Type inference for polymorphic recursion is undecidable, 这意味着即使一个 function 是 well-typed，编译器也无法推导出其类型

For example:

data Nested a = Cons a (Nested [a]) | Epsilon

length' Epsilon = 0
length' (Cons _ rest) = 1 + length' rest

会产生一个 type error，因为 length 被应用到了两个不同的类型上 Nested a and Nested [a].

The solution is to assert that the type is indeed Nested a -> Int.

length' :: Nested a -> Int
length' Epsilon = 0
length' (Cons _ rest) = 1 + length' rest

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https://stackoverflow.com/questions/40247339/polymorphic-recursion-syntax-and-uses