (2, 19), (5, 3), (20, 67), (1, 75), (38, 11), (1, 43), (4, 35), (7, 3), (57, 67), (2, 3), (5, 19), (17, 3), (6, 19), (8, 3), (6, 67), (7, 83), (59, 3), (9, 19), (1, 51), (17, 35), (20, 3), (14, 35), (7, 3), (2, 35), (11, 43), (6, 11), (7, 3), (26, 67), (7, 3), (13, 35), (5, 43), (7, 11), (2, 3), (5, 67), (1, 83), (2, 19), (2, 3), (10, 67), (21, 3), (147, 0)] Much nicer, don't you think? And the best part is, we should be able to reuse this run-length decoding even if we replace the arbitrary numbers by more descriptive type constructors! (Many thanks to the good denizens of #haskell, and Don Stewart's blog entry on run-length encoding/decoding using Arrows <http://cgi.cse.unsw.edu.au/~dons/blog/2007/07>.) -- gwern

11 19

Re: Haskell-Cafe Digest, Vol 53, Issue 2
by Jens Blanck 07 Jan '08

07 Jan '08

> The normal view taken by Haskellers is that the denotations of > > Haskell types are CPPOs. > > CPPO? > > > So: > > > > (1) Must be monotone > > (2) Must be continuous > > Could you please define what you mean by those terms > in this context? > > > (Needn't be strict, even though that messes up the resulting category > > substantially). > > I'm not convinced that the category is all that "messed up". > > The extra P would stand for "pointed" (has a least element, bottom), this is common in some communities. To me though, a cpo (complete partial order) is closed under directed suprema and the empty set is directed so bottom is already required. The category of cpos in not cartesian closed. For denotational semantics I believe the subcategory of Scott domains are what is usually considered. Continuous functions on cpos are by definition monotone and they respect directed suprema. Jens

5 10

Newbie question related to list evaluation
by Sai Hemanth K 07 Jan '08

07 Jan '08

Hi, I am new to functional and lazy programming languages ( that's correct, my life has been pretty pathetic so far) and am not able to understand GHC's behaviour for a particular function. Can someone help me please? I am trying to write a function which would compare two strings (from reverse) and return the position of first mismatch. This is part of the right-to-left scan of bayer-moore algorithm.. and it is essential for me to do it from reverse. Since my goal is to learn haskell, I am not using Data.ByteString. My function is as follows: matchReverse :: String -> String ->Int->(Bool,Int) matchReverse [] [] pos = (True,pos) matchReverse _ [] pos = (False,pos) matchReverse [] _ pos = (False,pos) matchReverse (x:xs) (y:ys) pos = let (matched,pos) = matchReverse xs ys (pos +1) in if matched then ((x==y),pos) else (False,pos) The behaviour I expected in four scenarios is as below: 1.matchReverse "kapilash" "kapilash" 0 --should return (True,0) 2.matchReverse "kapilash" "kapilast" 0 --should return (False,8) 3.matchReverse str1 str2 0 --should return (False,0) 4.matchReverse str1 str1 0 --should return (True,0) where str1 and str2 are defined as below: str1 = replicate 1000 'a' str2 = 'b':(replicate 999 'a') what confounds me is that it is able to identify the first element of the tuple in ALL the cases. Invoking fst on the each of the four calls instantly returns the expected value.(even for the cases 3 and 4 where, there are thousand elements) But it seems to go into an infinite loop while calculating the 'snd' of the tuple. Even for strings containing just one element each. can someone throw some light on this please? Why does it go into an infinite loop? Many thanks Kapilash -- I drink I am thunk.

