September 2010 - Beginners

Selecting Arguments in Function to Feed Map
by Lorenzo Isella 13 Sep '10

13 Sep '10

Dear All, Suppose you have the function f x y z = x*y +z and that you want to iterate it on a list z=[1,2,3,4], with x=4 and y=3 then you would do the following map (f x y) z. Now consider the case in which the list is given by y e.g. y=[1,2,3,4], with x=4 and z=3. How can you iterate f on y (i.e. its second argument) while keeping x and y fixed? Lorenzo

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how to implement accesor function for given data type
by Martin Tomko 13 Sep '10

13 Sep '10

Dear all, apologies for a newbie question, but I am returning to Haskell after years of hiatus and Java... I have the following data type (representing geometries): data Geometry = Point FID Coordinate | LineSegment FID (Coordinate,Coordinate) | Polyline FID [Coordinate] | Polygon FID [Coordinate] deriving (Show,Eq) type Coordinate = (Float, Float) And this is where I fail: I have no idea how to type getCoordinates, in order to match against the three different combinations -> it can result into a Coordinate, or a typle, or a List. I am sure it is possible! class Geoms a where getCoordinates :: a -> b instance Geoms Geometry where getCoordinates (Point _ a) = a getCoordinates (Polygon _ a) = a And I want to access the Coordinates for each given geometry. Note that I tried to define it using records: say, Polygon {getID::FID,gcoords::[Coordinate]}, but the coords function failed to match, due to having different return types for Geometry. Thank you very much, Martin

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Haskell for POS (business) apps
by glpunzi＠lordzealon.com 13 Sep '10

13 Sep '10

Hi all, I work in a company thath needs to develop some applications. As IT Manager, I'm on the decision to choose language and platforms. For the moment, We are developing a POS system. The first prototype is beeing developed under Windev(1), but, I would like to move to opensource options as soon as posible. For this reason I'm here. I'm looking to all this languages thath take my attention to take a look, and I'm very curious about Haskell, for this I would like to learn a little about it. For our projects, we will need database access (RDBMS or Objects database), reports, serial ports access to communicate with external devices (serial display, ticket printers, cash drawers, and so on..)..web services..well...all thath a POS application will need. I would like to develop for it too, a Desktop and Web BackOffice, some application to take orders (PocketPC, Android) and so on..It suppose, all of this, will work under Linux or Windows (it's decision of my customers), and I would like to do it like a chameleon, to adapt to many type of customers as possible. Then, my main question is: I know (suppose), with Haskell you can develop whath you want, and I readed Scala is a good option, but I don't like the idea of depend of JVM, and Haskell do native executables. Is worth the effort for this type of applications? There are libs for this things? It's easy to maintain large projects? Sorry for my english and lack of knowledge :S PD: I come from a Delphi and Smalltalk background. (1)http://www.windev.com

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Again on List Manipulation
by Lorenzo Isella 12 Sep '10

12 Sep '10

Dear All, First of all, thanks to the whole list for their help. I am still struggling with things I used to be able to do quite easily (though maybe not efficiently) with other languages. I still have to get used to the immutable nature of lists and the absence of for loops. Let us say you have two lists l = [1,1,1,1,2,2,2,2,2,2,2,3,3,5,6,7] (already sorted) and a list of corresponding values for every entry in l m= [2,2,2,2,4,4,4,4,6,6,4,4,4,10,12,14]. Also consider the list of the unique values in l l_un = nub l Now, what I would like to do is to get a third list, let us call it q, such that its length is equal to the length of l_un. On top of that, its i-th entry should be the sum of the entries of m corresponding to the i-th values of l_un. To fix the ideas, q should be q = [8,32, 8,10,12,14] . How would you do that? Cheers Lorenzo P.S.: I will need this function both with Integer and Double numbers (I may have situation in which the entries of one list or both are real numbers, though the goal stays the same)

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Arrays in Haskell
by Lorenzo Isella 11 Sep '10

