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  • Why is the Clojure Hello World program so slow compared to Java and Python?

    - by viksit
    Hi all, I'm reading "Programming Clojure" and I was comparing some languages I use for some simple code. I noticed that the clojure implementations were the slowest in each case. For instance, Python - hello.py def hello_world(name): print "Hello, %s" % name hello_world("world") and result, $ time python hello.py Hello, world real 0m0.027s user 0m0.013s sys 0m0.014s Java - hello.java import java.io.*; public class hello { public static void hello_world(String name) { System.out.println("Hello, " + name); } public static void main(String[] args) { hello_world("world"); } } and result, $ time java hello Hello, world real 0m0.324s user 0m0.296s sys 0m0.065s and finally, Clojure - hellofun.clj (defn hello-world [username] (println (format "Hello, %s" username))) (hello-world "world") and results, $ time clj hellofun.clj Hello, world real 0m1.418s user 0m1.649s sys 0m0.154s Thats a whole, garangutan 1.4 seconds! Does anyone have pointers on what the cause of this could be? Is Clojure really that slow, or are there JVM tricks et al that need to be used in order to speed up execution? More importantly - isn't this huge difference in performance going to be an issue at some point? (I mean, lets say I was using Clojure for a production system - the gain I get in using lisp seems completely offset by the performance issues I can see here). The machine used here is a 2007 Macbook Pro running Snow Leopard, a 2.16Ghz Intel C2D and 2G DDR2 SDRAM. BTW, the clj script I'm using is from here and looks like, #!/bin/bash JAVA=/System/Library/Frameworks/JavaVM.framework/Versions/1.6/Home/bin/java CLJ_DIR=/opt/jars CLOJURE=$CLJ_DIR/clojure.jar CONTRIB=$CLJ_DIR/clojure-contrib.jar JLINE=$CLJ_DIR/jline-0.9.94.jar CP=$PWD:$CLOJURE:$JLINE:$CONTRIB # Add extra jars as specified by `.clojure` file if [ -f .clojure ] then CP=$CP:`cat .clojure` fi if [ -z "$1" ]; then $JAVA -server -cp $CP \ jline.ConsoleRunner clojure.lang.Repl else scriptname=$1 $JAVA -server -cp $CP clojure.main $scriptname -- $* fi

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  • How to print "Hello, world!" (in every possible way)

    - by Attila Oláh
    Here's what I', trying to do: 1 language: (Python < 3): print "Hello, world!" 2 languages: (Python < 3 & Windows Shell, aka .bat file): rem=""" echo "Hello, world!" exit """ print "Hello, world!" Next step could be something like bash. Since the above one raises an exception, I tried to make it not raise exceptions, like this: rem=""" echo "Hello, world!" exit """ exit="" exit print "Hello, world!" The only issue is, of course, it won't print the hello world. And I really want it to print that hello world for me. Anyone can help with this? Also, any other language would do it, just don't break the previous ones (i.e. the answer still has to be valid Python code and print out the nice hello world greeting when run with Python.) Any ideas are welcome. I'm making this a community wiki so feel free to add ideas to the list.

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  • many1 no longer works with Parsec 3.x

    - by Zak
    After updating to Parsec 3.1 from 2.x, code using many1, such as word = many1 letter fails with No instance for (Stream s m Char) arising from a use of `letter' I found a mailing list post claiming that adding {-#LANGUAGE NoMonomorphismRestriction #-} to the top of the source file would solve the problem, but it did not.

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  • Parsec: backtracking not working

    - by Nathan Sanders
    I am trying to parse F# type syntax. I started writing an [F]Parsec grammar and ran into problems, so I simplified the grammar down to this: type ::= identifier | type -> type identifier ::= [A-Za-z0-9.`]+ After running into problems with FParsec, I switched to Parsec, since I have a full chapter of a book dedicated to explaining it. My code for this grammar is typeP = choice [identP, arrowP] identP = do id <- many1 (digit <|> letter <|> char '.' <|> char '`') -- more complicated code here later return id arrowP = do domain <- typeP string "->" range <- typeP return $ "("++domain++" -> "++range++")" run = parse (do t <- typeP eof return t) "F# type syntax" The problem is that Parsec doesn't backtrack by default, so > run "int" Right "int" -- works! > run "int->int" Left "F# type syntax" unexpected "-" expecting digit, letter, ".", "`" or end of input -- doesn't work! The first thing I tried was to reorder typeP: typeP = choice [arrowP, identP] But this just stack overflows because the grammar is left-recursive--typeP never gets to trying identP because it keeps trying arrowP over and over. Next I tried try in various places, for example: typeP = choice [try identP, arrowP] But nothing I do seems to change the basic behaviours of (1) stack overflow or (2) non-recognition of "-" following an identifier. My mistake is probably obvious to anybody who has successfully written a Parsec grammar. Can somebody point it out?

