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• why Haskell can deduce [] type in this function

  - by Sili

  rho x = map (((flip mod) x).(\a -> a^2-1)) (rho x) This function will generate an infinite list. And I tested in GHCi, the function type is *Main> :t rho rho :: Integral b => b -> [b] If I define a function like this fun x = ((flip mod) x).(\a -> a^2-1) The type is *Main> :t fun fun :: Integral c => c -> c -> c My question is, how can Haskell deduce the function type to b - [b]? We don't have any [] type data in this function. Thanks!

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• Is there any working implementation of reverse mode automatic differentiation for Haskell?

  - by Ian Fiske

  The closest-related implementation in Haskell I have seen is the forward mode at http://hackage.haskell.org/packages/archive/fad/1.0/doc/html/Numeric-FAD.html. The closest related related research appears to be reverse mode for another functional language related to Scheme at http://www.bcl.hamilton.ie/~qobi/stalingrad/. I see reverse mode in Haskell as kind of a holy grail for a lot of tasks, with the hopes that it could use Haskell's nested data parallelism to gain a nice speedup in heavy numerical optimization.

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• Haskell: 'No instance for' arising from a trivial usage of Regex library

  - by artemave

  Following the (accepted) answer from this question, I am expecting the following to work: Prelude Text.Regex.Posix Text.Regex.Base.RegexLike Text.Regex.Posix.String> makeRegex ".*" (makeRegex is a shortcut for makeRegexOpts with predefined options) However, it doesn't: <interactive>:1:0: No instance for (RegexMaker regex compOpt execOpt [Char]) arising from a use of `makeRegex' at <interactive>:1:0-13 Possible fix: add an instance declaration for (RegexMaker regex compOpt execOpt [Char]) In the expression: makeRegex ".*" In the definition of `it': it = makeRegex ".*" Prelude Text.Regex.Posix Text.Regex.Base.RegexLike Text.Regex.Posix.String> make Regex ".*"::Regex <interactive>:1:0: No instance for (RegexMaker Regex compOpt execOpt [Char]) arising from a use of `makeRegex' at <interactive>:1:0-13 Possible fix: add an instance declaration for (RegexMaker Regex compOpt execOpt [Char]) In the expression: makeRegex ".*" :: Regex In the definition of `it': it = makeRegex ".*" :: Regex And I really don't understand why. EDIT Haskell Platform 2009.02.02 (GHC 6.10.4) on Windows EDIT2 Prelude Text.Regex.Base.RegexLike Text.Regex.Posix.String> :i RegexMaker class (RegexOptions regex compOpt execOpt) => RegexMaker regex compOpt execOpt source | regex -> compOpt execOpt, compOpt -> regex execOpt, execOpt -> regex compOpt where makeRegex :: source -> regex makeRegexOpts :: compOpt -> execOpt -> source -> regex makeRegexM :: (Monad m) => source -> m regex makeRegexOptsM :: (Monad m) => compOpt -> execOpt -> source -> m regex -- Defined in Text.Regex.Base.RegexLike

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• How do i convert String to Integer/Float in Haskell

  - by Ranhiru

  data GroceryItem = CartItem ItemName Price Quantity | StockItem ItemName Price Quantity makeGroceryItem :: String -> Float -> Int -> GroceryItem makeGroceryItem name price quantity = CartItem name price quantity I want to create a GroceryItem when using a String or [String] createGroceryItem :: [String] -> GroceryItem createGroceryItem (a:b:c) = makeGroceryItem a b c The input will be in the format ["Apple","15.00","5"] which i broke up using words function in haskell. I get this error which i think is because the makeGroceryItem accepts a Float and an Int. But how do i make b and c Float and Int respectively? *Type error in application *** Expression : makeGroceryItem a read b read c *** Term : makeGroceryItem *** Type : String -> Float -> Int -> GroceryItem *** Does not match : a -> b -> c -> d -> e -> f* Thanx a lot in advance :)

