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  • The reason for MonadState get and put?

    - by CiscoIPPhone
    I'm reading the Monads chapter in Real World Haskell (chapter 14). A function is defined as follows: type RandomState a = State StdGen a getRandom :: Random a => RandomState a getRandom = get >>= \gen -> let (val, gen')= random gen in put gen' >> return val I don't really understand the purpose of the get and put functions here. I rewrote the function as following which seems to do the same thing and is more concise: getRandom2 :: Random a => RandomState a getRandom2= State $ \ s -> random s So my question is: What is the purpose of get and put?

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  • Consing lists with user-defined type in Haskell

    - by user1319603
    I have this type I defined myself: data Item = Book String String String Int -- Title, Author, Year, Qty | Movie String String String Int -- Title, Director, Year, Qty | CD String String String Int deriving Show -- Title, Artist, Year, Qty I've created an empty list all_Items = [] With the following function I am trying to insert a new book of type Item (Book) into the all_Items addBook all_Items = do putStrLn "Enter the title of the book" tit <- getLine putStrLn "Enter the author of the book" aut <- getLine putStrLn "Enter the year this book was published" yr <- getLine putStrLn "Enter quantity of copies for this item in the inventory" qty <- getLine Book tit aut yr (read qty::Int):all_Items return(all_Items) I however am receiving this error: Couldn't match expected type `IO a0' with actual type `[a1]' The error points to the line where I am using the consing operator to add the new book to the list. I can gather that it is a type error but I can't figure out what it is that I am doing wrong and how to fix it. Thanks in Advance!

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  • Haskell optimization of the following function

    - by me2
    Profiling of some code of mine showed that about 65% of the time I was running the following code. What it does is use the Data.Binary.Get monad to walk through a bytestring looking for the terminator. If it detects 0xff, it checks if the next byte is 0x00. If it is, it drops the 0x00 and continues. If it is not 0x00, then it drops both bytes and the resulting list of bytes is converted to a bytestring and returned. Any obvious ways to optimize this code? I can't see it. parseECS = f [] False where f acc ff = do b <- getWord8 if ff then if b == 0x00 then f (0xff:acc) False else return $ L.pack (reverse acc) else if b == 0xff then f acc True else f (b:acc) False

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  • Haskell: Pattern Matching with Lists

    - by user1670032
    I'm trying to make a function that takes in a list, and if one of the elements is negative, then any elements in that list that are equal to its positive counterpart should be changed to 0. Eg, if there is a -2 in a list, then all 2's in that list should be changed to 0. Any ideas why it only works for some cases and not others? I'm not understanding why this is, I've looked it over several times. changeToZero [] = [] changeToZero [x] = [x] changeToZero (x:zs:y:ws) | (x < 0) && ((-1)*(x) == y) = x : zs : 0 : changeToZero ws changeToZero (x:xs) = x : changeToZero xs *Main changeToZero [-1,1,-2,2,-3,3] [-1,1,-2,2,-3,3] *Main changeToZero [-2,1,2,3] [-2,1,0,3] *Main changeToZero [-2,1,2,3,2] [-2,1,0,3,2] *Main changeToZero [1,-2,2,2,1] [1,-2,2,0,1]

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  • Haskell maps returning a monad

    - by sabauma
    The lookup function in Data.Map and Data.IntMap currently return values wrapped in Maybe with the type signature lookup :: Ord k => k -> Map k a -> Maybe a It used to have the more general type of lookup :: (Monad m, Ord k) => k -> Map k a -> m a I realize the former likely reduces the need of extra type specification, but the latter would make it much more general and allow lookup to be used in list comprehensions. Is there any way to mimic this behavior with the newer version, or would I have to use an older version of the library?

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  • Manipulating source packages from Hackage how to easy deploy to several windowsboxes?

    - by Jonke
    Recently when I have found good sources packages for ghc 6.12/6.10 on Hackage I've been forced to do some minor or major changes to the cabal files to make those packages to work under windows. Besides to fork and merge my fixes with github, what seems to be the best way/ good enough practice to take these modified builds to a couple of other windows boxes that only has a basic haskell platform installed? I should prefer if I somehow could work with the cabal-install because that is what one normally use. Should one put the modfied build dirs on a shared/networked dir and mount from the targeted windows box? Say something like this: on machine prepare cabal fetch foo cabal unpack foo cd foo edit .cabal and .hs cabal configure cabal build On machine useanddevelopnormal cd machinepreparemount cd foo cabal install

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  • Creating a Haskell Empty Set

    - by mvid
    I am attempting to pass back a Node type from this function, but I get the error that empty is out of scope: import Data.Set (Set) import qualified Data.Set as Set data Node = Vertex String (Set Node) deriving Show toNode :: String -> Node toNode x = Vertex x empty What am I doing wrong?

