Haskell 字符串旋转 Function

Question

I need to write a Haskell function that that takes a String , s , and an Integer , i , then performs i rotations right on s .

rotate:: String -> Integer -> String

I have been successful in getting it to work with String -> Int -> String but I am being asked to do it with Integer

This is what I have implemented with Int :

rotate str n = take (length str) $ drop (negate n `mod` length str) $ cycle str

Answer 1

The problem is that drop:: Int -> [a] -> [a] takes an Int as type parameter. Furthermore length:: Foldable f => fa -> Int for example produces an Int . So that will not work.

There are however generic variants that work with Integral types or Num types, for example genericDrop:: Integral i => i -> [a] -> [a] and genericLength:: Num i => [a] -> i :

import Data.List(genericDrop, genericLength, genericTake)

rotate :: Integral i => [a] -> i -> [a]
rotate str n = genericTake l (genericDrop ((-n) `mod` l) (cycle str))
    where l = genericLength str