- Haskell-Cafe - Haskell.org

For class Monoid; better names than mempty & mappend might have been: mid (mident) & mbinop
by KC 08 Aug '11

08 Aug '11

It would be easier for beginners to "grok". -- -- Regards, KC

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Is there a program to check if a matrix is totally unimodular?
by KC 08 Aug '11

08 Aug '11

-- -- Regards, KC

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Re: [Haskell-cafe] [Haskell-beginners] Conciseness question
by KC 08 Aug '11

08 Aug '11

Sorry, I haven't read all the replies so I don't know if you have completely described the problem you're trying to solve; however, it might be better for people in future to say they want O(1) for insertions, O(lg n) for searching, etc. If they don't want to completely describe what their problem is. -- -- Regards, KC

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Scalable Erlang PostDoc and 2 PhD Studentships
by Trinder, Philip W 08 Aug '11

08 Aug '11

The RELEASE project - A High-level Paradigm for Reliable Large-scale Server Software - is funded by the EU Framework 7 programme for 36 months from October 2011. Its aim is to scale the radical concurrency-oriented programming paradigm to build reliable general-purpose software, such as server-based systems, on massively parallel machines (100 000 cores). We seek * A post-doctoral researcher, and * Two part-time PhD students all working in the area of scalable programming language design, implementation, and evaluation. Further information on each post is attached. Phil -------------------------------------------- Phil Trinder School of Mathematical and Computer Sciences Heriot-Watt University Edinburgh, EH14 4AS E: P.W.Trinder(a)hw.ac.uk T: +44 (0)131 451 3435 F: +44 (0)131 451 3327 I: www.macs.hw.ac.uk/~trinder -- Heriot-Watt University is a Scottish charity registered under charity number SC000278.

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Re: [Haskell-cafe] Github support for cabal files
by Greg Weber 08 Aug '11

08 Aug '11

I think the haddock description field is a great barrior to documentation. I don't want to clutter my cabal file with lengthy documentation. Michael Snoyberg and I could not figure out how to document the Hamlet syntax because there is no way (as far as I know) to have literal unescaped, uninterpreted text. [1] When I go to a github repo, I expect that I can scroll down to the README and I will get a good overview. When I go to hackage, I expect nothing more than a synopsis, and I hope I can follow several links to find the information I am looking for. It is suggested that the description field should point to a top level module with all the documentation, but although haddock is good for documenting code, it is not an ideal overview style, and not all packages lend themselves well to having one top-level module. I would like a description field that just points to an external (README) file. Cabal could either use the pandoc library, or to avoid that dependency directly it could use the pandoc executable (that would have to be installed first by those of us that want this feature). Alternatively it could just allow an html file for the description and make me responsible for writing a script that runs pandoc and then invokes sdist. [1] http://hackage.haskell.org/package/hamlet

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Simple Parsec example
by michael rice 08 Aug '11

08 Aug '11

What other imports must I add to get this to run. I can't seem to get it right. Michael ========= import Text.ParserCombinators.Parsec.Prim main = case (parse numbers "" "11, 2, 43") of Left err -> print err Right xs -> print (sum xs) numbers = commaSep integer ============== [michael@sabal ~]$ ghc --make parsetest.hs[1 of 1] Compiling Main ( parsetest.hs, parsetest.o ) parsetest.hs:7:11: Not in scope: `commaSep' parsetest.hs:7:20: Not in scope: `integer'[michael@sabal ~]$

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Rotating calipers
by mukesh tiwari 08 Aug '11

08 Aug '11

Hello all , I am trying to understand rotating calipers [ http://en.wikipedia.org/wiki/Rotating_calipers ] but i am not sure if understood this algorithm correctly . I tried to use the almost same algorithm given on wiki but with four calipers to solve the problem [ http://cgm.cs.mcgill.ca/~orm/rotcal.html ]. My approach is find xminP, xmaxP, yminP ymaxP and their corresponding calipers will be ( 0 * i - j ) , ( o * i + j ) , ( i + 0 * j ) and ( -i + 0 * j ). I implemented the algorithm in Haskell but its not working . I am not sure if i have followed the wiki algorithm correctly and could some one please tell me what is wrong with implementation. It would be great if some one can explain this algorithm in pseudo code which explains the rotating caliper and their implementation details . In case of indentation , see here [ http://hpaste.org/49907 ] . Thank you Mukesh Tiwari import Data.List import Data.Array import Data.Maybe import Data.Function import Text.Printf import qualified Data.ByteString.Char8 as BS data Point a = P a a deriving ( Show , Ord , Eq ) data Vector a = V a a deriving ( Show , Ord , Eq ) data Turn = S | L | R deriving ( Show , Eq , Ord , Enum ) --start of convex hull compPoint :: ( Num a , Ord a ) => Point a -> Point a -> Ordering compPoint ( P x1 y1 ) ( P x2 y2 ) | compare x1 x2 == EQ = compare y1 y2 | otherwise = compare x1 x2 findMinx :: ( Num a , Ord a ) => [ Point a ] -> [ Point a ] findMinx xs = sortBy ( \x y -> compPoint x y ) xs compAngle ::(Num a , Ord a ) => Point a -> Point a -> Point a -> Ordering compAngle ( P x1 y1 ) ( P x2 y2 ) ( P x0 y0 ) = compare ( ( y1 - y0 ) * ( x2 - x0 ) ) ( ( y2 - y0) * ( x1 - x0 ) ) sortByangle :: ( Num a , Ord a ) => [ Point a ] -> [ Point a ] sortByangle (z:xs) = z : sortBy ( \x y -> compAngle x y z ) xs findTurn :: ( Num a , Ord a , Eq a ) => Point a -> Point a -> Point a - > Turn findTurn ( P x0 y0 ) ( P x1 y1 ) ( P x2 y2 ) | ( y1 - y0 ) * ( x2- x0 ) < ( y2 - y0 ) * ( x1 - x0 ) = L | ( y1 - y0 ) * ( x2- x0 ) == ( y2 - y0 ) * ( x1 - x0 ) = S | otherwise = R findHull :: ( Num a , Ord a ) => [ Point a ] -> [ Point a ] -> [ Point a ] findHull [x] ( z : ys ) = findHull [ z , x ] ys --incase of second point on line from x to z findHull xs [] = xs findHull ( y : x : xs ) ( z : ys ) | findTurn x y z == R = findHull ( x : xs ) ( z:ys ) | findTurn x y z == S = findHull ( x : xs ) ( z:ys ) | otherwise = findHull ( z : y : x : xs ) ys convexHull ::( Num a , Ord a ) => [ Point a ] -> [ Point a ] convexHull xs = reverse . findHull [ y , x ] $ ys where ( x : y : ys ) = sortByangle . findMinx $ xs --end of convex hull --start of rotating caliper part http://en.wikipedia.org/wiki/Rotating_calipers --dot product for getting angle angVectors :: ( Num a , Ord a , Floating a ) => Vector a -> Vector a - > a angVectors ( V ax ay ) ( V bx by ) = theta where dot = ax * bx + ay * by a = sqrt $ ax ^ 2 + ay ^ 2 b = sqrt $ bx ^ 2 + by ^ 2 theta = acos $ dot / a / b --rotate the vector x y by angle t rotVector :: ( Num a , Ord a , Floating a ) => Vector a -> a -> Vector a rotVector ( V x y ) t = V ( x * cos t - y * sin t ) ( x * sin t + y * cos t ) --dist between two parallel vectors distVec :: ( Num a , Ord a , Floating a ) => Vector a -> Vector a -> a distVec ( V x1 y1 ) ( V x2 y2 ) = sqrt $ ( x1 - x2 ) ^ 2 + ( y1 - y2 ) ^ 2 --rotating caliipers rotCal :: ( Num a , Ord a , Floating a ) => Array Int ( Point a ) -> a -> [ Int ] -> [ Vector a ] -> a -> Int -> a rotCal arr ang [ pa , pb , qa , qb] [ cpa , cpb , cqa , cqb ] area n | 2 * ang > pi = area | otherwise = rotCal arr ang' [ pa' , pb' , qa' , qb' ] [ cpa' , cpb' , cqa' , cqb' ] area' n where P x1 y1 = arr ! pa P x2 y2 = arr ! ( mod ( pa + 1 ) n ) P x3 y3 = arr ! pb P x4 y4 = arr ! ( mod ( pb + 1 ) n ) P x5 y5 = arr ! qa P x6 y6 = arr ! ( mod ( qa + 1 ) n ) P x7 y7 = arr ! qb P x8 y8 = arr ! ( mod ( qb + 1 ) n ) t1 = angVectors cpa ( V ( x2 - x1 ) ( y2 - y1 ) ) t2 = angVectors cpb ( V ( x4 - x3 ) ( y4 - y3 ) ) t3 = angVectors cqa ( V ( x6 - x5 ) ( y6 - y5 ) ) t4 = angVectors cqb ( V ( x8 - x7 ) ( y8 - y7 ) ) t = minimum [ t1 , t2 , t3 , t4 ] cpa' = rotVector cpa t cpb' = rotVector cpb t cqa' = rotVector cqa t cqb' = rotVector cqb t ang' = ang + t ( pa' , pb' , qa' , qb' ) = fN [ t1 , t2 , t3 , t4 ] t where fN [ t1 , t2 , t3 , t4 ] t | t == t1 = ( mod ( pa + 1 ) n , pb , qa , qb ) | t == t2 = ( pa , mod ( pb + 1 ) n , qa , qb ) | t == t3 = ( pa , pb , mod ( qa + 1 ) n , qb ) | otherwise = ( pa , pb , qa , mod ( qb + 1 ) n ) width = distVec cpa' cpb' length = distVec cqa' cqb' area' = min area $ length * width solve :: ( Num a , Ord a , Floating a ) => [ Point a ] -> a solve [] = 0 solve [ p ] = 0 solve [ p1 , p2 ] = 0 solve [ p1 , p2 , p3 ] = 0 solve arr = rotCal arr' 0 [ pa , pb , qa , qb ] [ cpa , cpb , cqa , cqb ] area n where y1 = minimumBy ( on compare fN1 ) arr y2 = maximumBy ( on compare fN1 ) arr x1 = minimumBy ( on compare fN2 ) arr x2 = maximumBy ( on compare fN2 ) arr pa = fromJust . findIndex ( == y1 ) $ arr pb = fromJust . findIndex ( == y2 ) $ arr qa = fromJust . findIndex ( == x1 ) $ arr qb = fromJust . findIndex ( == x2 ) $ arr cpa = V 1 0 cpb = V ( -1 ) 0 cqa = V 0 ( -1 ) cqb = V 0 1 area = 1e9 n = length arr arr' = listArray ( 0 , n ) arr fN1 ( P x y ) = y fN2 ( P x y ) = x --end of rotating caliper final :: ( Num a , Ord a , Floating a ) => [ Point a ] -> a final [] = 0 final [ p ] = 0 final [ p1 , p2 ] = 0 final [ p1 , p2 , p3 ] = 0 final arr = solve . convexHull $ arr

