- Haskell-Cafe - Haskell.org

Re: [Haskell] ANN: data-ordlist-0.0.1 and NumberSieves-0.0
by Henning Thielemann 17 Jul '09

17 Jul '09

Leon Smith schrieb: > Two new packages have been uploaded to Hackage, one that implements bag > (multiset) and set operations on ordered lists, and another that offers > three different number theoretic sieves. > > http://hackage.haskell.org/package/data-ordlist > > Data.OrdList offers many of the same kinds of operations as Data.Set, > although Data.Set is likely to often be a better choice. However, > this library is not intended to be used as an abstract datatype for sets > and multisets, rather it is intended to be a convenient way for > efficiently dealing with lists that you happen to know are ordered. You could also implement that in terms of Map a Int.

1 0

ANNOUNCE: first Grapefruit release
by Wolfgang Jeltsch 17 Jul '09

17 Jul '09

Dear friends of Haskell and Functional Reactive Programming, its my pleasure to announce the first official release of Grapefruit, a library for Functional Reactive Programming (FRP) with a focus on user interfaces. With Grapefruit, you can implement reactive and interactive systems in a declarative style. User interfaces are described as networks of communicating widgets and windows. Communication is done via different kinds of signals which describe temporal behavior. Grapefruit consists of five packages with several interesting features: grapefruit-frp: Functional Reactive Programming core * The FRP implementation uses a hybrid push/pull-based approach. * Signals can be memoized by binding them to variables – like ordinary data structures. * Merging of event streams combines simultaneous events properly. * Signals cannot behave differently by starting them at different times. The type system is used to enforce proper aging of signals. grapefruit-records: A record system for Functional Reactive Programming * Input records can specify fields as being required or optional. * Uninteresting output fields can be ignored. The set of interesting fields is specified by pattern matching. grapefruit-ui: Declarative user interface programming * The interface for UI programming is platform-independent and can be implemented on top of different UI toolkits. * A concrete implementation can be selected at compile time or runtime. grapefruit-ui-gtk: GTK+-based implementation of the general UI interface * Gtk2Hs is used internally. grapefruit-examples: examples demonstrating features of Grapefruit * Circular communication and switching are demonstrated. Support for the following is planned for the future: * signals with incremental updates * user interfaces with changing structure * more kinds of UI components * graphical animations Further information about Grapefruit is available on its wiki page at <http://haskell.org/haskellwiki/Grapefruit>. If you have questions, applause or criticism, please get in touch with me. Wolfgang Jeltsch Principal Grapefruit developer

6 10

Re: [Haskell] ANNOUNCE: first Grapefruit release
by Wolfgang Jeltsch 17 Jul '09

17 Jul '09

[redirecting to haskell-cafe] Am Sonntag, 15. Februar 2009 00:25 schrieben Sie: > Hi Wolfgang, > > I was wondering if I can use FLTK as GUI backend for Grapefruit? This should be possible in principal. It just could be that my assumptions about how widgets are created and composed were too tight so that Grapefruit’s general interface doesn’t fit FLTK. In this case, please just tell me and I will try to make the interface more general. > I believe for this to make it happen, I would have to output FLTK's C++ > into C then create bindings for Haskell (via FFI). Is that doable or an > quite tall order? Recently, a student of mine has written a program which generates a Haskell Qt binding fully automatically from Qt header files. The generated binding consists of three layers. The first layer is C++ code which reexports Qt’s functionality as a pure C interface. The C interface is ugly for humans and not type safe (because C doesn’t know classes). The second layer consists of a couple of FFI declarations. The third layer is Haskell code which provides a nice interface similar to the original C++ interface. I still have to get the source code of the binding generator from that student but I hope this will happen soon. I want to publish it then on the web. It hope that it is possible to reuse this binding generator for other C++ libraries. > Jamie Clark Best wishes, Wolfgang

2 2

What is the point of the 'What the bleep' names in haskell?
by Daryoush Mehrtash 17 Jul '09

17 Jul '09

Why do Haskell programmers (and libraries) name their function like "<@<" or "###"? Why not use a more descriptive label for functions? Daryoush

5 4

excercise - a completely lazy sorting algorithm
by Petr Pudlak 17 Jul '09

17 Jul '09

Hi all, about a month ago, we were discussing sorting in Haskell with a friend. We realized a nice property of lazy merge-sort (which is AFAIK the implementation of Data.List.sort): Getting the first element of the sorted list actually requires O(n) time, not O(n * log n) as in imperative languages. And immediately we asked: Would it be possible to create a lazy selection/sorting algorithm so that getting any element of the sorted list/array by its index would require just O(n) time, and getting all the elements would still be in O(n * log n)? More precisely: The result of sorting an n-element list or array should be a structure that allows to ask i-th element of the result (for example, a lazy array). Asking k arbitrary elements should take no more than O(min(n * log n, k * n)) I believe that this could be done, but so far I wasn't able to implement and show it myself. I think the solution would be somewhat modified lazy quicksort (or "Median of Medians algorithm" if we want to be sure that we get good pivots). Perhaps somebody else would also like to give it a try? Or perhaps explain me why it's not possible? Best regards, Petr

7 18

Oops in Haskell
by CK Kashyap 17 Jul '09

17 Jul '09

Hi, Can someone please send me a working example based on the contents posted in the URL below? http://yi-editor.blogspot.com/2008/12/prototypes-encoding-oo-style.html Thanks, Kashyap

2 1

laziness blowup exercise
by Thomas Hartman 17 Jul '09

17 Jul '09

Challenge: change one function in the following pipeline so that t doesn't blow up when executed in ghci. import qualified Data.Set as S t = last . take (10^6) . iterate f $ S.empty f s = S.delete 1 . S.insert 1 $ s Please suggest more of these types of exercises if you have them and maybe we can collect the folk wisdom into a wiki page and/or exercise page for beginners. spoiler for the above problem below :) myiterate f x = let nxt = f x in nxt `seq` x : myiterate f nxt

4 12

What's the status with unicode characters on haddock ?
by david48 17 Jul '09

17 Jul '09

Hello all, I made a small program for my factory and I wanted to try to document it using haddock. The thing is, the comments are in French and the resulting html pages are unreadable because the accentuated letters are mangled. It's not acceptable to use HTML entities, as I'd like the comments to remain readable when/if I edit the code. Anyone has had the same problem ? Found a workaround ? Thanks, David.

5 9

Jane Street
by Matthias Görgens 17 Jul '09

17 Jul '09

If one of you knows something about working at Jane Street, I'd be happy to have exchange some mails. I am considering applying there. Thanks!

