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• Right rotate of tree in Haskell: how is it work?

  - by Roman

  I don't know haskell syntax, but I know some FP concepts (like algebraic data types, pattern matching, higher-order functions ect). Can someone explain please, what does this code mean: data Tree ? = Leaf ? | Fork ? (Tree ?) (Tree ?) rotateR tree = case tree of Fork q (Fork p a b) c -> Fork p a (Fork q b c) As I understand, first line is something like Tree-type declaration (but I don't understand it exactly). Second line includes pattern matching (I don't understand as well why do we need to use pattern matching here). And third line does something absolutely unreadable for non-haskell developer. I've found definition of Fork as fork (f,g) x = (f x, g x) but I can't move further anymore.

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• Pass Types as arguments to a function in Haskell?

  - by Charles Peng

  The following two functions are extremely similar. They read from a [String] n elements, either [Int] or [Float]. How can I factor the common code out? I don't know of any mechanism in Haskell that supports passing types as arguments. readInts n stream = foldl next ([], stream) [1..n] where next (lst, x:xs) _ = (lst ++ [v], xs) where v = read x :: Int readFloats n stream = foldl next ([], stream) [1..n] where next (lst, x:xs) _ = (lst ++ [v], xs) where v = read x :: Float I am at a beginner level of Haskell, so any comments on my code are welcome.

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• How do you get 100% code coverage with guards in Haskell?

  - by dan_waterworth

  I'm trying to get (and prove) 100% test coverage for some code I'm writing in Haskell using HPC. However if I write something like this: fac n | n > 0 = n * (fac (n - 1)) | otherwise = 1 Then the second expression of the guard statement has always True tagged to it. What is the easiest way to overcome this in the general case? edit: Just to clarify. This code: fac n = if n > 0 then n * (fac (n - 1)) else 1 Works fine with HPC, (running it gives 100% code coverage). I'm basically suffering from this problem: http://hackage.haskell.org/trac/ghc/ticket/3175

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• Is Haskell worth learning?

  - by Jason K

  I am looking at this question primarily from a career point of view, so I hope you answer it accordingly. I am fairly proficient with Python, can write C++ and I am a final year student of computer science engineering I am looking to learn Haskell because I have heard a lot about it. My question is: apart from learning it because of all the good I have heard about it, is it any good for my career? Is it used in the industry? I am curious to learn it but unless it helps me somehow in my career, I am not willing to make that change at this stage. Looking for some personal experiences here. Thanks!

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• Python: how to run several scripts (or functions) at the same time under windows 7 multicore processor 64bit

  - by Gianni

  sorry for this question because there are several examples in Stackoverflow. I am writing in order to clarify some of my doubts because I am quite new in Python language. i wrote a function: def clipmyfile(inFile,poly,outFile): ... # doing something with inFile and poly and return outFile Normally I do this: clipmyfile(inFile="File1.txt",poly="poly1.shp",outFile="res1.txt") clipmyfile(inFile="File2.txt",poly="poly2.shp",outFile="res2.txt") clipmyfile(inFile="File3.txt",poly="poly3.shp",outFile="res3.txt") ...... clipmyfile(inFile="File21.txt",poly="poly21.shp",outFile="res21.txt") I had read in this example Run several python programs at the same time and i can use (but probably i wrong) from multiprocessing import Pool p = Pool(21) # like in your example, running 21 separate processes to run the function in the same time and speed my analysis I am really honest to say that I didn't understand the next step. Thanks in advance for help and suggestion Gianni

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• Optimal Sharing of heavy computation job using Snow and/or multicore

  - by James

  Hi, I have the following problem. First my environment, I have two 24-CPU servers to work with and one big job (resampling a large dataset) to share among them. I've setup multicore and (a socket) Snow cluster on each. As a high-level interface I'm using foreach. What is the optimal sharing of the job? Should I setup a Snow cluster using CPUs from both machines and split the job that way (i.e. use doSNOW for the foreach loop). Or should I use the two servers separately and use multicore on each server (i.e. split the job in two chunks, run them on each server and then stich it back together). Basically what is an easy way to: 1. Keep communication between servers down (since this is probably the slowest bit). 2. Ensure that the random numbers generated in the servers are not highly correlated.

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• Want to add a functional language to my toolchest. Haskell or Erlang?