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• "Programming In Haskell" error in sat function

  - by Matt Ellen

  I'm in chapter 8 of Graham Hutton's Programming in Haskell and I'm copying the code and testing it in GHC. See the slides here: http://www.cis.syr.edu/~sueo/cis352/chapter8.pdf in particular slide 15 The relevant code I've copied so far is: type Parser a = String -> [(a, String)] pih_return :: a -> Parser a pih_return v = \inp -> [(v, inp)] failure :: Parser a failure = \inp -> [] item :: Parser Char item = \inp -> case inp of [] -> [] (x:xs) -> [(x,xs)] parse :: Parser a -> String -> [(a, String)] parse p inp = p inp sat :: (Char -> Bool) -> Parser Char sat p = do x <- item if p x then pih_return x else failure I have changed the name of the return function from the book to pih_return so that it doesn't clash with the Prelude return function. The errors are in the last function sat. I have copied this directly from the book. As you can probably see p is a function from Char to Bool (e.g. isDigit) and x is of type [(Char, String)], so that's the first error. Then pih_return takes a value v and returns [(v, inp)] where inp is a String. This causes an error in sat because the v being passed is x which is not a Char. I have come up with this solution, by explicitly including inp into sat sat :: (Char -> Bool) -> Parser Char sat p inp = do x <- item inp if p (fst x) then pih_return (fst x) inp else failure inp Is this the best way to solve the issue?

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• Monads and custom traversal functions in Haskell

  - by Bill

  Given the following simple BST definition: data Tree x = Empty | Leaf x | Node x (Tree x) (Tree x) deriving (Show, Eq) inOrder :: Tree x -> [x] inOrder Empty = [] inOrder (Leaf x) = [x] inOrder (Node root left right) = inOrder left ++ [root] ++ inOrder right I'd like to write an in-order function that can have side effects. I achieved that with: inOrderM :: (Show x, Monad m) => (x -> m a) -> Tree x -> m () inOrderM f (Empty) = return () inOrderM f (Leaf y) = f y >> return () inOrderM f (Node root left right) = inOrderM f left >> f root >> inOrderM f right -- print tree in order to stdout inOrderM print tree This works fine, but it seems repetitive - the same logic is already present in inOrder and my experience with Haskell leads me to believe that I'm probably doing something wrong if I'm writing a similar thing twice. Is there any way that I can write a single function inOrder that can take either pure or monadic functions?

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• Compiled Haskell libraries with FFI imports are invalid when imported into GHCI

