[英]Can I use a list of functions, as input, for a different function in Haskell?
我正在努力弄清楚如何制作一个将函数列表作为输入来生成某些输出的函数。 例如,假设我像这样创建了一个名为Func10的类型同义词:
type Func10 = Int -> Int
我可以创建一组将值加,减,除或乘以10的函数,如下所示:
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
现在,假设我要创建一个函数,该函数将包含4个值的列表以及要应用于它们的函数的列表:add10,subtract10,multipli10和divid10。
最初我以为可以在需要时为列表中的函数输入整数参数,如下所示:
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])
这导致以下错误:
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])
| ^^^^^^^^
我认为这可能是因为test_function需要整数输入作为调用test_function的一部分,如下所示:
test_function :: [Func10] -> Int
test_function function = function[0]
main = do
print("print 5 add 10")
print(test_function ([add10, subtract10] 5))
但是,这导致了类似的错误:
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))
| ^^^^^^^^^^^^^^^^^^^^^
正确的做法是什么? 我试图列出函数并将它们应用于其他一些值,但是很难在线找到有关此主题的信息。 谢谢。
就像Zpalmtree在评论中说的那样,该错误未使用!! 调用索引。 编写代码的正确方法是像这样将其写出:
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])
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