  - by sean.johnson

  I've been an OO/procedural guy my whole career except in school where I did a lot of logic programming (Prolog). I work on an amazing variety of projects (freelancer) and so I don't want the tools I know and understand to hold me back from using the right tool for the job. I've decided I should know a functional programming language. I've narrowed the field to Haskell and Erlang. What are the pros and cons, advantages and disadvantages, and major trade offs of Haskell and Erlang? How do I decide in a rational way, which is the better path? This is a big time investment, so I'd like to chose wisely. Is there a good case to be made for something else entirely? F#, Scala Ocaml? (BTW, I'm normally a Ruby/C/Obj.C guy, so I'm not terribly impressed or dependent on the JVM as a runtime. It's completely neutral to me. It's a fine runtime, I don't hold it for or against a language. I don't use Microsoft products though, so a .NET runtime would be a negative.)

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• Choosing a CMS to use with backend modules involving haskell and python [on hold]

  - by Butterflycode

  Hi I am trying to decide on a CMS to use for a new project. Security is the most important element of the CMS. I am looking to use a PHP based CMS such as Joomla or Drupal however, PHP has many security flaws which worries me. The data which needs to be secure will be inside a database and relate to account information. I am wondering what is the best way to do this? What I am wanting is a frontend which is made in php/js(joomla) and then I have a backend api which is written in Haskell to handle money transfers ensuring nothing goes wrong. In between the two I want a controller written in perhaps Python or C. I never want the php to touch the database. I want it to relay messages to the controller that's written in python or C and then it inputs to the database, sanitising data etc Am I perhaps thinking too deeply about this? Just wondering if anyone has any ideas on what I should do.... I can't quite explain what the project is as I don't want the idea to be stolen, but it has a lot money transactions involved so security is essential.

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• Creating GUI desktop applications that call into either OCaml or Haskell -- Is it a fool's errand?

  - by Rob Lachlan

  In both Haskell and OCaml, it's possible to call into the language from C programs. How feasible would it be to create Native applications for either Windows, Mac, or Linux which made extensive use of this technique? (I know that there are GUI libraries like wxHaskell, but suppose one wanted to just have a portion of your application logic in the foreign language.) Or is this a terrible idea?

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• Why are side-effects modeled as monads in Haskell?

  - by bodacydo

  Could anyone give some pointers on why the unpure computations in Haskell are modeled as monads? I mean monad is just an interface with 4 operations, so what was the reasoning as modeling side-effects in it?

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• What factors could determine whether Clojure, Scala or Haskell will gain traction?

  - by i_like_monkeys

  Given that it's impossible to see into the future, what factors related to Clojure, Scala or Haskell are likely to determine whether one of them catches on? Are there cultural or economic issues that could give one of these languages an advantage over the others? Or are none of these languages likely to gain traction because of their conceptual complexity?

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• Why is the recursion idiom in Haskell "'n+1' and 'n'" and not "'n' and 'n-1'"?

  - by rulfzid

  I'm working my way through Graham Hutton's Haskell book, and in his recursion chapter, he often pattern-matches on "n+1", as in: myReplicate1 0 _ = [] myReplicate1 (n+1) x = x : myReplicate1 n x Why that and not the following, which (1) seems functionally identical and (2) more intuitive in terms of understanding what's happening with the recursion: myReplicate2 0 _ = [] myReplicate2 n x = x : myReplicate2 (n-1) x Is there something I'm missing here? Or is it just a matter of style?

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• What's the Erlang/Haskell job market like in the U.S.?

  - by Krystof

  I've heard that Telecoms are the big source of Erlang jobs but I'm not sure how much of a market there is. How likely is it that someone could find a job in Erlang/Haskell if they decided to learn it? In my case I have a lot of programming experience in Java but am tired of Java and want to try something different.

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• How do I convert a list to a tuple in Haskell?

  - by Alex

  How can I best convert a list to a tuple in Haskell: [1,2,3,4,5,6] -> (1,2,3,4,5,6)

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• Why isn't there a typeclass for functions?

