我可以在Haskell中使用一系列函数作为输入吗？

Question

I am struggling to figure out how to make a function that takes a list of functions as input to generate some output. For example, let's say that I make a type synonym called Func10 like so:

type Func10 = Int -> Int

I can create a set of functions that add, subtract, divide or multiply a value to 10 like so:

add10 :: Func10
add10 input = 10 + input

subtract10 :: Func10
subtract10 input = 10 - input

times10 :: Func10
times10 input = 10 * input

divide10 :: Func10
divide10 input = 10 `div` input

Now let's say that I want to make a function that will take a list of 4 values and a list of functions that I want to apply to them: add10, subtract10, multiply10 and divide10.

Initially I thought that I could give the function in the list input the integer argument when I need it, like so:

test_function :: [Func10] -> [Int] -> Int
test_function function input = function[0] (input[1])

main = do
    print("print 5 add 10")
    print(test_function [add10, subtract10] [3,5,7,9])

This resulted in the following error:

function_lists2.hs:17:32: error:
    * Couldn't match expected type `[Integer] -> t0 -> Int'
                  with actual type `[Func10]'
    * The function `function' is applied to two arguments,
      but its type `[Func10]' has none
      In the expression: function [0] (input [1])
      In an equation for `test_function':
          test_function function input = function [0] (input [1])
   |
17 | test_function function input = function[0] (input[1])
   |                                ^^^^^^^^^^^^^^^^^^^^^^

function_lists2.hs:17:45: error:
    * Couldn't match expected type `[Integer] -> t0'
                  with actual type `[Int]'
    * The function `input' is applied to one argument,
      but its type `[Int]' has none
      In the second argument of `function', namely `(input [1])'
      In the expression: function [0] (input [1])
   |
17 | test_function function input = function[0] (input[1])
   |                                             ^^^^^^^^

I thought that this could be because test_function needs the integer input as part of calling test_function, like so:

test_function :: [Func10] -> Int
test_function function = function[0]

main = do
    print("print 5 add 10")
    print(test_function ([add10, subtract10] 5))

However, this resulted in a similar error:

function_lists2.hs:26:26: error:
    * Couldn't match expected type `[Integer] -> Int'
                  with actual type `[Func10]'
    * The function `function' is applied to one argument,
      but its type `[Func10]' has none
      In the expression: function [0]
      In an equation for `test_function':
          test_function function = function [0]
   |
26 | test_function function = function[0]
   |                          ^^^^^^^^^^^

function_lists2.hs:30:30: error:
    * Couldn't match expected type `Integer -> [Func10]'
                  with actual type `[Func10]'
    * The function `[add10, subtract10]' is applied to one argument,
      but its type `[Func10]' has none
      In the first argument of `test_function', namely
        `([add10, subtract10] 5)'
      In the first argument of `print', namely
        `(test_function ([add10, subtract10] 5))'
   |
30 |         print(test_function ([add10, subtract10] 5))
   |                              ^^^^^^^^^^^^^^^^^^^^^

What would be the correct way of doing this? I am trying to take a list of functions and apply them to some other values but it's hard to find information on this topic online. Thanks.

Answer 1

Like Zpalmtree said in the comments, the error was not using !! to call indexes. The correct way to write the code was to have it written out like so:

test_function :: [Func10] -> [Int] -> Int
test_function function input = (function !! 0) (input !! 1)

main = do
    print("print 5 add 10")
    print(test_function [add10, subtract10] [3,5,7,9])