5 5

Haskell Weekly News: Issue 68 - January 05, 2008
by Don Stewart 06 Jan '08

06 Jan '08

--------------------------------------------------------------------------- Haskell Weekly News Issue 68 - January 05, 2008 --------------------------------------------------------------------------- Welcome to issue 68 of HWN, a newsletter covering developments in the [1]Haskell community. This HWN features new releases of the GHC and nhc98 Haskell compilers, a pre-release of darcs 2.0, several new user groups formed, and of course, more than 100 updated and new libraries 1. http://haskell.org/ Announcements GHC 6.8.2. The GHC Team [2]announced the release of GHC 6.8.2, featuring optimisation improvements, improvements to ghci and fixes to standalone deriving. 2. http://www.haskell.org/ghc/download_ghc_682.html nhc98 1.2 released. Malcolm Wallace [3]announced the release of nhc98 1.2. 1.20 is a refreshed release with many of the current core library packages included, and a variety of small bugfixes since the last release. It successfully compiles and runs more programs from the nobench suite than jhc, hbc, Hugs, or yhc. It generates an interpreted bytecode that, on the whole runs faster than that generated by Hugs or yhc, and in many cases is also faster than ghci. Although nhc98 is written in Haskell, you don't need an existing Haskell compiler on your platform to build nhc98 - a C compiler will do. Hence, it is portable to almost any unix-like machine with a 32-bit compatibility mode. Many useful build tools come included: hmake (the inspiration for ghc --make), hi (interactive read-eval-print, like Hugs or ghci), cpphs (Haskell-aware replacement for cpp) and hsc2hs (preprocessor for FFI code) 3. http://article.gmane.org/gmane.comp.lang.haskell.general/15770 darcs 2.0.0pre2. David Roundy [4]announced the availability of the second prerelease of [5]darcs two, darcs 2.0.0pre2. This release fixes several severe performance bugs that were present in the first prerelease. These issues were identified and fixed thanks to the helpful testing of Simon Marlow and Peter Rockai. We also added support for compilation under ghc 6.4, so even more users should be able to test this release. 4. http://article.gmane.org/gmane.comp.lang.haskell.cafe/33483 5. http://darcs.net/ The Monad.Reader Issue 9: SoC special. Wouter Swierstra [6]announced a new issue of The Monad.Reader, a 'Summer of Code Special' - it consists of three articles from student participants of Google's Summer of Code, describing the projects they worked on. 6. http://article.gmane.org/gmane.comp.lang.haskell.cafe/31848 What's happening with Haskell? The 13th HCAR. Andres Loeh [7]announced the 13th edition of the Haskell Communities and Activities Report 7. http://www.haskell.org/communities/ Teach yourself gtk2hs in 21 hours. Hans van Thiel [8]announced a Gtk2Hs basics tutorial, based on the Tony Gale and Ian Main GTK+2.0 tutorial, is now available for review and comment. 8. http://article.gmane.org/gmane.comp.lang.haskell.cafe/32671 Minimalistic Haskell blog framework. Paul Brown [9]announced a lightweight, experimental blog publishing application, [10]perpubplat 9. http://article.gmane.org/gmane.comp.lang.haskell.cafe/34503 10. http://datapr0n.com/repos/perpubplat atom. Tom Hawkins [11]announced the release of atom 2007.12; atom is a domain-specific language embedded in Haskell for describing real-time control applications 11. http://www.haskell.org/pipermail/haskell-cafe/2007-December/035742.html Hackage New and updated libraries in [12]the Hackage library database. 12. http://hackage.haskell.org/ * bytestring 0.9.0.4. Uploaded by DonaldStewart. [13]bytestring: Fast, packed, strict and lazy byte arrays with a list interface. * uuagc 0.9.5. Uploaded by ArieMiddelkoop. [14]uuagc: Attribute Grammar System of Universiteit Utrecht. * uulib 0.9.5. Uploaded by ArieMiddelkoop. [15]uulib: Haskell Utrecht Tools Library. * llvm 0.0.2. Uploaded by BryanOSullivan. [16]llvm: Bindings to the LLVM compiler toolkit. * HDBC-sqlite3 1.1.3.1. Uploaded by JohnGoerzen. [17]HDBC-sqlite3: Sqlite v3 driver for HDBC. * HDBC-odbc 1.1.3.1. Uploaded by JohnGoerzen. [18]HDBC-odbc: ODBC driver for HDBC. * dimensional 0.7.2. Uploaded by BjornBuckwalter. [19]dimensional: Statically checked physical dimensions.. * uulib 0.9.5. Uploaded by ArieMiddelkoop. [20]uulib: Haskell Utrecht Tools Library. * hsc3 0.1. Uploaded by RohanDrape. [21]hsc3: Haskell SuperCollider. * hosc 0.1. Uploaded by RohanDrape. [22]hosc: Haskell Open Sound Control. 13. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/bytestring-0.9.0… 14. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/uuagc-0.9.5 15. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/uulib-0.9.5 16. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/llvm-0.0.2 17. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/HDBC-sqlite3-1.1… 18. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/HDBC-odbc-1.1.3.1 19. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/dimensional-0.7.2 20. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/uulib-0.9.5 21. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hsc3-0.1 22. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hosc-0.1 * GLFW 0.2. Uploaded by PaulLiu. [23]GLFW: A binding for GLFW, An OpenGL Framework. * control-timeout 0.1. Uploaded by AdamLangley. [24]control-timeout: Timeout handling. * hiccup 0.35. Uploaded by KyleConsalus. [25]hiccup: Relatively efficient Tcl interpreter with support for basic operations. * phooey 2.0. Uploaded by ConalElliott. [26]phooey: Functional user interfaces. * reactive 0.0. Uploaded by ConalElliott. [27]reactive: Simple foundation for functional reactive programming. * phooey 1.4. Uploaded by ConalElliott. [28]phooey: Functional user interfaces. * hburg 1.1.1. Uploaded by IgorBohm. [29]hburg: Haskell Bottom Up Rewrite Generator. * hinotify 0.2. Uploaded by LennartKolmodin. [30]hinotify: Haskell binding to INotify. * cabal-rpm 0.3.3. Uploaded by BryanOSullivan. [31]cabal-rpm: RPM package builder for Haskell Cabal source packages.. * codec-libevent 0.1. Uploaded by AdamLangley. [32]codec-libevent: Cross-platform structure serialisation. 23. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/GLFW-0.2 24. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/control-timeout-… 25. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hiccup-0.35 26. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/phooey-2.0 27. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/reactive-0.0 28. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/phooey-1.4 29. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hburg-1.1.1 30. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hinotify-0.2 31. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/cabal-rpm-0.3.3 32. