April 2012 - Beginners

Issue installing reactive-banana-5.0.0.1
by Miguel Negrao 08 May '12

08 May '12

Hi It appears I can’t install reactive-banana-5.0.0.1: cabal install reactive-banana Resolving dependencies... cabal: cannot configure mtl-2.1. It requires transformers ==0.3.* For the dependency on transformers ==0.3.* there are these packages: transformers-0.3.0.0. However none of them are available. transformers-0.3.0.0 was excluded because reactive-banana-0.5.0.1 requires transformers ==0.2.* How can I solve this ? thank you, Miguel

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Haskell as a useful practical 'tool' for intelligent non-programmers
by Nicholas Kormanik 06 May '12

06 May '12

I am not a programmer, and have no intention of becoming one. I'm a stock and options trader. MetaStock is one of the primary programs I use. Other statistical and mathematical programs as well. Very often when some small need arises, I Google-search for a solution. There seems to be any number of freeware utilities out there in cyberland -- and more all the time -- that do pretty much whatever is needed. Additionally, Mathematica (as one example) has a powerful programming language built in. So, my question is: Does it make practical sense to spend time learning Haskell for the purpose of adding it to my assortment of 'tools' -- to quickly do this or that, as the need arises? Is there any better general practical 'tool' (or, if you want, 'programming language') to add to my arsenal. Thanks for your comments and suggestions. Nicholas Kormanik

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How to deal with library dependencies?
by Lorenzo Bolla 27 Apr '12

27 Apr '12

Hi all, I've recently installed the latest yesod-platform with cabal and now I'd like to install yesod-markdown, too. Problem is, yesod-markdown refuses to install because it depends on libraries older than the ones installed by yesod-platform on my system: for example, yesod-markdown requires 0.4 <= blaze-html < 0.5, but latest yesod-platform depends on blaze-html 0.5.0. This problem is obviously more general, and so is my question: How to handle packages that depend of different versions of the same library? Would you just install the "highest common version" across all the packages that depend on it? What if it doesn't exist? Would you manually patch the cabal files for all the packages in the dependency tree, hoping that newest libraries are backward compatible? Would you wait for the library developers to update their cabal file every time a library they rely upon changes? To me, it looks like dependencies in .cabal files are usually too strict. Or, packages should agree on the meaning of "minor" version numbers. For example, if blaze-html's API did not change from 0.4 to 0.5 (supposing that API changes are identified by a change in the major version number), then there is no point in the libraries that depend on it to require a dependency <0.5. Is this rule of minor/major version numbers followed/agreed upon in Haskell libraries? Thanks, L. -- Lorenzo Bolla http://lbolla.info

3 3

Cyclic structures
by David Tchepak 25 Apr '12

25 Apr '12

I'm currently reading the first edition of "Introduction to Functional Programming" by Richard Bird and Philip Wadler. Section 7.6 is on cyclic structures, and has the following example: ones = forever 1 forever x = x : forever x Which, after 5 evaluations, produces a graph like: 1 : 1 : 1 : 1 : 1 : forever 1 The book then has a re-written version of `forever` that looks like this: forever x = zs where zs = x : zs Which instead produces a cyclic structure: * 1 : ^-----| So it takes up a fixed amount of space, despite being an infinite list. I was hoping someone could explain the difference between the first and second `forever`s for me. Does the second version become a cyclic graph because the `zs` intermediate expression has no additional arguments so doesn't need to be reduced further? Or do both end up being optimised the same by GHC? (my newbie attempts to profile show the second takes up a bit less space, but I'm not sure if I'm measuring it right). Appreciate any help. Regards, David

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Re: [Haskell-beginners] [Haskell-cafe] I am having trouble with the type declaration for creating an identity matrix.
by KC 24 Apr '12

24 Apr '12

Thank you. One is ONLY supposed to supply the list elements for "newListArray" which fill the array in increasing order. On Tue, Apr 24, 2012 at 3:21 PM, Kevin Charter <kcharter(a)gmail.com> wrote: > On Tue, Apr 24, 2012 at 3:20 PM, KC <kc1956(a)gmail.com> wrote: > >> initIdentityMat :: Int -> ST s (STUArray s (Int,Int) ((Int, Int), Double)) >> initIdentityMat m = newListArray ((1,m),(1,m)) ([((i,j), if i == j then >> 1.0 else 0.0) | i <- [1..m], j <- [1..m]] :: [((Int,Int), Double)]) >> >> Doesn't seem to compile, nor do minor variations of the type declaration. >> > > If you use Hoogle to find the type and API docs for 'newListArray', I > believe you'll be able to figure out what's wrong, but I'll give you a > hint. The list you're giving to 'newListArray' contains too much; and the > pair you give it is only half correct. > > Kevin > > -- > Kevin Charter > kevin.charter(a)acm.org > -- -- Regards, KC

