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我在C#中实现了自己的Promise结构,并想在Haskell中测试这个概念,所以经过一些严重的大脑训练(对我来说还是很新的),我制作了

data Promise f a =  PendingPromise f | ResolvedPromise a | BrokenPromise deriving( Show )

class Future p where
        later :: (b -> c) -> p (a -> b) b -> p (a -> c) c
        resolve :: p (a -> b) a -> a -> p (a -> b) b

instance Future Promise where
        later g (PendingPromise f) = PendingPromise (g . f)
        later g (ResolvedPromise a) = ResolvedPromise (g a)
        resolve (PendingPromise f) a = ResolvedPromise (f …