  - by John Millikin

  I am using GHC 6.12.1, in Ubuntu 10.04 When I try to use the FFI syntax for static storage, only modules running in interpreted mode (ie GHCI) work properly. Compiled modules have invalid pointers, and do not work. I'd like to know whether anybody can reproduce the problem, whether this an error in my code or GHC, and (if the latter) whether it's a known issue. I'm using sys_siglist because it's present in a standard library on my system, but I don't believe the actual storage used matters (I discovered this while writing a binding to libidn). If it helps, sys_siglist is defined in <signal.h> as: extern __const char *__const sys_siglist[_NSIG]; I thought this type might be the problem, so I also tried wrapping it in a plain C procedure: #include<stdio.h> const char **test_ffi_import() { printf("C think sys_siglist = %X\n", sys_siglist); return sys_siglist; } However, importing that doesn't change the result, and the printf() call prints the same pointer value as show siglist_a. My suspicion is that it's something to do with static and dynamic library loading. Update: somebody in #haskell suggested this might be 64-bit specific; if anybody tries to reproduce it, can you mention your architecture and whether it worked in a comment? Code as follows: -- A.hs {-# LANGUAGE ForeignFunctionInterface #-} module A where import Foreign import Foreign.C foreign import ccall "&sys_siglist" siglist_a :: Ptr CString -- -- B.hs {-# LANGUAGE ForeignFunctionInterface #-} module B where import Foreign import Foreign.C foreign import ccall "&sys_siglist" siglist_b :: Ptr CString -- -- Main.hs {-# LANGUAGE ForeignFunctionInterface #-} module Main where import Foreign import Foreign.C import A import B foreign import ccall "&sys_siglist" siglist_main :: Ptr CString main = do putStrLn $ "siglist_a = " ++ show siglist_a putStrLn $ "siglist_b = " ++ show siglist_b putStrLn $ "siglist_main = " ++ show siglist_main peekSiglist "a " siglist_a peekSiglist "b " siglist_b peekSiglist "main" siglist_main peekSiglist name siglist = do ptr <- peekElemOff siglist 2 str <- maybePeek peekCString ptr putStrLn $ "siglist_" ++ name ++ "[2] = " ++ show str I would expect something like this output, where all pointer values identical and valid: $ runhaskell Main.hs siglist_a = 0x00007f53a948fe00 siglist_b = 0x00007f53a948fe00 siglist_main = 0x00007f53a948fe00 siglist_a [2] = Just "Interrupt" siglist_b [2] = Just "Interrupt" siglist_main[2] = Just "Interrupt" However, if I compile A.hs (with ghc -c A.hs), then the output changes to: $ runhaskell Main.hs siglist_a = 0x0000000040378918 siglist_b = 0x00007fe7c029ce00 siglist_main = 0x00007fe7c029ce00 siglist_a [2] = Nothing siglist_b [2] = Just "Interrupt" siglist_main[2] = Just "Interrupt"

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• Zipping with padding in Haskell

  - by Travis Brown

  A couple of times I've found myself wanting a zip in Haskell that adds padding to the shorter list instead of truncating the longer one. This is easy enough to write. (Monoid works for me here, but you could also just pass in the elements that you want to use for padding.) zipPad :: (Monoid a, Monoid b) => [a] -> [b] -> [(a, b)] zipPad xs [] = zip xs (repeat mempty) zipPad [] ys = zip (repeat mempty) ys zipPad (x:xs) (y:ys) = (x, y) : zipPad xs ys This approach gets ugly when trying to define zipPad3. I typed up the following and then realized that of course it doesn't work: zipPad3 :: (Monoid a, Monoid b, Monoid c) => [a] -> [b] -> [c] -> [(a, b, c)] zipPad3 xs [] [] = zip3 xs (repeat mempty) (repeat mempty) zipPad3 [] ys [] = zip3 (repeat mempty) ys (repeat mempty) zipPad3 [] [] zs = zip3 (repeat mempty) (repeat mempty) zs zipPad3 xs ys [] = zip3 xs ys (repeat mempty) zipPad3 xs [] zs = zip3 xs (repeat mempty) zs zipPad3 [] ys zs = zip3 (repeat mempty) ys zs zipPad3 (x:xs) (y:ys) (z:zs) = (x, y, z) : zipPad3 xs ys zs At this point I cheated and just used length to pick the longest list and pad the others. Am I overlooking a more elegant way to do this, or is something like zipPad3 already defined somewhere?

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• Printing a field with additional dots in haskell

  - by Frank Kluyt

  I'm writing a function called printField. This function takes an int and a string as arguments and then then prints a field like this "Derp..." with this: printField 7 "Derp". When the field consists of digits the output should be "...3456". The function I wrote looks like this: printField :: Int -> String -> String printField x y = if isDigit y then concat(replicate n ".") ++ y else y ++ concat(replicate n ".") where n = x - length y This obviously isn't working. The error I get from GHC is: Couldn't match type `[Char]' with `Char' Expected type: Char Actual type: String In the first argument of `isDigit', namely `y' In the expression: isDigit y In the expression: if isDigit y then concat (replicate n ".") ++ y else y ++ concat (replicate n ".") I can't get it to work :(. Can anyone help me out? Please keep in mind that I'm new to Haskell and functional programming in general.

