August 2011 - Haskell-Cafe

Declarative configuration languages?
by Richard O'Keefe 09 Aug '11

09 Aug '11

A colleague just asked me I'm trying to kick off some work into middleware for configuration of large-scale, distributed systems. Have you come across any elegant, declarative configuration languages used for this sort of job? I've found a couple of papers, but I've never _used_ any of the systems. Any recommendations?

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Couldn't match expected type `Integer' against inferred type `Int'
by Paul Reiners 09 Aug '11

09 Aug '11

Why am I getting this error: > Couldn't match expected type `Integer' against inferred type `Int' > In the expression: foldl step 0 xs > In the definition of `asInt_foldAux': > asInt_foldAux xs > = foldl step 0 xs > where > step acc '.' = error "cannot handle decimal numbers" > step acc x = acc * 10 + digitToInt x for this code? > import Data.Char (digitToInt) > > asInt_fold :: String -> Integer > asInt_fold ('-':xs) = -(asInt_foldAux xs) > asInt_fold xs = asInt_foldAux xs > > asInt_foldAux :: String -> Integer > asInt_foldAux xs = foldl step 0 xs > where step acc '.' = error "cannot handle decimal numbers" > step acc x = acc * 10 + digitToInt x Note that I'm using Int, rather than Integer, to avoid silent overflow errors.

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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 07 Aug '11

07 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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