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xmonad on xkcd
by Jon Fairbairn 07 Aug '11

07 Aug '11

http://xkcd.com/934/ (and look at the “hover text”) — so who’s going to implement it? -- Jón Fairbairn Jon.Fairbairn(a)cl.cam.ac.uk

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library-profiling default
by Tom Doris 07 Aug '11

07 Aug '11

Hi Is there a good reason that the default for library-profiling in .cabal/config is set to False? It seems a lot of people hit the problem of trying to profile for the first time, finding it doesn't work because profiling libraries haven't been installed, then they have to walk the dependencies reinstalling everything. Is there a major cost or problem with just defaulting this to True? Apologies if this is answered elsewhere, I saw various discussions on why it is difficult to automatically build required libs with profiling on demand, but nothing that discussed changing the default so that they are always built. Tom

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ANN: Netwire AFRP library
by Ertugrul Soeylemez 07 Aug '11

07 Aug '11

Hi there, after a few weeks of development time, I have released the first official version of my arrowized FRP library called Netwire: http://hackage.haskell.org/package/netwire I have written it with networking applications in mind, but it is a general purpose library, so it can be used in all of the classic fields like gaming, animation, user interfaces and others. Its basic idea is the same as in Yampa, but its internal structure is much simpler and hence it's easier to extend. Also like in most of my libraries I have specifically decided to expose the internals, so you can extend Netwire without having to fork. Features not found in Yampa/Animas: * switching by ArrowChoice, * suspended signal transformers, * signal inhibition, * combination through ArrowZero and ArrowPlus, * impure signal transformers. There exists a wiki page, which should give you a quickstart: http://haskell.org/haskellwiki/Netwire I would be grateful for any constructive feedback. Greets, Ertugrul -- nightmare = unsafePerformIO (getWrongWife >>= sex) http://ertes.de/

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trouble using the aeson package
by anonymous 07 Aug '11

07 Aug '11

I recently installed the aeson package using cabal-install without any trouble: ❲~❳ uname -rspi; cabal -V; ghc -V; ghc-pkg list | grep "aeson\|double-conversion" Linux 2.6.38-10-generic x86_64 x86_64 cabal-install version 0.10.2 using version 1.10.1.0 of the Cabal library The Glorious Glasgow Haskell Compilation System, version 7.0.3 aeson-0.3.2.9 double-conversion-0.2.0.1 However when I try to use it I receive this error message: λ> let f = Data.Aeson.String Loading package double-conversion-0.2.0.1 ... can't load .so/.DLL for: stdc++ (libstdc++.so: cannot open shared object file: No such file or directory) This is the version I have installed: ❲~❳ aptitude search stdc++ | grep ^i i libstdc++6 - The GNU Standard C++ Library v3 i A libstdc++6-4.4-dev - The GNU Standard C++ Library v3 (developme i libstdc++6-4.5-dev - The GNU Standard C++ Library v3 (developme ❲~❳ aptitude show libstdc++6-4.4-dev | head -n 4 Package: libstdc++6-4.4-dev State: installed Automatically installed: yes Version: 4.4.5-15ubuntu1

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Prelude> read "1234" :: String -> "*** Exception: Prelude.read: no parse
by michael rice 07 Aug '11

07 Aug '11

Prelude> read "1234" :: Int1234Prelude> read "1234" :: String"*** Exception: Prelude.read: no parse Why? Michael

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Re: [Haskell-cafe] Deriving instances with GADTs
by Daniel Schüssler 05 Aug '11

05 Aug '11

Hi, you can declare an instance instance Show (OrderType s o) where ... this makes sense since it means "for every type s and o, OrderType s o is showable", which is vacuously true if (s,o) isn't in {Buy,Sell} x {Market, Limit} (because in that case, there /is/ no non-bottom value of type 'OrderType s o'). Obviously, this will make the newtype's 'deriving' clause work. Furthermore, you can use {-# LANGUAGE StandaloneDeriving #-} and write deriving instance Show (OrderType s o) (Standalone deriving works differently from a 'deriving' clause; the former generates the code regardless of how weird the type is and lets the typechecker decide. For your GADT, this succeeds). Cheers, Daniel Schüssler On 2011-August-04 Thursday 08:57:46 Tim Cowlishaw wrote: > Hi all, > > I've been writing a DSL to describe securities orders, and after a lot > of help from the kind folk of this list and #haskell have come up with > the following implementation, using generalised algebraic data types: > > https://gist.github.com/1124621 > > Elsewhere in my application, I make use of the order type defined > therein in the following newtype declaration: > > newtype OrderListLevel s = OrderListLevel {orders :: [Order s Limit]} > deriving (Eq, Show) > > However, the 'deriving' clause here fails: > > src/Simulation/OrderList.hs:9:82: > No instance for (Eq (Order s Limit)) > arising from the 'deriving' clause of a data type declaration > at src/Simulation/OrderList.hs:9:82-83 > > src/Simulation/OrderList.hs:9:86: > No instance for (Show (Order s Limit)) > arising from the 'deriving' clause of a data type declaration > at src/Simulation/OrderList.hs:9:86-89 > > > > I don't fully understand this - the error is correct that there is no > instance of either Eq or Show for (Order s Limit), however, instances > are defined for Order Buy Limit and Order Sell Limit, and since these > are the only possible types that a value can be constructed with (the > type constructor is 'closed' over these types in some sense I guess), > it seems to me that this should provide enough information to derive > the Eq and Show instances. Am I making unreasonable expectations of > ghci's instance-deriving mechanism here, or missing something obvious? > > Many thanks in advance, > > Tim > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe

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weird type signature in Arrow Notation
by bob zhang 05 Aug '11

05 Aug '11

hi, all testB :: (ArrowChoice t1, Num a1, Num a) => (a -> a1 -> t2) -> t1 a t3 -> t1 a1 t3 -> t1 (a, a1) t testB f g h = proc (x,y) -> do if (f x y)then g -< x + 1 else h -< y + 2 it's very strange that the type of _f_ is (a->a1->t2) which I thought should be a -> a1 -> Bool, btw, is there any way to get the output of preprocessing using -XArrow extensions, Thanks a lot best, bob

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One place available at CamHac
by Simon Marlow 04 Aug '11

04 Aug '11

Hi Folks, There is a place available at CamHac next Friday-Sunday (12-14 August). First come first served! For instructions on how to register, see the wiki page: http://www.haskell.org/haskellwiki/CamHac#Registration Also a reminder, if you have already registered but won't be able to make it, please let us know so that we can reallocate your place. Cheers, Simon

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Deriving instances with GADTs
by Tim Cowlishaw 04 Aug '11

