haskell

问题 Why does the following fail to compile (on GHC 7.4.2)? {-# LANGUAGE TemplateHaskell #-} f1 = $([| id |]) main = print $ (f1 (42 :: Int), f1 (42 :: Integer)) Note that the following compiles fine: {-# LANGUAGE TemplateHaskell #-} f1 = id -- Don't use template Haskell here. main = print $ (f1 (42 :: Int), f1 (42 :: Integer)) Is there a language extension I can use to make the former compile? I know the Template Haskell seems silly in this example, but it's a simplified version of a more complex

问题 I tried to create a Nix-style local build (v2-build) project on Cabal 3.0. But several development tools (flycheck-haskell and doctest) don't work. They worked on new-build project on Cabal 2.4. Error message says they cannot find dependencies, as far as I read. $ cabal v2-clean $ cabal v2-build $ cabal v2-test Build profile: -w ghc-8.8.1 -O1 In order, the following will be built (use -v for more details): - hstest9-0.1.0.0 (test:doctestd) (first run) Configuring test suite 'doctestd' for

问题 We want to program the gauss-elimination to calculate a basis (linear algebra) as exercise for ourselves. It is not homework. I thought first of [[Int]] as structure for our matrix. I thought then that we can sort the lists lexicographically. But then we must calculate with the matrix. And there is the problem. Can someone give us some hints. 回答1: Consider using matrices from the hmatrix package. Among its modules you can find both a fast implementation of a matrix and a lot of linear algebra

问题 I'm learning about operator precedence in Haskell. Several places across the web mention that function application has higher precedence than operators, but I couldn't find a definitive source for that. Here is one such mention from A Gentle Introduction To Haskell: Function application has higher precedence than any infix operator There is a section in the Haskell 98 Report that alludes to it: normal constructor application has higher precedence than infix constructor application Where is a

问题 How does history management work in GHCI or other Haskell-based REPLs? Since Haskell is a pure language, I guess it's implemented using a monad, perhaps the state monad. Kindly note I'm a beginner in Haskell, so please provide a detailed explanation rather than just linking to the source. 回答1: This is a simplified example of how a program might keep a history of commands entered by the user. It basically has the same structure as the number guessing game, so once you understand that you

问题 How does history management work in GHCI or other Haskell-based REPLs? Since Haskell is a pure language, I guess it's implemented using a monad, perhaps the state monad. Kindly note I'm a beginner in Haskell, so please provide a detailed explanation rather than just linking to the source. 回答1: This is a simplified example of how a program might keep a history of commands entered by the user. It basically has the same structure as the number guessing game, so once you understand that you

问题 I've configured logging like so: import System.Environment import System.Log.Logger import System.Log.Handler (setFormatter) import System.Log.Handler.Simple (streamHandler) import System.Log.Formatter import System.IO (getLine, stdout) main = do stdOutHandler <- streamHandler stdout DEBUG >>= \lh -> return $ setFormatter lh (simpleLogFormatter "[$time : $loggername : $prio] $msg") updateGlobalLogger "Linker" (setLevel DEBUG . setHandlers [stdOutHandler]) infoM "Linker" "Hello world!"

问题 Toying a bit with the github gist API while trying to get down with the Aeson JSON library. I've run into a problem with the generated ToJSON instance, and I don't know exactly how to solve it. I need to contain a value inside and the key that is associated to the value also needs to be a value and not a predefined key name. It's a bit easier to show. The desired output is, { "public": true, "description": "Something..", "files": {"This Thing.md": {"content": "Here we go!"}} } where the value

问题 I want to write a function for calculating the average using the State Monad in haskell this is the code I wrote as far import Control.Monad.State type MyState = (Double,Double) media s (a,n)= ((a*n+s)/(n+1),n+1) getAverage:: Double ->State MyState s1-> Double getAverage s c=get >>= \s0 -> let (x,s1) =media s s0 in put s1 >> return x I got this error when compile in GHCI, and I stuck there can you help me to understand what is wrong, thank you in advance 回答1: The code you provided gives this

问题 I'm a beginner trying to learn more about Haskell and Aeson by parsing some json files I find online. I have a .json that looks like this "Abilities": { "Prime": { "Ammo": 210, "Available": true, "Diposition": 3, "Type": "Secondary", "Class": "Pistols", "NormalAttack": { "Chance": 0.25, "Streak": 2, "Rate": 2.67, "ShotType": "Hit-Scan", "Damage": { "Front": 15, "Back": 15, "Up": 120, "Down": 40 }, "Status": 0.25 } "Accuracy": 9.8, "Reload": 3, "Name": "Prime", "Magazine": 16, }, "Dual": {