1 0

Alternative IO
by Cristiano Paris 17 Jul '09

17 Jul '09

As a joke, I wrote an instance of Alternative for IO actions: {-# LANGUAGE ScopedTypeVariables #-} module Main where import Control.Applicative import Control.Exception instance Alternative IO where empty = undefined x <|> y = handle (\ (_ :: SomeException) -> y) x This would allow to write IO code which failsafes to a value if the previous computation failed, i.e.: *Main Control.Applicative> undefined <|> print "Hello" "Hello" *Main Control.Applicative> print "Hello" <|> undefined "Hello" It seems a neat way to catch exception in some scenarios. What do you think? Why is not Alternative IO defined in Control.Applicative? Thanks, Cristiano

9 11

Python vs Haskell in tying the knot
by Cristiano Paris 17 Jul '09

17 Jul '09

On Fri, Jul 17, 2009 at 12:41 PM, Cristiano Paris<frodo(a)theshire.org> wrote: > ... > Now, to confirm my hypothesis, I wrote a slight different version of > fib, like follows: > > fib' n = 1:1:(fib' n) `plus` (tail $ fib' n) where plus = zipWith (+) > > i.e. I inserted a fictious argument n in the definition of fib'. > > Now, if I try "take 30 $ fib' 100", it takes significntly longer than > "take 30 fib": specifically, the latter is instantaneous, while the > former takes about 5 seconds to complete on my MacBook Pro. Is this an > evidence that the "tying the knot" process is going on in the first > version? BTW, after a -O2 compilation, fib' is apparently as fast a fib. Cristiano

2 1

Line drawing algorithm
by CK Kashyap 17 Jul '09

17 Jul '09

Hi All, I am working on a diagraming utility in Haskell. I started with line drawing. I am doing the basic stuff using the y = mx + c formula to draw a line between (x1,y1) and (x2,y2) Here's what I need to do - if dx > dy where dx = (x2 - x1) and dy = (y2 - y1) then I need to vary x between x1 and x2 and find the various y's however if dy > dx then I need to vary y beteen y1 and y2 and get various x's In the code below, I've only taken care of the situation where dx > dy - I was thinking if there was a better way to do it that takes care of the other condition as well without repeating the code. type Point = (Integer,Integer) line :: Point -> Point -> [Point] -- get all the points in the line line p1@(x1,y1) p2@(x2,y2) = line' start end start slope where (start,end) = reorderPoints p1 p2 slope = ((fromIntegral (y2-y1)) / (fromIntegral (x2-x1))) reorderPoints (px1,py1) (px2,py2) | px1 < px2 = (p1,p2) | otherwise = (p2,p1) line' :: Point -> Point -> Point -> Double -> [Point] line' start@(x1,y1) end@(x2,y2) point@(x3,y3) slope | x3 == x2 = [end] | otherwise = [point] ++ line' start end (newX,newY) slope where newX = x3 + 1 newY = y1 + round (slope * (fromIntegral (newX - x1))) hello = line (1,1) (10,10) Regards, Kashyap

2 2

Python vs Haskell in tying the knot
by Cristiano Paris 17 Jul '09

17 Jul '09

Hi, as pointed out in this list, it seems that a "tying the knot" example would be the one better explaining the power of Haskell's lazy-by-default approach against Python+Iterator's approach to laziness. So, in these days I'm trying to grasp the meaning of this "tying the knot" concept which seems to be quite hard to understand for me (at least as much as Monads and Delimited Continuations were). Specifically, I was looking for a very basic example of TTK and came up with this implementation of Fibonacci (again!) which might possibly be a TTK-flavored way for generation: fib = 1:1:fib `plus` (tail fib) where plus = zipWith (+) Of course, this was something I already encountered when exploring the Y-combinator. Anyhow, I tried to translate this implementation to Python using Iterators and this is what I wrote: def fib(): yield 1 yield 1 p = plus(fib(),tail(fib())) while True: yield p.next() def plus(x,y): while True: yield x.next() + y.next() print take(5,fib()) I've omitted the implementation for tail() and take() for brevity. Apart from the iterator machinery, this is an direct translation of the Haskell's fib implementation. More, it's quite modular because fib lives by itself and is composed with take to get the final result. The only caveat in the Python code is that it maintains an O(n^2) Iterator's states, thus making it a very poor implementation. So my considerations are: 1 - The Fibonacci generator is not an example of TTK at all and then it can be translated to Python. 2 - The Fibonacci generator is a too simple example of TTK, easy to be translated to Python. 3 - The O(n^2) state caveat is THE point making the difference between Haskell and Python, for which Haksell is much more efficient that Python while remaining very expressive and idiomatic (but that may not be the case for other TTK examples). I'm trying to implement myself the doubly linked list example from the Wikipage, which is "certified" to be a TTK example, but I'd like to have your comments on this. Thank you. Cristiano

6 6

Parsec for C or C++
by Roy Lowrance 17 Jul '09

17 Jul '09

I am working on a research language that is a variant of C. I'd like to use Parsec as the parser. Is there an existing Parsec parser for C or C++ (or Java) that could serve as a starting point? Thanks, Roy

4 6

Parallellizing array-based function
by Jon Harrop 17 Jul '09

17 Jul '09

Is it possible to parallelize array-based functions such as in-place quicksort in Haskell? If so, does anyone have a working implementation? -- Dr Jon Harrop, Flying Frog Consultancy Ltd. http://www.ffconsultancy.com/?e

1 0

PEPM'10 - Call for Papers (Deadline: 6 Oct 09) - Invited Speakers announced
by Janis Voigtlaender 17 Jul '09