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  • Type error while trying to implement the (>>=) function in order to create a custom monad transforme

    - by CharlieP
    Hello, I'm trying to create a monad transformer for a future project, but unfortunately, my implementation of the Monad typeclasse's (=) function doesn't work. First of all, here is the underlying monad's implementation : newtype Runtime a = R { unR :: State EInfo a } deriving (Monad) Here, the implementation of the Monad typeclasse is done automatically by GHC (using the GeneralizedNewtypeDeriving language pragma). The monad transformer is defined as so : newtype RuntimeT m a = RuntimeT { runRuntimeT :: m (Runtime a) } The problem comes from the way I instanciate the (=) function of the Monad typeclasse : instance (Monad m) => Monad (RuntimeT m) where return a = RuntimeT $ (return . return) a x >>= f = runRuntimeT x >>= id >>= f The way I see it, the first >>= runs in the underlying m monad. Thus, runRuntimeT x >>= returns a value of type Runtime a (right ?). Then, the following code, id >>=, should return a value of type a. This value is the passed on to the function f of type f :: (Monad m) => a -> RuntimeT m b. And here comes the type problem : the f function's type doesn't match the type required by the (=) function. Jow can I make this coherent ? I can see why this doesn't work, but I can't manage to turn it into something functionnal. Thank you for you help, and do not hesitate to correct any flaws in my message, Charlie P.

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  • Removing the contents of a Chan or MVar in a single discrete step

    - by Bill
    I'm writing a discrete simulation where request values from multiple threads accumulate in a centralized queue. Every n milliseconds, a manager wakes up to process requests. When the manager wakes up, it should retrieve all of the contents of the central queue in a single discrete step. While processing these, any client threads attempting to submit to the queue should block. When processing completes, the queue reopens and the manager goes back to sleep. What's the best way to do this? The retry behavior of STM isn't really what I want. If I use a Chan or MVar, there's no way to prevent clients from enqueuing additional requests during processing. One approach is to use an MVar as a mutex on a Chan holding the queue. Are there other ways to do this?

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  • I'm working on Peano Axioms in Agda and I've hit a bit of a sticking point

    - by Schroedinger
    PA6 : ?{m n} -> m = n -> n = m is the axiom I am trying to solve and support, I've tried using a cong (from the core library) but am having troubles with the cong constructor PA6 = cong gets me nowhere, I know for cong I am required to supply a refl for equality and a type, but I'm, not sure what type I'm supposed to supply. Ideas? This is for a small assignment at University, so I'd rather someone demonstrate what I've missed rather than write the acutual answer, but I'd appreciate any degree of support.

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  • How to Convert Type in Tuples

    - by Pradeep
    how to convert a String type to a Int i have a tuple and i want to convert it to a tuple which has different types tupletotuple :: (String,String,String) ->(String,Int,Int) tupletotuple (a,b,c) = (a,read(b),read(c)) i get this Error Msg Project tupletotuple ("cha",4,3) ERROR - Cannot infer instance * Instance : Num [Char] * Expression : tupletotuple ("cha",4,3)

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  • Getting Cabal to work with GHC 6.12.1

    - by Dan Dyer
    I've installed the latest GHC package (6.12.1) on OS X, but I can't get Cabal to work. I've removed the version I had previously that worked with GHC 6.10 and tried to re-install from scratch. The latest Cabal version available for download is 1.6.0.2. However, when I try to build this I get the following error: Configuring Cabal-1.6.0.2... Setup: failed to parse output of 'ghc-pkg dump' From what I've found searching, this seems to suggest that the version of Cabal is too old for the version of GHC. Is there any way to get Cabal to work with GHC 6.12.1 yet? EDIT: To be clear, I'm trying to set-up cabal-install.

