Haskell非穷尽模式在功能上与“否则” [英] Haskell non-exhaustive patterns in function with `otherwise`

问题描述

我正在使用以下功能：

combinations :: Int -> [a] -> [[a]]
combinations k xs = combinations' (length xs) k xs
  where combinations' n k' l@(y:ys)
          | k' == 0   = [[]]
          | k' >= n   = [l]
          | null l    = []
          | otherwise = Prelude.map (y :) (combinations' (n - 1) (k' - 1) ys) ++ combinations' (n - 1) k' ys 

对于我能想到的任何示例它都可以正常工作，但是当我从较大项目中的其他函数调用它时，在某些情况下，我会得到例外：

It works just fine for just about any example i can come up with, but when i call it from other functions in my larger project, for some cases I get an exception:

Exception: projekt.hs:(34,9)-(38,108): Non-exhaustive patterns in function combinations'

以上定义是否有误？是否缺少某些情况？我以为否则可以处理不属于先前情况的任何事情。

Is there something wrong with the above definition? Is it missing some case? I thought otherwise handles anything that doesn't fall into previous cases.

推荐答案

由于 l'（@：ys）中的 l @（x：xs）您缺少此案组合_ _ [[] 。

后卫 null l 始终为 False 。

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