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  • Haskell type classes and type families (cont'd)

    - by Giuseppe Maggiore
    I need some help in figuring a compiler error which is really driving me nuts... I have the following type class: infixl 7 --> class Selectable a s b where type Res a s b :: * (-->) :: (CNum n) => (Reference s a) -> (n,(a->b),(a->b->a)) -> Res a s b which I instance twice. First time goes like a charm: instance Selectable a s b where type Res a s b = Reference s b (-->) (Reference get set) (_,read,write) = (Reference (\s -> let (v,s') = get s in (read v,s')) (\s -> \x -> let (v,s') = get s v' = write v x (_,s'') = set s' v' in (x,s''))) since the type checker infers (-->) :: Reference s a -> (n,a->b,a->b->a) -> Reference s b and this signature matches with the class signature for (--) since Res a s b = Reference s b Now I add a second instance and everything breaks: instance (Recursive a, Rec a ~ reca) => Selectable a s (Method reca b c) where type Res a s (Method reca b c) = b -> Reference s c (-->) (Reference get set) (_,read,write) = \(x :: b) -> from_constant( Constant(\(s :: s)-> let (v,s') = get s :: (a,s) m = read v ry = m x :: Reference (reca) c (y,v') = getter ry (cons v) :: (c,reca) v'' = elim v' (_,s'') = set s' v'' in (y,s''))) :: Reference s c the compiler complains that Couldn't match expected type `Res a s (Method reca b c)' against inferred type `b -> Reference s c' The lambda expression `\ (x :: b) -> ...' has one argument, which does not match its type In the expression: \ (x :: b) -> from_constant (Constant (\ (s :: s) -> let ... in ...)) :: Reference s c In the definition of `-->': --> (Reference get set) (_, read, write) = \ (x :: b) -> from_constant (Constant (\ (s :: s) -> ...)) :: Reference s c reading carefully the compiler is telling me that it has inferred the type of (--) thusly: (-->) :: Reference s a -> (n,a->(Method reca b c),a->(Method reca b c)->a) -> (b -> Reference s c) which is correct since Res a s (Method reca b c) = b -> Reference s c but why can't it match the two definitions? Sorry for not offering a more succint and standalone example, but in this case I cannot figure how to do it...

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  • Haskell Write Computation result to file

    - by peterwkc
    Hello to all, i have function which create a tuple after computation but i would like to write it to file. I know how to write file using writeFile but did not know how to combine computation and monads IO together in the type signature This is my code. invest :: ([Char]->Int->Int->([Char], Int) ) -> [Char]->Int->Int->([Char], Int) invest myinvest x y = myinvest x y myinvest :: [Char]->Int->Int->([Char], Int) myinvest w x y | y > 0 = (w, x + y) | otherwise = error "Invest amount must greater than zero" where I have a function which computes the maximum value from list but i want to these function receive input from file then perform the computation of maximum value. maximuminvest :: (Ord a) => [a] -> a maximuminvest [] = error "Empty Invest Amount List" maximuminvest [x] = x maximuminvest (x:xs) | x > maxTail = x | otherwise = maxTail where maxTail = maximuminvest xs Please help. Thanks.

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  • Haskell: writing the result of a computation to file

    - by peterwkc
    I have a function which creates a tuple after computation, but I would like to write it to file. I know how to write to a file using writeFile, but do not know how to combine the computation and monads IO together in the type signature. This is my code. invest :: ([Char]->Int->Int->([Char], Int) ) -> [Char]->Int->Int->([Char], Int) invest myinvest x y = myinvest x y myinvest :: [Char]->Int->Int->([Char], Int) myinvest w x y | y > 0 = (w, x + y) | otherwise = error "Invest amount must greater than zero" where I have a function which computes the maximum value from a list, but I want this function to receive input from a file, and then perform the computation of maximum value. maximuminvest :: (Ord a) => [a] -> a maximuminvest [] = error "Empty Invest Amount List" maximuminvest [x] = x maximuminvest (x:xs) | x > maxTail = x | otherwise = maxTail where maxTail = maximuminvest xs Please help. Thanks.

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  • Explain Type Classes in Haskell

    - by Tsubasa Gomamoto
    Hi, I am a C++ / Java programmer and the main paradigm I happen to use in everyday programming is OOP. In some thread I read a comment that Type classes are more intuitive in nature than OOP. Can someone explain the concept of type classes in simple words so that an OOP guy like me can understand it?