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/codec-libevent-0… * irc 0.4. Uploaded by TrevorElliott. [33]irc: A small library for parsing IRC messages.. * dlist 0.4. Uploaded by DonaldStewart. [34]dlist: Differences lists. * AutoForms 0.4.0. Uploaded by MadsLindstroem. [35]AutoForms: GUI library based upon generic programming (SYB3). * bktrees 0.2.1. Uploaded by JosefSvenningsson. [36]bktrees: A set data structure with approximate searching. * bktrees 0.2. Uploaded by JosefSvenningsson. [37]bktrees: A set data structure with approximate searching. * binary-strict 0.1. Uploaded by AdamLangley. [38]binary-strict: Binary deserialisation using strict ByteStrings. * haddock 0.9. Uploaded by SimonMarlow. [39]haddock: Haddock is a documentation-generation tool for Haskell libraries. * bytestring-mmap 0.2.0. Uploaded by DonaldStewart. [40]bytestring-mmap: mmap support for strict ByteStrings. * bytestring 0.9.0.3. Uploaded by DonaldStewart. [41]bytestring: Fast, packed, strict and lazy byte arrays with a list interface. * hiccup 0.3. Uploaded by KyleConsalus. [42]hiccup: Added by KyleConsalus, Wed Dec 19 17:00:42 PST 2007.. 33. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/irc-0.4 34. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/dlist-0.4 35. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/AutoForms-0.4.0 36. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/bktrees-0.2.1 37. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/bktrees-0.2 38. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/binary-strict-0.1 39. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/haddock-0.9 40. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/bytestring-mmap-… 41. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/bytestring-0.9.0… 42. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hiccup-0.3 * cedict 0.1.1. Uploaded by JasonDusek. [43]cedict: Convenient Chinese character lookup.. * TypeCompose 0.3. Uploaded by ConalElliott. [44]TypeCompose: Type composition classes & instances. * bytestring-mmap 0.1.2. Uploaded by DonaldStewart. [45]bytestring-mmap: mmap support for strict ByteStrings. * bytestring 0.9.0.2. Uploaded by DonaldStewart. [46]bytestring: Fast, packed, strict and lazy byte arrays with a list interface. * bytestring-mmap 0.1.1. Uploaded by DonaldStewart. [47]bytestring-mmap: mmap support for strict ByteStrings. * mkcabal 0.3. Uploaded by DonaldStewart. [48]mkcabal: Generate cabal files for a Haskell project. * terminfo 0.1. Uploaded by Judah Jacobson. [49]terminfo: Haskell bindings to the terminfo library.. * Cabal 1.2.3.0. Uploaded by Duncan Coutts. [50]Cabal: A framework for packaging Haskell software. * hxt 7.4. Uploaded by UweSchmidt. [51]hxt: A collection of tools for processing XML with Haskell.. * X11 1.4.1. Uploaded by Spencer Janssen. [52]X11: A binding to the X11 graphics library. 43. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/cedict-0.1.1 44. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/TypeCompose-0.3 45. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/bytestring-mmap-… 46. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/bytestring-0.9.0… 47. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/bytestring-mmap-… 48. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/mkcabal-0.3 49. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/terminfo-0.1 50. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/Cabal-1.2.3.0 51. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hxt-7.4 52. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/X11-1.4.1 * dataenc 0.10.1. Uploaded by Magnus Therning. [53]dataenc: Data encoding library currently providing Uuencode, Base64, Base64Url, Base32, Base32Hex, and Base16.. * bytestringreadp 0.1. Uploaded by Gracjan Polak. [54]bytestringreadp: A ReadP style parser library for ByteString. * encoding 0.3. Uploaded by HenningGuenther. [55]encoding: A library for various character encodings. * hslua 0.2. Uploaded by Gracjan Polak. [56]hslua: A Lua language interpreter embedding in Haskell. * xmonad-contrib 0.5. Uploaded by Spencer Janssen. [57]xmonad-contrib: Third party extensions for xmonad. * xmonad 0.5. Uploaded by SpencerJanssen. [58]xmonad: A tiling window manager. * pandoc 0.45. Uploaded by John MacFarlane. [59]pandoc: Conversion between markup formats. * markov-chain 0.0.1. Uploaded by Henning Thielemann. [60]markov-chain: Markov Chains for generating random sequences with a user definable behaviour.. * parsedate 3000.0.0. Uploaded by Bjorn Bringert. [61]parsedate: Data and time parsing for CalendarTime. * hackage2hwn 0.1. Uploaded by Don Stewart. [62]hackage2hwn: Convert hackage = Hackage RSS feeds to Haskell Weekly News format. 53. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/dataenc-0.10.1 54. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/bytestringreadp-… 55. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/encoding-0.3 56. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hslua-0.2 57. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/xmonad-contrib-0… 58. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/xmonad-0.5 59. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/pandoc-0.45 60. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/markov-chain-0.0… 61. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/parsedate-3000.0… 62. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hackage2hwn-0.1 * hask-home 2007.12.6. Uploaded by BjornBringert. [63]hask-home: Generate homepages for cabal packages. * hmarkup 3000.0.1. Uploaded by BjornBringert. [64]hmarkup: Simple wikitext-like markup format implementation.. * hspread 0.2. Uploaded by AndreaVezzosi. [65]hspread: A client library for the spread toolkit. * pcap 0.4.2. Uploaded by BryanOSullivan. [66]pcap: A system-independent interface for user-level packet capture. * hogg 0.3.0. Uploaded by ConradParker. [67]hogg: Library and tools to manipulate the Ogg container format. * Finance-Quote-Yahoo 0.4.1. Uploaded by BradClawsie. [68]Finance-Quote-Yahoo: Obtain quote data from finance.yahoo.com. * Monadius 0.9.20071204. Uploaded by GwernBranwen. [69]Monadius: 2-D arcade scroller. * Shu-thing 1.0.20071203. Uploaded by GwernBranwen. [70]Shu-thing: A vector shooter game. * hmatrix 0.1.1.0. Uploaded by AlbertoRuiz. [71]hmatrix: Linear algebra and numerical computations. * HTTP 3001.0.3. Uploaded by BjornBringert. [72]HTTP: Added by BjornBringert, Fri Nov 30 14:50:55 PST 2007.. 63. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hask-home-2007.1… 64. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hmarkup-3000.0.1 65. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hspread-0.2 66. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/pcap-0.4.2 67. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hogg-0.3.0 68. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/Finance-Quote-Ya… 69. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/Monadius-0.9.200… 70. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/Shu-thing-1.0.20… 71. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hmatrix-0.1.1.0 72. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/HTTP-3001.0.3 * rss 3000.0.1. Uploaded by BjornBringert. [73]rss: A library for generating RSS 2.0 feeds.. * haxr 3000.0.1. Uploaded by BjornBringert. [74]haxr: XML-RPC client and server library.. * fitsio 0.1. Uploaded by EricSessoms. [75]fitsio: A library for reading and writing data files in the FITS data format.. * YamlReference 0.6. Uploaded by Oren Ben Kiki. [76]YamlReference, YAML reference implementation * LambdaShell 0.9.1. Uploaded by Robert Dockins. [77]LambdaShell, simple shell for evaluating lambda expressions * Shellac 0.9.1. Uploaded by Robert Dockins. [78]Shellac, a framework for creating shell envinronments * EdisonCore 1.2.1.1. Uploaded by Robert Dockins. [79]EdisonCore, a library of efficent, purely-functional data structures (Core Implementations) * hmatrix 0.1.0.0. Uploaded by Alberto Ruiz. [80]hmatrix, linear algebra and numerical computations * strict-concurrency 0.1. Uploaded by Don Stewart. [81]strict-concurrency, strict concurrency abstractions * X11 1.4.0. Uploaded by Don Stewart. [82]X11, binding to the X11 graphics library 73. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/rss-3000.0.1 74. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/haxr-3000.0.1 75. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/fitsio-0.1 76. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/YamlReference-0.6 77. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/LambdaShell-0.9.1 78. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/Shellac-0.9.1 79. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/EdisonCore-1.2.1… 80. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hmatrix-0.1.0.0 81. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/strict-concurren… 82. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/X11-1.4.0 * safecopy 0.3. Uploaded by David Himmelstrup. [83]safecopy, binary serialization with version control * HaXml 1.13.3. Uploaded by Malcolm Wallace. [84]HaXml, utilities for manipulating XML documents * c2hs 0.15.1. Uploaded by Duncan Coutts. [85]c2hs, C->Haskell Interface Generator * calc 0.1. Uploaded by Austin Seipp. [86]calc, small compiler for arithmetic expressions. * miniplex 0.2.1. Uploaded by Lukas Mai. [87]miniplex, simple 1-to-N interprocess communication * sat 1.1.1. Uploaded by Andrii Zvorygin. [88]sat, CNF SATisfier * dimensional 0.7.1. Uploaded by Bjorn Buckwalter. [89]dimensional, statically checked physical dimensions * hxt 7.4. Uploaded by Uwe Schmidt. [90]hxt, collection of tools for processing XML with Haskell. * dlist 0.3.2. Uploaded by Don Stewart. [91]dlist, difference lists. A list type supporting fast append. * mkcabal 0.2. Uploaded by Don Stewart. [92]mkcabal, generate cabal files for a Haskell project 83. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/safecopy-0.3 84. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/HaXml-1.13.3 85. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/c2hs-0.15.1 86. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/calc-0.1 87. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/miniplex-0.2.1 88. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/sat-1.1.1 89. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/dimensional-0.7.1 90. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hxt-7.4 91. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/dlist-0.3.2 92. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/mkcabal-0.2 * Chart 0.5. Uploaded by Tim Docker. [93]Chart, a library for generating 2D Charts and Plots * MaybeT 0.1.1. Uploaded by Don Stewart. [94]MaybeT, MaybeT monad transformer * regex-pcre 0.93. Uploaded by Chris Kuklewicz. [95]regex-pcre, replaces Text.Regex * fixpoint 0.1. Uploaded by Roman Leshchinskiy. [96]fixpoint, data types as fixpoints * ChasingBottoms 1.2.2. Uploaded by Nils Anders Danielsson. [97]ChasingBottoms, support for testing partial and infinite values * GrowlNotify 0.3. Uploaded by Nicholas Burlett. [98]GrowlNotify, notification utility for Growl * pcap 0.4.1. Uploaded by Bryan OSullivan. [99]pcap, a system-independent interface for user-level packet capture * bencode 0.3. Uploaded by David Himmelstrup. [100]bencode, parser and printer for bencoded data. * stream-fusion 0.1.1. Uploaded by Don Stewart. [101]stream-fusion, provides the standard Haskell list library reimplemented to allow stream fusion. This should in general provide faster list operations, and faster code for list-heavy programs. * HTTP 3001.0.2. Uploaded by Bjorn Bringert. [102]HTTP, library for client-side HTTP 93. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/Chart-0.5 94. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/MaybeT-0.1.1 95. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/regex-pcre-0.93 96. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/fixpoint-0.1 97. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/ChasingBottoms-1… 98. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/GrowlNotify-0.3 99. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/pcap-0.4.1 100. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/bencode-0.3 101. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/stream-fusion-0.… 102. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/HTTP-3001.0.2 * X11-xft 0.2. Uploaded by Clemens Fruhwirth. [103]X11-xft, bindings to the Xft, X Free Type interface library, and some Xrender parts * GrowlNotify 0.1. Uploaded by Nicholas Burlett. [104]GrowlNotify, notification utility for Growl. * HsHaruPDF 0.0.0. Uploaded by Audrey Tang. [105]HsHaruPDF, Haskell binding to libharu * unicode-normalization 0.1. Uploaded by Reinier Lamers. [106]unicode-normalization, Unicode normalization using the ICU library * uniplate 1.0.1. Uploaded by Neil Mitchell. [107]uniplate, uniform type generic traversals * lax-0.0.0. Uploaded by Wolfgang Jeltsch. [108]lax, Lax arrows are variants of other arrows which are ?less strict? than the original arrows. They can be used, for example, to produce I/O fixpoints in situations where fixIO would fail. * fastcgi 3001.0.1. Uploaded by Bjorn Bringert. [109]fastcgi, a Haskell library for writing FastCGI programs 103. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/X11-xft-0.2 104. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/GrowlNotify-0.1 105. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/HsHaruPDF-0.0.0 106. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/unicode-normaliz… 107. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/uniplate-1.0.1 108. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/lax-0.0.0 109. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/fastcgi-3001.0.1 Conference roundup New user groups * [110]Portland Functional Programmers Group * [111]FPSIG @ Southampton * [112]SingHaskell 110. http://groups.google.com/group/pdxfunc 111. http://article.gmane.org/gmane.comp.lang.haskell.cafe/32753 112. http://article.gmane.org/gmane.comp.lang.haskell.cafe/32268 Jobs Prototyping. Peter Verswyvelen [113]announced a job using Haskell for prototyping computer animation and games 113. http://article.gmane.org/gmane.comp.lang.haskell.cafe/33377 Blog noise [114]Haskell news from the [115]blogosphere. 114. http://planet.haskell.org/ 115. http://haskell.org/haskellwiki/Blog_articles * [116]Small shots of lambda calculus * [117]Pattern Matching in Ruby * [118]Haskell 'words' and Perl 'split' * [119]Word numbers, Part 4: Sort the words, sum the numbers * [120]Improve Your C#! Borrow from F#... * [121]Sun battling Microsoft (over F#?) * [122]zip in F# and Haskell * [123]Random numbers in Haskell * [124]A survey of Haskell unicode support * [125]Using Haskell for scripting tasks 116. http://fmota.tk/2007/11/small-shots-of-lambda-calculus-part-iii.html 117. http://blog.pretheory.com/arch/2007/11/pattern_matching_in_ruby_1.php 118. http://osfameron.vox.com/library/post/haskell-words-and-perl-split.html 119. http://conway.rutgers.edu/~ccshan/wiki/blog/posts/WordNumbers4/ 120. http://diditwith.net/2007/11/14/ImproveYourCBorrowFromF.aspx 121. http://vanirsystems.com/danielsblog/?p=227 122. http://jyliao.blogspot.com/2007/11/learning-wpf-with-f-dock-and-grid.html 123. http://monadicheadaches.blogspot.com/2007/11/random-numbers-where-it-all-st… 124. http://blog.kfish.org/2007/10/survey-haskell-unicode-support.html 125. http://pinderkent.blogsavy.com/archives/142 * [126]Princeton lost the DARPA Grand Challenge because of a C# memory leak * [127]Laziness in C#/LINQ * [128]Getting functional programming: currying * [129]The magic foldr * [130]Nested Parallel List Comprehensions * [131]FFI in Haskell * [132]Broadening ones horizons * [133]Monad Wars - 1: the Prompt * [134]Monad Wars - 2: the command line * [135]Holy Shmoly, GHC Haskell 6.8 smokes Python and Ruby away! 126. http://www.codeproject.com/showcase/IfOnlyWedUsedANTSProfiler.asp 127. http://gavinbierman.blogspot.com/2007/11/laziness-in-clinq.html 128. http://overwatering.blogspot.com/2007/11/currying.html 129. http://caos.di.uminho.pt/~ulisses/blog/2007/11/20/foldr-the-magic-function/ 130. http://tomschrijvers.blogspot.com/2007/11/nested-parallel-list-comprehensio… 131. http://notes-on-haskell.blogspot.com/2007/02/ffi-in-haskell.html 132. http://ms-os.com/showthread.php?t=116050 133. http://osfameron.vox.com/library/post/monad-wars---1-the-prompt.html 134. http://osfameron.vox.com/library/post/monad-wars---2-the-command-line.html 135. http://www.cse.unsw.edu.au/~dons/blog/2007/11/29#smoking * [136]Use those extra cores and beat C today! (Parallel Haskell redux) * [137]Parallelizing Haskell * [138]Python, Haskell, Ruby Smackdown * [139]Legitimate uses of micro-benchmarks: parameter passing and function call costs * [140]Holy Shmoly, GHC does some magic all by itself! * [141]PARE - PARallel Execution in Erlang - a response to Haskell * [142]Haskell design patterns are (probably) needed * [143]Deriving a Virtual Machine * [144]Games, cores, and functional languages * [145]Structure of a functional Java, er, method 136. http://www.cse.unsw.edu.au/~dons/blog/2007/11/29#smoking-4core 137. http://cmssphere.blogspot.com/2007/11/parallelizing-haskell.html 138. http://feedingthesnake.wordpress.com/2007/11/30/python-haskell-ruby-smackdo… 139. http://eigenclass.org/hiki/legitimate-microbenchmarks 140. http://geekrant.wordpress.com/2007/11/29/holy-shmoly-ghc-does-some-magic-al… 141. http://chlorophil.blogspot.com/2007/11/pare-parallel-execution-in-erlang.ht… 142. http://jpmoresmau.blogspot.com/2007/11/haskell-design-patterns-are-probably… 143. http://www.iis.sinica.edu.tw/~scm/?p=42 144. http://www.antipode.ca/2007/games-cores-and-functional-languages/ 145. http://jpmoresmau.blogspot.com/2007/11/structure-of-functional-java-er-meth… * [146]Current Fixation: Haskell * [147]Multicores, F# and SPJ: leading people to Haskell * [148]The IO Monad for People who Simply Don't Care * [149]Why I chose to learn Haskell * [150]Some Playing with Derivatives * [151]Solve Ball Clock Puzzle in Python and Haskell * [152]Back to functional languages... at least for a while? * [153]Overloading Semicolon, or, monads from 10,000 Feet * [154]Getting started with Haskell * [155]Arrows first encounter 146. http://onerousmachinations.blogspot.com/2007/11/current-fixation-haskell.ht… 147. http://nxtgenug.spaces.live.com/Blog/cns!59F714755D6E2A69!221.entry 148. http://sigfpe.blogspot.com/2007/11/io-monad-for-people-who-simply-dont.html 149. http://offthelip.org/?p=91 150. http://cdsmith.wordpress.com/2007/11/29/some-playing-with-derivatives/ 151. http://the-little-haskeller.blogspot.com/2007/11/solve-ball-clock-puzzle-in… 152. http://afurtado1980.spaces.live.com/Blog/cns!63514833CE40C143!518.entry 153. http://osteele.com/archives/2007/12/overloading-semicolon 154. http://talkingcode.co.uk/2007/12/03/getting-started-with-haskell/ 155. http://monadicheadaches.blogspot.com/2007/12/arrows-first-encounter.html * [156]Arithmetic for lists * [157]Functional Programming on .NET - Part 1 * [158]N-Queens in Haskell * [159]N-Queens in the writer monad * [160]Unit testing in Haskell * [161]Calculating the reflect-rotate-translate normal form for an isometry of the plane in Haskell, and verifying it with QuickCheck * [162]Visualizing 2D convex hull using Gtk and OpenGL in Haskell * [163]Hugs for the Nintendo DS * [164]Comparative terseness of Perl and Haskell * [165]Note on point-free programming style 156. http://wadler.blogspot.com/2007/12/arithmetic-for-lists.html 157. http://zerogradient.wordpress.com/2007/12/05/functional-programming-on-net-… 158. http://therning.org/magnus/archives/330 159. http://cdsmith.wordpress.com/2007/12/09/some-basic-stuff-the-writer-monad/ 160. http://hitesh-jasani.blogspot.com/2007/12/unit-testing-with-hunit-in-haskel… 161. http://www.kennknowles.com/blog/2007/12/03/calculating-the-reflect-rotate-t… 162. http://www.kennknowles.com/blog/2007/11/20/visualizing-2d-convex-hull-using… 163. http://closuretohome.blogspot.com/2007/12/hugs-for-nintendo-ds.html 164. http://blog.plover.com/2007/11/30/#pow-sqrt 165. http://blog.plover.com/prog/haskell/pointfree.html * [166]Haskell Fibonacci Revisited * [167]PXSL Tools 1.0: Your ticket out of XML Hell * [168]Haskell is kind of cool * [169]Learning Haskell with ProjectEuler * [170]Infinite lazy Knuth-Bendix completion for monoids in Haskell * [171]Hugs for the Nintendo DS * [172]My Type of Language... * [173]Catching pods with hpodder * [174]Hamming's problem * [175]Sharper function operators 166. http://chasethedevil.blogspot.com/2007/12/haskell-fibonacci-revisited.html 167. http://blog.moertel.com/articles/2007/12/17/pxsl-tools-1-0-your-ticket-out-… 168. http://www.goesping.org/archives/2007/12/18/haskell-is-kind-of-cool/ 169. http://www.robdickerson.net/?p=12 170. http://www.kennknowles.com/blog/2007/12/20/infinite-lazy-knuth-bendix-compl… 