为什么这些表达有不同程度的歧义？

Question

我正在写这个函数：

||| Returns the ten largest values in the list.
top_ten : Ord a => List a -> List a

我的第一次尝试是使用函数组合实现pointfree：

top_ten = take 10 . reverse . sort

但这给出了以下错误：

Main.idr:3:9:When checking right hand side of top_ten with expected type
        List a -> List a

Can't disambiguate name: Prelude.List.take, Prelude.Stream.take

我的第二次尝试是一个简单直接的实现：

top_ten xs = take 10 (reverse (sort xs))

这行得通，就像这些：

top_ten xs = take 10 $ reverse $ sort xs
top_ten xs = take 10 (reverse $ sort xs)
top_ten xs = take 10 $ reverse . sort $ xs
top_ten xs = take 10 (reverse . sort $ xs)

但是，这些不是：

top_ten xs = take 10 . reverse $ sort xs
top_ten xs = take 10 . reverse . sort $ xs
top_ten xs = take 10 $ (reverse . sort) xs
top_ten xs = (take 10 . reverse) (sort xs)
top_ten xs = take 10 ((reverse . sort) xs)

这里到底发生了什么？ 是什么导致这些等价表达式具有不同程度的歧义？

Answer 1

根据您在范围内的导入（或前奏曲中的函数），Idris 无法选择正确的函数，正如 András Kovács 在他的评论中所说的那样。

不过，您可以帮助 idris：

top_ten: Ord a => List a -> List a
top_ten = with Prelude.List take 10 . reverse . sort