Search Results

Search found 762 results on 31 pages for 'haskell'.

Page 8/31 | < Previous Page | 4 5 6 7 8 9 10 11 12 13 14 15  | Next Page >

• 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!

  Read the article

• 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 !!!!

  Read the article

• 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

  Read the article

• 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.

  Read the article

• 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.

  Read the article

• 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!)?

  Read the article

• 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.

  Read the article

• 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

  Read the article

• 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?

  Read the article

• 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

  Read the article

• haskell network io hgetline

  - by Alex

  I want to read all the data on a handle, and then block waiting for more data. listen1 stops when there is a '\n' character in the stream. listen2 works and could be made completely general by imitating the code for hGetNonBlocking. What is the best way to do this? import qualified Data.ByteString as B loop = sequence_ . repeat listen1 :: Handle -> TChan B.ByteString -> IO() listen1 sock chan = do loop ( do s <- B.hGetLine sock atomically (writeTChan chan s) ) listen2 :: Handle -> TChan B.ByteString -> IO() listen2 sock chan = do loop ( do s <- B.hGet sock 1 s1 <- B.hGetNonBlocking sock 65000 atomically (writeTChan chan (B.append s s1)) )

  Read the article

• Haskell Parsec Numeration

  - by Martin

  I'm using Text.ParserCombinators.Parsec and Text.XHtml to parse an input like this: - First type A\n -- First type B\n - Second type A\n -- First type B\n --Second type B\n And my output should be: <h11 First type A\n</h1 <h21.1 First type B\n</h2 <h12 Second type A\n</h2 <h22.1 First type B\n</h2 <h22.2 Second type B\n</h2 I have come to this part, but I cannot get any further: title1= do{ ;(count 1 (char '-')) ;s <- many1 anyChar newline ;return (h1 << s) } title2= do{ ;(count 2 (char '--')) ;s <- many1 anyChar newline ;return (h1 << s) } text=do { ;many (choice [try(title1),try(title2)]) } main :: IO () main = do t putStr "Error: " print err Right x - putStrLn $ prettyHtml x This is ok, but it does not include the numbering. Any ideas? Thanks!

  Read the article

• Haskell: Left-biased/short-circuiting function

  - by user2967411

  Two classes ago, our professor presented to us a Parser module. Here is the code: module Parser (Parser,parser,runParser,satisfy,char,string,many,many1,(+++)) where import Data.Char import Control.Monad import Control.Monad.State type Parser = StateT String [] runParser :: Parser a -> String -> [(a,String)] runParser = runStateT parser :: (String -> [(a,String)]) -> Parser a parser = StateT satisfy :: (Char -> Bool) -> Parser Char satisfy f = parser $ \s -> case s of [] -> [] a:as -> [(a,as) | f a] char :: Char -> Parser Char char = satisfy . (==) alpha,digit :: Parser Char alpha = satisfy isAlpha digit = satisfy isDigit string :: String -> Parser String string = mapM char infixr 5 +++ (+++) :: Parser a -> Parser a -> Parser a (+++) = mplus many, many1 :: Parser a -> Parser [a] many p = return [] +++ many1 p many1 p = liftM2 (:) p (many p) Today he gave us an assignment to introduce "a left-biased, or short-circuiting version of (+++)", called (<++). His hint was for us to consider the original implementation of (+++). When he first introduced +++ to us, this was the code he wrote, which I am going to call the original implementation: infixr 5 +++ (+++) :: Parser a -> Parser a -> Parser a p +++ q = Parser $ \s -> runParser p s ++ runParser q s I have been having tons of trouble since we were introduced to parsing and so it continues. I have tried/am considering two approaches. 1) Use the "original" implementation, as in p +++ q = Parser $ \s - runParser p s ++ runParser q s 2) Use the final implementation, as in (+++) = mplus Here are my questions: 1) The module will not compile if I use the original implementation. The error: Not in scope: data constructor 'Parser'. It compiles fine using (+++) = mplus. What is wrong with using the original implementation that is avoided by using the final implementation? 2) How do I check if the first Parser returns anything? Is something like (not (isNothing (Parser $ \s - runParser p s) on the right track? It seems like it should be easy but I have no idea. 3) Once I figure out how to check if the first Parser returns anything, if I am to base my code on the final implementation, would it be as easy as this?: -- if p returns something then p <++ q = mplus (Parser $ \s -> runParser p s) mzero -- else (<++) = mplus Best, Jeff

  Read the article

• Haskell Parse Paragraph and em element with Parsec

  - by Martin

  I'm using Text.ParserCombinators.Parsec and Text.XHtml to parse an input like this: this is the beginning of the paragraph --this is an emphasized text-- and this is the end\n And my output should be: <p>this is the beginning of the paragraph <em>this is an emphasized text</em> and this is the end\n</p> This code parses and returns an emphasized element em = do{ ;count 2 (char '-') ; ;s <- manyTill anyChar (count 2 (char '-')) ;return (emphasize << s) } But I don't know how to get the paragraphs with emphasized items Any ideas? Thanks!!

  Read the article

• Custom whiteSpace using Haskell Parsec

  - by fryguybob

  I would like to use Parsec's makeTokenParser to build my parser, but I want to use my own definition of whiteSpace. Doing the following replaces whiteSpace with my definition, but all the lexeme parsers still use the old definition (e.g. P.identifier lexer will use the old whiteSpace). ... lexer :: P.TokenParser () lexer = l { P.whiteSpace = myWhiteSpace } where l = P.makeTokenParser myLanguageDef ... Looking at the code for makeTokenParser I think I understand why it works this way. I want to know if there are any workarounds to avoid completely duplicating the code for makeTokenParser?