171. http://blog.closuretohome.com/2007/12/hugs-for-nintendo-ds.html 172. http://lambdalounge.blogspot.com/2007/12/my-type-of-language.html 173. http://lorenzod8n.wordpress.com/2007/12/22/catching-pods-with-hpodder/ 174. http://conway.rutgers.edu/~ccshan/wiki/blog/posts/Hamming/ 175. http://blog.closuretohome.com/2008/01/sharper-function-operators.html * [176]Xiangqiboard: play Xiangqi against a computer opponent * [177]Haskell is kind of cool * [178]FParsec - A Parser Combinator Library for F# * [179]RSA-Haskell * [180]LLVM bindings for Haskell * [181]perpubplat 0.9 - You're Looking at It: blog framework for Haskell * [182]The point of pointfree * [183]qtHaskell * [184]Web Objects and the underappreciated recursive do * [185]Equality operators in PHP and Haskell 176. http://xiangqiboard.blogspot.com/2007/12/gnuxiangqi-angekndigt.html 177. http://www.goesping.org/archives/2007/12/18/haskell-is-kind-of-cool/ 178. http://www.quanttec.com/fparsec/ 179. http://netsuperbrain.com/rsa-haskell.html 180. http://www.serpentine.com/blog/2008/01/03/llvm-bindings-for-haskell/ 181. http://mult.ifario.us/p/perpubplat-0-9-you-re-looking-at-it 182. http://www.vex.net/~trebla/weblog/pointfree.html 183. http://qthaskell.sourceforge.net/ 184. http://www.alpheccar.org/en/posts/show/89 185. http://www.syntaxpolice.org/index.php/?q=node%2F419 * [186]Pushing Haskell's type system to the limits: A reflective JSON serializer * [187]A Wake Up Call for the Logic Programming Community * [188]Design your functions for partial application * [189]Taxicab Numbers * [190]Composing Contracts * [191]HTTP content-type comparison at the type level * [192]Haskell-Join-Rules * [193]Exploring JPEG * [194]What's interesting to me about SquirrelMail attack * [195]Haskell and F#: Language Design 186. http://lingnerd.blogspot.com/2007/12/pushing-haskells-type-system-to-limits… 187. http://www.cs.kuleuven.ac.be/~dtai/projects/ALP/newsletter/dec07/content/Ar… 188. http://www.serpentine.com/blog/2007/12/26/design-your-functions-for-partial… 189. http://notes-on-haskell.blogspot.com/2007/12/taxicab-numbers.html 190. http://contracts.scheming.org/ 191. http://www.alpheccar.org/en/posts/show/88 192. http://taichi.ddns.comp.nus.edu.sg/taichiwiki/HaskellJoinRules 193. http://www.imperialviolet.org/binary/jpeg/ 194. http://www.syntaxpolice.org/index.php/?q=node%2F418 195. http://corsis.blogspot.com/2008/01/haskell-and-f-language-design.html * [196]An example of Haskell's beauty * [197]My resolve to learn Haskell has become stronger after having watched 'A Taste of Haskell' * [198]Emacs love w/ Haskell * [199]Finding my way * [200]Why not Scala? * [201]A Different Kind of Obscurity * [202]Languages that save you some typing 196. http://blog.whoop.as/?p=1668 197. http://rmathew.blogspot.com/2008/01/taste-of-haskell.html 198. http://blog.whoop.as/?p=1665 199. http://nointernalmonologue.blogspot.com/2008/01/finding-my-way.html 200. http://unenterprise.blogspot.com/2008/01/why-not-scala.html 201. http://ddvlad.wordpress.com/2007/12/14/a-different-kind-of-obscurity/ 202. http://daltonic.blogspot.com/2008/01/language-that-saves-you-some-typing.ht… Quotes of the Week * Conal: For me, the heart of functional programming is exactly this separation between model and presentation. The former is naturally functional and compositional, while the latter is often imperative/sequential and not-so-compositional. IO belongs with the latter. * ddarius: has programmed too much in Haskell. He now produces code that -compiles- and works the first time * disspy: If all you know is C, everything begins to look like a segmentation fault. * markedtrees: (On the city of Haskell) Ah yes, Haskell. Where all the types are strong, all the men carry arrows, and all the children are above average. * ola-bini: Haskell's type system is really nice, for example, but OCaml's really feels like half of it exists just to cover up holes in the other half, I'm half way into Erlang, but for several reasons the language feels very primitive. * so1i.warazd: I'm more and more comfortable spending time with Haskell these days. Haskell may not be the next thing, but whatever the next big thing is, it's probably going to have Haskelly fingerprints all over it?.. * sylvan: think that the perceived difficulty in using purely functional programming is probably a bit exaggerated these days, as all it means is 'we're explicit about where side effects occur' * falvo: I really wish that someone would come up with a type-safe replacement for the likes of Python. Oh, wait, it's called Haskell! Unfortunately, I'm forbidden from using Haskell at work. Sigh * Tyler Spaulding: eventually even 'simple' programs will nned multiple threads. Does that mean developers will suddenly flock to Haskell? Again, no. Language designers are well aware of the situation. Sun and Microsoft are constantly working on improving the Java and .NET frameworks. And by the time the average programmer needs it, both will have plenty of support for easy threading * ricky clarkson: Haskell is full of Aha and Hah moments for me * five9a2: Concurrency aside, I find it common to write Haskell code that is as fast as C. It is true that for most things, the C can be tweaked to go a bit faster, but that tweaking needs to be done on a case-by-case basis. In Haskell, it is easier to compose optimized components. Better algorithms beat an optimized compiler any day and using the best algorithms everywhere in C code tends to be painful, error-prone, and usually disparaged as premature optimization. * Jeff Moser: It's been this fear of skills rot that has pushed me to look into Lisp, Haskell, F#, Erlang, and other languages to avoid The Blub Paradox. * NFJS 2008 predictions: If you've never programmed in Haskell, now's a good time to learn, because those concepts and syntax are fast making their way towards you... * The honey monster: With the advent of multi-core CPUs and the promise of many core processors in the near future it occurrs to me that my interest in functional programming languages could not of happened at a more opportune time. It is not that imperative programming languages are not as capable, merely that functional programming languages seem to be more natural fit About the Haskell Weekly News New editions are posted to [203]the Haskell mailing list as well as to [204]the Haskell Sequence and [205]Planet Haskell. [206]RSS is also available, and headlines appear on [207]haskell.org. Headlines are available as [208]PDF. To help create new editions of this newsletter, please see the [209]contributing information. Send stories to dons at galois.com. The darcs repository is available at darcs get [210]http://code.haskell.org/~dons/code/hwn/ 203. http://www.haskell.org/mailman/listinfo/haskell 205. http://planet.haskell.org/ 207. http://haskell.org/ 208. http://code.haskell.org/~dons/code/hwn/archives/20080105.pdf 209. http://haskell.org/haskellwiki/HWN 210. http://code.haskell.org/~dons/code/hwn/

