Haskell语言学习笔记（64）Lens（4）

Prisms

data NewTask =
  SimpleTask String |
  HarderTask String Int |
  CompoundTask String [NewTask]
  deriving (Show)

makePrisms ''NewTask
*Main> a ^? _SimpleTask
Just "Clean"
*Main> b ^? _HarderTask
Just ("Clean Kitchen",15)
*Main> b ^? _HarderTask._2
Just 15
*Main> b & _SimpleTask .~ "Clean Garage"
HarderTask "Clean Kitchen" 15
*Main> b & _HarderTask._2 .~ 30
HarderTask "Clean Kitchen" 30

手动计算

_Left = prism Left $ either Right (Left . Right)

prism bt seta = dimap seta (either pure (fmap bt)) . right'

data Market a b s t = Market (b -> t) (s -> Either t a)
instance Choice (Market a b) where
  right' (Market bt seta) = Market (Right . bt) $ \cs -> case cs of
    Left c -> Left (Left c)
    Right s -> case seta s of
      Left t -> Left (Right t)
      Right a -> Right a

preview l = getFirst . foldMapOf l (First . Just)

foldMapOf l f = getConst . l (Const . f)
Prelude Control.Lens> preview _Left (Left 5)
Just 5

参考链接

Haskell语言学习笔记（38）Lens（1）
Haskell语言学习笔记（44）Lens（2）
Haskell语言学习笔记（56）Lens（3）
Taking a Close look at Lenses

时间： 2024-10-27 19:08:56

Haskell语言学习笔记（64）Lens（4）的相关文章

Haskell语言学习笔记（69）Yesod

Yesod Yesod 是一个使用 Haskell 语言的 Web 框架. 安装 Yesod 首先更新 Haskell Platform 到最新版 (Yesod 依赖的库非常多,版本不一致的话很容易安装失败) $ cabal install yesod Installed yesod-1.4.5 Hello World -- helloworld.hs {-# LANGUAGE OverloadedStrings #-} {-# LANGUAGE QuasiQuotes #-} {-# LANG

Haskell语言学习笔记（44）Lens（2）

preview, review Prelude Control.Lens> view _Left (Left "abc") "abc" Prelude Control.Lens> view _Right (Right "abc") "abc" Prelude Control.Lens> view _Just (Just "abc") "abc" Prelude Cont

Haskell语言学习笔记（65）Lens（5）

手动计算 set mapped 5 [1,2,3] set l b = runIdentity . l (\_ -> Identity b) mapped = sets fmap sets f g = taintedDot (f (untaintedDot g)) instance Settable Identity where untainted = runIdentity untaintedDot = (runIdentity #.) taintedDot = (Identity #.) s

Haskell语言学习笔记（19）File IO

关于IO Action 类型为IO t. 运算时不执行,因而没有任何效果,只有执行时才会有效果,产生副作用. 一个IO Action只有在其他IO Action中才能被执行. 类型为IO t的IO Action被执行后的结果类型为t. File IO 函数 openFile hClose

Haskell语言学习笔记（49）ByteString Text

Data.ByteString String 是 [Char] 的同义词,在使用上存在List的惰性所带来的性能问题. 在处理大型二进制文件时,可以使用 ByteString 来代替 String. ByteString 包含 Lazy 模块 Data.ByteString.Lazy 和 Strict 模块 Data.ByteString. 其中 Lazy 模块内部使用 chunks(64K数据块). Prelude> import qualified Data.ByteString.Lazy

Haskell语言学习笔记（55）Data.Vector

Data.Vector Construction Prelude V> import Data.Vector as V Prelude V> V.empty [] Prelude V> V.singleton 1 [1] Prelude V> V.replicate 5 1 [1,1,1,1,1] Prelude V> generate 5 (+1) [1,2,3,4,5] Prelude V> iterateN 5 (*2) 1 [1,2,4,8,16]

Haskell语言学习笔记（57）Parsec（4）

Parser 类型 data ParsecT s u m a type Parsec s u = ParsecT s u Identity type Parser = Parsec String () data ParsecT s u m a ParsecT 带四个类型参数:数据流类型 s,用户状态类型 u,底层Monad类型 m,返回类型 a. ParsecT 是一个Monad转换器. type Parsec s u = ParsecT s u Identity Parsec 类型是 Pars

Haskell语言学习笔记（58）Bifoldable

Bifoldable class Bifoldable p where bifold :: Monoid m => p m m -> m bifold = bifoldMap id id bifoldMap :: Monoid m => (a -> m) -> (b -> m) -> p a b -> m bifoldMap f g = bifoldr (mappend . f) (mappend . g) mempty bifoldr :: (a ->

Haskell语言学习笔记（59）Bitraversable

Bitraversable class (Bifunctor t, Bifoldable t) => Bitraversable t where bitraverse :: Applicative f => (a -> f c) -> (b -> f d) -> t a b -> f (t c d) bitraverse f g = bisequenceA . bimap f g bisequenceA :: (Bitraversable t, Applicati