17 Jul '09

=============================================================== CALL FOR PAPERS ACM SIGPLAN 2010 Workshop on Partial Evaluation and Program Manipulation (PEPM'10) Madrid, January 18-19, 2010 (Affiliated with POPL'10) http://www.program-transformation.org/PEPM10 =============================================================== INVITED SPEAKERS: * Lennart Augustsson (Standard Chartered Bank, UK) * Jeremy Siek (University of Colorado at Boulder, USA) IMPORTANT DATES: * Paper submission: Tue, October 6, 2009, 23:59, Apia time * Author notification: Thu, October 29, 2009 * Camera-ready papers: Mon, November 9, 2009 To facilitate smooth organization of the review process, authors are asked to submit a short abstract by October 1, 2009. SUBMISSION CATEGORIES: * Regular research papers (max. 10 pages in ACM Proceedings style) * Tool demonstration papers (max. 4 pages plus max. 6 pages appendix) TRAVEL SUPPORT: Students and other attendants in need can apply for a SIGPLAN PAC grant to help cover expenses. For details, see http://www.sigplan.org/PAC.htm. SCOPE: The PEPM Symposium/Workshop series aims to bring together researchers and practitioners working in the areas of program manipulation, partial evaluation, and program generation. PEPM focuses on techniques, theories, tools, and applications of analysis and manipulation of programs. The 2010 PEPM workshop will be based on a broad interpretation of semantics-based program manipulation in a continued effort to expand the scope of PEPM significantly beyond the traditionally covered areas of partial evaluation and specialization and include practical applications of program transformations such as refactoring tools, and practical implementation techniques such as rule-based transformation systems. In addition, it covers manipulation and transformations of program and system representations such as structural and semantic models that occur in the context of model-driven development. In order to reach out to practitioners, there is a separate category of tool demonstration papers. Topics of interest for PEPM'10 include, but are not limited to: * Program and model manipulation techniques such as transformations driven by rules, patterns, or analyses, partial evaluation, specialization, program inversion, program composition, slicing, symbolic execution, refactoring, aspect weaving, decompilation, and obfuscation. * Program analysis techniques that are used to drive program/model manipulation such as abstract interpretation, static analysis, binding-time analysis, dynamic analysis, constraint solving, type systems, automated testing and test case generation. * Analysis and transformation for programs/models with advanced features such as objects, generics, ownership types, aspects, reflection, XML type systems, component frameworks, and middleware. * Techniques that treat programs/models as data objects including meta-programming, generative programming, deep embedded domain-specific languages, program synthesis by sketching and inductive programming, staged computation, and model-driven program generation and transformation. * Application of the above techniques including experimental studies, engineering needed for scalability, and benchmarking. Examples of application domains include legacy program understanding and transformation, DSL implementations, visual languages and end-user programming, scientific computing, middleware frameworks and infrastructure needed for distributed and web-based applications, resource-limited computation, and security. We especially encourage papers that break new ground including descriptions of how program/model manipulation tools can be integrated into realistic software development processes, descriptions of robust tools capable of effectively handling realistic applications, and new areas of application such as rapidly evolving systems, distributed and web-based programming including middleware manipulation, model-driven development, and on-the-fly program adaptation driven by run-time or statistical analysis. PROCEEDINGS: There will be formal proceedings published by ACM Press. In addition to printed proceedings, accepted papers will be included in the ACM Digital Library. Selected papers may later on be invited for a journal special issue dedicated to PEPM'10. SUBMISSION GUIDELINES: Papers should be submitted electronically via the workshop web site. Regular research papers must not exceed 10 pages in ACM Proceedings style. Tool demonstration papers must not exceed 4 pages in ACM Proceedings style, and authors will be expected to present a live demonstration of the described tool at the workshop (tool papers should include an additional appendix of up to 6 extra pages giving the outline, screenshots, examples, etc. to indicate the content of the proposed live demo at the workshop). Authors using Latex to prepare their submissions should use the new improved SIGPLAN proceedings style (sigplanconf.cls). PROGRAM CO-CHAIRS: * John Gallagher (Roskilde University, Denmark, and IMDEA Software, Spain) * Janis Voigtländer (Technische Universität Dresden, Germany) PROGRAM COMMITTEE MEMBERS: * Wei-Ngan Chin (National University of Singapore, Singapore) * Michael Codish (Ben-Gurion University of the Negev, Israel) * Jim Cordy (Queen's University, Canada) * Nate Foster (University of Pennsylvania, USA) * Haifeng Guo (University of Nebraska at Omaha, USA) * Patricia Johann (University of Strathclyde, UK) * Oleg Kiselyov (FNMOC, USA) * Günter Kniesel (University of Bonn, Germany) * Viktor Kuncak (Ecole Polytechnique Fédérale de Lausanne, Switzerland) * Yanhong Annie Liu (State University of New York at Stony Brook, USA) * Andres Löh (Utrecht University, Netherlands) * Jan Midtgaard (Roskilde University, Denmark) * David Monniaux (National Center for Scientific Research (CNRS) and VERIMAG laboratory, France) * Akimasa Morihata (University of Tokyo, Japan) * Alberto Pettorossi (Università di Roma Tor Vergata, Italy) * João Saraiva (Universidade do Minho, Portugal) * Ganesh Sittampalam (Credit Suisse, UK) * Fausto Spoto (Università di Verona, Italy) * Harald Søndergaard (University of Melbourne, Australia) * Walid Taha (Rice University, USA)

1 0

A voyage of undiscovery
by Bernie Pope 17 Jul '09

17 Jul '09

2009/7/17 Andrew Coppin <andrewcoppin(a)btinternet.com>: > I've been working hard this week, and I'm stumbled upon something which is > probably of absolutely no surprise to anybody but me. > > Consider the following expression: > > (foo True, foo 'x') > > Is this expression well-typed? > > Astonishingly, the answer depends on where "foo" is defined. If "foo" is a > local variable, then the above expression is guaranteed to be ill-typed. > However, if we have (for example) > > foo :: x -> x > > as a top-level function, then the above expression becomes well-typed. Some useful reading material: Section 22.7 of the book "Types and Programming Languages" by Benjamin Pierce. The classic paper "Basic Polymorphic Typechecking" by Luca Cardelli: http://lucacardelli.name/Papers/BasicTypechecking.pdf Both of these are very readable introductions to the let-style polymorphism found in the Hindley/Milner type system. Haskell's type system is essentially an elaboration of that idea. Pierce's book shows how to achieve let-polymorphism by inlining non-recursive let bindings during type checking/inference, which is a nice way to understand what is going on.

1 0

Atom resources
by Thomas DuBuisson 17 Jul '09

17 Jul '09

The wiki [1] where Atom [2] once lived has been gone for some time now. The only resources I know for Atom are a couple blog posts, the haddock docs, and the Atom source on patchtag. Is anyone aware of other resources? Tom [1] http://funhdl.org/wiki/doku.php [2] http://hackage.haskell.org/package/atom

3 3

another laziness blowup (this one unsolved)
by Thomas Hartman 17 Jul '09

17 Jul '09

Is it possible to fix alternate' (the second version), or otherwise define a fast stepwise alternate that doesn't blow up on long lists? alternate just breaks up [1,2,3,4,5] into ([1,3,5],[2,4]) Thanks! {-# LANGUAGE BangPatterns #-} import Data.List import Control.Arrow import Control.Parallel.Strategies t = (last *** last) $! alternate $ [1..(10^6)] t' = (last *** last) $! alternate' $ [1..(10^6)] t'' = (last *** last) $! alternate'' $ [1..(10^6)] -- finishes reasonably fast, but does a separate computation for the list and its tail rather than just rip through it alternate x = (skip1 x,(skip1 $ tail x)) skip1 = skip 1 skip n xs = let (a,b) = splitAt (n+1) xs in case a of [] -> [] x:_ -> x : skip n b -- this one overflows on million element list, even after fiddling with strictness on input args. can this be fixed? alternate' xs = let f3 :: Int -> (([Int],[Int]),Int) -> (([Int],[Int]),Int) f3 x ((a,b),n) = -- rnf (x,((a,b),n)) `seq` let nxtn = n+1 in if n `mod` 2 == 0 then ((x:a,b),nxtn) else ((a,x:b),nxtn) in fst . foldr f3 (([],[]),0) $ xs -- no overflow, goes through the list stepwise, but it's actually slightly slower than the first alternate because of the reverses alternate'' xs = let f3 ((a,b),n) x = (let nxtn = n+1 in if n `mod` 2 == 0 then ((x:a,b),nxtn) else ((a,x:b),nxtn) ) in (reverse *** reverse) . fst . foldl' f3 (([],[]),0) $ xs

