haskell

问题 My site is built in Haskell/Servant and Wai/Warp. I need to redirect all requests from my domain.com to www.domain.com with the the 301 or 302 status. I know I can do that with the help of Wai/Warp somehow. How exactly? startApp :: IO () startApp = run 1234 app 回答1: The package wai-util has a convenience function redirect' to create such a Response , so you should be able to do something like app :: Application app req respond = respond =<< redirect' status302 [] uri where Just uri = parseURI

问题 When I run $ cabal install OpenGLRaw I get the following errors. ... ... ... cbits/HsOpenGLRaw.c:78:20: error: stdlib.h: No such file or directory cbits/HsOpenGLRaw.c:79:19: error: dlfcn.h: No such file or directory cbits/HsOpenGLRaw.c: In function ‘hs_OpenGLRaw_getProcAddress’: cbits/HsOpenGLRaw.c:97:0: error: ‘NULL’ undeclared (first use in this function) cbits/HsOpenGLRaw.c:97:0: error: (Each undeclared identifier is reported only once cbits/HsOpenGLRaw.c:97:0: error: for each function it

问题 to calculate the value of the expression on the fly at the production rules in happy doesn't work if I'm using the lambda expressions. For example this code Exp : let var '=' Exp in Exp { \p -> $6 (($2,$4 p):p) } | Exp1 { $1 } Exp1 : Exp1 '+' Term { \p -> $1 p + $3 p } | Exp1 '-' Term { \p -> $1 p - $3 p } | Term { $1 } Term : Term '*' Factor { \p -> $1 p * $3 p } | Term '/' Factor { \p -> $1 p `div` $3 p } | Factor { $1 } Factor : int { \p -> $1 } | var { \p -> case lookup $1 p of Nothing ->

问题 I have a list of strings as follows; ["75","95 64","17 47 82"] How can I convert this to a list of Ints; [75,95,64,17,47,82] My instinct is to use map and an anonymous function? 回答1: Haskell has a few useful functions read :: String -> Int -- Restricted for clarity map :: (a -> b) -> [a] -> [b] words :: String -> [String] concatMap :: (a -> [b]) -> [a] -> [b] So you give map a function and a list and it will apply your function to each element of the list. Now knowing this, you should be able

问题 I am still trying to grasp the way Haskell and Functional Programming works, and I need help understanding why my function is not working. I am trying to create a function that takes a list of integers as a parameter and filters out/returns a sublist which contains any multiples of 3 from the first list. Here is my code: module Main where sublist = [] myFunc :: [Int] -> [Int] myFunc [] = [] myFunc [t] = do if t `mod` 3 == 0 then t : sublist else myFunc [] myFunc (h:t) = do if h `mod` 3 /= 0

问题 Merged with How can I simplify a basic arithmetic expression?. how will i define the function 'simplify' using primitive recursion? simplify :: Expr -> Expr ... simplify Simplify an expression using basic arithmetic, e.g. simplify (Plus (Var "x") (Const 0)) = Var "x" 来源： https://stackoverflow.com/questions/324003/primitive-recursion

问题 all_nat x = [ls| sum ls == x] I'd like to write a function that given an integer x it returns all the lists that the result of their elements when summed is the integer x but I always get the error "not in scope: 'ls' " for both times it apperas. I'm new to haskell. What's the syntax error here? 回答1: The problem is that you need to define all used variables somewhere, but ls is undefined. Moreover, it can't be defined automatically, because the compiler doesn't know about the task — how the

问题 I've been using this page on the Haskell website all day and its been really helpful with learning list functions: http://www.haskell.org/haskellwiki/How_to_work_on_lists My task at the moment is to write a single line statement that returns the number of characters (a-Z) that are used in a string. I can't seem to find any help on the above page or anywhere else on the internet I know how to count characters in a string by using length nameoflist, but I'm not sure how I would go about

问题 I was playing around with parsing (PostgreSQL) logs which can have entries that are multi-line. 2016-01-01 01:01:01 entry1 2016-01-01 01:01:02 entry2a entry2b 2016-01-01 01:01:03 entry3 So - with a Perl or Python script I'd just grab the next line and if it wasn't starting with a timestamp append it to the previous log entry. What is a sensible way to approach this with attoparsec hooked up to io-streams ? I clearly want to do something with lookAhead and failing to match a timestamp but my

问题 I'm trying to define the Show function for the polymorphic Tree type. Could anyone help me? import Char data Tree t = NilT | Node t (Tree t) (Tree t) class Mar t where maior :: t -> String instance Mar Tree where maior (NilT) = "a" maior (Node t a b) = "b" instance Show Tree where show = maior Thanks a Lot! Solution (given by ivanm): import Char data Tree t = NilT | Node t (Tree t) (Tree t) class Mar t where maior :: t -> String instance Mar (Tree t) where maior (NilT) = "a" maior (Node t a b