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• Project euler problem 3 in haskell

  - by shk

  I'm new in Haskell and try to solve 3 problem from http://projecteuler.net/. The prime factors of 13195 are 5, 7, 13 and 29. What is the largest prime factor of the number 600851475143 ? My solution: import Data.List getD :: Int -> Int getD x = -- find deviders let deriveList = filter (\y -> (x `mod` y) == 0) [1 .. x] filteredList = filter isSimpleNumber deriveList in maximum filteredList -- Check is nmber simple isSimpleNumber :: Int -> Bool isSimpleNumber x = let deriveList = map (\y -> (x `mod` y)) [1 .. x] filterLength = length ( filter (\z -> z == 0) deriveList) in case filterLength of 2 -> True _ -> False I try to run for example: getD 13195 > 29 But when i try: getD 600851475143 I get error Exception: Prelude.maximum: empty list Why? Thank you @Barry Brown, I think i must use: getD :: Integer -> Integer But i get error: Couldn't match expected type `Int' with actual type `Integer' Expected type: [Int] Actual type: [Integer] In the second argument of `filter', namely `deriveList' In the expression: filter isSimpleNumber deriveList Thank you.

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• Haskell quiz: a simple function

  - by levy

  I'm not a Haskell programmer, but I'm curious about the following questions. Informal function specification: Let MapProduct be a function that takes a function called F and multiple lists. It returns a list containing the results of calling F with one argument from each list in each possible combination. Example: Call MapProduct with F being a function that simply returns a list of its arguments, and two lists. One of the lists contains the integers 1 and 2, the other one contains the strings "a" and "b". It should return a list that contains the lists: 1 and "a", 1 and "b", 2 and "a", 2 and "b". Questions: How is MapProduct implemented? What is the function's type? What is F's type? Can one guess what the function does just by looking at its type? Can you handle inhomogeneous lists as input? (e.g. 1 and "a" in one of the input lists) What extra limitation (if any) do you need to introduce to implement MapProduct?

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• Types in Haskell

  - by Linda Cohen

  I'm kind of new in Haskell and I have difficulty understanding how inferred types and such works. map :: (a -> b) -> [a] -> [b] (.) :: (a -> b) -> (c -> a) -> c -> b What EXACTLY does that mean? foldr :: (a -> b -> b) -> b -> [a] -> b foldl :: (a -> b -> a) -> a -> [b] -> a foldl1 :: (a -> a -> a) -> [a] -> a What are the differences between these? And how would I define the inferred type of something like foldr map THANKS!

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• Body Mass Index program in haskell

  - by user364996

  Hi there. I'm trying to write a simple program in Haskell that can determine someone's body mass index. Here's what I have written: type Height = Float type Weight = Float type PeopleStats = [(String, Height, Weight)] and... bmi :: Height -> Weight -> Float bmi heightCm weightKg = weightKg/(heightCm)^2 healthy :: Height -> Weight -> Bool healthy heightCm weightKg | 25 > index && 18 < index = True | otherwise = False where index = bmi heightCm weightKg So far, the function "healthy" can calculate someone's BMI, and the function "healthyPeople" returns a boolean statement determining if the person's BMI falls within the limits which is considered normal for a healthy person. I want to write a function called "healthyPeople". healthyPeople :: PeopleStats -> [String] This function needs to take a list of PeopleStats and returns a list of names (Strings) of people who are deemed to be "healthy" from the "healthy" function. For example: If I input [("Lee", 65, 185), ("Wang", 170, 100), ("Tsu", 160, 120)] I will get a list of the names of the people whose BMI returns true form the boolean function in "healthy". Please help !!!!