1 1

Re: [Haskell-cafe] Leaner Haskell.org frontpage
by haskell＠kudling.de 16 Jul '09

16 Jul '09

I never said we should only expose 7 links. Take for example the task "Find out more about this Haskell i heared about". You would need to scan the right half of the front page and you need to scan the left part of the page. There you need to scan "About", it could be explained under "Why use Haskell?" or "Language definition" or "Haskell in 5 steps" or "Learning Haskell" or "Wiki articles" or "Blog articles and news". Where should i look? I have to scan a lot of text, i have to keep a lot of options in mind and for my taste the load is too much. Be my limit 7 or 20 links. haskell(a)kudling.de wrote: > Most people feel overwhelmed when confronted with more than 7+-2 items: > http://www.smashingmagazine.com/2007/10/09/30-usability-issues-to-be-aware-… This refers to the number of items/things people can remember in their short-time memory. This has nothing to do with the maximum number of menu items you should use. There is of course a limit, but there is no reason to limit it to 7+-2. Cheers, -- Jochem Berndsen | jochem(a)functor.nl GPG: 0xE6FABFAB

11 16

Applying different functions to different types for lists
by Szekeres István 16 Jul '09

16 Jul '09

Hi, I want to build a framework to apply different functions to different types of lists and I want to generalize the solution as much as possible but my Haskell knowledge is not really enough to do this (I'm a haskell novice). The problem has two dimensions: 1. Lets say I have two functions: r13, ul : Char -> Char where r13 returns the rot13ed character, and ul converts upper case chars to lower case and vice versa. Later I want a rot13 function that can operate on different types of lists and an uxl function that can also operate on different types of lists. 2. Lets say I have two kinds of lists to handle: String and Data.ByteString.Lazy as L. For Strings I want the updater function to be used for each character and for L I want all items to be converted to Char, the updater function to be updated and the result to be converted back to the original item type. Therefore I define a common class for handling these lists: class ListUpdater a where updateFn :: Char -> Char update :: a -> a so I can define the update function for the different types of lists: instance ListUpdater String where update = map updateFn instance ListUpdater L.ByteString where update = L.pack . (map updateFn) . L.unpack But now how should I glue these 2 dimensions together? How to say that I want a rot13 function that applies to any data type instantiated from ListUpdater and that is using the r13 function to do the update? And also that I want an uxl function that also applies to any data type instantiated from ListUpdater and that is using the ul function to do the update? Thanks, Istvan

2 1

IFL 2009: Third Call for Papers
by IFL 2009 16 Jul '09

16 Jul '09

1 0

Pattern matching does not work like this?
by Magicloud Magiclouds 16 Jul '09

16 Jul '09

Hi, I do not notice this before. "fun ([0, 1] ++ xs) = .." in my code could not be compiled, parse error. -- 竹密岂妨流水过山高哪阻野云飞

11 13

Re: Problems with nested Monads
by John Lato 16 Jul '09

16 Jul '09

If M is a monad transformer (e.g. StateT) and able to be parameterised over an arbitrary monad, you can do something related, which may actually be more useful to you. Given the definition of StateT from mtl: newtype StateT s m a = StateT { runStateT :: s -> m (a, s) } you could define this join-like function: join2 :: (Monad n) => n (StateT s n a) -> StateT s n a join2 m = StateT (\state -> m >>= flip runStateT state) (I don't know a good name for this function; it's called "joinIM" in iteratee) Perhaps this would apply to your situation? Cheers, John Lato > > Message: 18 > Date: Fri, 10 Jul 2009 17:25:55 -0400 > From: Job Vranish <jvranish(a)gmail.com> > Subject: Re: [Haskell-cafe] Problems with nested Monads > To: Edward Kmett <ekmett(a)gmail.com> > Cc: Haskell Cafe mailing list <haskell-cafe(a)haskell.org> > Message-ID: > <ed4cefb50907101425w118990e2x5d6585582157e08f(a)mail.gmail.com> > Content-Type: text/plain; charset="iso-8859-1" > > Yeah, I think the problem with my case is that while M is a specific monad > (essentially StateT), N can be an arbitrary monad, which I think destroys my > changes of making a valid joinInner/joinOuter/distribute. > Maybe someday Haskell will infer valid joinInner/joinOuter for simple cases > :D > Thanks for you help. I'll definitely have to see if I can find that paper. > > - Job Vranish > > On Fri, Jul 10, 2009 at 3:09 PM, Edward Kmett <ekmett(a)gmail.com> wrote: > >> The problem you have is that monad composition isn't defined in general. >> You would need some form of distributive law either for your monads in >> general, or for your particular monads wrapped around this particular kind >> of value. >> >> What I would look for is a function of the form of one of: >> >> distribute :: N (M a) -> M (N a) >> joinInner :: M (N (M a)) -> M (N a) >> joinOuter :: N (M (N a)) -> M (N a) >> >> that holds for your partiular monads M and N. >> >> IIRC Mark P. Jones wrote a paper or a lib back around '93 that used these >> forms of distributive laws to derive monads from the composition of a monad >> and a pointed endofunctor. >> >> -Edward Kmett >> >> On Fri, Jul 10, 2009 at 11:34 AM, Job Vranish <jvranish(a)gmail.com> wrote: >> >>> I'm trying to make a function that uses another monadic function inside a >>> preexisting monad, and I'm having trouble. >>> Basically my problem boils down to this. I have three monadic functions >>> with the following types: >>> f :: A -> M B >>> g :: B -> N C >>> h :: C -> M D >>> (M and N are in the monad class) >>> I want a function i where >>> i :: A -> M (N D) >>> >>> the best I can come up with is: >>> i :: A -> M (N (M D)) >>> i a = liftM (liftM h) =<< (return . g) (f a) >>> >>> I'm starting to feel pretty sure that what I'm going for is impossible. Is >>> this the case? >>> _______________________________________________ >>> Haskell-Cafe mailing list >>> Haskell-Cafe(a)haskell.org >>> http://www.haskell.org/mailman/listinfo/haskell-cafe >>>