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  • displaying database content in wxHaskell

    - by snorlaks
    Hello, Im using tutorials from wxHaskell and want to display content of table movies in the grid. HEre is my code : {-------------------------------------------------------------------------------- Test Grid. --------------------------------------------------------------------------------} module Main where import Graphics.UI.WX import Graphics.UI.WXCore hiding (Event) import Database.HDBC.Sqlite3 (connectSqlite3) import Database.HDBC main = start gui gui :: IO () gui = do f <- frame [text := "Grid test", visible := False] -- grids g <- gridCtrl f [] gridSetGridLineColour g (colorSystem Color3DFace) gridSetCellHighlightColour g black appendColumns g (movies) -- Here is error: -- Couldn't match expected type [String]' -- against inferred typeIO [[String]]' --appendRows g (map show [1..length (tail movies)]) --mapM_ (setRow g) (zip [0..] (tail movies)) gridAutoSize g -- layout set f [layout := column 5 [fill (dynamic (widget g))] ] focusOn g set f [visible := True] -- reduce flicker at startup. return () where movies = do conn <- connectSqlite3 "Spop.db" r <- quickQuery' conn "SELECT id, title, year, description from Movie where id = 1" [] let myResult = map convRow r return myResult setRow g (row,values) = mapM_ ((col,value) - gridSetCellValue g row col value) (zip [0..] values) {-------------------------------------------------------------------------------- Library?f --------------------------------------------------------------------------------} gridCtrl :: Window a - [Prop (Grid ())] - IO (Grid ()) gridCtrl parent props = feed2 props 0 $ initialWindow $ \id rect - \props flags - do g <- gridCreate parent id rect flags gridCreateGrid g 0 0 0 set g props return g appendColumns :: Grid a - [String] - IO () appendColumns g [] = return () appendColumns g labels = do n <- gridGetNumberCols g gridAppendCols g (length labels) True mapM_ ((i,label) - gridSetColLabelValue g i label) (zip [n..] labels) appendRows :: Grid a - [String] - IO () appendRows g [] = return () appendRows g labels = do n <- gridGetNumberRows g gridAppendRows g (length labels) True mapM_ ((i,label) - gridSetRowLabelValue g i label) (zip [n..] labels) convRow :: [SqlValue] - [String] convRow [sqlId, sqlTitle, sqlYear, sqlDescription] = [intid, title, year, description] where intid = (fromSql sqlId)::String title = (fromSql sqlTitle)::String year = (fromSql sqlYear)::String description = (fromSql sqlDescription)::String What should I do to get rif of error in code above (24th line)

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  • "Ambigous type variable" error when defining custom "read" function

    - by Tener
    While trying to compile the following code, which is enhanced version of read build on readMay from Safe package. readI :: (Typeable a, Read a) => String -> a readI str = case readMay str of Just x -> x Nothing -> error ("Prelude.read failed, expected type: " ++ (show (typeOf > (undefined :: a))) ++ "String was: " ++ str) I get an error from GHC: WavefrontSimple.hs:54:81: Ambiguous type variable `a' in the constraint: `Typeable a' arising from a use of `typeOf' at src/WavefrontSimple.hs:54:81-103 Probable fix: add a type signature that fixes these type variable(s)` I don't understand why. What should be fixed to get what I meant? EDIT: Ok, so the solution to use ScopedTypeVariables and forall a in type signature works. But why the following produces very similar error to the one above? The compiler should infer the right type since there is asTypeOf :: a -> a -> a used. readI :: (Typeable a, Read a) => String -> a readI str = let xx = undefined in case readMay str of Just x -> x `asTypeOf` xx Nothing -> error ("Prelude.read failed, expected type: " ++ (show (typeOf xx)) ++ "String was: " ++ str)

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  • Continuation monad "interface"

    - by sdcvvc
    The state monad "interface" class MonadState s m where get :: m s put :: s -> m () (+ return and bind) allows to construct any possible computation with State monad without using State constructor. For example, State $ \s -> (s+1, s-1) can be written as do s <- get put (s-1) return (s+1) Similarily, I never have to use Reader constructor, because I can create that computation using ask, return and (>>=). Precisely: Reader f == ask >>= return . f. Is it the same true for continuations - is it possible to write all instances of Cont r a using callCC (the only function in MonadCont), return and bind, and never type something like Cont (\c -> ...)?