11 Sep '10

Dear All, The recent feedback I got on the mailing list led me to think about the data structure I need for my computations and array manipulations (loosely speaking, let us say that I need indexing and slicing tables of data). Coming from Python, I am a bit confused: let me say that in my Python scripts I almost never use lists, but rather NumPy arrays. In that case, it is an easy choice (almost every decent software for numerics/visualization etc... in Python relies explicitly or implicitly on NumPy). On top of that, NumPy is fast, has a lot of inbuilt functions and interfaces nicely with SciPy, matplotlib, Mayavi2 etc... It seems to me (please correct me if I am mistaken) that the situation in Haskell is a bit more 'open' to choices. At least I think so when I look at http://en.wikibooks.org/wiki/Haskell/Hierarchical_libraries/Arrays I may want to drop lists at some point for performance reasons but also because in my work I really have tables of numbers I find convenient to think of as arrays/matrices (again, loosely speaking, I mean matrices as arrays + linear algebra like taking the inverse, the determinant and so on). Bottom line, I would need a data type that (1) is decently fast (OK, there is more than performance to scripting, but you see my point) (2) allows slicing/indexing (e.g. take 3rd row, second column, flatten it out, take every element larger than 34 in a row, find its unique elements, sort them etc...) without having to re-invent the wheel myself. This is more on the manipulation side than the linear algebra. As you can see, I would like to be able to find something similar to the very useful functions in Data.List for an array. (3) it would be nice if these data type could have either integers of real numbers as entries. If the original data is a made up of integer numbers, conversion to real number is always suspicious (I have horrible memories (at least in other languages) of numbers that were equal as integers and no longer when read as real numbers because of one last digit changing...which can give you a headache in some cases. (4) again it would be nice if I could feed these arrays/vectors to tools that take integrals, derivatives, inverse etc... I am considering for this reason several possibilities http://www.haskell.org/tutorial/arrays.html http://users.skynet.be/jyp/html/base/Data-Array.html http://code.haskell.org/hmatrix/hmatrix.pdf hmatrix is probably what I am looking for (linear algebra, interface to gnuplot for plotting, ODE solver etc...) but there is no possibility of using arrays of integers, so I am concerned about using it to read and compare data files filled with integers where I check if certain entries are equal or not. Also I wonder if I can find any extra documentation other than the well written tutorial (which explains a lot, but cannot do everything in less than 30 pages and I would have plenty of questions about array manipulations there). Any suggestion/clarification is more than welcome. Cheers Lorenzo

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Stack space overflow with foldl'
by Ryan Prichard 11 Sep '10

11 Sep '10

Hi, I see a stack overflow with this code, but I don't understand why. Test.hs -------------- main :: IO () main = do let list = ([1..3*1000*1000] :: [Int]) let !x = foldl'2 (flip seq) () list return x -- foldl'2 is just the __GLASGOW_HASKELL__ version of Data.List.foldl'. -- http://www.haskell.org/ghc/docs/6.12.2/html/libraries/base-4.2.0.1/ -- src/Data-List.html -- The test case behaves the same if I call foldl' instead of foldl'2. foldl'2 :: (a -> b -> a) -> a -> [b] -> a foldl'2 f z0 xs0 = lgo z0 xs0 where lgo z [] = z lgo z (x:xs) = let z' = f z x in z' `seq` (lgo z' xs) $ ghc Test.hs -o Test $ time ./Test real 0m0.087s user 0m0.084s sys 0m0.000s $ ghc Test.hs -o Test -O $ time ./Test Stack space overflow: current size 8388608 bytes. Use `+RTS -Ksize -RTS' to increase it. real 0m1.780s user 0m1.764s sys 0m0.004s $ ghc --version The Glorious Glasgow Haskell Compilation System, version 6.12.3 I looked at the Core output with ghc-core, with or without optimizations, and I see a $wlgo recursive function that doesn't appear to end in a tail call. I don't see any let expressions in the folding code, so I assume no thunks are being created. I can make a tail call appear by doing either of two things: 1. Replace "lgo z []" with "lgo !z []". This suggestion came from an email on haskell-beginners that I can't find right now. It was a few months ago. 2. Instead of using the __GLASGOW_HASKELL__ version of foldl', use the other version: foldl' f a [] = a foldl' f a (x:xs) = let a' = f a x in a' `seq` foldl' f a' xs My test case is contrived. Originally, I had a program that read lines from a file as Data.ByteString values, and I was trying to debug a stack overflow. I added the foldl' call to force the evaluation of the ByteString lines, but the foldl' call itself overflowed the stack. I might have fixed my original stack overflow problem. I was applying sum to a large list of integers, and sum is lazy. I don't think I have any real code anymore that overflows the stack, but I'm uncomfortable because I don't know why my test case doesn't work. Is the foldl' call in my test case allowed to have linear stack usage? Thanks, -Ryan