1 0

homomorphic encryption and monads?
by Jason Dagit 15 Jul '09

15 Jul '09

Hello, I have just a very vague understanding of what homomorphic encryption is, but I was wondering if homomorphic encryption behaves as a one-way monad (like IO). So for example, suppose you wrote a function that worked on encrypted email to determine if it spam, maybe it looks like this: isSpam :: Cypher String -> Cypher Bool I could be mistaken about the way this works, but it seems that isSpam can't return a plain 'Bool' because then you could know things about the encrypted data without having to decrypt it first. So that is why I think it has to return "Cypher Bool". And because it's a homomorphic encryption, you could have something like doWork: doWork :: Cypher a -> (a -> b) -> Cypher b Which we could use to implement isSpam: isSpam s = doWork s spamTest where spamTest :: String -> Bool I initially thought that doWork should be (>>=), but it seems the type of the function should either be (a -> b) or (Cypher a -> Cypher b). That is, return :: a -> Cypher a, is not available. All the data should already be encrypted. Thinking about it a bit more, since we never have to decrypt the data to work with it, it seems that (a -> b) is wrong as well, because we don't have the key or whatever to do the decrypting. So, then it seems reasonable that the only type for doWork is this: doWork :: Cypher a -> (Cypher a -> Cypher b) -> Cypher b Which doesn't really seem all that useful now. On the other hand, maybe we have an algorithm which can take a function on normal values and gives us a function on Cypher values: enCypher :: (a -> b) -> Cypher a -> Cypher b Or perhaps, it should be: enCypher :: (a -> b) -> Cypher (a -> b) Which would match the type of return. If that is the type of return, then we probably want: (>>=) :: Cypher (a -> b) -> ((a -> b) -> Cypher (c -> d)) -> Cypher (c -> d) At this point, I'm not actually sure if the second type for enCypher makes any sense. But, if it does, then I think it says the purpose of the monad is to act on the transformations and not the data. That is, instead of focusing on the data as being held in the cypher, we are thinking of the functions as being transformed into a cypher space. Has anyone else been thinking about this? Jason

3 2

ANNOUNCE: generator
by Yair Chuchem 15 Jul '09

15 Jul '09

A new "generator" package has been uploaded to Hackage. It implements an alternative list monad transformer, a list class, and related functions. The difference from mtl/transformers's ListT is that mtl is a monadic action that returns a list: newtype ListT m a = ListT { runListT :: m [a] } generator's is a monadic list: data ListItem l a = Nil | Cons { headL :: a, tailL :: l a } newtype ListT m a = ListT { runListT :: m (ListItem (ListT m) a) } A short example program which reads numbers from the user and interactively sums them up: import Control.Monad.ListT (ListT) import Data.List.Class (execute, joinM, repeat, scanl, takeWhile) import Prelude hiding (repeat, scanl, takeWhile) main = execute . joinM . fmap print . scanl (+) 0 . fmap (fst . head) . takeWhile (not . null) . fmap reads . joinM $ (repeat getLine :: ListT IO (IO String)) I also wrote an example/blog-post about using ListT to add an undo option to the classic game of "hamurabi": http://mashebali.blogspot.com/2009/07/charlemagne-disraeli-and-jefferson.ht… Another interesting observation is that "ListT [] a" is a tree of "a"s. The module Data.List.Tree includes functions to prune and search such trees (dfs, bfs, bestFirstSearchOn, etc). This can be useful for modularizing code that uses the list monad for combinatoric search by decoupling tree creation from processing and pruning.

1 0

ANN: shelltestrunner 0.6 released
by Simon Michael 15 Jul '09

15 Jul '09

(and, the original which I didn't cc to -cafe.) I'm pleased to announce the first release of shelltestrunner: a small tool for testing any command-line program by running it through "shell" tests defined with a simple file format. Each test can specify the command-line arguments, input, expected output, expected stderr output, and/or expected exit code. This was extracted from the hledger project, inspired by the tests in John Wiegley's ledger project, and uses Max Bolingbroke's test- framework. In some cases you will get a big speedup by using test- framework's parallelising feature. Example: $ shelltestrunner shelltestrunner *.test -- -j8 :args.test:1: [OK] :args.test:2: [OK] :args.test:3: [OK] :args.test:4: [OK] :early-j-option.test: [OK] :help-flag.test: [OK] etc. I hope you find it useful. Feedback, patches, or alternate packaging suggestions are welcome! Best, -Simon

1 0

Re: ANN: shelltestrunner 0.6 released
by Simon Michael 15 Jul '09

15 Jul '09

And the urls: home - http://hackage.haskell.org/package/shelltestrunner darcs repo - http://joyful.com/repos/shelltestrunner

1 0

Problems with nested Monads
by Job Vranish 15 Jul '09

15 Jul '09

I'm trying to make a function that uses another monadic function inside a preexisting monad, and I'm having trouble. Basically my problem boils down to this. I have three monadic functions with the following types: f :: A -> M B g :: B -> N C h :: C -> M D (M and N are in the monad class) I want a function i where i :: A -> M (N D) the best I can come up with is: i :: A -> M (N (M D)) i a = liftM (liftM h) =<< (return . g) (f a) I'm starting to feel pretty sure that what I'm going for is impossible. Is this the case?

5 5

ANN: darcs 2.3 beta 4
by Petr Rockai 15 Jul '09

15 Jul '09

Hi again! We have decided to delay the release cycle slightly for 2.3 and release another beta. The primary reason for this is our Windows support -- I would like to invite all Windows users to install darcs-beta and give it a ride. If you have a working cabal-install, all you need to do is run: $ cabal update $ cabal install darcs-beta (this works on all platforms; you may need to use -f-curl, if you don't have the cURL headers available -- this is often the case on Windows). If you do not have cabal-install, please follow the instructions near the end of this message, "Installing on Windows". In addition to using cabal-install, you can also download a tarball from http://repos.mornfall.net/darcs/darcs-2.2.98.4.tar.gz and build manually (see the build instructions in README inside the tarball). Feedback -------- To make the darcs 2.3 release a good one, we still need testing feedback: please drop a note to darcs-users@ if you have installed darcs-beta, or failed to install it. If you run into any bugs, we need to know about them. Thanks! The question of GHC 6.8 ----------------------- Using GHC 6.10.3 or newer is *strongly recommended*. You may compile darcs with GHC 6.8, but there are several caveats. If you are using 6.8.2 or older, please disable mmap support (pass -f-mmap to cabal install or runghc Setup configure below). Note that the GHC 6.8.2 that ships with Debian Lenny is not affected and it should be safe to keep mmap enabled. It is also recommended to disable use of Hackage zlib when compiling with GHC 6.8.2 (including the Debian Lenny version): pass -f-zlib to cabal. When using zlib, we have seen occasional crashes with error messages like "openBinaryFile: file locked" -- this is a known GHC 6.8.2 bug (and is fixed in GHC 6.8.3). Last, if you are using a 64-bit system, darcs may hang when you exit a pager when compiled with GHC older than 6.10.3. Although this is harmless, it is quite inconvenient. Overall, the status of GHC 6.8 is "semi-supported": for many cases, things will work just fine, especially if you take a little extra caution with compilation flags. Installing on Windows --------------------- To install darcs on Windows systems from scratch, please download the Haskell Platform and MSYS: * http://hackage.haskell.org/platform/2009.2.0.1/HaskellPlatform-2009.2.0.1-s… * http://sourceforge.net/projects/mingw/files/MSYS+Base+System/MSYS-1.0.11-rc… After installing both, you should have an "MSYS" icon: run MSYS and in the terminal window type: $ cabal update $ cabal install darcs-beta -f-curl This should download, compile and install all required dependencies and darcs-beta itself. The resulting darcs executable will be placed into the Haskell Platform executables folder, and should be accessible from the MSYS shell (just type "darcs --version" to check). Yours, Petr.