04 Aug '11

Hi all, I've been writing a DSL to describe securities orders, and after a lot of help from the kind folk of this list and #haskell have come up with the following implementation, using generalised algebraic data types: https://gist.github.com/1124621 Elsewhere in my application, I make use of the order type defined therein in the following newtype declaration: newtype OrderListLevel s = OrderListLevel {orders :: [Order s Limit]} deriving (Eq, Show) However, the 'deriving' clause here fails: src/Simulation/OrderList.hs:9:82: No instance for (Eq (Order s Limit)) arising from the 'deriving' clause of a data type declaration at src/Simulation/OrderList.hs:9:82-83 src/Simulation/OrderList.hs:9:86: No instance for (Show (Order s Limit)) arising from the 'deriving' clause of a data type declaration at src/Simulation/OrderList.hs:9:86-89 I don't fully understand this - the error is correct that there is no instance of either Eq or Show for (Order s Limit), however, instances are defined for Order Buy Limit and Order Sell Limit, and since these are the only possible types that a value can be constructed with (the type constructor is 'closed' over these types in some sense I guess), it seems to me that this should provide enough information to derive the Eq and Show instances. Am I making unreasonable expectations of ghci's instance-deriving mechanism here, or missing something obvious? Many thanks in advance, Tim

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Haskell Weekly News: Issue 193
by Daniel Santa Cruz 04 Aug '11

04 Aug '11

Welcome to issue 193 of the HWN, a newsletter covering developments in the Haskell community. This release covers the week of July 24 to 30, 2011. [1] http://goo.gl/8hDku You can find an HTML rendition of this issue at: http://contemplatecode.blogspot.com/2011/08/haskell-weekly-news-issue-193.h… Announcements Wouter Swierstra issued a call for participation to the 16th ACM SIGPLAN International Conference on Function Programming (ICFP 2011) to be held in Tokyo, Japon on September 19-21, 2011. [2] http://goo.gl/ob6Tq New and Updated Projects These projects were announced in the mailing lists: * syntactic (Emil Axelsson): Update to version 0.5 [3] http://goo.gl/sKutg * Yi (Jeff Wheeler): Update to version 0.6.4.0 [4] http://goo.gl/Tr2Ns Quotes of the Week * sipa: it can optimize non-terminating programs into programs that output "<<loop>>" and quit; that's an optimization of infinity% ! * fazzone: People generally seem to react the same way to "My first assembly was x86" as "My dog died" -- "Oh my god, I'm so sorry" * [danharaj] I hate myself [edwardk] happy to help * bos: the number of people who grok rank-K types is inversely proportional to ackermann(K) * copumpkin: holy shit, I'm unsafeCoerce * quicksilver: edwardk had this brilliant idea of how he could stress-test our package architecture * chrisdone: monoid sounds like a name for a hairless alien species that communicates telepathically from star trek * [copumpkin] I basically translated repa to agda and made it all safe n shit. also all slow n shit [thoughtpolice] safe n shit, like, type systems yo * theorbtwo: Well, that's one way to respond to ghc having overly-long error messages. Just replace all error messges with "that's the stupidist shit I've ever heard". * acowley: I'm 101% sure I never would have figured it out. I was told all hope was lost, then simonmar swung in on his web shooters and figured it out * quicksilver: agda is an alluring trap for haskell programmers who hope to write better code. instead they end up writing no code. Top Reddit Stories * my humble rant about the Haskell learning curve Domain: willowbend.cx, Score: 66, Comments: 134 On Reddit: [5] http://goo.gl/g1buw Original: [6] http://goo.gl/3xDxl * GHC 7.2.* release candidates are now available Domain: haskell.org, Score: 28, Comments: 17 On Reddit: [7] http://goo.gl/0C0gx Original: [8] http://goo.gl/cvec5 * oleg compares iteratee with python's yield Domain: haskell.org, Score: 27, Comments: 12 On Reddit: [9] http://goo.gl/8YQKq Original: [10] http://goo.gl/BafGo * Report from HacPDX-II about progress on the new Hackage server Domain: well-typed.com, Score: 26, Comments: 0 On Reddit: [11] http://goo.gl/vBr7N Original: [12] http://goo.gl/QXy9a * Arrow = Category + Applicative? (Part I) Domain: cdsmith.wordpress.com, Score: 26, Comments: 34 On Reddit: [13] http://goo.gl/Zs5Yg Original: [14] http://goo.gl/AsfL1 * Profunctors in Haskell - A Neighborhood of Infinity Domain: blog.sigfpe.com, Score: 19, Comments: 11 On Reddit: [15] http://goo.gl/58CbB Original: [16] http://goo.gl/IKI9H * BlockedIndefinitelyOnMVar : Inside 1712B Domain: blog.ezyang.com, Score: 19, Comments: 3 On Reddit: [17] http://goo.gl/3Xtn0 Original: [18] http://goo.gl/4osPH * The Great Yesod Reorganization of 2011 Domain: yesodweb.com, Score: 18, Comments: 8 On Reddit: [19] http://goo.gl/ZGNoa Original: [20] http://goo.gl/V4buP * Is there a Haskell code formatter? Domain: stackoverflow.com, Score: 16, Comments: 32 On Reddit: [21] http://goo.gl/lQOX3 Original: [22] http://goo.gl/AthoZ * Building a Rate of Return System in Haskell – Part 3, Annualized Returns Domain: blog.virtucal.com, Score: 9, Comments: 7 On Reddit: [23] http://goo.gl/0QGPB Original: [24] http://goo.gl/R6am5 Top StackOverflow Questions * Is there a Haskell code formatter? votes: 18, answers: 1 Read on SO: [25] http://goo.gl/AthoZ * Help in understanding pointfree code votes: 17, answers: 1 Read on SO: [26] http://goo.gl/Ryy5c * Haskell: What is Weak Head Normal Form? votes: 17, answers: 5 Read on SO: [27] http://goo.gl/3CHAB * Understanding GHC assembly output votes: 13, answers: 1 Read on SO: [28] http://goo.gl/ujlve * What are Haskell's monad transformers in categorical terms? votes: 13, answers: 1 Read on SO: [29] http://goo.gl/vMPdO * Managing a stateful computation system in Haskell votes: 12, answers: 1 Read on SO: [30] http://goo.gl/HABkL * Am I using reactive-banana right? votes: 11, answers: 1 Read on SO: [31] http://goo.gl/7jiKr * Is there an object-identity-based, thread-safe memoization library somewhere? votes: 11, answers: 2 Read on SO: [32] http://goo.gl/7vGvl * Is there a practical way of using natural numbers in Haskell? votes: 11, answers: 2 Read on SO: [33] http://goo.gl/hJBK9 * Current state of integrating unit tests with Haskell's Cabal? votes: 10, answers: 1 Read on SO: [34] http://goo.gl/nm8Zc About the Haskell Weekly News To help create new editions of this newsletter, please send stories to dstcruz(a)gmail.com. Until next time, Daniel Santa Cruz References 1. http://haskell.org/ 2. http://permalink.gmane.org/gmane.comp.science.types.announce/2446 3. http://permalink.gmane.org/gmane.comp.lang.haskell.cafe/91427 4. http://permalink.gmane.org/gmane.editors.yi.devel/5550 5. http://willowbend.cx/2011/07/22/whats-wrong-with-learning-haskell/ 6. http://www.reddit.com/r/haskell/comments/j0say/my_humble_rant_about_the_has… 7. http://www.haskell.org/ghc/dist/7.2.1-rc1/ 8. http://www.reddit.com/r/haskell/comments/j3yly/ghc_72_release_candidates_ar… 9. http://www.haskell.org/pipermail/haskell-cafe/2011-July/094384.html 10. http://www.reddit.com/r/haskell/comments/j32c4/oleg_compares_iteratee_with_… 11. http://www.well-typed.com/blog/57 12. http://www.reddit.com/r/haskell/comments/iznjs/report_from_hacpdxii_about_p… 13. http://cdsmith.wordpress.com/2011/07/30/arrow-category-applicative-part-i/ 14. http://www.reddit.com/r/haskell/comments/j4h6a/arrow_category_applicative_p… 15. http://blog.sigfpe.com/2011/07/profunctors-in-haskell.html 16. http://www.reddit.com/r/haskell/comments/iyima/profunctors_in_haskell_a_nei… 17. http://blog.ezyang.com/2011/07/blockedindefinitelyonmvar/ 18. http://www.reddit.com/r/haskell/comments/j1ld6/blockedindefinitelyonmvar_in… 19. http://www.yesodweb.com/blog/2011/07/yesod-reorg 20. http://www.reddit.com/r/haskell/comments/iz2ew/the_great_yesod_reorganizati… 21. http://stackoverflow.com/questions/6870148/is-there-a-haskell-code-formatter 22. http://www.reddit.com/r/haskell/comments/j31f4/is_there_a_haskell_code_form… 23. http://blog.virtucal.com/cyclical/2011/7/28/building-a-rate-of-return-syste… 24. http://www.reddit.com/r/haskell/comments/j2gyp/building_a_rate_of_return_sy… 25. http://stackoverflow.com/questions/6870148/is-there-a-haskell-code-formatter 26. http://stackoverflow.com/questions/6797247/help-in-understanding-pointfree-… 27. http://stackoverflow.com/questions/6872898/haskell-what-is-weak-head-normal… 28. http://stackoverflow.com/questions/6808867/understanding-ghc-assembly-output 29. http://stackoverflow.com/questions/6854303/what-are-haskells-monad-transfor… 30. http://stackoverflow.com/questions/6835777/managing-a-stateful-computation-… 31. http://stackoverflow.com/questions/6812533/am-i-using-reactive-banana-right 32. http://stackoverflow.com/questions/6878193/is-there-an-object-identity-base… 33. http://stackoverflow.com/questions/6884220/is-there-a-practical-way-of-usin… 34. http://stackoverflow.com/questions/6787231/current-state-of-integrating-uni…

