小编Coo*_*ity的帖子

我正在尝试制作一个包含多种可能类型的集合.这是我希望它看起来的一个例子:

type Position = Vector2
type Velocity = Vector2
type Appearance = String

type Component = Position | Velocity | Appearance

let components = List<Dictionary<string, Component>>()
let pos = Dictionary<string, Position>()

components.Add(pos) // Type "Position" does not match with type "Component"

我想声明仍然适合一般类型的特定类型.有没有办法可以像这样写我的代码？有没有比较惯用的方法呢？

我坚持使用foldr实现groupBy.出于某种原因,当我改变一个保护条件时,类型签名就会对我产生影响.

我可以编译这个,虽然它不正确:

groupBy' :: (a -> a -> Bool) -> [a] -> [[a]]

groupBy' f xs = foldr step [] xs
    where    step x [] = [x] : []
             step x (y:ys)
                 | True = []:(y:ys)
                 | otherwise = (x:y):ys

但这不编译:

groupBy' :: (a -> a -> Bool) -> [a] -> [[a]]

groupBy' f xs = foldr step [] xs
    where    step x [] = [x] : []
             step x (y:ys)
                 | f x y = []:(y:ys)
                 | otherwise = …