Haskell 求无穷流的函子
我关注的是F-代数 它解释说 终端coalgebra在编程中通常被解释为一个配方 用于生成(可能无限)数据结构或转换 系统 他说 余代数的一个典型例子是基于一个函子,其 point是e类型元素的无限流。这是 函子: 这是它的固定点:Haskell 求无穷流的函子,haskell,functor,recursive-datastructures,Haskell,Functor,Recursive Datastructures,我关注的是F-代数 它解释说 终端coalgebra在编程中通常被解释为一个配方 用于生成(可能无限)数据结构或转换 系统 他说 余代数的一个典型例子是基于一个函子,其 point是e类型元素的无限流。这是 函子: 这是它的固定点: data Stream e = Stream e (Stream e) 我试过这个密码 有关部分 newtype Fix f = Fix (f (Fix f)) unFix :: Fix f -> f (Fix f) unFix (Fix x) = x c
data Stream e = Stream e (Stream e)
我试过这个密码
有关部分
newtype Fix f = Fix (f (Fix f))
unFix :: Fix f -> f (Fix f)
unFix (Fix x) = x
cata :: Functor f => (f a -> a) -> Fix f -> a
cata alg = alg . fmap (cata alg) . unFix
ana :: Functor f => (a -> f a) -> a -> Fix f
ana coalg = Fix . fmap (ana coalg) . coalg
data StreamF e a = StreamF e a
deriving Functor
data Stream e = Stream e (Stream e)
era :: [Int] -> StreamF Int [Int]
era (p : ns) = StreamF p (filter (notdiv p) ns)
where notdiv p n = n `mod` p /= 0
primes = ana era [2..]
我犯了这个错误
main.hs:42:14: error:
• Can’t make a derived instance of ‘Functor (StreamF e)’:
You need DeriveFunctor to derive an instance for this class
• In the data declaration for ‘StreamF’
我错在哪里了?
在Haskell中,如果不使用语言扩展,派生
非常有限。由于编译器不能始终计算出函子
实例应该是什么,因此派生函子不是标准的Haskell
但是,有一个语言扩展允许这样做,即-XDeriveFunctor
。要启用此扩展,请执行以下操作之一:
- 使用标志
编译。(例如:编译时运行-XDeriveFunctor
)ghc-XDeriveFunctor Main.hs
- 在文件顶部编写pragma
{-#LANGUAGE DeriveFunctor#-}
{-# LANGUAGE DeriveFunctor #-}
newtype Fix f = Fix (f (Fix f))
unFix :: Fix f -> f (Fix f)
unFix (Fix x) = x
cata :: Functor f => (f a -> a) -> Fix f -> a
cata alg = alg . fmap (cata alg) . unFix
ana :: Functor f => (a -> f a) -> a -> Fix f
ana coalg = Fix . fmap (ana coalg) . coalg
data StreamF e a = StreamF e a
deriving Functor
data Stream e = Stream e (Stream e)
era :: [Int] -> StreamF Int [Int]
era (p : ns) = StreamF p (filter (notdiv p) ns)
where notdiv p n = n `mod` p /= 0
primes = ana era [2..]
如果您计划使用GHCi,请在加载文件之前使用:set-XDeriveFunctor
。在vanilla Haskell中不能使用派生(Functor)
。您需要在文件顶部使用pragma{-#LANGUAGE DeriveFunctor#-}
。更多信息。相反,编译器可以派生一个函子
实例(这样做很简单),在Haskell 2010中不需要这样做。
{-# LANGUAGE DeriveFunctor #-}
newtype Fix f = Fix (f (Fix f))
unFix :: Fix f -> f (Fix f)
unFix (Fix x) = x
cata :: Functor f => (f a -> a) -> Fix f -> a
cata alg = alg . fmap (cata alg) . unFix
ana :: Functor f => (a -> f a) -> a -> Fix f
ana coalg = Fix . fmap (ana coalg) . coalg
data StreamF e a = StreamF e a
deriving Functor
data Stream e = Stream e (Stream e)
era :: [Int] -> StreamF Int [Int]
era (p : ns) = StreamF p (filter (notdiv p) ns)
where notdiv p n = n `mod` p /= 0
primes = ana era [2..]