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ANNOUNCE: vector-algorithms 0.5.2
by Dan Doel 04 Aug '11

04 Aug '11

Greetings, Following some work at hac-phi, I've finally put together a new release of vector-algorithms. It should now be available via hackage, or you can pull from code.haskell.org if you prefer: hackage: http://hackage.haskell.org/package/vector-algorithms/ latest: darcs get http://code.haskell.org/~dolio/vector-algorithms/ The highlights of the new release are as follows: - Some bit rot in the test suite has been fixed - Some strictness has been added to the merge sort to improve unboxing on current GHCs - A couple out-of-bounds errors have been caught and fixed - An entirely new sort---American flag sort---has been added to the line-up * American flag sort is an in-place bucket sort, which only uses a constant amount of auxiliary heap space (determined by the element type). And unlike a typical radix sort, it is actually suitable for sorting arrays of variable-length (O(1) lookup) strings. To this end, there is a (strict) ByteString instance for the relevant class, and possibly more such instances to come. Bug reports, patches, requests, etc. can be sent to me at this address. Enjoy, -- Dan

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Fwd: shootout
by Ben 04 Aug '11

04 Aug '11

hello cafe-istas -- for those of you who are into these things, a lot of the shootout programs are suffering from "make errors" and thus do not have benchmarks. http://shootout.alioth.debian.org/u64q/haskell.php best, ben Begin forwarded message: > From: Don Stewart <dons00(a)gmail.com> > Date: July 30, 2011 9:52:12 AM PDT > To: Ben <midfield(a)gmail.com> > Cc: Don Bruce Stewart <dons(a)cse.unsw.edu.au> > Subject: Re: shootout > > Best to bring this up on haskell-cafe@ > > On Sun, Jul 31, 2011 at 12:34 AM, Ben <midfield(a)gmail.com> wrote: >> FYI, a lot of the haskell programs on the shootout suffer from "make errors" and thus do not have benchmarks. >> >> best, ben

3 2

Difference between class and instance contexts
by Patrick Browne 03 Aug '11

03 Aug '11

3 2

Fractional Part
by Ata Jafari 03 Aug '11

03 Aug '11

Hi everyone, I'm totally new to Haskell and functional programming. I try to solve some problems from Proejct Euler with Haskell so that I can improve myself in functional programming. In the first step I want to write a little code that can give me only the decimal part of a float. For instance: if the number is (0.123) I want to obtain only the (123) part to do some processes on that. (I mean the result of this code must be an integer.) I don't expect any complete code, any suggestion and help will be appreciated. Thanks Ata

4 3

QuickCheck Questions
by Mark Spezzano 02 Aug '11

02 Aug '11

Hi all, I would appreciate it if someone can point me in the right direction with the following problem. I'm deliberately implementing a naive Queues packages that uses finite lists as the underlying representation. I've already read through Hughes' paper and the article in The Fun of Programming, but I'm still having some difficulties. Specifically: 1. I have a newtype Queue a = Queue [a] and I want to generate Queues of random Integers that are also of random size. How do I do this in QuickCheck? I guess that I need to write a generator and then make my "Queue a" concrete type an instance of Arbitrary? How? 2. If I wanted to specify/constrain the ranges of random Integers generated, how would I do this? 3. If I wanted to specify/constrain the Queue sizes how would I do this? On a separate issue, I also see that QuickCheck 2 has some features not discussed anywhere. For example "Coarbitrary" and "shrink" are a bit of a mystery to me. Any code examples for any of the above questions would help greatly. I've been stuck on this problem for hours. :) Cheers, Mark

4 4

difference between class context and deriving
by Patrick Browne 02 Aug '11

02 Aug '11

What is the difference between using a class context and deriving in data type declaration? Are there certain situations in which one or the other is preferred? data Eq a => Set1 a = NilSet1 | ConsSet1 a (Set1 a) data Set2 a = NilSet2 | ConsSet2 a (Set2 a) deriving Eq (NilSet1) == (NilSet1) -- no instance, error (NilSet2) == (NilSet2) -- True -- seems OK type Type1 = Set1 Integer type Type2 = Set2 Integer -- seems OK data Data1 = Set1 | Set2 -- Seems to have same type :t ConsSet2 1 NilSet2 ConsSet2 1 NilSet2 :: forall t. (Num t) => Set2 t :t ConsSet1 1 NilSet1 ConsSet1 1 NilSet1 :: forall t. (Num t) => Set1 t -- Pat This message has been scanned for content and viruses by the DIT Information Services E-Mail Scanning Service, and is believed to be clean. http://www.dit.ie

4 3

State of play with Haskell-Cocoa (Objective-C) bindings?
by Luke Evans 01 Aug '11

01 Aug '11

I'm planning to start an Objective-C/Cocoa project and would like to write it in Haskell as much as possible. Of course, I can contrive to split the project into app logic (Haskell) and UI (Objective-C) with some sort of minimal interface between them; possibly just plain old C FFI. However, I would like an interface that's as 'automatic' as possible - one that preferably means that I don't have to spend a lot of time maintaining a somewhat heavy-weight special interface layer as I grow my code on both sides of the divide. Preferably the suture line is as far over on the Objective-C side as possible, so that I can write pretty much everything in Haskell. Now, I see a couple of references to some options: - HOC seems very close to what I'm looking for, but the project looks pretty 'dormant'. - I see some mention of an Objective-C FFI, but am uncertain if this is in any current build of GHC. Are there any other options up anyone's sleeves? Anybody working on something interesting I could play with? Until recently, I was concerned about being able to use GC on the Objective-C side, but with Apple's new ARC memory management model, that's no longer an issue. Most Haskell-ObjC bindings that already exist are surely designed for Retain-Release, which I assume is not disrupted by ARC as this is just RR at runtime. -- Luke

3 2

Unbelievable parallel speedup
by John D. Ramsdell 01 Aug '11

01 Aug '11

I've enjoyed reading Simon Marlow's new tutorial on parallel and concurrent programming, and learned some surprisingly basic tricks. I didn't know about the '-s' runtime option for printing statistics. I decided to compute speedups for a program I wrote just as Simon did, after running the program on an unloaded machine with four processors. When I did, I found the speedup on two processors was 2.4, on three it was 3.2, and on four it was 4.4! Am I living in a dream world? I ran the test nine more times, and here is a table of the speedups. 2.35975 3.42595 4.39351 1.57458 2.18623 2.94045 1.83232 2.77858 3.41629 1.58011 2.37084 2.94913 2.36678 3.63694 4.42066 1.58199 2.29053 2.95165 1.57656 2.34844 2.94683 1.58143 2.3242 2.95098 2.36703 3.36802 4.41918 1.58341 2.30123 2.93933 That last line looks pretty reasonable to me, and is what I expected. Let's look at a table of the elapse times. 415.67 176.15 121.33 94.61 277.52 176.25 126.94 94.38 321.37 175.39 115.66 94.07 277.72 175.76 117.14 94.17 415.63 175.61 114.28 94.02 277.75 175.57 121.26 94.10 277.68 176.13 118.24 94.23 277.51 175.48 119.40 94.04 415.58 175.57 123.39 94.04 277.62 175.33 120.64 94.45 Notice that the elapse times for two and four processors is pretty consistent, and the one for three processors is a little inconsistent, but the times for the single processor case are all over the map. Can anyone explain all this variance? I have enclosed the raw output from the runs and the script that was run ten times to produce the output. John

5 4

A language that runs on the JVM or .NET has the advantage of Oracle & Microsoft making those layers more parallelizable.
by KC 01 Aug '11

01 Aug '11

Are there plans a foot (or under fingers) to make a version of Haskell that runs on the JVM? -- -- Regards, KC