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  • Confusion regarding laziness

    - by Abhinav Kaushik
    I have a function myLength = foldl (\ x _ -> x + 1) 0 which fails with stack overflow with input around 10^6 elements (myLength [1..1000000] fails). I believe that is due to the thunk build up since when I replace foldl with foldl', it works. So far so good. But now I have another function to reverse a list : myReverse = foldl (\ acc x -> x : acc) [] which uses the lazy version foldl (instead of foldl') When I do myLength . myReverse $ [1.1000000]. This time it works fine. I fail to understand why foldl works for the later case and not for former?

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  • "Pattern matching" of algebraic type data constructors

    - by jetxee
    Let's consider a data type with many constructors: data T = Alpha Int | Beta Int | Gamma Int Int | Delta Int I want to write a function to check if two values are produced with the same constructor: sameK (Alpha _) (Alpha _) = True sameK (Beta _) (Beta _) = True sameK (Gamma _ _) (Gamma _ _) = True sameK _ _ = False Maintaining sameK is not much fun, it is potentially buggy. For example, when new constructors are added to T, it's easy to forget to update sameK. I omitted one line to give an example: -- it’s easy to forget: -- sameK (Delta _) (Delta _) = True The question is how to avoid boilerplate in sameK? Or how to make sure it checks for all T constructors? The workaround I found is to use separate data types for each of the constructors, deriving Data.Typeable, and declaring a common type class, but I don't like this solution, because it is much less readable and otherwise just a simple algebraic type works for me: {-# LANGUAGE DeriveDataTypeable #-} import Data.Typeable class Tlike t where value :: t -> t value = id data Alpha = Alpha Int deriving Typeable data Beta = Beta Int deriving Typeable data Gamma = Gamma Int Int deriving Typeable data Delta = Delta Int deriving Typeable instance Tlike Alpha instance Tlike Beta instance Tlike Gamma instance Tlike Delta sameK :: (Tlike t, Typeable t, Tlike t', Typeable t') => t -> t' -> Bool sameK a b = typeOf a == typeOf b

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  • Examples of attoparsec in parsing binary file formats?

    - by me2
    Previously attoparsec was suggested to me for parsing complex binary file formats. While I can find examples of attoparsec parsing HTTP, which is essentially text based, I cannot find an example parsing actual binary, for example, a TCP packet, or image file, or mp3. Can someone post some code or pointer to some code which does this using attoparsec?

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  • Unsure of how to get the right evaluation order

    - by Matt Fenwick
    I'm not sure what the difference between these two pieces of code is (with respect to x), but the first one completes: $ foldr (\x y -> if x == 4 then x else x + y) 0 [1,2 .. ] 10 and the second one doesn't (at least in GHCi): $ foldr (\x (y, n) -> if x == 4 then (x, n) else (x + y, n + 1)) (0, 0) [1,2 .. ] ....... What am I doing wrong that prevents the second example from completing when it hits x == 4, as in the first one? I've tried adding bang-patterns to both the x and to the x == 4 (inside a let) but neither seems to make a difference.

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  • Is there a good way to QuickCheck Happstack.State methods?

    - by Paul Kuliniewicz
    I have a set of Happstack.State MACID methods that I want to test using QuickCheck, but I'm having trouble figuring out the most elegant way to accomplish that. The problems I'm running into are: The only way to evaluate an Ev monad computation is in the IO monad via query or update. There's no way to create a purely in-memory MACID store; this is by design. Therefore, running things in the IO monad means there are temporary files to clean up after each test. There's no way to initialize a new MACID store except with the initialValue for the state; it can't be generated via Arbitrary unless I expose an access method that replaces the state wholesale. Working around all of the above means writing methods that only use features of MonadReader or MonadState (and running the test inside Reader or State instead of Ev. This means forgoing the use of getRandom or getEventClockTime and the like inside the method definitions. The only options I can see are: Run the methods in a throw-away on-disk MACID store, cleaning up after each test and settling for starting from initialValue each time. Write the methods to have most of the code run in a MonadReader or MonadState (which is more easily testable), and rely on a small amount of non-QuickCheck-able glue around it that calls getRandom or getEventClockTime as necessary. Is there a better solution that I'm overlooking?