1 0

Build 32-bit apps on 64-bit Windows?
by Lyle Kopnicky 15 Jul '09

15 Jul '09

If I am running GHC on 64-bit Windows, do I have a choice of building a 32-bit or 64-bit app? On a cursory glance through the command-line options, I didn't find anything. - Lyle

2 1

Circular pure data structures?
by John Ky 15 Jul '09

15 Jul '09

Hello, Is it possible to create a circular pure data structure in Haskell? For example: a :: Data let b = getNext a let c = getNext b c == a -- Gives True Thanks, -John

10 11

Re: iteratee enumHandle (and parsing)
by John Lato 15 Jul '09

15 Jul '09

Hello, This explanation is entirely correct. I'm not happy with this situation, because I think it's confusing for people just starting with iteratee. However, I do want something that will be unicode (and other possible encodings) aware. Currently, I'm waiting to see how unicode support in the base libraries works out. What seems the best possibility at this time is writing codecs that could be used with mapStream or convStream to handle this appropriately, but I'd prefer to re-use core unicode support if possible. I'm open to suggestions on this issue. At the very least, I should probably include the ASCII encoding in the iteratee package. I'd like to make the common case of reading ASCII text easier than it currently is. Until then, one approach is using mapStream as suggested below. Another is using a wrapped ByteString instead of the List chunk. The ByteString chunk forces the element to be an 8-bit ASCII character instead of using the Char storable instance. If the full polymorphism of the List chunk is required, the conversion can be done with either mapStream or convStream. convStream is probably more efficient for this at the moment. Regarding using Parsec with iteratees: unfortunately I don't really do much in the way of parsing myself so I'm not much help here. I think that Oleg intended for parsers to be written directly using the iteratee primitives, in particular the "heads" function would be useful (at least he found it so). I don't see an efficient way to generate a continuation on a partial parse with Parsec, using iteratees or not. I think you could just load data into a parser with setInput, but if you don't have enough data for a complete parse you'll just need to hold onto all the data until you've loaded enough to run it. If you tell me what functions you find lacking from iteratee, I'll look into implementing them. I'd rather not re-implement all of Parsec, but the text parsers should be simple enough, as well as some of the combinators. Cheers, John Lato > From: Echo Nolan <hellish(a)comcast.net> > > Hi Paolino. > > What's happening is reading [Char] uses the Storable instance for Char > which is 32-bit. Thus, you get gibberish. The below does what you want, > by reading Word8s and converting them. > > import Control.Exception > import Data.Char > import Data.Iteratee.IO > import Data.Iteratee.Base > import Data.Word > import System.IO > > main :: IO () > main = do > h <- openFile "mamma23" ReadWriteMode > hPutStr h "ciao" > hSeek h AbsoluteSeek 0 > l <- enumHandle h readString >>= run > print $ assert (l == "ciao") () > > -- This is declared on its own so I can give a type signature without making > -- any of the above lines unmanageably long. > readString :: IterateeG [] Word8 IO String > readString = joinI $ mapStream (chr . fromIntegral) stream2list > > This only works for ASCII, of course. Someone should write some > enumerators for the other encodings. > > Regards, > Echo Nolan

1 0

How to "Read" this?
by Magicloud Magiclouds 15 Jul '09

15 Jul '09

Hi, I have a data structure, which shows like this: AttrBgColor {bgColor = Color 0 0 0} And the following is my Read code. But it failed parsing.... 31 instance Read Attribute where 32 readsPrec _ str = [ (mkAttr attr_ color, rest) | (attr_, rest1) <- lex str 33 , (color, rest) <- if (isPrefixOf " {bgColor = " rest1) 34 || (isPrefixOf " {fgColor = " rest1) 35 then case reads $ drop 12 rest1 of 36 [(color_, rest_)] -> [(color_, rest_)] 37 _ -> [(Color 0 0 0, rest1)] 38 else [(Color 0 0 0, rest1)] ] 39 where mkAttr "AttrFgColor" color = AttrFgColor color 40 mkAttr "AttrBgColor" color = AttrBgColor color 41 mkAttr "AttrInverse" _ = AttrInverse 42 mkAttr "AttrWeak" _ = AttrWeak 43 mkAttr "AttrUnderline" _ = AttrUnderline -- 竹密岂妨流水过山高哪阻野云飞

2 1

Re: [Haskell] ANNOUNCE: StateVar 1.0.0.0
by Felipe Lessa 14 Jul '09

14 Jul '09

On Tue, Jul 14, 2009 at 07:16:41PM +0200, Sven Panne wrote: > To further modularize the OpenGL/OpenAL packages, a new > StateVar package has been released to Hackage. It contains > state variables, which are references in the IO monad, like > IORefs or parts of the OpenGL state. That's just great, thanks a lot! I was just going to ask if you had any plans of releasing it in the near future. Now, do you have any plans of releasing OpenGL using OpenGLRaw in the near future :)? I'm asking only because using StateVar right now would actually give more headaches than solve them as we would have two different versions of get and ($=) in scope. Thanks for your work, -- Felipe.

1 0

Goldman Sachs - your home for OCaml and Erlang?
by Max Cantor 14 Jul '09

14 Jul '09

I know that this is a bit off topic, but thought it would interest several readers. Apparently, GS, which. AFAIK, doesn't make a lot of noise in the FP space compared to some other banks, does use some FP: http://www.zerohedge.com/article/aleynikov-code-dump-uncovered

2 1

Re: [Haskell-cafe] Haskell as a first language?
by Gareth Smith 14 Jul '09

14 Jul '09

> On Tue, 2009-07-14 at 03:01 -0700, Michael Vanier wrote: > > Haskell is a wonderful language (my favorite language by far) but it is > > pretty difficult for a beginner. In fact, it is pretty difficult for > > anyone to learn in my experience, because it has so many advanced > > concepts that simply don't exist in other languages, and trying to > > absorb them all at once will likely be overwhelming. Then Duncan said: > As a contrary data-point, at Oxford we teach functional programming > (using Haskell) as the first course at the very beginning of the > computer science degree. I know several other universities also use FP > and Haskell very early on in their CS courses. At Imperial College, Haskell's also the first language the undergrads touch and people seem to like it. I think it's easy to forget how reassuring it is to be surrounded by lots of other beginners when one is learning something completely new - like programming. A lot of people here seem to be suggesting that haskell is an advanced language for advanced programmers. I think it more likely that people whose first experience of haskell is online are likely to come into contact with mostly advanced programmers in here and on #haskell. This is great, because they can teach us things - but it can also be off-putting, because they can seem so far beyond us. Imagine trying to learn basic arithmetic in a university common room full of category theorists. Personally, I think that haskell is a great beginners language. Just don't feel that you have to be able to keep up with all the #haskellers. It's absolutely fine to not understand what a monad transformer is. G

1 0

Example for formatted show in HStringTemplate
by Kemps-Benedix Torsten 14 Jul '09

14 Jul '09

Hello, is there a working example of how to use the format clause with HStringTemplate, e.g. for Data.Time.Day? I think, if there is a parameter $day$, a reasonable template might contain e.g.: $day;format="%d.%b.%Y"$ But I only get "toModifiedJulianDay: [54960]" as the result which corresponds to the unformatted show. Regards, Torsten

3 4

What to say about Haskell?
by Patai Gergely 14 Jul '09

14 Jul '09

Hello all, I was asked to give a one-hour 'introductory' seminar on Haskell. The audience is a bunch of very clever people with a wider than usual perspective on programming and mathematics, and my talk should be rather informational than evangelistic. Which topics do you think I should touch by all means given the short time? Gergely -- http://www.fastmail.fm - Same, same, but different...

