haskell tutorial

Source Address: Haskell Learning-functorWhat is functor functor is an object that can perform map operations, and functor is like an expression with a semantic appended to it, which can be likened to a box. The definition of functor can be interpreted like this: given a mapping of A to B function and a box, the result will return to the box loaded with B. Fmap can be thought of as a function that accepts functions and a functor , which apply function

Today, let's meet Yesod-bin.Take the following installation steps from a clean environment, for example in Windows Install Haskell-platform (you can manually install cabal and GHC) to update cabal. Cabal Update updates caball-install. Cabal Install–global Cabal-install Remember to take the global parameter, otherwise your update is only in the current cabal environment, and there is nothing luan to switch to a directory with

Y-conbinator "Is there any use"? No, in most of the languages that support functional programming, you are free to use recursion, and the goods are scattered in the flesh and blood of functional programming as theoretical cornerstones. This is a math note, which is a math note, which is a math note, and the computer has no relationship with half a dime, Important words to say three times (fled 1.λ-calculus Turing's life opened a lot of brain holes, of which λ-calculus is one of them. (Turing sai

Talk about my own background, I am a dabbler programmer, do anything like to know more and try again, I have been exposed to many languages, most of them as tools to useMost of my work now lies in data mining and machine learning, and web development, and the first language I've worked with is Python, where I learned about the concept of functional programming.Speaking of Lisp is said to be a copy of Lisp Python, and learned that the language is from the hacker and painter This book, is said to

Data ListLike an array in C, you can only save data of the same type.Two basic operations + + and:, + + for connecting two lists of course two list values must be of the same type: cons, used to construct the list, the first element must be a value, cannot be a list StringYou can see that the top and bottom results are identical, "denotes characters," "denotes strings, and C uses a pointer array to represent strings, and Haskell's string is a list of characters, so all operations of the

In Haskell, functions are generally performed using prefixes. But there is a way to change the prefix to infix.All we need to do is wrap the function together, and we can turn a function with 2 parameters into infix form.Prelude> "A" ' Notelem ' ["B"] true prelude> Notelem "a" ["B"] Truefunction infix, not only can have 2 parameters of the function into infix form, can also be more than 2 functions into infix form.But it looks pretty ugly.Prelude> FOL

I accidentally found this black hole question in the blog Park, which is suitable for Haskell practice. I tried to implement it with Haskell. Original question description: Sort the four numbers in a four-digit manner into a new number and a new number in a large to small arrangement. Then, the two numbers are subtracted and repeat this step, as long as the four digits in four digits are not repeated, t

In fact, this difference is just like the difference between a common function and a higher-order function. Isn't that easy to understand? Well, if you say you don't know what a high-order function is, don't read this article. Let's take a look at how I compared them. Let's take a look at how EQ is defined in Haskell. I call it "normal typeclasses" (to distinguish the functor typeclasses, I call it P ), here we define a typeclasses and define an acti

Prismsdata NewTask = SimpleTask String | HarderTask String Int | CompoundTask String [NewTask] deriving (Show)makePrisms ''NewTask*Main> a ^? _SimpleTaskJust "Clean"*Main> b ^? _HarderTaskJust ("Clean Kitchen",15)*Main> b ^? _HarderTask._2Just 15*Main> b _SimpleTask .~ "Clean Garage"HarderTask "Clean Kitchen" 15*Main> b _HarderTask._2 .~ 30HarderTask "Clean Kitchen" 30Manual calculation_Left = prism Left $ either Right (Left . Right)prism bt seta = dimap seta (either pure (fmap bt)) . righ

Manual calculation of Set mapped 5 [[+]set l b = runIdentity . l (\_ -> Identity b)mapped = sets fmapsets f g = taintedDot (f (untaintedDot g))instance Settable Identity where untainted = runIdentity untaintedDot = (runIdentity #.) taintedDot = (Identity #.)set mapped 5 [1,2,3]= runIdentity . (sets fmap) (\_ -> Identity 5) $ [1,2,3]= runIdentity $ sets fmap (\_ -> Identity 5) $ [1,2,3]= runIdentity $ taintedDot (fmap (untaintedDot (\_ -> Identity 5))) $ [1,2,3]= runIdentity $ (Identity .) (fm