9 9

ANNOUNCE: HROOT-0.5.0.1 - haskell binding to ROOT
by Ian-Woo Kim 01 Aug '11

01 Aug '11

Hi, everyone, I am pleased to announce a haskell binding to the ROOT analysis package, HROOT: http://hackage.haskell.org/package/HROOT >From wikipedia: ROOT (http://root.cern.ch) is an object-oriented program and library developed by CERN. It was originally designed for particle physics data analysis and contains several features specific to this field, but it is also used in other applications such as astronomy and data mining. ROOT includes * histogramming and graphing to view and analyze distributions and functions, * curve fitting (regression analysis) and minimization of functionals, * statistics tools used for data analysis, * matrix algebra, * four-vector computations, as used in high energy physics, * standard mathematical functions, * multivariate data analysis, e.g. using neural networks, * image manipulation, used, for instance, to analyze astronomical pictures, * access to distributed data (in the context of the Grid), * distributed computing, to parallelize data analyses, * persistence and serialization of objects, which can cope with changes in class definitions of persistent data, access to databases, * 3D visualizations (geometry) * creating files in various graphics formats, like PostScript, PNG, SVG, * interfacing Python and Ruby code in both directions, * interfacing Monte Carlo event generators. Current implementation of HROOT is very limited to histogramming and graphing, but complete binding will be planned to be provided soon. This will help haskell community for doing numerical analysis. To install HROOT, you have to install ROOT prior to HROOT. It's fairly easy to install. The homepage of HROOT is now on http://ianwookim.org/HROOT Enjoy this new addition. I'd love to hear feedback. Part of this work was done in my "HacPhi 2011" activity. Thank you for HacPhi organizers and all the participants. Ian-Woo

1 0

Re: [Haskell-cafe] XCode Dependency for HP on Mac
by Tom Murphy 31 Jul '11

31 Jul '11

On 7/30/11, Mark Lentczner <mark(a)glyphic.com> wrote: > Hiho - I'm the maintainer of the Mac installer for HP. I thought I'd > chime in a bit: > An expert :) > On Mac OS X, "developer tools" is essentially synonymous with "Xcode". > That is, to get the set of standard utilities needed for development > on compiled executables (notably the "binutils"), you install Xcode. > True, it also includes the IDE called Xcode, but the vast bulk [...] Remember everyone: install just the system tools and dev tools (only haskell required parts), and your install is 1 or 2 GB instead of 11GB. > As several have pointed out, you can download Xcode for free. It turns out this is true. It's very hard to find the dl link to an XCode that doesn't require Lion, but it actually is there. > (I haven't bought a new Air... > yet... but perhaps someone can check to see if the Xcode installer is > one the SSD volume already?) > It doesn't, and it's not on the USB restore disk either. > It is conceivably possible to build and distribute some of those > tools, but not the whole bundle. But the difficulty of getting such a > build just right, and all the pieces in the right place, seems absurd > to attempt to recreate when Apple has done it, and gives it away for > free. Apple's versions of bintools also includes many extensions extra > options for the OS X environment (like supporting multi-arch binaries) > Finally, there is also licensing questions regarding the parts > supplied by the OS vendor (headers, stub libs, debug libs, etc....) > So in response to the question "is there a way to install the HP without XCode?" the answer is pretty much no. Ok. It's a shame, but thanks for the insight! Thanks, Tom

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Re: [Haskell-cafe] [Haskell-beginners] No instance for (Show a)
by Mark Spezzano 31 Jul '11

31 Jul '11

Hi, You might need a class constraint. instance (Show a) => Show (Stack a) where This basically lets Haskell know that if your "a" type is Showable then "Stack a" is also Showable. Let me know if this works. Cheers, Mark Spezzano On 31/07/2011, at 6:49 PM, Ovidiu Deac wrote: > For some reason ghc complains about not being able to call show on an > Integer (?!?!?) > > Please enlighten me! > > ovidiu > > This is the hspec: > it "shows one element" > ( show (push 1 EmptyStack) ≡ "Stack(1)") > > ...this is the code: > > data Stack a = > EmptyStack | > StackEntry a (Stack a) > deriving(Eq) > > > instance Show (Stack a) where > show s = > "Stack(" ⊕ (showImpl s) ⊕ ")" > where > showImpl EmptyStack = "" > showImpl (StackEntry x _) = show x > > ...and this is the error: > > src/Stack.hs:12:22: > No instance for (Show a) > arising from a use of `showImpl' > In the first argument of `(++)', namely `(showImpl s)' > In the second argument of `(++)', namely `(showImpl s) ++ ")"' > In the expression: "Stack(" ++ (showImpl s) ++ ")" > > _______________________________________________ > Beginners mailing list > Beginners(a)haskell.org > http://www.haskell.org/mailman/listinfo/beginners

2 1

Properties for Foldable
by Conal Elliott 30 Jul '11

30 Jul '11

Is there a collection of laws associated with the Foldable class? Or for Traversable? - Conal

5 6

Function returning the type of its argument.
by Grigory Sarnitskiy 29 Jul '11

29 Jul '11

It is not possible in Haskell, moreover it seems to be too alien to Haskell. Nevertheless, is there an extension which would allow to write a function that returns the type of its argument?

3 2

Contract opportunity with Biosimilarity LLC
by Greg Meredith 29 Jul '11

29 Jul '11

Dear Haskellians, No desire to spam, but some folks on this list might find this of interest. Best wishes, --greg Biosimilarity LLC is looking for a the next Merlin! We need an artiste with a certain taste and technical instinct that have attracted them to next generation functional programming languages and other high magic. We need a technologist who can make the demons and dragons of web front ends do their bidding while making sound architectural and platform choices. Naturally, comfort with the Open Source ecosystem from git and GitHub to mvn and/or sbt to Jenkins is a given. For such a candidate we don't have to talk about familiarity with Scala or Haskell or ML or JavaScript or Clojure because they eat languages for breakfast and graphics libraries for lunch. The initial engagement is for a limited contract. However, if things go well, who knows what adventures await? The innovations and market potential of the project are guaranteed to keep any sorcerer fascinated. And, you'll get a chance to work with a team of like-minded magicians with long track records of delivering landmark software. While Biosimilarity is a Seattle-based company, telecommuting is definitely an option for the right candidate. If you feel like this might be you, please contact me directly at this email or via the number provided below. Sending along a CV in advance of a more personable communication is likely to be the most effective protocol, but we're always open to creativity. An online portfolio of web projects could also be an advantage, but again raw creativity and can-do carry the day here. -- L.G. Meredith Managing Partner Biosimilarity LLC 1219 NW 83rd St Seattle, WA 98117 +1 206.650.3740 http://biosimilarity.blogspot.com

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Re: [Haskell-cafe] compare iteratee with python's yield
by oleg＠okmij.org 29 Jul '11

29 Jul '11

Your question is insightful. The posted answers are excellent. I merely wish to illustrate one, already made, point on a concrete example. The complete code is in the file http://okmij.org/ftp/Haskell/Iteratee/IterDemo.hs The file implements a series of progressively more complex examples -- from wc to grep, -- stressing compositionality. The whole program is assembled from independent building blocks; as our task changes, we replace one building block with another. The building blocks, iteratees, may be stateful. We never have to worry about writing out the whole state of the program and properly initializing it. The overall state is implicitly composed from the state of each building block; each iteratee manages its own state without leaking it. The particular example to illustrate the state encapsulation is grep1_word > grep1_word word fname = > let search = IterateeM.dropWhile (/= word) >> IterateeM.head > runI' = runI . en_handle (\_ -> "not found:" ++ word) in > print =<< run =<< enum_file fname > (runI =<< en_lines > ((runI' =<< en_words search) > `en_pair` (count_i `en_pair` en_last))) which implements "grep -w -n -m 1 <word> <filename>". It searches for the first occurrence of the given word in the file and returns the line of that occurrence and the line number. (The example also illustrates exception handling: The exception is raised by Iteratee.head when IterateeM.dropWhile dropped the whole stream while looking for the given word.) A few comments: en_lines is the enumeratee, transforming a stream of characters into a stream of lines; en_words is the enumeratee that transforms the stream of lines to the stream of words. The stateful iteratee count_i counts the elements in any stream; en_last, the analogue of List.last, returns the last seen element. The combinator en_pair pairs two iteratees to run in parallel, processing the same stream chunk-by-chunk. The processing stops as soon as one of the iteratees in the pair stopped. That behavior does the trick: "en_words search" stops when the word is found. The whole pair is stopped too. The other component of the pair, (count_i `en_pair` en_last), will receive EOF and tell us the line and the line count they have last seen. I guess the example can be written in Python-like style as follows (simplifying and assuming that the file is already parsed into lines): counter = 0 for line in file_lines: counter = counter + 1 if contains line word: print counter print line break Suppose we wish to change our example to print not only the found line, but a few lines before it (the option "-B" of grep). We have to modify our Python-like code: -- add a piece of state to keep the context lines, and initialize it -- add code to update the state as we read lines -- add the finalization code, to convert the state to the desired result The code will look like the following (### mark the changes): counter = 0 lines_seen = [] ### for line in file_lines: counter = counter + 1 accumulate lines_seen line ### if contains line word: print counter print (finalize lines_seen) ### break In contrast, to modify the iteratee code for the new task, we make the _single_ change: replace en_last with (en_lastN 5) (for 5 lines of context). It should be stressed that the Python code has to be changed in three *distinct, disconnected places*. New state has to be defined and initialized, before the loop. The state has to be updated, somewhere in the loop. The state has to be finalized, in the loop termination part. When changing code, it is easy to overlook the needed changes. For example, it is easier to accumulate the context lines in reverse order. When printing the lines, we should not forget to reverse them. When changes are disconnected, it is hard to reason and test for them. With so many other things going on, it is hard to write a proper test (for example, when testing the context accumulation we don't care about word matching; we should test the accumulation in isolation of whatever else we are doing in the iteration). The iteratee en_lastN deals only with the context accumulation aspect: > -- Return N last received elements > -- The acc below is the list of N last elements in the reverse > -- order > en_lastN :: Monad m => Int -> Iteratee el m [el] > en_lastN n = ie_cont $ step [] > where > step acc (Chunk []) = ie_contM (step acc) > step acc (Chunk ls) = ie_contM (step $! Prelude.take n $ reverse ls ++ acc) > step acc stream = ie_doneM (reverse acc) stream The initialization of the state, the accumulation and the finalization are all in the same place. The en_lastN iteratee does not care of counting, searching, or whatever else is going on. In fact, the iteratee is pure and deals with arbitrary elements (not necessarily lines). We can test it using Quickcheck or HUnit. No IO is needed for the test. Thus in the iteratee style, the overall state of the program becomes distributed, across iteratees that encapsulate and manage only their own, small portion of the program state. The code IterDemo.hs shows other examples, including emulating generators (aka, unfold_stream, or the `while loop'), to build the stream of intermediate results of an iteratee. An iteratee in question does not even suspect that it is being used `incrementally', seduced to reveal the intermediate results. Thank you for the inspiring question.