10 15

Re: [Haskell-cafe] Program with profiling runs faster than without
by Grigory Sarnitskiy 14 Jul '09

14 Jul '09

14.07.09, 15:16, "Malcolm Wallace" <malcolm.wallace(a)cs.york.ac.uk>: > This is only a guess, but maybe there is a context-qualified CAF-like > value that is being re-evaluated multiple times in the non-profiling > case, but is appropriately monomorphised in the profiling case, so it > is only evaluated once? > Regards, > Malcolm And I guess you are quite right. I knew there is a poorly optimized place in my program, but I didn't touch it for some ideological reasons. But this morning I get how to optimize it in an ideological friendly manner, so now without profiling the program ~ 2 times faster. The speed of profiled version hasn't changed, hence I've manually done some work the compiler does during profiling.

1 0

Program with profiling runs faster than without
by Grigory Sarnitskiy 14 Jul '09

14 Jul '09

Hello! I can't understand why the following happens. ghc --make -fforce-recomp -O2 -fexcess-precision -fvia-C -optc-O2 Run.lhs -o Run -prof -auto-all and time ./Run TestSim seems to be much faster (I got ~4 times faster indeed) than just ghc --make -fforce-recomp -O2 -fexcess-precision -fvia-C -optc-O2 Run.lhs -o Run and time ./Run TestSim with ghc 6.8.2 on pentium 4 2.4 GHz under Ubuntu I was on my way to optimize the program when I've discovered this. The program is rather complicated and badly optimized but still I think it is quite strange to the profiled version to run faster. All necessary files I send with this letter (Tann.7z).

2 2

Re: [Haskell-cafe] Are GADTs what I need?
by Chris Eidhof 14 Jul '09

14 Jul '09

Then you could add a specific constructor for String. The main point is: the case construct only works for values, not for types. There is no typecase construct. If you want to have certain restrictions on the 'a', such as the Show class, you could also do something like this: > data Value where > VWrapper :: (Show a) => a -> Value If you could elaborate a bit on what you're trying to accomplish (from a higher viewpoint) then maybe we can help you some more. -chris On 13 jul 2009, at 17:42, Kev Mahoney wrote: > Thanks, that helps. > > I was hoping to not have to parametrize Value as there is a fair bit > of code to change, and it cascades down through the data structures > (maybe a forall a . Value a will help here?) > > I will have a go at this approach. In case anyone is interested the > code is at http://github.com/KMahoney > > > 2009/7/13 Chris Eidhof <chris(a)eidhof.nl>: >> Hey Kev, >> >> The types are "thrown away" during compile time. Therefore, if you >> have a >> constructor "VWrapper :: a -> Value" nothing is known about that >> "a" when >> you scrutinize it. >> >> What you could do, however, is something like this: >> >>> data Value a where >>> VInt :: Integer -> Value Integer >>> ... >>> VWrapper :: a -> Value a >> >> And then you can write a function doSomething: >> >>> doSomething :: Value String -> String >>> doSomething (VWrapper s) = s >> >> HTH, >> >> -chris >> >> On 13 jul 2009, at 12:41, Kev Mahoney wrote: >> >>> Hi there, >>> >>> I'm currently writing an interpreter that I would like to be able to >>> use with other haskell programs. I would like to be able to pass >>> along >>> arbitrary types though the interpreter. I've seen hints that GADTs >>> can >>> do this, but I am having trouble understanding them. >>> >>> So far, I've learnt you can do this: >>> >>> data Value where >>> VInt :: Integer -> Value >>> ... >>> VWrapper :: a -> Value >>> >>> which can let you encode arbitrary 'dynamic' types into Value. I was >>> hoping to be able to pattern match to get the value out again e.g. >>> >>> doSomething :: Value -> .... >>> doSomething (VWrapper String s) = ..... >>> >>> Also, anything that can help me out with GADTs in general will be >>> much >>> appreciated. >>> >>> Thanks, >>> Kevin. >>> _______________________________________________ >>> Haskell-Cafe mailing list >>> Haskell-Cafe(a)haskell.org >>> http://www.haskell.org/mailman/listinfo/haskell-cafe >> >>

6 6

Removing polymorphism from type classes (viz. Functor)
by George Pollard 14 Jul '09

14 Jul '09

Ok, so I have a small idea I'm trying to work on; call it a Prelude-rewrite if you want. For this I want to be able to have the hierarchy Functor → Applicative → Monad. For Functor, I would like to be able to implement it for a wider variety of types, as there are types which have aren't polymorphic which would also benefit from having an instance. My running example for this set of types is ByteString; the module contains the method: map ∷ (Word8 → Word8) → ByteString → ByteString However, we cannot use this for Functor because ByteString isn't polymorphic. To get around this, I devised the following: Introduce a type family which represents ‘points’ inside the type: type family Point f ∷ ★ For ByteString we have: type instance Point ByteString = Word8 For a polymorphic example (lists) we have: type instance Point [a] = a Now Functor becomes: class SimpleFunctor f where fmap ∷ (Point f → Point f) → (f → f) However, this doesn't allow for the existence of functions with the type (a → b). I need to introduce another type into the class: class Functor f g where fmap ∷ (Point f → Point g) → (f → g) But having two types isn't very nice (for one thing we can't introduce a fundep because for lists as it fails one of the coverage conditions), so introduce another type family to represent types which can be produced by giving a free variable: type Subst f a ∷ ★ type Subst [a] b = [b] type Subst ByteString b = ByteString class Functor f where fmap ∷ (Point f → Point (Subst f a)) → (f → Subst f a) I'm not sure how much of a hack this is, or if there is a better way. It seems to be OK... Now I want to implement Applicative. It would make sense to have ‘return’ be split out into a separate class, because this can be restricted in a similar way to Functor: class Pointed f where return ∷ Point f → f instance Pointed [a] where return x = [x] instance Pointed ByteString where return = BS.singleton Now, I want to be able to restrict Applicative to things which have [Pointed f, and forall a b. Point f ~ (a → b)]. At the moment I can't figure this out because I believe it would require something like the ‘quantified contexts’ proposal: class (Pointed f, ∀ a b. Point f ~ (a → b)) ⇒ Applicative f where ... I could have something like: class (Pointed f, Point f ~ (a → b)) ⇒ Applicative f a b where apply ∷ f → Subst f a → Subst f b This is still not very nice, because it requires two more type variables in the class, and the non-type-families version is far more straightforward... in fact, it makes sense for the Applicative class to have a polymorphic type because it must be able to have ‘return’ applied to arbitrary functions (remember [fmap f xs ≡ return f `apply` xs]). So back to: class Applicative f where apply ∷ f (a → b) → f a → f b But then ‘return’ cannot be added via a superclass restriction to Pointed! I seem to have painted myself into a corner. Does anyone see a better way to go about this? Thanks, - George