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I am having trouble with the type declaration for creating an identity matrix.
by KC 24 Apr '12

24 Apr '12

initIdentityMat :: Int -> ST s (STUArray s (Int,Int) ((Int, Int), Double)) initIdentityMat m = newListArray ((1,m),(1,m)) ([((i,j), if i == j then 1.0 else 0.0) | i <- [1..m], j <- [1..m]] :: [((Int,Int), Double)]) Doesn't seem to compile, nor do minor variations of the type declaration. -- -- Regards, KC

1 0

A strange expression in Quick Hull example
by Zhi-Qiang Lei 24 Apr '12

24 Apr '12

I copy the Quick Hull snippet from http://darcs.haskell.org/packages/dph/examples/quickhull/QuickHull.hs The "packed" expression in "hsplitList" is much confusing to me. Why does it have to zip points with cross? Is [ p | (c, p) <- cross, c > 0.0 ] not enough? Thanks. data Point = Point !Double !Double data Line = Line Point Point instance Show Point where show (Point x y) = show (x, y) distance :: Point -> Line -> Double distance (Point xo yo) (Line (Point x1 y1) (Point x2 y2)) = (x1 - xo) * (y2 - yo) - (y1 - yo) * (x2 - xo) upper :: (a -> a -> Bool) -> [(a, b)] -> b upper above = snd . foldl1 pick where pick left@(kl, _) right@(kr, _) | kl `above` kr = left | otherwise = right hsplitList :: [Point] -> Line -> [Point] hsplitList points line@(Line p1 p2) | length packed < 2 = p1:packed | otherwise = hsplitList packed (Line p1 pm) ++ hsplitList packed (Line pm p2) where cross = [ (distance p line, p) | p <- points ] packed = [ p | (p, (c, _)) <- zip points cross, c > 0.0 ] pm = upper (>) cross quickHullList :: [Point] -> [Point] quickHullList [] = [] quickHullList points = hsplitList points (Line minx maxx) ++ hsplitList points (Line maxx minx) where xs = [ (x, p) | p@(Point x y) <- points ] minx = upper (<) xs maxx = upper (>) xs Best regards, Zhi-Qiang Lei zhiqiang.lei(a)gmail.com

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Re: [Haskell-beginners] [Haskell-cafe] The last statement in a 'do' construct must be an expression: a <- readArray arr (1, 1)
by KC 23 Apr '12

23 Apr '12

Thank you, that was it. I was mixing up and tabs and spaces. I expected new versions of NotePad++ to keep my old settings. On Mon, Apr 23, 2012 at 1:54 PM, Ryan Yates <fryguybob(a)gmail.com> wrote: > Perhaps you are mixing tabs and spaces? > > On Mon, Apr 23, 2012 at 4:52 PM, KC <kc1956(a)gmail.com> wrote: > >> I'm getting the above error message and I cannot figure out why? >> >> >> buildPair = >> do >> arr <- newArray ((1,1),(1,10)) 37 :: ST s (STArray s (Int,Int) Int) >> a <- readArray arr (1,1) >> writeArray arr (1,1) 64 >> b <- readArray arr (1,1) >> return (a,b) >> >> >> main = print $ runST buildPair >> >> >> -- >> -- >> Regards, >> KC >> >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> http://www.haskell.org/mailman/listinfo/haskell-cafe >> >> > -- -- Regards, KC

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The last statement in a 'do' construct must be an expression: a <- readArray arr (1, 1)
by KC 23 Apr '12

23 Apr '12

I'm getting the above error message and I cannot figure out why? buildPair = do arr <- newArray ((1,1),(1,10)) 37 :: ST s (STArray s (Int,Int) Int) a <- readArray arr (1,1) writeArray arr (1,1) 64 b <- readArray arr (1,1) return (a,b) main = print $ runST buildPair -- -- Regards, KC

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(no subject)
by KC 23 Apr '12

23 Apr '12

buildPair = do arr <- newArray ((1,1),(1,10)) 37 :: ST s (STArray s (Int,Int) Int) a <- readArray arr (1,1) writeArray arr (1,1) 64 b <- readArray arr (1,1) return (a,b) main = print $ runST buildPair -- -- Regards, KC

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