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bytestring 0.9.0.4
by Don Stewart 06 Jan '08

06 Jan '08

2.5 years after the first release, bytestring 0.9.0.4 is now available on hackage, http://hackage.haskell.org/cgi-bin/hackage-scripts/package/bytestring-0.9.0… Changes since the 0.9 release include: * support for bytestring literals (use -XOverloadedStrings) * make Data.ByteString.Lazy.Char8.lines lazier * fixes Handle resource leak in getContents/readFile * faster isSpace operations * performance improvements The most notable change is the instance IsString for strict and lazy bytestrings, enabling bytestrings to be written as direct string literals, without needing 'pack'. That is, the following is valid: import Data.ByteString.Char8 main = print ("abcdef" :: ByteString) (and the compile will even turn the pack into a no-op at compile time). Or, using the (soon to be released pcre-light package), bytestring regex literals can be written as: compile "a(.*)b" where compile :: ByteString -> Either String Regex With -XOverloadedStrings, the 'pack'for the regex is inferred. -- Don & Duncan

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HsColour version confusion
by Adrian Hey 05 Jan '08

05 Jan '08

Hello, I'm confused about which HsColour version I should be using with Haddock/Cabal (on Windows). According to this page.. http://www.cs.york.ac.uk/fp/darcs/hscolour/ ..the latest version is 1.9. But the latest version in Hackage is 1.6, the latest version in the ftp downloads dir is 1.8, unless you want a pre-compiled windows version in which case you're stuck with 1.3 :-) Anyone know what's going on? Thanks -- Adrian Hey

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ANN / CFC: minimalistic haskell blogging web application (perpubplat)
by Paul Brown 05 Jan '08

05 Jan '08

All -- I've spent some time on cleaning up my hobby blog publishing software and setting up a darcs repository, so I'll throw it out there for criticism or suggestions: darcs get http://datapr0n.com/repos/perpubplat It's running my blog (http://mult.ifario.us) It's relatively lightweight in that it doesn't use a framework but still weighs in at <1500 LOC with comments, but it does rely on the (Thanks, Bjorn!) FastCGI and XHTML combinator libraries. I have some plans for handling comments and the like, but things I'd like to figure out: - Turn the blog data model into a monad. - An idiomatic approach to handling conditional GET and E-Tag, i.e., send a 304 response if things aren't modified. Thanks in advance. -- paulrbrown(a)gmail.com http://mult.ifario.us/

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ANN / CFP - LLVM bindings for Haskell
by Bryan O'Sullivan 05 Jan '08

05 Jan '08

This is an early release of Haskell bindings for the popular LLVM compiler infrastructure project. If you don't know what LLVM is, it's a wonderful toybox of compiler components, from a complete toolchain supporting multiple architectures through a set of well-defined APIs and IR formats that are designed for building interesting software with. The official LLVM home page is here: http://llvm.org/ The Haskell bindings are based on Gordon Henriksen's C bindings. The C bindings are almost untyped, but the Haskell bindings re-add type safety to prevent runtime crashes and general badness. Currently, the entire code generation system is implemented, with most LLVM data types supported (notably absent are structs). Also plugged in is JIT support, so you can generate code at runtime from Haskell and run it immediately. I've attached an example. Please join in the hacking fun! darcs get http://darcs.serpentine.com/llvm If you want a source tarball, fetch it from here: http://darcs.serpentine.com/llvm/llvm-0.0.2.tar.gz (Hackage can't host code that uses GHC 6.8.2's language extension names yet.) There's very light documentation at present, but it ought to be enough to get you going. <b