1 0

Binding of an integer variable
by Paul Reiners 28 Jul '11

28 Jul '11

I have a question about the following GHCi interaction: Prelude> let x = 23 Prelude> :show bindings x :: Integer = _ What is the meaning of the underscore in the third line? Why doesn't it say this, instead? x :: Integer = 23

6 7

ANN: syntactic-0.5
by Emil Axelsson 28 Jul '11

28 Jul '11

I've just uploaded a new version of syntactic: http://hackage.haskell.org/package/syntactic The most important change is that I've added observable sharing based on StableNames. The implementation and interface are conceptually quite similar to Andy Gill's data-reify. The library offers both referentially transparent common sub-expression elimination (that uses a combination of hashing and observable sharing under the hood), as well as direct observation of sharing. The difference between the two flavors is demonstrated in the accompanied file: Examples/NanoFeldspar/Test.hs (See test7_X.) Still missing a way to rebuild an expression with let bindings from the syntax graphs. This is ongoing work. / Emil

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Haskell Weekly News: Issue 192
by Daniel Santa Cruz 28 Jul '11

28 Jul '11

Welcome to issue 192 of the HWN, a newsletter covering developments in the Haskell community. This release covers the week of July 17 to 23, 2011. [1] http://goo.gl/8hDku You can find an HTML rendition of this issue at: http://contemplatecode.blogspot.com/2011/07/haskell-weekly-news-issue-192.h… Announcements Nicolas Wu announced the 4th edition of the Haskell Parallel Digest. The digest is made possible by the Parallel GHC Project. [2] http://goo.gl/lm5T1 Quotes of the Week * copumpkin: perhaps I should stop eating people I've never met * Saizan: agda is webscale in the sense that you need half of The Cloud to run the typechecker * kmc: my algorithm is O(1), but the constant factor grows quadratically as the problem size increases * cmccann: C++ is dual to Haskell in a sense: it's much too hard for the average programmer to use safely, but they do anyway with disastrous results; whereas Haskell isn't actually that difficult to use but people don't even try Top Reddit Stories * What Haskell Doesn't Have Domain: elaforge.blogspot.com, Score: 73, Comments: 58 On Reddit: [3] http://goo.gl/17UDu Original: [4] http://goo.gl/u4Ozy * Take the State of Haskell 2011 Survey Domain: blog.johantibell.com, Score: 55, Comments: 0 On Reddit: [5] http://goo.gl/Q7iip Original: [6] http://goo.gl/jq1Je * Economic Interpreter for Haskell Domain: pnyf.inf.elte.hu, Score: 40, Comments: 5 On Reddit: [7] http://goo.gl/BmBqf Original: [8] http://goo.gl/92mfS * Haskell on Heroku Domain: yesodweb.com, Score: 38, Comments: 5 On Reddit: [9] http://goo.gl/LYHZY Original: [10] http://goo.gl/Ly4n7 * Haskell Stickers! Domain: redbubble.com, Score: 33, Comments: 7 On Reddit: [11] http://goo.gl/WVXoH Original: [12] http://goo.gl/RpQ0f * Rewritten Agda introduction Domain: pnyf.inf.elte.hu, Score: 31, Comments: 34 On Reddit: [13] http://goo.gl/gz6TJ Original: [14] http://goo.gl/oTg5J * A plugin for GHC to turn Haskell into a strict language Domain: github.com, Score: 28, Comments: 18 On Reddit: [15] http://goo.gl/BK4os * What is the state of HackageDB 2.0? Domain: hackage.haskell.org, Score: 26, Comments: 8 On Reddit: [16] http://goo.gl/nrMTc Original: [17] http://goo.gl/tui2x * wxHaskell: The road to the Haskell Platform? Domain: sourceforge.net, Score: 26, Comments: 33 On Reddit: [18] http://goo.gl/vqfPC Original: [19] http://goo.gl/KNa16 * Just do It: Simple Monadic Equational Reasoning [pdf] Domain: cs.ox.ac.uk, Score: 24, Comments: 2 On Reddit: [20] http://goo.gl/GG0cs Original: [21] http://goo.gl/7MHGw * Cabal-dev 0.8 is out with improvements to: flag parsing, user cabal settings, cabal-dev ghci and more! Domain: github.com, Score: 24, Comments: 15 On Reddit: [22] http://goo.gl/YO6Q6 * Functors, applicative functors, monads – their relationship by example Domain: heh.fi, Score: 23, Comments: 0 On Reddit: [23] http://goo.gl/bkJ4Y Original: [24] http://goo.gl/bf6zo * Snap 0.5.2 released Domain: snapframework.com, Score: 23, Comments: 0 On Reddit: [25] http://goo.gl/0RCh3 Original: [26] http://goo.gl/jeFU5 Top StackOverflow Questions * Help in understanding pointfree code votes: 14, answers: 1 Read on SO: [27] http://goo.gl/Ryy5c * How can I combine Handles in Haskell? votes: 12, answers: 2 Read on SO: [28] http://goo.gl/25W4E * Current state of integrating unit tests with Haskell's Cabal? votes: 9, answers: 1 Read on SO: [29] http://goo.gl/nm8Zc * Does iteratee I/O make sense in non-functional languages? votes: 9, answers: 2 Read on SO: [30] http://goo.gl/f8YhP * Literate Haskell: References And Indexing votes: 8, answers: 1 Read on SO: [31] http://goo.gl/QYPBK * Call C Functions from Haskell at runtime votes: 7, answers: 1 Read on SO: [32] http://goo.gl/oj0R7 * Type-class instances for types with 2 parameters when the type-class has only one votes: 7, answers: 2 Read on SO: [33] http://goo.gl/JmxBr * List of options: equivalent of sequence in Scala? votes: 7, answers: 4 Read on SO: [34] http://goo.gl/MnZg8 * Is it necessary to specify every superclass in a class context of a class declaration? votes: 7, answers: 1 Read on SO: [35] http://goo.gl/OxoSN * Partial decoding of ByteStrings to Text votes: 7, answers: 1 Read on SO: [36] http://goo.gl/9JSKw * Haskell, algorithms and school votes: 7, answers: 1 Read on SO: [37] http://goo.gl/bsM5v About the Haskell Weekly News To help create new editions of this newsletter, please send stories to dstcruz(a)gmail.com. Until next time, Daniel Santa Cruz References 1. http://haskell.org/ 2. http://permalink.gmane.org/gmane.comp.lang.haskell.parallel/14 3. http://www.reddit.com/r/haskell/comments/iuhc0/what_haskell_doesnt_have/ 4. http://elaforge.blogspot.com/2011/07/what-haskell-doesnt-have.html 5. http://www.reddit.com/r/haskell/comments/irxtf/take_the_state_of_haskell_20… 6. http://blog.johantibell.com/2011/07/its-time-for-this-years-state-of.html 7. http://www.reddit.com/r/haskell/comments/iuswk/economic_interpreter_for_has… 8. http://pnyf.inf.elte.hu/fp/Economic_en.xml?yes 9. http://www.reddit.com/r/haskell/comments/iuoun/haskell_on_heroku/ 10. http://www.yesodweb.com/blog/2011/07/haskell-on-heroku 11. http://www.reddit.com/r/haskell/comments/iwm3r/haskell_stickers/ 12. http://www.redbubble.com/products/configure/6390818-sticker 13. http://www.reddit.com/r/haskell/comments/itkii/rewritten_agda_introduction/ 14. http://pnyf.inf.elte.hu/fp/Overview_en.xml?fuckyoureddit#agda 15. http://www.reddit.com/r/haskell/comments/iur1n/a_plugin_for_ghc_to_turn_has… 16. http://www.reddit.com/r/haskell/comments/isqmc/what_is_the_state_of_hackage… 17. http://hackage.haskell.org/trac/hackage/wiki/HackageDB/2.0 18. http://www.reddit.com/r/haskell/comments/itwl5/wxhaskell_the_road_to_the_ha… 19. http://sourceforge.net/mailarchive/forum.php?thread_name=alpine.DEB.2.00.11… 20. http://www.reddit.com/r/haskell/comments/it13r/just_do_it_simple_monadic_eq… 21. http://www.cs.ox.ac.uk/jeremy.gibbons/publications/mr.pdf 22. http://www.reddit.com/r/haskell/comments/it50b/cabaldev_08_is_out_with_impr… 23. http://www.reddit.com/r/haskell/comments/it3lh/functors_applicative_functor… 24. http://heh.fi/haskell/functors/ 25. http://www.reddit.com/r/haskell/comments/iv6g3/snap_052_released/ 26. http://snapframework.com/blog/2011/07/20/snap-0.5.2-released 27. http://stackoverflow.com/questions/6797247/help-in-understanding-pointfree-… 28. http://stackoverflow.com/questions/6736790/how-can-i-combine-handles-in-has… 29. http://stackoverflow.com/questions/6787231/current-state-of-integrating-uni… 30. http://stackoverflow.com/questions/6795384/does-iteratee-i-o-make-sense-in-… 31. http://stackoverflow.com/questions/6736482/literate-haskell-references-and-… 32. http://stackoverflow.com/questions/6740850/call-c-functions-from-haskell-at… 33. http://stackoverflow.com/questions/6747083/type-class-instances-for-types-w… 34. http://stackoverflow.com/questions/6750609/list-of-options-equivalent-of-se… 35. http://stackoverflow.com/questions/6782389/is-it-necessary-to-specify-every… 36. http://stackoverflow.com/questions/6786328/partial-decoding-of-bytestrings-… 37. http://stackoverflow.com/questions/6797216/haskell-algorithms-and-school