  - by Steve314

  I already tried this on Reddit, but there's no sign of a response - maybe it's the wrong place, maybe I'm too impatient. Anyway... In a learning problem I've been messing around with, I realised I needed a typeclass for functions with operations for applying, composing etc. Reasons... It can be convenient to treat a representation of a function as if it were the function itself, so that applying the function implicitly uses an interpreter, and composing functions derives a new description. Once you have a typeclass for functions, you can have derived typeclasses for special kinds of functions - in my case, I want invertible functions. For example, functions that apply integer offsets could be represented by an ADT containing an integer. Applying those functions just means adding the integer. Composition is implemented by adding the wrapped integers. The inverse function has the integer negated. The identity function wraps zero. The constant function cannot be provided because there's no suitable representation for it. Of course it doesn't need to spell things as if it the values were genuine Haskell functions, but once I had the idea, I thought a library like that must already exist and maybe even using the standard spellings. But I can't find such a typeclass in the Haskell library. I found the Data.Function module, but there's no typeclass - just some common functions that are also available from the Prelude. So - why isn't there a typeclass for functions? Is it "just because there isn't" or "because it's not so useful as you think"? Or maybe there's a fundamental problem with the idea? The biggest possible problem I've thought of so far is that function application on actual functions would probably have to be special-cased by the compiler to avoid a looping problem - in order to apply this function I need to apply the function application function, and to do that I need to call the function application function, and to do that...

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• FP for simulation and modelling

  - by heaptobesquare

  I'm about to start a simulation/modelling project. I already know that OOP is used for this kind of projects. However, studying Haskell made me consider using the FP paradigm for modelling a system of components. Let me elaborate: Let's say I have a component of type A, characterised by a set of data (a parameter like temperature or pressure,a PDE and some boundary conditions,etc.) and a component of type B, characterised by a different set of data(different or same parameter, different PDE and boundary conditions). Let's also assume that the functions/methods that are going to be applied on each component are the same (a Galerkin method for example). If I were to use an OOP approach, I would create two objects that would encapsulate each type's data, the methods for solving the PDE(inheritance would be used here for code reuse) and the solution to the PDE. On the other hand, if I were to use an FP approach, each component would be broken down to data parts and the functions that would act upon the data in order to get the solution for the PDE. This approach seems simpler to me assuming that linear operations on data would be trivial and that the parameters are constant. What if the parameters are not constant(for example, temperature increases suddenly and therefore cannot be immutable)? In OOP, the object's (mutable) state can be used. I know that Haskell has Monads for that. To conclude, would implementing the FP approach be actually simpler,less time consuming and easier to manage (add a different type of component or new method to solve the pde) compared to the OOP one? I come from a C++/Fortran background, plus I'm not a professional programmer, so correct me on anything that I've got wrong.

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• Haskell's type system treats a numerical value as function?

  - by Long

  After playing around with haskell a bit I stumbled over this function: Prelude Data.Maclaurin> :t ((+) . ($) . (+)) ((+) . ($) . (+)) :: (Num a) => a -> (a -> a) -> a -> a (Data.Maclaurin is exported by the package vector-space.) So it takes a Num, a function, another Num and ultimately returns a Num. What magic makes the following work? Prelude Data.Maclaurin> ((+) . ($) . (+)) 1 2 3 6 2 is obviously not a function (a-a) or did I miss out on something?

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• How do laziness and I/O work together in Haskell?

  - by Bill

  I'm trying to get a deeper understanding of laziness in Haskell. I was imagining the following snippet today: data Image = Image { name :: String, pixels :: String } image :: String -> IO Image image path = Image path <$> readFile path The appeal here is that I could simply create an Image instance and pass it around; if I need the image data it would be read lazily - if not, the time and memory cost of reading the file would be avoided: main = do image <- image "file" putStrLn $ length $ pixels image But is that how it actually works? How is laziness compatible with IO? Will readFile be called regardless of whether I access pixels image or will the runtime leave that thunk unevaluated if I never refer to it? If the image is indeed read lazily, then isn't it possible I/O actions could occur out of order? For example, what if immediately after calling image I delete the file? Now the putStrLn call will find nothing when it tries to read.

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• How do I write a constant-space length function in Haskell?

  - by Bill

  The canonical implementation of length :: [a] -> Int is: length [] = 0 length (x:xs) = 1 + length xs which is very beautiful but suffers from stack overflow as it uses linear space. The tail-recursive version: length xs = length' xs 0 where length' [] n = n length' (x:xs) n = length xs (n + 1) doesn't suffer from this problem, but I don't understand how this can run in constant space in a lazy language. Isn't the runtime accumulating numerous (n + 1) thunks as it moves through the list? Shouldn't this function Haskell to consume O(n) space and lead to stack overflow? (if it matters, I'm using GHC)

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• In Haskell, how can you sort a list of infinite lists of strings?