3 3

Questions about haskell CPP macros
by Matthew Elder 13 Jul '09

13 Jul '09

Hello Cafe, I am trying to improve the error reporting in my sendfile library, and I know I can find out the current file name and line number with something like this: {-# LANGUAGE CPP #-} main = putStrLn (__FILE__ ++ ":" ++ show __LINE__) This outputs: test.hs:2 Unfortunately, if your file is in a hierarchy of folders, this flat file name doesn't give much context. Is there a macro to find out the current module? IE if I had a module Foo.Bar.Car.MyModule, I would like to be able to output something like this on error: Foo.Bar.Car.MyModule:2 Any help is appreciated! Thanks, Matt -- Need somewhere to put your code? http://patch-tag.com Want to build a webapp? http://happstack.com

4 4

Are GADTs what I need?
by Kev Mahoney 13 Jul '09

13 Jul '09

Hi there, I'm currently writing an interpreter that I would like to be able to use with other haskell programs. I would like to be able to pass along arbitrary types though the interpreter. I've seen hints that GADTs can do this, but I am having trouble understanding them. So far, I've learnt you can do this: data Value where VInt :: Integer -> Value ... VWrapper :: a -> Value which can let you encode arbitrary 'dynamic' types into Value. I was hoping to be able to pattern match to get the value out again e.g. doSomething :: Value -> .... doSomething (VWrapper String s) = ..... Also, anything that can help me out with GADTs in general will be much appreciated. Thanks, Kevin.

4 4

Re: [Haskell-cafe] Leaner Haskell.org frontpage
by Don Stewart 13 Jul '09

13 Jul '09

ttencate: > On Thu, Jul 9, 2009 at 18:33, Don Stewart<dons(a)galois.com> wrote: > > ttencate: > >> Are there any kind of hard statistics and analytics that we can base > >> this discussion upon? There is always room for improvement, but > >> stumbling around in the dark making blind guesses may not be the best > >> way to go. Although I personally feel that Lenny's proposed page is an > >> improvement, statistics could tell us what actual people actually use > >> the site for. > > Thanks Don, I should have thought of that. It's a start! No matter what we decide, I'd like to advocate for maintaining the RSS feed of Hackage uploads on the front page -- this is what makes the page seem alive and active, and was introduced in response to John Hughes complaining the page was always out of date. (Similar to the Arch Linux package updates: http://www.archlinux.org/) -- Don

11 15

Type families and polymorphism
by Jeremy Yallop 13 Jul '09

13 Jul '09

Why does compiling the following program give an error? > {-# LANGUAGE TypeFamilies, RankNTypes #-} > > type family TF a > > identity :: (forall a. TF a) -> (forall a. TF a) > identity x = x GHC 6.10.3 gives me: Couldn't match expected type `TF a1' against inferred type `TF a' In the expression: x In the definition of `identity': identity x = x -- The University of Edinburgh is a charitable body, registered in Scotland, with registration number SC005336.

5 5

Hylomorphisms (was: [Haskell-cafe] excercise - a completely lazy sorting algorithm)
by Raynor Vliegendhart 13 Jul '09

13 Jul '09

On 7/12/09, Heinrich Apfelmus <apfelmus(a)quantentunnel.de> wrote: > Raynor Vliegendhart wrote: > > On 7/9/09, Heinrich Apfelmus <apfelmus(a)quantentunnel.de> wrote: > >> Of course, some part of algorithm has to be recursive, but this can be > >> outsourced to a general recursion scheme, like the hylomorphism > >> > >> hylo :: Functor f => (a -> f a) -> (f b -> b) -> (a -> b) > >> hylo f g = g . fmap (hylo f g) . f > >> > > > > Is that definition of hylo actually usable? A few on IRC tried to use > > that definition for a few examples, but the examples failed to > > terminate or blew up the stack. > > The implementation of quicksort with hylo works fine for me, given > medium sized inputs like for example quicksort (reverse [1..1000]) . > > What were the examples you tried? > One of the examples I tried was: hylo (unfoldr (\a -> Just (a,a))) head $ 42 This expression fails to determinate. Here are two examples copumpkin tried on IRC: <copumpkin> > let hylo f g = g . fmap (hylo f g) . f in hylo (flip replicate 2) length 5 <lambdabot> 5 <copumpkin> > let hylo f g = g . fmap (hylo f g) . f in hylo (flip replicate 2) sum 5 <lambdabot> * Exception: stack overflow

3 2

Why this cannot be compiled?
by Magicloud Magiclouds 13 Jul '09

13 Jul '09

Hi, The following code failed to compiled, with error: Attribute.hs:46:91: Couldn't match expected type `[t]' against inferred type `(a, String)' In the expression: (color_, rest_) In a case alternative: [(color_, rest_)] -> (color_, rest_) In the expression: case reads rest1 of { [(color_, rest_)] -> (color_, rest_) _ -> (Color 0 0 0, rest1) } 43 instance Read Attribute where 44 readsPrec _ str = [ (mkAttr attr_ color, rest) | (attr_, rest1) <- lex str 45 , (color, rest) <- case reads rest1 of 46 [(color_, rest_)] -> (color_, rest_) 47 _ -> (Color 0 0 0, rest1) ] 48 where mkAttr "AttrFgColor" color = AttrFgColor color 49 mkAttr "AttrBgColor" color = AttrBgColor color 50 mkAttr "AttrInverse" _ = AttrInverse 51 mkAttr "AttrWeak" _ = AttrWeak 52 mkAttr "AttrUnderline" _ = AttrUnderline -- 竹密岂妨流水过山高哪阻野云飞

2 2

Could C libraries be part of Haskell Platform?
by Maurício 13 Jul '09

13 Jul '09

Probably not, but... There are stable and portable libraries that implement functionality missing in Haskell. It could be nice to be able to list then in a cabal file (using, say, pkgconfig-depends) and know for sure that any system with Haskell Platform will have then available for use with FFI. An example would be sqlite3. To me, this seems to be ugly in theory and good in practice. Best, Maurício

3 6