如何以递归方式调用haskell中的子序列

Question

我试图制作调用子序列的递归函数,但我遇到了一些错误.

我的代码:

recursive 1 list = subsequences list
recursive n list = subsequences (recursive (n-1) list)

错误:

Occurs check: cannot construct the infinite type: a1 ~ [a1]
    Expected type: [a1]
      Actual type: [[a1]]

    Relevant bindings include
      recursive :: a -> t -> [[a1]] (bound at p.hs:6:1)
    In the first argument of ‘subsequences’, namely
      ‘(recursive (n - 1) list)’
    In the expression: subsequences (recursive (n - 1) list)

你能帮我解决一下这个问题,还是找另一种方法来调用子序列n次？

对不起,我的英语不好

Answer 1

我没有太多的多态递归,所以我想试试这个.这是我得到的:

{-# LANGUAGE DeriveFunctor #-}

import Data.List (subsequences)

-- Any multiply-nested list such as a, [a], [[a]], [[[a]]], ...
data MultiList a
  = Leaf a
  | Nest (MultiList [a])
  deriving (Show, Functor)

recursive :: Int -> [a] -> MultiList [[a]]
recursive 1 list = Leaf (subsequences list)
recursive n list = Nest (fmap subsequences (recursive (n-1) list))