haskell

问题 I'm working on Project Euler Problem 14. Here's my solution. import Data.List collatzLength :: Int->Int collatzLength 1 = 1 collatzLength n | odd n = 1 + collatzLength (3 * n + 1) | even n = 1 + collatzLength (n `quot` 2) maxTuple :: (Int, Int)->(Int, Int)->Ordering maxTuple (x1, x2) (y1, y2) | x1 > y1 = GT | x1 < y1 = LT | otherwise = EQ I'm running the following out of GHCi maximumBy maxTuple [(collatzLength x, x) | x <- [1..1000000]] I know that if Haskell evaluated strictly, the time on

问题 If I have a function f :: State Int () , is it possible to use it within another function g :: StateT Int IO () ? Nesting it with f = do { something; g } fails to typecheck with Couldn't match type 'Data.Functor.Identity.Identity' with 'IO' . 回答1: Yes, this operation is usually called "hoisting". For the State monad, it could be defined as hoistState :: Monad m => State s a -> StateT s m a hoistState = state . runState Unfortunately, it is not defined in the Control.Monad.State module. 来源：

问题 I am starting to test Haskell for linear algebra. Does anyone have any recommendations for the best package for this purpose? Any other good resources for doing basic matrix manipulation with Haskell? The haskell wiki lists several resources for this. My current focus in on hmatrix and bindings-gsl, both of which look promising. 回答1: The hmatrix and hmatrix-static libraries are excellent. Hunt around on Hackage some more: http://hackage.haskell.org/package/vect 来源： https://stackoverflow.com

问题 I am starting to test Haskell for linear algebra. Does anyone have any recommendations for the best package for this purpose? Any other good resources for doing basic matrix manipulation with Haskell? The haskell wiki lists several resources for this. My current focus in on hmatrix and bindings-gsl, both of which look promising. 回答1: The hmatrix and hmatrix-static libraries are excellent. Hunt around on Hackage some more: http://hackage.haskell.org/package/vect 来源： https://stackoverflow.com

问题 the type of fmap in Functor is: fmap :: Functor f => (a -> b) -> f a -> f b it looks like ,first apply function (a -> b) to the parameter of f a to create a result of type b, then apply f to it, and result is f b using Maybe a for example : fmap show (Just 1) result is : Just "1" same as saying: Just (show 1) but when (->) is used as a Functor (in Control.Monad.Instances) import Control.Monad.Instances (fmap show Just) 1 result is : "Just 1" that is, Just is apply first, then show is applied.

问题 the type of fmap in Functor is: fmap :: Functor f => (a -> b) -> f a -> f b it looks like ,first apply function (a -> b) to the parameter of f a to create a result of type b, then apply f to it, and result is f b using Maybe a for example : fmap show (Just 1) result is : Just "1" same as saying: Just (show 1) but when (->) is used as a Functor (in Control.Monad.Instances) import Control.Monad.Instances (fmap show Just) 1 result is : "Just 1" that is, Just is apply first, then show is applied.

问题 For example, I need a function: gather :: Int -> [a] -> [[a]] gather n list = ??? where gather 3 "Hello!" == ["Hel","ell","llo","ol!"] . I have a working implementation: gather :: Int-> [a] -> [[a]] gather n list = unfoldr (\x -> if fst x + n > length (snd x) then Nothing else Just (take n (drop (fst x) (snd x)), (fst x + 1, snd x))) (0, list) but I am wondering if there is something already built into the language for this? I scanned Data.List but didn't see anything. 回答1: You could use

问题 If I have defined a datatype with, let's say, 5 attributes. How would I be able to create a list of one of the attributes. Ex : data Person = Person name fname sexe age height Marie, John, Jessie :: Person Marie = Person "Marie" _ _ _ John = Person "John" _ _ _ Jessie = Person "Jessie" _ _ _ How can I return a list containing all the names : ( Marie , John , Jessie ) 回答1: Your code isn't valid Haskell. You could have data Person = Person FName LName Sexe Age Height type FName = String type

问题 Writing a Oberon-like language parser, I'm having troubles compiling the parser after I've updated it to be able to define more procedures on the same level, and not only nested one inside the other. This is my lexer: { module Lexer where } %wrapper "basic" $alpha = [a-zA-Z] $digit = [0-9] $validChar = [^\"] tokens :- $white+ ; "PROCEDURE" { \s -> KW_TokenProcedure } "END" { \s -> KW_TokenEnd } ";" { \s -> KW_TokenSemiColon } $alpha [$alpha $digit \_]* { \s -> TokenVariableIdentifier s } { --

问题 Stack has many threads on overlapping instances, and while these are helpful in explaining the source of the problem, I am still not clear as to how to redesign my code for the problem to go away. While I will certain invest more time and effort in going through the details of existing answers, I will post here the general pattern which I have identified as creating the problem, in the hope that a simple and generic answer exists: I typically find myself defining a class such as: {-# LANGUAGE