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• urgent..haskell mini interpreter

  - by mohamed elshikh

  i'm asked to implement this project and i have problems in part b which is the eval function this is the full describtion of the project You are required to implement an interpreter for mini-Haskell language. An interpreter is dened in Wikipedia as a computer program that executes, i.e. performs, instructions written in a programming language. The interpreter should be able to evaluate functions written in a special notation, which you will dene. A function is dened by: Function name Input Parameters : dened as a list of variables. The body of the function. The body of the function can be any of the following statements: a) Variable: The function may return any of the input variables. b) Arithmetic Expressions: The arithmetic expressions include input variables and addition, sub- traction, multiplication, division and modulus operations on arithmetic expressions. c) Boolean Expressions: The Boolean expressions include the ordering of arithmetic expressions (applying the relationships: <, =<, , = or =) and the anding, oring and negation of Boolean expressions. d) If-then-else statements: where the if keyword is followed by a Boolean expression. The then and else parts may be followed by any of the statements described here. e) Guarded expressions: where each case consists of a boolean expression and any of the statements described here. The expression consists of any number of cases. The rst case whose condition is true, its body should be evaluated. The guarded expression has to terminate with an otherwise case. f) Function calls: the body of the function may have a call to another function. Note that all inputs passed to the function will be of type Int. The output of the function can be of type Int or Bool. To implement the interpreter, you are required to implement the following: a) Dene a datatype for the following expressions: Variables Arithmetic expressions Boolean expressions If-then-else statements Guarded expressions Functions b) Implement the function eval which evaluates a function. It takes 3 inputs: The name of a function to be evaluated represented as a string. A list of inputs to that function. The arguments will always be of datatype Int. A list of functions. Each function is represented as instance of the datatype that you have created for functions. c) Implement the function get_type that returns the type of the function (as a string). The input to this function is the same as in part b. here is what i've done data Variable = v(char) data Arth= va Variable | Add Arth Arth | Sub Arth Arth | Times Arth Arth | Divide Arth Arth data Bol= Great Arth Arth | Small Arth Arth | Geq Arth Arth | Seq Arth Arth | And Bol Bol | Or Bol Bol | Neg Bol data Cond = data Guard = data Fun =cons String [Variable] Body data Body= bodycons(String) |Bol |Cond |Guard |Arth

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• Avoiding explicit recursion in Haskell

  - by Travis Brown

  The following simple function applies a given monadic function iteratively until it hits a Nothing, at which point it returns the last non-Nothing value. It does what I need, and I understand how it works. lastJustM :: (Monad m) => (a -> m (Maybe a)) -> a -> m a lastJustM g x = g x >>= maybe (return x) (lastJustM g) As part of my self-education in Haskell I'm trying to avoid explicit recursion (or at least understand how to) whenever I can. It seems like there should be a simple non-explicitly recursive solution in this case, but I'm having trouble figuring it out. I don't want something like a monadic version of takeWhile, since it could be expensive to collect all the pre-Nothing values, and I don't care about them anyway. I checked Hoogle for the signature and nothing shows up. The m (Maybe a) bit makes me think a monad transformer might be useful here, but I don't really have the intuitions I'd need to come up with the details (yet). It's probably either embarrassingly easy to do this or embarrassingly easy to see why it can't or shouldn't be done, but this wouldn't be the first time I've used self-embarrassment as a pedagogical strategy. Background: Here's a simplified working example for context: suppose we're interested in random walks in the unit square, but we only care about points of exit. We have the following step function: randomStep :: (Floating a, Ord a, Random a) => a -> (a, a) -> State StdGen (Maybe (a, a)) randomStep s (x, y) = do (a, gen') <- randomR (0, 2 * pi) <$> get put gen' let (x', y') = (x + s * cos a, y + s * sin a) if x' < 0 || x' > 1 || y' < 0 || y' > 1 then return Nothing else return $ Just (x', y') Something like evalState (lastJustM (randomStep 0.01) (0.5, 0.5)) <$> newStdGen will give us a new data point.

