在使用高阶函数的参数中对haskell进行排序 [英] Sorting in haskell with parameter using higher order function

问题描述

你好，我是一名Haskell初学者，我真的迷失了。
这是我的任务，它要求我使用更高的顺序功能来做类似下面的事情：

$ $ p $ Main> mySort（<）[1,5,3,6,4,1,3,3,2]
[1,1,2,3,3,3,4,5,6]
Main> mySort（>）[1,5,3,6,4,1,3,3,2]
[6,5,4,3,3,3,2,1,1]
Main> mySort longerWord [Hello，The，a，Daniel，Declarative]
[Declarative，Daniel，Hello，The，a]


首先，我认为我应该创建一个函数来区分它是否是< ，>或moreWord

  checkConditionStr :: String-> Int 
 checkConditionStr str 
 | str == （<）= 1 
 | str ==（>）= 2 
 | str ==longerWord= 3 
  pre> 
 
 
但这个例子没有引号（即mysort（<）不是我的排序（<），所以这是我的第一个问题。这个函数，但它不是编译。否则是forWordWord 
  checkCondition :: Ordering-> Int 
 checkCondition ord 
 | ord ==（<）= 1 
 | ord ==（>）= 2 
 |否则= 2 
  
其次，我仍然难以理解高阶函数，这是否合理？ 
 
 
  mySort i列表
 | i == 1 map（sortBy compare）list 
 | i == 2 map（sortBy（flip compare））list 
  
 
 
解决方案
你不应该匹配这些功能sp特别地，它首先破坏了使用高阶函数的目的。事实上，你不能这样写，因为没有比较函数的一般方法。
 
 
 
相反，直接使用传递的函数进行排序。这样，它将适用于任何适合的比较功能，而不仅仅是您明确写入代码的功能。 
 
 
 
例如，假设任务是使用传递的运算符合并两个值： 
 
 
  combine（+）2 3 = 5 
 combine（*）3 5 = 15 
 combine max 10 100 = 100 
  
您可以像这样解决： 
 
 
  combine op xy = x `op` y 
  
您可以使用类似的方法解决排序问题吗？
 
 
 
提示：您可能需要定义一个辅助函数来将传递的比较函数转换为适合 sortBy 的形式：
  compareUsing ::（a  - > a  - > Bool） - > （a  - > a  - >订购）
 compareUsing op x y = ... 
  
 
Hi I'm a Haskell beginner and I'm really lost.
This is for my assignment, and it asks me to do something like below using higer order function
Main> mySort (<) [1,5,3,6,4,1,3,3,2] 
[1,1,2,3,3,3,4,5,6] 
Main> mySort (>) [1,5,3,6,4,1,3,3,2] 
[6,5,4,3,3,3,2,1,1] 
Main> mySort longerWord ["Hello", "The", "a", "Daniel", "Declarative"]
["Declarative", "Daniel", "Hello", "The", "a"]
First of all, I thought I should make a function that distinguish whether it's < , > or longerWord
checkConditionStr::String->Int
checkConditionStr str
    |str=="(<)" =1
    |str=="(>)" =2
    |str=="longerWord" =3
but the example doesn't have quotation mark (i.e. mysort (<) not my sort"(<)" so here is my first problem. I worte this function but it's not compiling. otherwise is for longerWord
checkCondition::Ordering->Int
checkCondition ord
    |ord==(<) =1
    |ord==(>) =2
    |otherwise =2
secondly I still have difficulty understanding higher order function. would this make sense?
mySort::(String->Int)->[a]->[a]
mySort i list
    |i==1 map (sortBy compare) list
    |i==2 map (sortBy(flip compare)) list

解决方案
You're not supposed to match against those functions specifically. It defeats the purpose of using a higher-order function in the first place. In fact, you can't write it like this, since there is no general way of comparing functions.

Instead, use the passed function directly for the sorting. That way, it will work for any suitable comparison function, not just the ones you've explicitly written code for. 

For example, imagine the task was to combine two values using a passed operator:
combine (+) 2 3 = 5
combine (*) 3 5 = 15
combine max 10 100 = 100
You would solve it like this:
combine op x y = x `op` y
Can you use a similar approach to solving the sorting problem?

Hint: You may want to define a helper function to transform the passed comparison function into a form suitable for sortBy:
compareUsing :: (a -> a -> Bool) -> (a -> a -> Ordering)
compareUsing op x y = ...


                        
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