1 0

Retaining functions in memory
by Siddhartha Gadgil 28 Jul '11

28 Jul '11

I have been making programs for mathematical applications (low-dimensional topology) in Haskell, which I find a delight to code in. However, the execution is slow, and this seems to be because recursively defined functions seem to be recomputed. For example f(100) needs f(15) which needs f(7) ... The dependencies are not obvious to the compiler. I was looking for a way to retain the values of a specific function in memory. Is there some way to do this. Thanks, Siddhartha

4 3

Input redirection in WinGHCi
by Paul Reiners 28 Jul '11

28 Jul '11

I know I can do the following from a command prompt: $ runghc WC < quux.txt How do I do this in WinGHCi? I know I have to first load the file like this: Prelude> :load WC But then what? This doesn't work: *Main> WC < quux.txt <interactive>:1:1: Not in scope: data constructor `WC' <interactive>:1:6: Not in scope: `quux' <interactive>:1:11: Not in scope: `txt'

2 1

Please help me spot where space leak occur.
by Olexander Kozlov 28 Jul '11

28 Jul '11

Greetings Haskell community, I stuck with a space leak problem in simple task. I have a function described with a list of pairs: type Fn a b = [(a, b)] mapping values from a to b. And I have a composition operation which accepts two functions and returns theirs composition: compose :: Eq b => Fn a b -> Fn b c -> Fn a c Finding composition of 1,000,000 functions takes about 240M with the following implementation: type Fn a b = [(a, b)] lkup :: Eq a => a -> Fn a b -> b lkup x ((a, b) : abs) = if x /= a then lkup x abs else b compose :: Eq b => Fn a b -> Fn b c -> Fn a c compose ((a, b) : abs) bcs = (a, lkup b bcs) : compose abs bcs compose [] _ = [] fldl :: (a -> b -> a) -> a -> [b] -> a fldl f z (x:xs) = fldl f (f z x) xs fldl _ z [] = z fna = [(1, 2), (2, 3), (3, 1)] fni = [(1, 1), (2, 2), (3, 3)] test = fldl compose fni (replicate 1000000 fna) main = print test I build with: ghc --make -prof -auto-all -caf-all -fforce-recomp -rtsopts Test.hs Profile results: Test.exe +RTS -K50M -p -RTS total time = 0.34 secs (17 ticks @ 20 ms) total alloc = 240,003,820 bytes (excludes profiling overheads) COST CENTRE MODULE %time %alloc compose Main 58.8 80.0 lkup Main 35.3 0.0 test Main 5.9 11.7 fldl Main 0.0 8.3 After reading 'High-Performance Haskell' by Johan Tibell I tried to made my compose function strict: compose :: Eq b => Fn a b -> Fn b c -> Fn a c compose ((a, b) : abs) bcs = let c = lkup b bcs in c `seq` (a, c) : compose abs bcs compose [] _ = [] and memory consuption reduced down to 180M. Good achievement but still too much! Profile results: Test.exe +RTS -K50M -p -RTS total time = 0.24 secs (12 ticks @ 20 ms) total alloc = 180,003,820 bytes (excludes profiling overheads) COST CENTRE MODULE %time %alloc compose Main 83.3 73.3 lkup Main 8.3 0.0 fldl Main 8.3 11.1 test Main 0.0 15.6 I tried to make fldl strict as well: fldl :: (a -> b -> a) -> a -> [b] -> a fldl f z (x:xs) = let z' = f z x in z' `seq` fldl f z' xs fldl _ z [] = z and end up with 160M of total allocations. Profile results: Test.exe +RTS -K32M -p -RTS total time = 0.30 secs (15 ticks @ 20 ms) total alloc = 160,003,820 bytes (excludes profiling overheads) COST CENTRE MODULE %time %alloc compose Main 46.7 82.5 lkup Main 40.0 0.0 test Main 6.7 17.5 fldl Main 6.7 0.0 Please help me spot where space leak occur, I see no other places for space leaks. I'll appreciate any help. (I intentionally wrote my own implementations of lookup and foldl to have direct control on them.)

2 3

Ur tutorial, and a challenge
by Adam Chlipala 27 Jul '11

27 Jul '11

Last week, I posted a message to this list looking for people interested in joining projects using my domain-specific language Ur/Web. Some responses rightly chastised me for the lack of documentation on the core Ur language's novel type system features. I'm sure many Haskellers have had the experience of quickly grokking the interfaces of new libraries by reading their type signatures, and I think the same is true for Ur. So, to provide that initial bump of background that should help folks get started, I've begun an Ur tutorial: http://www.impredicative.com/ur/tutorial/ The chapters that are already there are intended to be sufficient to help any experienced Haskell programmer get started quickly with Ur/Web. In particular the second chapter on type-level programming may be of interest as a mind expander, even for folks who don't want to use Ur/Web. I'd love feedback about weaknesses in the tutorial! I also want to attach a challenge to this tutorial, as an expansion to an answer I gave earlier about why Ur/Web needs a new programming language and can't just be implemented as a Haskell library. Consider this online Ur/Web demo: http://www.impredicative.com/ur/demo/crud1.html The example involves a library component encapsulating functionality like that of Ruby on Rails's scaffolding: automatic generation of a standard web-based "admin interface" to an SQL database table. The Ur/Web version uses static typing to guarantee that any applications generated by this component are free of injection attacks and other generic problems. The guarantees apply both to app communication with server-side pieces (e.g., static type-checking of SQL) and client-side pieces (e.g., static type-checking of HTML). This is not done by type-checking individual invocations of the admin-interface component. Rather, the component is checked at a static type which guarantees correct operation for _any_specialization_parameters_. So, the challenge is, can this functionality be implemented in Haskell (GHC extensions fair game, any web framework allowed)? If so, how pretty is it? :)

4 5

Idiomatic ways to make all instances of a certain class also instances of another?
by Tim Cowlishaw 27 Jul '11

27 Jul '11

Hi all, I'm currently embarking on my first major project in Haskell, after dabbling with it for several years, and seem to keep finding myself in situations where I create a typeclass that seems to be some sort of specialisation of another, more general typeclass. Given that this is the case, I've then decided that all instances of the specific class should therefore also be instances of the general class, and arrived at the following method of doing so, using the FlexibleInstances and UndecidableInstances extensions to GHC: {-# LANGUAGE FlexibleInstances, UndecidableInstances #-} class Max a where maximum :: a -> a -> a instance (Ord a) => Max a where maximum = max (Obviously, this is a very trivial, and rather silly example - I'm not really trying to implement a class called 'Max'). However, I'd be curious to know if (a) There are better or more idiomatic ways of achieving the same effect, and (b) Whether or not I should be doing this at all; It did occur to me that this seems rather trying to re-implement OOP-style inheritance with typeclasses, and therefore perhaps not a very Haskellish approach to designing software. Therefore - are there better ways to achieve this, or should I not be doing this at all, and, if the latter, what would be the best means of achieving a similar result (i.e. a typeclass that implements all the functionality of one or more others, optionally with some additional specialism)? Many thanks, Tim