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• Writing a mini-language with haskell, trouble with "while" statements and blocks { }

  - by Nibirue

  EDIT: problem partially solved, skip to the bottom for update. I'm writing a small language using haskell, and I've made a lot of progress, but I am having trouble implementing statements that use blocks, like "{ ... }". I've implemented support for If statements like so in my parser file: stmt = skip +++ ifstmt +++ assignment +++ whilestmt ifstmt = symbol "if" >> parens expr >>= \c -> stmt >>= \t -> symbol "else" >> stmt >>= \e -> return $ If c t e whilestmt = symbol "while" >> parens expr >>= \c -> symbol "\n" >> symbol "{" >> stmt >>= \t -> symbol "}" >> return $ While c t expr = composite +++ atomic And in the Syntax file: class PP a where pp :: Int -> a -> String instance PP Stmt where pp ind (If c t e) = indent ind ++ "if (" ++ show c ++ ") \n" ++ pp (ind + 2) t ++ indent ind ++ "else\n" ++ pp (ind + 2) e pp ind (While c t) = indent ind ++ "while (" ++ show c ++") \n" ++ "{" ++ pp (ind + 2) t ++ "}" ++ indent ind Something is wrong with the while statement, and I don't understand what. The logic seems correct, but when I run the code I get the following error: EDIT: Fixed the first problem based on the first reply, now it is not recognizing my while statment which I assume comes from this: exec :: Env -> Stmt -> Env exec env (If c t e) = exec env ( if eval env c == BoolLit True then t else e ) exec env (While c t) = exec env ( if eval env c == BoolLit True then t ) The file being read from looks like this: x = 1; c = 0; if (x < 2) c = c + 1; else ; -- SEPARATE FILES FOR EACH x = 1; c = 1; while (x < 10) { c = c * x; x = x + 1; } c I've tried to understand the error report but nothing I've tried solves the problem.

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• Is Haskell "mainstream", or is it mainly used by hobbyists and academics?

  - by Asher

  I came across a post where someone wrote something inaccurate about Haskell (won't go into it) and he got flammed for it. Which (pleasantly) surprised me. About 3 years ago I read this joke about Haskell: All the haskell programmers in the world can fit into a 747 and if that plane were to crash no one would care... or something along those lines. Which brings me to my question: how healthy is the Haskell community, anyway? Is Haskell "mainstream"? Is it mainly used by hobbiest and academics or someone making some serious money from it (which is the true yardstick of how good a language is - just kidding, geez!)?

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• Cross platform mobile development VS Native Mobile Development: Present And Future.

  - by MobileDev123

  I just completed one year in Smart phone development, working on BlackBerry and Android and also developed one application exclusively targeted to nokia feature phones. And just a month ago I come to know about Titanium Appcelerator tool that enables cross platform development, but there are some developers who complain about it's sub-par functionalities. Even a little bit experience of mine says that developing in native environment rather than these cross platform tools will give you more advantages by giving a developer a chance to add more features with better performance. Do you have same experience? Or you find such cross development tools really useful regarding to advance functionality and performance? As porting (or co developing) same application to different mobile platform is common thing nowadays, what do you think will these cross platform tools evolve and force developers to get a hands on approach on them or majority will stick to the native development environment?

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• Unity in C# for Platform Specific Implementations