  - by HaskellNoob

  So basically, if I have a (finite or infinite) list of (finite or infinite) lists of strings, is it possible to sort the list by length first and then by lexicographic order, excluding duplicates? A sample input/output would be: Input: [["a", "b",...], ["a", "aa", "aaa"], ["b", "bb", "bbb",...], ...] Output: ["a", "b", "aa", "bb", "aaa", "bbb", ...] I know that the input list is not a valid haskell expression but suppose that there is an input like that. I tried using merge algorithm but it tends to hang on the inputs that I give it. Can somebody explain and show a decent sorting function that can do this? If there isn't any function like that, can you explain why? In case somebody didn't understand what I meant by the sorting order, I meant that shortest length strings are sorted first AND if one or more strings are of same length then they are sorted using < operator. Thanks!

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• How do I create two mutual producer/consumers with internal state in Haskell?

  - by Keith

  I've got an agent that takes in states and returns actions, while keeping an internal representation of the utility of state/action pairs. I've also got an environment that takes in actions and returns state/reward pairs. I need to be able to set the agent up with a start state and then continuously go from agent -(action)- environment -(state, reward)- agent -(action)-... However, the internal states (which need to be updated every iteration) need to stay private (that is, within the agent or the environment). This means that I can't simply call environment as a function within the agent using state and action as arguments. I'm somewhat of a Haskell noobie, so I'm not even sure if this is possible.

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• Haskell: Why is it saying my function type is off?

  - by linkmaster03

  I wrote a little Haskell program to find the area of a triangle, primarily to practice custom types, but it keeps throwing the following error on compile: areafinder.hs:7:4: Couldn't match expected type 'Triangle' against inferred type 'm b' In a stmt of a 'do' expression: putStr "Base: " In the expression: do { putStr "Base: "; baseStr I'm not sure where 'm b' comes from, so I'm at a loss here. Why is it throwing this error, and what can I do to fix it? Here is my code: module Main where data Triangle = Triangle Double Double -- base, height getTriangle :: Triangle getTriangle = do putStr "Base: " baseStr Double calcTriangle (Triangle base height) = base * height main = putStrLn ("Area = " ++ show (calcTriangle getTriangle)) Thanks. :)

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• How would you write this Clojure snippet in Ruby and/or Haskell?

  - by dnolen

  I was working on a Rails template and was trying to write a bit of code that allows me to populate a table or multiple columns of ul tags "top-to-bottom" and "left-to-right" across however many columns I specify. I'm just getting the hang of Ruby so I couldn't figure this out. I'm also curious about an idiomatic Haskell version for this useful snippet. Improvements to Clojure version appreciated: (defn table [xs & {:keys [cols direction] :or {cols 1 direction 'right}}] (into [] (condp = direction 'down (let [c (count xs) q (int (/ c cols)) n (if (> (mod c q) 0) (inc q) q)] (apply map vector (partition n n (repeat nil) xs))) 'right (map vec (partition cols cols (repeat nil) xs))))) With this bit of code I can then do the following: (table (range 10) :cols 3) Printed out this would look like so: 0 1 2 3 4 5 6 7 8 9 And the trickier one: (table (range 10) :cols 3 :direction 'down) Looks like so: 0 4 8 1 5 9 2 6 3 7

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• Will Haskell be a good choice for my task?

  - by Narzanka

  I'm starting a new project and don't know which language to use. My 'must have' requirements are: Be able to run on Windows/LinuxMacOs natively (native executable) - user should be able to just run the exe (when on Windows for example) and see the results. No runtimes/interpreters (no jvm, clr, etc) - one file download should be enough to run the application. Full unicode support. Be able to manipulate OS threads (create them, run multiple tasks in parallel on multi-core CPUs, etc). Be reasonably fast (Python level performance and better). To have some kind of standard library that does low-level, mundane tasks. Not very niche and have some community behind it to be able to ask questions. My 'nice to have' requirements are: Language should be functional It should have good string manipulation capabilities (not necessarily regex) Not extremely hard to learn I'm thinking about Haskell now but keeping in mind OCaml as well. Please advice if my choice is correct.

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• Does functional programming mandate new naming conventions?

  - by Jakob

  I recently started studying functional programming using Haskell and came upon this article on the official Haskell wiki: How to read Haskell. The article claims that short variable names such as x, xs, and f are fitting for Haskell code, because of conciseness and abstraction. In essence, it claims that functional programming is such a distinct paradigm that the naming conventions from other paradigms don't apply. What are your thoughts on this?

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