Haskell语言学习笔记（19）File IO

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>

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

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

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

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

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]

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 ->

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

Biapplicative class Bifunctor p => Biapplicative p where bipure :: a -> b -> p a b (<<*>>) :: p (a -> b) (c -> d) -> p a c -> p b d (*>>) :: p a b -> p c d -> p c d a *>> b = bimap (const id) (const id) &