  - by DxCK

  My program has heavy interaction with the operating system through Win32API functions. now i want to migrate my program to run under Mono under Linux (No wine), and this requires different implementations to the interaction with the operating system. I started designing a code that can have different implementation for difference platform and is extensible for new future platforms. public interface ISomeInterface { void SomePlatformSpecificOperation(); } [PlatformSpecific(PlatformID.Unix)] public class SomeImplementation : ISomeInterface { #region ISomeInterface Members public void SomePlatformSpecificOperation() { Console.WriteLine("From SomeImplementation"); } #endregion } public class PlatformSpecificAttribute : Attribute { private PlatformID _platform; public PlatformSpecificAttribute(PlatformID platform) { _platform = platform; } public PlatformID Platform { get { return _platform; } } } public static class PlatformSpecificUtils { public static IEnumerable<Type> GetImplementationTypes<T>() { foreach (Assembly assembly in AppDomain.CurrentDomain.GetAssemblies()) { foreach (Type type in assembly.GetTypes()) { if (typeof(T).IsAssignableFrom(type) && type != typeof(T) && IsPlatformMatch(type)) { yield return type; } } } } private static bool IsPlatformMatch(Type type) { return GetPlatforms(type).Any(platform => platform == Environment.OSVersion.Platform); } private static IEnumerable<PlatformID> GetPlatforms(Type type) { return type.GetCustomAttributes(typeof(PlatformSpecificAttribute), false) .Select(obj => ((PlatformSpecificAttribute)obj).Platform); } } class Program { static void Main(string[] args) { Type first = PlatformSpecificUtils.GetImplementationTypes<ISomeInterface>().FirstOrDefault(); } } I see two problems with this design: I can't force the implementations of ISomeInterface to have a PlatformSpecificAttribute. Multiple implementations can be marked with the same PlatformID, and i dont know witch to use in the Main. Using the first one is ummm ugly. How to solve those problems? Can you suggest another design?

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• What's the proper term for a function inverse to a constructor - to unwrap a value from a data type?

  - by Petr Pudlák

  Edit: I'm rephrasing the question a bit. Apparently I caused some confusion because I didn't realize that the term destructor is used in OOP for something quite different - it's a function invoked when an object is being destroyed. In functional programming we (try to) avoid mutable state so there is no such equivalent to it. (I added the proper tag to the question.) Instead, I've seen that the record field for unwrapping a value (especially for single-valued data types such as newtypes) is sometimes called destructor or perhaps deconstructor. For example, let's have (in Haskell): newtype Wrap = Wrap { unwrap :: Int } Here Wrap is the constructor and unwrap is what? The questions are: How do we call unwrap in functional programming? Deconstructor? Destructor? Or by some other term? And to clarify, is this/other terminology applicable to other functional languages, or is it used just in the Haskell? Perhaps also, is there any terminology for this in general, in non-functional languages? I've seen both terms, for example: ... Most often, one supplies smart constructors and destructors for these to ease working with them. ... at Haskell wiki, or ... The general theme here is to fuse constructor - deconstructor pairs like ... at Haskell wikibook (here it's probably meant in a bit more general sense), or newtype DList a = DL { unDL :: [a] -> [a] } The unDL function is our deconstructor, which removes the DL constructor. ... in The Real World Haskell.

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• Refernce platform specific System.Data.SQLite

  - by Dmitriy Nagirnyak

  Hi, I am using SQLite for the unit testing and might use it as a database for local development/staging. The System.Data.SQLite has basically 2 versions: x86 and x64. Correct one should be used for the specific platform. I have 64 bit Win7, other guys in the team might use 32-bit OSs. The server's platform is not known at this stage. If I use 32-bit version of the assembly on 64-bit platform I get BadImageFormatException: Could not load file or assembly 'System.Data.SQLite'. I believe similar will happen trying to use 64-bit assembly on 32-bit platform. So my question is what is the best way to reference the SQLite assembly so that it does not depend on the platform and people can just use it? It is ok to use 32-bit version of assembly on a 64-bit platform (Maybe there is a switch for that somewhere?). Thanks, Dmitriy.

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• Reference platform specific System.Data.SQLite

  - by Dmitriy Nagirnyak

  I am using SQLite for the unit testing and might use it as a database for local development/staging. The System.Data.SQLite has basically 2 versions: x86 and x64. Correct one should be used for the specific platform. I have 64 bit Win7, other guys in the team might use 32-bit OSs. The server's platform is not known at this stage. If I use 32-bit version of the assembly on 64-bit platform I get BadImageFormatException: Could not load file or assembly 'System.Data.SQLite'. I believe similar will happen trying to use 64-bit assembly on 32-bit platform. So my question is what is the best way to reference the SQLite assembly so that it does not depend on the platform and people can just use it? It is ok to use 32-bit version of assembly on a 64-bit platform (Maybe there is a switch for that somewhere?).

