Can I use a list of functions, as input, for a different function in Haskell?

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.

To take an item from a list you need to use !!. Your first function would be written like testfunction function input = (function !! 0) (input !! 1) but generally manually taking items from lists is a bad idea, you should pattern match or loop through the list — Zpalmtree
– Zpalmtree, Commented Nov 18, 2017 at 22:24
why are you using list in this way... i never had the need to index a list, so what are you trying to do here... — Jason Hu
– Jason Hu, Commented Nov 18, 2017 at 22:25
Ah yes, @Zpalmtree you're right. I completely forgot that to call an nth index form a list you use !!. That solves the error. HuStmpHrrr I was trying to create some AI "players" and I needed to use the list of players in a functioni that would calculate the next state of the program. — s4342re41
– s4342re41, Commented Nov 18, 2017 at 22:33
What result exactly you want test_function [add10, subtract10] [3,5,7,9] to return? — Redu
– Redu, Commented Nov 19, 2017 at 16:37

s4342re41 · Accepted Answer · 2017-11-18 22:36:25Z

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])

answered Nov 18, 2017 at 22:36

s4342re41

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1 Comment

Will Ness Over a year ago

or (function <*> input) !! (0*4 + 1).

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