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Again on list manipulations
by Lorenzo Isella 10 Sep '10

10 Sep '10

Dear All, I know this must be a one-liner, but it am banging my head against the wall. I am trying my hands at list manipulation in Haskell and a lot of useful function are making my life easier but I cannot achieve something really simple. Let us say you have the lists ml and sel ml=[23,44,55,8,98] and sel=[1,2] . Now, I would like simply to get a new list whose entries are the elements of ml in position sel. In my case things might be a little more complicated because all the elements are Integer and not Int (I noticed that sometimes this means I have to resort to generic functions). Cheers Lorenzo

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Simple Math Function
by Lorenzo Isella 10 Sep '10

10 Sep '10

Dear All, I am trying to translate some codes of mine into haskell. It looks like Haskell can shorten quite a bit the implementation of mathematical formulas. Let us say you have two list of real numbers; let us call them y and t. The list t is increasing (a sort of time). Now consider the couples (t_a, y_a) and (t_b,y_b) where t_a<t_b i.e. two (non-simultaneous) values of t and the corresponding y_a and y_b. There are no assumptions on y. What I would like to write is a function which checks the existence (t_c,y_c), where y_a<y_c<y_b (i.e. y_c is some intermediate time) such that y_c<y_b+(y_a-y_b)*(t_b-t_c)/(t_b-t_a). I have already implemented this in other languages, but I ended up with quite lengthy functions (what if a list is empty, what if it has fewer than 2-3 elements and so on...). Also, it would be nice to iterate this function on all possible combinations (t_a, y_a) and (t_b,y_b), t_a<t_b I can form from my data. I am not asking someone to 'do my homework', rather I would like to see how much more compact things can get in haskell. Many thanks Lorenzo

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Cabal and .chs modules dependencies
by John Obbele 10 Sep '10

10 Sep '10

Hi everyone ! I'm stuck with a cabal quirk concerning modules build with c2hs: the modules declared in {exposed,others}-modules are build sequentially *and* the exposed modules are processed first, followed by the "other-modules". So if I have a Foobar.Quux.MyModule which depends of Foobar.MyInternal.Foo1(.chs) and write in *.cabal: > library > Other-Modules: Foobar.MyInternal.Foo0 > Foobar.MyInternal.Foo1 > > Exposed-Modules: Foobar.Quux.MyModule or > library > Other-Modules: Foobar.MyInternal.Foo0 > > Exposed-Modules: Foobar.Quux.MyModule > Foobar.MyInternal.Foo1 I get the following error: bash $ cabal clean && cabal configure && cabal build cabal clean cleaning... cabal configure Resolving dependencies... Configuring foobar-0.0.0.1 cabal build Preprocessing library foobar-0.0.0.1... c2hs: Foobar.MyInternal.Foo1.chi not found in: dist/build The only solution I have for now is to write: > library > Exposed-Modules: Foobar.MyInternal.Foo1 > Foobar.Quux.MyModule > > Other-Modules: Foobar.MyInternal.Foo0 ... which unfortunately exposes "Foobar.MyInternal.Foo1". Does anyone know how *not* to expose Foobar.MyInternal.* ? (by the way I am using those versions: cabal-install version 0.8.2 using version 1.8.0.2 of the Cabal library C->Haskell Compiler, version 0.16.2 Crystal Seed, 24 Jan 2009 build platform is "i386-linux" <1, True, True, 1>) regards, /John

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Network programming (select(2)) for Haskell?
by Patrick LeBoutillier 10 Sep '10

10 Sep '10

Hi, Recently I've written a Perl program that "bridges" 2 rsync commands together. It basically allows remote-to-remote rsyncs from a "control" server . The code is here: http://hpaste.org/fastcgi/hpaste.fcgi/view?id=29852#a29852 In order to get better at network programming using haskell, I'd like to try and port it to Haskell, but I can't seem to locate the equivalent to select(2) in Haskell. Perhaps a different idiom is to be used? These kinds of scripts a pretty much the bread and butter of my daily work, and understanding network programming of this type in Haskell would really help me integrate Haskell into my workplace. Can anyone offer pointers for me to get started? Thanks a lot, Patrick -- ===================== Patrick LeBoutillier Rosemère, Québec, Canada

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