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• backtracking in haskell

  - by dmindreader

  I have to traverse a matrix and say how many "characteristic areas" of each type it has. A characteristic area is defined as a zone where elements of value n or n are adjacent. For example, given the matrix: 0 1 2 2 0 1 1 2 0 3 0 0 There's a single characteristic area of type 1 which is equal to the original matrix: 0 1 2 2 0 1 1 2 0 3 0 0 There are two characteristic areas of type 2: 0 0 2 2 0 0 0 0 0 0 0 2 0 0 0 0 0 0 0 0 0 3 0 0 And one characteristic area of type 3: 0 0 0 0 0 0 0 0 0 3 0 0 So, for the function call: countAreas [[0,1,2,2],[0,1,1,2],[0,3,0,0]] The result should be [1,2,1] I haven't defined countAreas yet, I'm stuck with my visit function when it has no more possible squares in which to move it gets stuck and doesn't make the proper recursive call. I'm new to functional programming and I'm still scratching my head about how to implement a backtracking algorithm here. Take a look at my code, what can I do to change it? move_right :: (Int,Int) -> [[Int]] -> Int -> Bool move_right (i,j) mat cond | (j + 1) < number_of_columns mat && consult (i,j+1) mat /= cond = True | otherwise = False move_left :: (Int,Int) -> [[Int]] -> Int -> Bool move_left (i,j) mat cond | (j - 1) >= 0 && consult (i,j-1) mat /= cond = True | otherwise = False move_up :: (Int,Int) -> [[Int]] -> Int -> Bool move_up (i,j) mat cond | (i - 1) >= 0 && consult (i-1,j) mat /= cond = True | otherwise = False move_down :: (Int,Int) -> [[Int]] -> Int -> Bool move_down (i,j) mat cond | (i + 1) < number_of_rows mat && consult (i+1,j) mat /= cond = True | otherwise = False imp :: (Int,Int) -> Int imp (i,j) = i number_of_rows :: [[Int]] -> Int number_of_rows i = length i number_of_columns :: [[Int]] -> Int number_of_columns (x:xs) = length x consult :: (Int,Int) -> [[Int]] -> Int consult (i,j) l = (l !! i) !! j visited :: (Int,Int) -> [(Int,Int)] -> Bool visited x y = elem x y add :: (Int,Int) -> [(Int,Int)] -> [(Int,Int)] add x y = x:y visit :: (Int,Int) -> [(Int,Int)] -> [[Int]] -> Int -> [(Int,Int)] visit (i,j) vis mat cond | move_right (i,j) mat cond && not (visited (i,j+1) vis) = visit (i,j+1) (add (i,j+1) vis) mat cond | move_down (i,j) mat cond && not (visited (i+1,j) vis) = visit (i+1,j) (add (i+1,j) vis) mat cond | move_left (i,j) mat cond && not (visited (i,j-1) vis) = visit (i,j-1) (add (i,j-1) vis) mat cond | move_up (i,j) mat cond && not (visited (i-1,j) vis) = visit (i-1,j) (add (i-1,j) vis) mat cond | otherwise = vis

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• Understanding Haskell's fibonacci

  - by AR

  fibs :: [Int] fibs = 0 : 1 : [ a + b | (a, b) <- zip fibs (tail fibs)] This generates the Fibonacci sequence. I understand the behaviour of the guards, of :, zip and tail, but I don't understand <-. What is it doing here?

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• what is programming paradigm in haskell

  - by Pradeep

  im preparing for my exam. i got this question and i read several articles about this. but still i cant get a proper idea about this question what Paradigm means is that different programming styles ( as far as i think) in the question they ask explain it by taking two paradigms so this explanation should be done using two styles of programming "List Comprehnsions or Primitive Recursion, Higher order functions " is these styles are programming paradigms?? please help me

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