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An interesting monad: "Prompt"
by Ryan Ingram 04 Jan '08

04 Jan '08

(This message is a literate haskell file. Code for the "Prompt" monad is preceded by ">"; code for my examples is preceded by "]" and isn't complete, but intended for illustration.) I've been trying to implement a few rules-driven board/card games in Haskell and I always run into the ugly problem of "how do I get user input"? The usual technique is to embed the game in the IO Monad: ] type Game = IO ] -- or ] type Game = StateT GameState IO The problem with this approach is that now arbitrary IO computations are expressible as part of a game action, which makes it much harder to implement things like replay, undo, and especially testing! The goal was to be able to write code like this: ] takeTurn :: Player -> Game () ] takeTurn player = do ] piece <- action (ChoosePiece player) ] attack <- action (ChooseAttack player piece) ] bonusTurn <- executeAttack piece attack ] when bonusTurn $ takeTurn player but be able to script the code for testing, allow undo, automatically be able to save replays, etc. While thinking about this problem earlier this week, I came up with the following solution: > {-# OPTIONS_GHC -fglasgow-exts -fallow-undecidable-instances #-} > -- undecidable instances is only needed for the MonadTrans instance below > > module Prompt where > import Control.Monad.Trans > import Control.Monad.Identity > class Monad m => MonadPrompt p m | m -> p where > prompt :: p a -> m a "prompt" is an action that takes a prompt type and gives you a result. A simple example: ] prompt [1,3,5] :: MonadPrompt [] m => m Int This prompt would ask for someone to pick a value from the list and return it. This would be somewhat useful on its own; you could implement a "choose" function that picked randomly from a list of options and gave non-deterministic (or even exhaustive) testing, but on its own this wouldn't be much better than the list monad. What really made this click for me was that the prompt type could be built on a GADT: ] newtype GamePrompt a = GP (GameState, GameChoice a) ] data GameChoice a where ] -- pick a piece to act with ] ChoosePiece :: Player -> GameChoice GamePiece ] -- pick how they should attack ] ChooseAttack :: Player -> GamePiece -> GameChoice AttackType ] -- etc. Now you can use this type information as part of a "handler" function: ] gameIO :: GamePrompt a -> IO a ] gameIO (GP (state, ChoosePiece player)) = getPiece state player ] gameIO (GP (state, ChooseAttack player piece)) = attackMenu player piece ] -- ... The neat thing here is that the GADT specializes the type of "IO a" on the right hand side. So, "getPiece state player" has the type "IO GamePiece", not the general "IO a". So the GADT is serving as a witness of the type of response wanted by the game. Another neat things is that, you don't need to embed this in the IO monad at all; you could instead run a pure computation to do AI, or even use it for unit testing! > -- unit testing example > data ScriptElem p where SE :: p a -> a -> ScriptElem p > type Script p = [ScriptElem p] > > infix 1 --> > (-->) = SE ] gameScript :: ScriptElem GameChoice -> GameChoice a -> Maybe a ] gameScript (SE (ChoosePiece _) piece) (ChoosePiece _) = Just piece ] gameScript (SE (ChooseAttack _ _) attack) (ChooseAttack _ _) = Just attack ] gameScript _ _ = Nothing ] ] testGame :: Script GameChoice ] testGame = ] [ ChoosePiece P1 --> Knight ] , ChooseAttack P1 Knight --> Charge ] , ChoosePiece P2 --> FootSoldier ] , ... ] ] So, how to implement all of this? > data Prompt (p :: * -> *) :: (* -> *) where > PromptDone :: result -> Prompt p result > -- a is the type needed to continue the computation > Prompt :: p a -> (a -> Prompt p result) -> Prompt p result This doesn't require GADT's; it's just using existential types, but I like the aesthetics better this way. Intuitively, a (Prompt p result) either gives you an immediate result (PromptDone), or gives you a prompt which you need to reply to in order to continue the computation. This type is a MonadPrompt: > instance Functor (Prompt p) where > fmap f (PromptDone r) = PromptDone (f r) > fmap f (Prompt p cont) = Prompt p (fmap f . cont) > > instance Monad (Prompt p) where > return = PromptDone > PromptDone r >>= f = f r > Prompt p cont >>= f = Prompt p ((>>= f) . cont) > > instance MonadPrompt p (Prompt p) where > prompt p = Prompt p return > > -- Just for fun, make it work with StateT as well > -- (needs -fallow-undecidable-instances) > instance (Monad (t m), MonadTrans t, MonadPrompt p m) => MonadPrompt p (t m) where > prompt = lift . prompt The last bit to tie it together is an observation function which allows you to run the game: > runPromptM :: Monad m => (forall a. p a -> m a) -> Prompt p r -> m r > runPromptM _ (PromptDone r) = return r > runPromptM f (Prompt pa c) = f pa >>= runPromptM f . c > > runPrompt :: (forall a. p a -> a) -> Prompt p r -> r > runPrompt f p = runIdentity $ runPromptM (Identity . f) p > > runScript :: (forall a. ScriptElem p -> p a -> Maybe a) > -> Script p -> Prompt p r -> Maybe r > runScript _ [] (PromptDone r) = Just r > runScript s (x:xs) (Prompt pa c) = case s x pa of > Nothing -> Nothing > Just a -> runScript s xs (c a) > runScript _ _ _ = Nothing > -- script & computation out of sync My original goal is now achievable: ] type Game = StateT GameState (Prompt GamePrompt) ] ] action :: GameChoice a -> Game a ] action p = do ] state <- get ] prompt $ GP (state, p) ] runGameScript :: Script GameChoice -> GameState -> Game a -> Maybe (GameState, a) ] runGameScript script initialState game ] = runScript scriptFn script' (runStateT game initialState) ] where ] script' = map sEmbed script ] scriptFn s (GP (s,p)) = gameScript (sExtract s) p ] sEmbed (SE p a) = SE (GP (undefined, p)) a ] sExtract (SE (GP (_,p)) a) = SE p a Any comments are welcome! Thanks for reading this far. -- ryan

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