Category class Category cat where id :: cat a a (.) :: cat b c -> cat a b -> cat a c instance Category (->) where id = GHC.Base.id (.) = (GHC.Base..) (<<<) :: Category cat => cat b c -> cat a b -> cat a c (<<<) = (.) (&…
GADTs GADTs(Generalised Algebraic Data Types,广义代数数据类型)是对代数数据类型的一种扩展. 它允许在定义数据类型时明确指定类型参数的类型并使用模式匹配. --ADT data Maybe a = Nothing | Just a --GADT data Maybe a where Nothing :: Maybe a Just :: a -> Maybe a 实例 {-#LANGUAGE GADTs #-} data Expr a where I :…
安装 free 包 $ cabal install free Installed free-5.0.2 Free Monad data Free f a = Pure a | Free (f (Free f a)) instance Functor f => Functor (Free f) where fmap f = go where go (Pure a) = Pure (f a) go (Free fa) = Free (go <$> fa) instance Functor f…
自定义 Lens 和 Isos -- Some of the examples in this chapter require a few GHC extensions: -- TemplateHaskell is needed for makeLenses; RankNTypes is needed for -- a few type signatures later on. {-# LANGUAGE TemplateHaskell, RankNTypes #-} import Control…