5 7

Fwd: C9 video in the Monadic Design Patterns for the Web series
by Greg Meredith 27 Jul '11

27 Jul '11

Dear Haskellians, A new C9 video in the series! So, you folks already know most of this... except for maybe the generalization of the Conway construction! Best wishes, --greg ---------- Forwarded message ---------- From: Charles Torre <...> Date: Tue, Jul 26, 2011 at 1:12 PM Subject: C9 video in the Monadic Design Patterns for the Web series To: Meredith Gregory <lgreg.meredith(a)gmail.com> Cc: Brian Beckman <...> And we’re live!**** ** ** http://channel9.msdn.com/Shows/Going+Deep/C9-Lectures-Greg-Meredith-Monadic… **** C**** ** ** *From:* Charles Torre *Sent:* Tuesday, July 26, 2011 11:51 AM *To:* 'Meredith Gregory' *Cc:* Brian Beckman *Subject:* C9 video in the Monadic Design Patterns for the Web series**** ** ** Here it ‘tis:**** ** ** Greg Meredith <http://biosimilarity.blogspot.com/>, a mathematician and computer scientist, has graciously agreed to do a C9 lecture series covering monadic design principles applied to web development. You've met Greg before in a Whiteboard jam session with Brian Beckman<http://channel9.msdn.com/shows/Going+Deep/E2E-Whiteboard-Jam-Session-with-B…> .**** The fundamental concept here is the monad, and Greg has a novel and conceptually simplified explanation of what a monad is and why it matters. This is a very important and required first step in the series since the whole of it is about the application of monadic composition to real world web development.**** In *part 4, *Greg primarily focuses on the idea that *a monad is really an API* -- it's a view onto the organization of data and control structures, not those structures themselves. In OO terms, it's an *interface*. To make this point concrete Greg explores one of the simplest possible data structures that supports at least two different, yet consistent interpretations of the same API. The structure used, Conway's partisan games<http://mathworld.wolfram.com/ConwayGame.html>, turned out to be tailor-made for this investigation. Not only does this data structure have the requisite container-like shape, it provided opportunities to see just what's necessary in a container to implement the monadic interface. ** ** Running throughout the presentation is a more general comparison of reuse between an OO approach versus a more functional one. When the monadic API is "mixed into" the implementing structure we get less reuse than when the implementing structure is passed as a type parameter. Finally, doing the work put us in a unique position to see not just how to generalize Conway's construction, *monadically*, but the underlying pattern which allows the generalization to suggest itself.**** See *part 1 <http://channel9.msdn.com/Shows/Going+Deep/C9-Lectures-Greg-Meredith-Monadic…> *See *part 2<http://channel9.msdn.com/Shows/Going+Deep/C9-Lectures-Greg-Meredith-Monadic…> ** *See* part 3<http://channel9.msdn.com/Shows/Going+Deep/C9-Lectures-Greg-Meredith-Monadic…> ***** ** -- L.G. Meredith Managing Partner Biosimilarity LLC 7329 39th Ave SW Seattle, WA 98136 +1 206.650.3740 http://biosimilarity.blogspot.com -- L.G. Meredith Managing Partner Biosimilarity LLC 1219 NW 83rd St Seattle, WA 98117 +1 206.650.3740 http://biosimilarity.blogspot.com

3 5

Approximate String Matching
by dokondr 27 Jul '11

27 Jul '11

Hi, I am looking for Haskell libraries to do approximate string matching: http://en.wikipedia.org/wiki/Approximate_string_matching I need this to reduce a set of English word variants with spelling errors to a single canonical dictionary entry. Any libraries to work with spelling will also help. Thanks, Dmitri

2 1

Stack space overflow in HaskellNet
by Manfred Lotz 27 Jul '11

27 Jul '11

Hi there, If I take example imap.hs import System.IO import Network.HaskellNet.IMAP import Text.Mime import qualified Data.ByteString.Char8 as BS import Control.Monad -- the next lines were changed to fit to my local imap server imapServer = "imap.mail.org" user = "" pass = "" main = do con <- connectIMAP imapServer login con user pass mboxes <- list con mapM print mboxes select con "INBOX" msgs <- search con [ALLs] mapM_ (\x -> print x) (take 4 msgs) forM_ (take 4msgs) (\x -> fetch con x >>= print) and change the last line (in order to print all messages of the mailbox) into: forM_ msgs (\x -> fetch con x >>= print) I get Stack space overflow: current size 8388608 bytes. Use `+RTS -Ksize -RTS' to increase it. Is this something to be fixed in the HaskellNet code, or in the example code? -- Manfred

6 13

Question re SMTPClient
by Jayakumar Ramanathan 27 Jul '11

27 Jul '11

How does one pass authentication info to the SMTP server when using the package STMPClient? Thanks! Jay

1 0

Typeclasses, data, constructors
by Sergiy Nazarenko 27 Jul '11

27 Jul '11

Hi everyone! I have data declaration like this: data MyTable = MyTableOne Int String | MyTableTwo Int String String and function that insert new row in tables: newRow :: MyTable -> IO Int newRow (MyTableOne fld1 fld2 ) = ..... newRow (MyTableTwo fld1 fld2 fld3 ) = ...... That work perfectly, I enjoy it. But now I want to get rows, and I don't know what to do :( I want func something like this: getRow :: Int -> [MyTable] .... - what should I write here ? Possibly use this function: getRow 23 :: MyTableTwo (like haskell function Read for example: read "7" :: Int) I try to use typeclasses, class GetRow a where hasID :: Int -> IO a instance GetRow MyTableOne where hasID myid = return [(MyTableOne 1 "name")] instance GetRow MyTableTwo where hasID myid = return [(MyTableTwo 1 "name" "path")] But I have error: Not in scope: type constructor or class `MyTableOne' make: *** [Foo.so] Error 1 Not in scope: type constructor or class `MyTableTwo' make: *** [Foo.so] Error 1 I ask you to suggest me right way in my efforts Cheers!

2 1

file splitter with enumerator package
by Eric Rasmussen 27 Jul '11

27 Jul '11

Hi everyone, A friend of mine recently asked if I knew of a utility to split a large file (4gb in his case) into arbitrarily-sized files on Windows. Although there are a number of file-splitting utilities, the catch was it couldn't break in the middle of a line. When the standard "why don't you use Linux?" response proved unhelpful, I took this as an opportunity to write my first program using the enumerator package. If anyone has time, I'm really interested in knowing if there's a better way to take the incoming stream and output it directly to a file. The basic steps I'm taking are: 1) Data.Enumerator.Binary.take -- grabs the user-specified number of bytes, then (because it returns a lazy ByteString) I use Data.ByteString.Lazy.hPut to output the chunk 2) Data.Enumerator.Binary.head -- after using take for the big chunk, it inspects and outputs individual characters and stops after it outputs the next newline character 3) I close the handle that steps 1&2 used to output the data and then repeat 1&2 with the next handle (an infinite lazy list of filepaths like part1.csv, part2.csv, and so on) The full code is pasted here: http://hpaste.org/49366, and while I'd like to get any other feedback on how to make it better, I want to note that I'm not planning to release this as a utility so I wouldn't want anyone to spend extra time performing a full code review. Thanks! Eric

5 13

Library for Sparse Vectors?
by dokondr 27 Jul '11

27 Jul '11

Hi, Can't find on hackage any sparse vector library. Does such thing exist? I need efficient storage and dot product calculation for very sparse vectors with about 10 out of 40 000 non-zero components. One solution would be to represent Sparse Vector as Data.Map with (component_index, component_value) pairs to store non-zero components of the vector. In this case, for example, calculating cosine similarity ( http://en.wikipedia.org/wiki/Cosine_similarity) for for every pair of 10 000 vectors, would not be very nice and efficient, I am afraid. Thanks!

3 5

Effects of Monads in Parallelisation
by Burak Ekici 26 Jul '11

26 Jul '11

Dear List, I am currently trying to understand the effects of monads in the sense of parallelisation in Haskell. Could somebody please explain the difference between 'rpar' and 'par'? I mean, what has been changed after the encapsulation of 'par' function by Eval monad? If you asked to compare the parallelisation via monads with non-monadic manner of it, what could you say? Many thanks in advance. All the best, Burak.

2 1

Fwd: Re: How to select last inserted record from Table Using Database.HSQL.MySQL
by Steffen Schuldenzucker 26 Jul '11

26 Jul '11

Forwarding to list -------- Original Message -------- Subject: Re: [Haskell-cafe] How to select last inserted record from Table Using Database.HSQL.MySQL Date: Tue, 26 Jul 2011 14:27:56 +0300 From: Sergiy Nazarenko <nazarenko.sergiy(a)gmail.com> To: Steffen Schuldenzucker <sschuldenzucker(a)uni-bonn.de> Thanx a lot! I could solve that problem in this way: cmd = " INSERT INTO mytable (bar,foo) VALUES ("val1","val2"); SELECT LAST_INSERT_ID() as id;" lst <- query connection cmd >>= collectRows (\st -> getFieldValue st "id") lst has required value Cheers!

1 0