Preview, reviewPrelude Control.Lens> view _Left (Left "abc")"abc"Prelude Control.Lens> view _Right (Right "abc")"abc"Prelude Control.Lens> view _Just (Just "abc")"abc"Prelude Control.Lens> preview _Left (Left "abc")Just "abc"Prelude Control.Lens> review _Left "abc"Left "abc" The preview and review functions handle either such and typesThe preview function walks up a branch.The review function goes down one branch. Preview, review version of the operatorPrelude Control.Lens> Left "a

Distributiveclass Functor g => Distributive g where distribute :: Functor f => f (g a) -> g (f a) distribute = collect id collect :: Functor f => (a -> g b) -> f a -> g (f b) collect f = distribute . fmap f distributeM :: Monad m => m (g a) -> g (m a) distributeM = fmap unwrapMonad . distribute . WrapMonad collectM :: Monad m => (a -> g b) -> m a -> g (m b) collectM f = distributeM . liftM fcotraverse :: (Distributive g, Functor f) => (f a -> b) -> f (g a) -> g bcotraverse f

Today I wrote a Haskell code, and found that let sometimes need to add in, sometimes do not need to add in. The main reason for this is the Do code block. Do {let And the rest of the time you need to use in. What do I do if I want let to use in the Do code block? Dolet x = in Foo x Indentation is required because the indentation of Haskell is meaningful. OK, and then say a question about let indentation. F

Because I have proofread and edited it multiple times, I can see myArticleThere is no excitement when I appear in a magazine. In addition, the content of my work has also changed recently.ProgramMember. I am much more busy than before, so I have little time to focus on development. However, I can regard programs as a hobby and a tool for daily work. I can use what I like more purely. Now, I often write one or two lines of shell or Python scripts to treat my computer as my own robot and do some

YesodYesod is a WEB framework that uses the Haskell language.Installing YesodFirst update Haskell Platform to the latest version(Yesod relies on a lot of libraries, the version is inconsistent, it is easy to install failure)$ cabal install yesodInstalled yesod-1.4.5Hello World-- helloworld.hs{-# LANGUAGE OverloadedStrings #-}{-# LANGUAGE QuasiQuotes #-}{-# LANGUAGE TemplateHaskell #-}{-#

Environment used: Windows Server 2008 + GHC 7.6.3 (Haskell Platform 2013.2.0.0 Self-band) + Pandoc 1.12.4 Operation Steps: 1. Install Haskell Platform, download address: http://www.haskell.org/platform/. 2. Install pandoc, install command: Cabal install Pandoc 3. Run ghci on the command line 4. Refer to the corresponding module of Pandoc and run at the Prelude command prompt: : module Text.pandoc T

0 reply content: No! First, the list of Python is different from the list of Haskell .. The latter is a linked list, and the former is a dynamic array .. Then the implementation of Haskell's list comprehension relies on converting to map/filter (fusion optimization is not considered), and finally implementing it through recursion; the implementation of list comprehension in Python depends on the _ iter _ method (that is, the meaning of the itera

{158Std::cout ' ';159Printlist (list()); the }161 162Template163 voidPrintlist (List)164 {165Std::cout V;166}(Sure enough, only the statically highlighted C + + code looks the same as Xiang)That's how it's used.1 listfoldleft0;, Add, Decltype (list) >:: Result xxx; 2 listscanleft3;, Add, Decltype (list) >:: Result yyy; 3 printlist (yyy); 4 printlist (XXX);FOLDL1,FOLDR,FOLDR1 and the corresponding SCANR ... Ah ah ah ah ah oh ah1 int Main () 2 {3 cout the whole person is feeling full of powe

Before understanding functor, it is necessary to understand the category theory of abstract algebra, the knowledge of category theory as the foreshadowing, the feeling functor is not so difficult to understand.A category C includes: an object-composed class Ob (C) objects. Each State shot F has a "source object" A and "target object" B, and both A and B are within OB (C). It is therefore written as f:a B, and the F is a state shot from a to B. A "state shot" of all A-t

Tags: blog HTTP for Io Div re 1 trizip: [a]-> [B]-> [c]-> [(a, B, c)] 2 trizip a B c 3 | null A = [] 4 | null B = [] 5 | null c = [] 6 trizip (X: XS) (Y: ys) (Z: ZS) = (++) [(x, y, z)] (trizip Xs ys ZS) [Email protected] ~ /Haskell $ ghcighci, version 7.6.3: http://www.haskell.org/ghc :? For helploading package ghc-prim... linking... done. loading package integer-GMP... linking... done. loading Package base... linking... done. prelude>: l trizip. H