小编Fre*_*y J的帖子

import qualified Prelude

class List list where
  toList :: list a -> [a]
  fromList :: [a] -> list a

data Deque a = Deque [a] [a]

instance List Deque where
  toList (Deque xs sy) = xs ++ reverse sy

  fromList xs = Deque ys (reverse zs)
    where (ys, zs) = splitAt (length xs `div` 2) xs

我收到如下复制的错误。GHCI 似乎没有检测ys为 Deque，而是检测为类 List 的一般实例。

ghci> xs = [2, 3, 4]
ghci> ys = fromList xs
ghci> (Deque a b) = ys …