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  • Use QuickCheck by generating primes

    - by Dan
    Background For fun, I'm trying to write a property for quick-check that can test the basic idea behind cryptography with RSA. Choose two distinct primes, p and q. Let N = p*q e is some number relatively prime to (p-1)(q-1) (in practice, e is usually 3 for fast encoding) d is the modular inverse of e modulo (p-1)(q-1) For all x such that 1 < x < N, it is always true that (x^e)^d = x modulo N In other words, x is the "message", raising it to the eth power mod N is the act of "encoding" the message, and raising the encoded message to the dth power mod N is the act of "decoding" it. (The property is also trivially true for x = 1, a case which is its own encryption) Code Here are the methods I have coded up so far: import Test.QuickCheck -- modular exponentiation modExp :: Integral a => a -> a -> a -> a modExp y z n = modExp' (y `mod` n) z `mod` n where modExp' y z | z == 0 = 1 | even z = modExp (y*y) (z `div` 2) n | odd z = (modExp (y*y) (z `div` 2) n) * y -- relatively prime rPrime :: Integral a => a -> a -> Bool rPrime a b = gcd a b == 1 -- multiplicative inverse (modular) mInverse :: Integral a => a -> a -> a mInverse 1 _ = 1 mInverse x y = (n * y + 1) `div` x where n = x - mInverse (y `mod` x) x -- just a quick way to test for primality n `divides` x = x `mod` n == 0 primes = 2:filter isPrime [3..] isPrime x = null . filter (`divides` x) $ takeWhile (\y -> y*y <= x) primes -- the property prop_rsa (p,q,x) = isPrime p && isPrime q && p /= q && x > 1 && x < n && rPrime e t ==> x == (x `powModN` e) `powModN` d where e = 3 n = p*q t = (p-1)*(q-1) d = mInverse e t a `powModN` b = modExp a b n (Thanks, google and random blog, for the implementation of modular multiplicative inverse) Question The problem should be obvious: there are way too many conditions on the property to make it at all usable. Trying to invoke quickCheck prop_rsa in ghci made my terminal hang. So I've poked around the QuickCheck manual a bit, and it says: Properties may take the form forAll <generator> $ \<pattern> -> <property> How do I make a <generator> for prime numbers? Or with the other constraints, so that quickCheck doesn't have to sift through a bunch of failed conditions? Any other general advice (especially regarding QuickCheck) is welcome.

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  • Weirdness with cabal, HTF, and HUnit assertions

    - by rampion
    So I'm trying to use HTF to run some HUnit-style assertions % cat tests/TestDemo.hs {-# OPTIONS_GHC -Wall -F -pgmF htfpp #-} module Main where import Test.Framework import Test.HUnit.Base ((@?=)) import System.Environment (getArgs) -- just run some tests main :: IO () main = getArgs >>= flip runTestWithArgs Main.allHTFTests -- all these tests should fail test_fail_int1 :: Assertion test_fail_int1 = (0::Int) @?= (1::Int) test_fail_bool1 :: Assertion test_fail_bool1 = True @?= False test_fail_string1 :: Assertion test_fail_string1 = "0" @?= "1" test_fail_int2 :: Assertion test_fail_int2 = [0::Int] @?= [1::Int] test_fail_string2 :: Assertion test_fail_string2 = "true" @?= "false" test_fail_bool2 :: Assertion test_fail_bool2 = [True] @?= [False] And when I use ghc --make, it seems to work correctly. % ghc --make tests/TestDemo.hs [1 of 1] Compiling Main ( tests/TestDemo.hs, tests/TestDemo.o ) Linking tests/TestDemo ... % tests/TestDemoA ... * Tests: 6 * Passed: 0 * Failures: 6 * Errors: 0 Failures: * Main:fail_int1 (tests/TestDemo.hs:9) * Main:fail_bool1 (tests/TestDemo.hs:12) * Main:fail_string1 (tests/TestDemo.hs:15) * Main:fail_int2 (tests/TestDemo.hs:19) * Main:fail_string2 (tests/TestDemo.hs:22) * Main:fail_bool2 (tests/TestDemo.hs:25) But when I use cabal to build it, not all the tests that should fail, fail. % cat Demo.cabal ... executable test-demo build-depends: base >= 4, HUnit, HTF main-is: TestDemo.hs hs-source-dirs: tests % cabal configure Resolving dependencies... Configuring Demo-0.0.0... % cabal build Preprocessing executables for Demo-0.0.0... Building Demo-0.0.0... [1 of 1] Compiling Main ( tests/TestDemo.hs, dist/build/test-demo/test-demo-tmp/Main.o ) Linking dist/build/test-demo/test-demo ... % dist/build/test-demo/test-demo ... * Tests: 6 * Passed: 3 * Failures: 3 * Errors: 0 Failures: * Main:fail_int2 (tests/TestDemo.hs:23) * Main:fail_string2 (tests/TestDemo.hs:26) * Main:fail_bool2 (tests/TestDemo.hs:29) What's going wrong and how can I fix it?