  Read the article

• Multiple Haskell cabal-packages in one directory

  - by aleator

  What is the recommended way of having several cabal packages in one directory? Why: I have an old project with many separable modules. Since originally they formed just one program it was, and still is, handy to have them in same directory for easy compiling. Options Just suffer and split everything, including VCS holding the stuff, into different directories? Hack cabal until it is happy with multiple .cabal files in same directory? Make another subdirectory for each module and put .cabal files there along with symlinks to original pieces of code? Something smarter? What?

  Read the article

• Haskell Convert List to List of Tuples

  - by peterwkc

  Hello, i have a list like this ["peter","1000","michell","2000","kelly","3000"] and i would like to convert to [("peter",1000),("michell", 2000),("kelly",3000)] Please help. Thanks.

  Read the article

• Level-order in Haskell

  - by brain_damage

  I have a structure for a tree and I want to print the tree by levels. data Tree a = Nd a [Tree a] deriving Show type Nd = String tree = Nd "a" [Nd "b" [Nd "c" [], Nd "g" [Nd "h" [], Nd "i" [], Nd "j" [], Nd "k" []]], Nd "d" [Nd "f" []], Nd "e" [Nd "l" [Nd "n" [Nd "o" []]], Nd "m" []]] preorder (Nd x ts) = x : concatMap preorder ts postorder (Nd x ts) = (concatMap postorder ts) ++ [x] But how to do it by levels? "levels tree" should print ["a", "bde", "cgflm", "hijkn", "o"]. I think that "iterate" would be suitable function for the purpose, but I cannot come up with a solution how to use it. Would you help me, please?

  Read the article

• [Haskell] Problem when mixing type classes and type families

  - by Giuseppe Maggiore

  Hi! This code compiles fine: {-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies, FlexibleInstances, UndecidableInstances, FlexibleContexts, EmptyDataDecls, ScopedTypeVariables, TypeOperators, TypeSynonymInstances, TypeFamilies #-} class Sel a s b where type Res a s b :: * instance Sel a s b where type Res a s b = (s -> (b,s)) instance Sel a s (b->(c,a)) where type Res a s (b->(c,a)) = (b -> s -> (c,s)) but as soon as I add the R predicate ghc fails: {-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies, FlexibleInstances, UndecidableInstances, FlexibleContexts, EmptyDataDecls, ScopedTypeVariables, TypeOperators, TypeSynonymInstances, TypeFamilies #-} class Sel a s b where type Res a s b :: * instance Sel a s b where type Res a s b = (s -> (b,s)) class R a where type Rec a :: * cons :: a -> Rec a elim :: Rec a -> a instance Sel a s (b->(c,Rec a)) where type Res a s (b->(c,Rec a)) = (b -> s -> (c,s)) complaining that: Illegal type synonym family application in instance: b -> (c, Rec a) In the instance declaration for `Sel a s (b -> (c, Rec a))' what does it mean and (most importantly) how do I fix it? Thanks

  Read the article

• Wrong IO actions order using putStr and getLine

  - by QWRp

  I have a code : main = do putStr "Test input : " content <- getLine putStrLn content And when I run it (with runhaskell) or compile it (ghc 6.10.4) result is like this: asd Test input : asd I'm new to haskell and in my opinion printing should be first. Am I right? In code sample on http://learnyouahaskell.com/ which used putStr then getLine presented output is different than mine (IMHO correct). When I use putStrLn program works as expected (print then prompt and print). Is it a bug in ghc, or it is the way that it should work?

  Read the article

• How to convert ByteString to [Word8] in Haskell?

  - by bodacydo

  I am dealing with ByteStrings and at this one place I need to use [Word8] but I have a ByteString. How do I convert a ByteString to [Word8] list? I tried unpack function from ByteString library but it returned a [Char] list rather than [Word8] list. Do I need to take this step and convert it first to [Char] list and only to [Word8] list? If so, how do I do that. Please advise the most efficient method! Thank you!

  Read the article

• simple text menu in Haskell

  - by snorlaks

  Hello, I would like to know what is the best solution to create simple menu with functionality described below (pseudo code) just like im used to: while (true){ x = readLine(); case (x): x == "1" then do sth1 function x == "2" then do sth2 function } thanks for help, maybe any other ideas on how to make menu not in pattern described above?

  Read the article

• Haskell IO russian symbols

  - by Anton

  Hi. I try process file which writen by russian symbols. When read and after write text to file i get something like: "\160\192\231\229\240\225\224\233\228\230\224\237" How i can get normal symbols ? Thanks

  Read the article

• Updating List Elements, Haskell

  - by Tom

  I have homework where I am to update a list using a function that takes two elements and returns a value of part of the first element given in the function. So it's required to update the entire listing by going through each element and update its value by applying the function against all other elements in the list (including itself). So far I've been trying to firstly map the list (so that each element is done the same) and then specifically update each elements value by mapping again just the value of the specified element however in trying to map just the specific value through: the function, the specific element and the entire list I keep getting complaints that I'm inferring the list of values made from the 'map function p@list list' rather than simply giving the value at p@list. Is this the correct method to try to update a list against the entire list itself? EDIT: spelling mistakes and grammar

  Read the article

• "Subclassing" show in Haskell?

  - by me2

  Lets say I have the following: data Greek = Alpha | Beta | Gamma | Phi deriving Show I want to use the default showing of all items except Beta, which I want to say "two". Can I do this?

  Read the article

< Previous Page | 4 5 6 7 8 9 10 11 12 13 14 15  | Next Page >