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  • Using an element against an entire list in Haskell

    - by Snick
    I have an assignment and am currently caught in one section of what I'm trying to do. Without going in to specific detail here is the basic layout: I'm given a data element, f, that holds four different types inside (each with their own purpose): data F = F Float Int, Int a function: func :: F -> F-> Q Which takes two data elements and (by simple calculations) returns a type that is now an updated version of one of the types in the first f. I now have an entire list of these elements and need to run the given function using one data element and return the type's value (not the data element). My first analysis was to use a foldl function: myfunc :: F -> [F] -> Q myfunc y [] = func y y -- func deals with the same data element calls myfunc y (x:xs) = foldl func y (x:xs) however I keep getting the same error: "Couldn't match expected type 'F' against inferred type 'Q'. In the first argument of 'foldl', namely 'myfunc' In the expression: foldl func y (x:xs) I apologise for such an abstract analysis on my problem but could anyone give me an idea as to what I should do? Should I even use a fold function or is there recursion I'm not thinking about?

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  • Haskell: Gluing a char and a list together?

    - by Vincent
    So I have this code here: toWords :: String - [a] toWords "" = [] toWords (nr1 : rest) | nr1 == ' ' = toWords rest | otherwise = nr1 : toWords rest The "toWords" function should simply remove all spaces and return a list with all the words. But I keep getting this error: test.hs:5:18: Couldn't match expected type a' against inferred typeChar' `a' is a rigid type variable bound by the type signature for `toWords' at test.hs:1:22 In the first argument of `(:)', namely `nr1' In the expression: nr1 : toWords rest In the definition of `toWords': toWords (nr1 : rest) | nr1 == ' ' = toWords rest | otherwise = nr1 : toWords rest Failed, modules loaded: none.

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  • List of divisors of an integer n (Haskell)

    - by Code-Guru
    I currently have the following function to get the divisors of an integer: -- All divisors of a number divisors :: Integer -> [Integer] divisors 1 = [1] divisors n = firstHalf ++ secondHalf where firstHalf = filter (divides n) (candidates n) secondHalf = filter (\d -> n `div` d /= d) (map (n `div`) (reverse firstHalf)) candidates n = takeWhile (\d -> d * d <= n) [1..n] I ended up adding the filter to secondHalf because a divisor was repeating when n is a square of a prime number. This seems like a very inefficient way to solve this problem. So I have two questions: How do I measure if this really is a bottle neck in my algorithm? And if it is, how do I go about finding a better way to avoid repetitions when n is a square of a prime?

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  • How and why is ap defined as liftM2 id in Haskell

    - by luke_randall
    Whilst trying to better understand Applicative, I looked at the definition of <*, which tends to be defined as ap, which in turn is defined as: ap :: (Monad m) => m (a -> b) -> m a -> m b ap = liftM2 id Looking at the type signatures for liftM2 and id, namely: liftM2 :: (Monad m) => (a1 -> a2 -> r) -> m a1 -> m a2 -> m r id :: a -> a I fail to understand how just by passing in id, the relevant part of the type signature seems to transform from (a1 -> a2 -> r) -> m a1 to m (a -> b). What am I missing here?

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  • Where can I learn advanced Haskell?

    - by FredOverflow
    In a comment to one of my answers, SO user sdcwc essentially pointed out that the following code: comb 0 = [[]] comb n = let rest = comb (n-1) in map ('0':) rest ++ map ('1':) rest could be replaced by: comb n = replicateM n "01" which had me completely stunned. Now I am looking for a tutorial, book or PDF that teaches these advanced concepts. I am not looking for a "what's a monad" tutorial aimed at beginners or online references explaining the type of replicateM. I want to learn how to think in monads and use them effectively, monadic "patterns" if you will.

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  • Haskell: Defaulting constraints to type

    - by yairchu
    Consider this example: applyKTimes :: Integral i => i -> (a -> a) -> a -> a applyKTimes 0 _ x = x applyKTimes k f x = applyKTimes (k-1) f (f x) applyThrice :: (a -> a) -> a -> a applyThrice = applyKTimes 3 The 3 in applyThrice is defaulted by GHC to an Integer as shown when compiling with -Wall: Warning: Defaulting the following constraint(s) to type 'Integer' 'Integral t' arising from a use of 'applyKTimes' So I guess that Integer is the default Integral a => a. Is there a way to define "default types" for other constraints too? Is using default types bad practice? (it does complain when using -Wall..)

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  • Haskell map function with predicate

    - by Paul
    I feel like this should be fairly obvious, or easy, but I just can't get it. What I want to do is apply a function to a list (using map) but only if a condition is held. Imagine you only wanted to divide the numbers which were even: map (`div` 2) (even) [1,2,3,4] And that would give out [1,1,3,2] since only the even numbers would have the function applied to them. Obviously this doesn't work, but is there a way to make this work without having to write a seperate function that you can give to map? Filter is almost there, except I also want to keep the elements which the condition doesn't hold for, and just not apply the function to them. Thanks

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