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  • Binding type variables that only occur in assertions

    - by Giuseppe Maggiore
    Hi! I find it extremely difficult to describe my problem, so here goes nothing: I have a bunch of assertions on the type of a function. These assertions rely on a type variable that is not used for any parameter of the function, but is only used for internal bindings. Whenever I use this function it does not compile because, of course, the compiler has no information from which to guess what type to bind my type variable. Here is the code: {-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies, FlexibleInstances, UndecidableInstances, FlexibleContexts, EmptyDataDecls, ScopedTypeVariables, TypeOperators, TypeSynonymInstances #-} class C a a' where convert :: a -> a' class F a b where apply :: a -> b class S s a where select :: s -> a data CInt = CInt Int instance S (Int,String) Int where select (i,_) = i instance F Int CInt where apply = CInt f :: forall s a b . (S s a, F a b) => s -> b f s = let v = select s :: a y = apply v :: b in y x :: Int x = f (10,"Pippo") And here is the generated error: FunctorsProblems.hs:21:4: No instances for (F a Int, S (t, [Char]) a) arising from a use of `f' at FunctorsProblems.hs:21:4-17 Possible fix: add an instance declaration for (F a Int, S (t, [Char]) a) In the expression: f (10, "Pippo") In the definition of `x': x = f (10, "Pippo") Failed, modules loaded: none. Prelude>

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  • When should I use $ (and can it always be replaced with parentheses)?

    - by J Cooper
    From what I'm reading, $ is described as "applies a function to its arguments." However, it doesn't seem to work quite like (apply ...) in Lisp, because it's a binary operator, so really the only thing it looks like it does is help to avoid parentheses sometimes, like foo $ bar quux instead of foo (bar quux). Am I understanding it right? Is the latter form considered "bad style"?

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  • Why this Either-monad code does not type check?

    - by pf_miles
    instance Monad (Either a) where return = Left fail = Right Left x >>= f = f x Right x >>= _ = Right x this code frag in 'baby.hs' caused the horrible compilation error: Prelude> :l baby [1 of 1] Compiling Main ( baby.hs, interpreted ) baby.hs:2:18: Couldn't match expected type `a1' against inferred type `a' `a1' is a rigid type variable bound by the type signature for `return' at <no location info> `a' is a rigid type variable bound by the instance declaration at baby.hs:1:23 In the expression: Left In the definition of `return': return = Left In the instance declaration for `Monad (Either a)' baby.hs:3:16: Couldn't match expected type `[Char]' against inferred type `a1' `a1' is a rigid type variable bound by the type signature for `fail' at <no location info> Expected type: String Inferred type: a1 In the expression: Right In the definition of `fail': fail = Right baby.hs:4:26: Couldn't match expected type `a1' against inferred type `a' `a1' is a rigid type variable bound by the type signature for `>>=' at <no location info> `a' is a rigid type variable bound by the instance declaration at baby.hs:1:23 In the first argument of `f', namely `x' In the expression: f x In the definition of `>>=': Left x >>= f = f x baby.hs:5:31: Couldn't match expected type `b' against inferred type `a' `b' is a rigid type variable bound by the type signature for `>>=' at <no location info> `a' is a rigid type variable bound by the instance declaration at baby.hs:1:23 In the first argument of `Right', namely `x' In the expression: Right x In the definition of `>>=': Right x >>= _ = Right x Failed, modules loaded: none. why this happen? and how could I make this code compile ? thanks for any help~

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